May/June 2016

ACS International School Hillingdon

Semester II Chemistry Study Guide

This study guide includes topics:

States of matter Atoms Moles Chemical bonding Different types of reactions Acids and bases Organic chemistry Rates of Reactions

In addition, please also review from your notes, textbook, assessments and resources on myHomework and Google drive.

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1. STATES OF MATTER

The particles in solids, liquids and gases have different amounts of energy. They are arranged differently and move in different ways. The table below summarises the arrangement and movement of the particles in solids, liquids and gases.

Solids have fixed shape and fixed volume.

Liquids have fixed volume but no fixed shape.

Gases have no fixed shape or volume. Gases are the only state if matter that can be compressed.

Separating solids from liquids – filtration

If a substance does not dissolve in a solvent, we say that it is insoluble. For example, sand does not dissolve in water – it is insoluble. Filtration is a method for separating an insoluble solid from a liquid. When a mixture of sand and water is filtered:

the sand stays behind in the filter paper (it becomes the residue) the water passes through the filter paper (it becomes the filtrate)

Separating solids from liquids – evaporation

Evaporation is used to separate a soluble solid from a liquid. For example, copper sulfate is soluble in water – its crystals dissolve in water to form copper sulfate solution. During evaporation, the water evaporates away leaving solid copper sulfate crystals behind.

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A solution is placed in an evaporating basin and heated with a Bunsen burner.

Separating the solvent from a solution – simple distillation

Simple distillation is a method for separating the solvent from a solution. For example, water can be separated from salt solution by simple distillation. This method works because water has a much lower boiling point than salt. When the solution is heated, the water evaporates. It is then cooled and condensed into a separate container. The salt does not evaporate and so it stays behind.

Salt solution is heated.

Every pure substance has its own particular melting point and boiling point. One way to check the purity of the separated liquid is to measure its boiling point. For example, pure water boils at 100°C. If it contains any dissolved solids, its boiling point will be higher than this.

Separating a liquid from a mixture – fractional distillation

Fractional distillation is a method for separating a liquid from a mixture of two or more liquids. For example, liquid ethanol can be separated from a mixture of ethanol and water by fractional distillation.

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May/June 2016A water and ethanol mixture is heated in a flask using an electric heater. Vapour forms in the air above the mixture in the flask. One way to check the purity of the separated liquids is to measure their boiling points. For example, pure ethanol boils at 78°C and pure water boils at 100°C.

Separating dissolved solids – chromatography

Paper chromatography is a method for separating dissolved substances from one another. It is often used when the dissolved substances are coloured, such as inks, food colourings and plant dyes. It works because some of the coloured substances dissolve in the solvent used better than others, so they travel further up the paper.

A pencil line is drawn, and spots of ink or plant dye are placed on it. There is a container of solvent, such as water or ethanol.

A pure substance will only produce one spot on the chromatogram during paper chromatography. Two substances will be the same if they produce the same colour of spot, and their spots travel the same distance up the paper. In the example below, red, blue and yellow are three pure substances. The sample on the left is a mixture of all three.

2. ATOMS IN THE PERIODIC TABLEThere are more than 100 different elements. The periodic table is a chart showing all the elements arranged in a particular way. The vertical columns in the periodic table are called groups. Each group contains elements that have similar properties.

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Similar chemical properties to other elements in the same column - in other words similar chemical reactions. Magnesium, for example, is placed in the alkali earths' column:

Elements in the same group have similar chemical properties because they have the same number of valence electrons. Valence electrons are used in forming bonds to other atoms. The group number represents the number of valence electrons.

You can use either of the periodic table given above and below.

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The periodic table has eight main groups. For example, group 1 contains very reactive metals such as sodium - Na - while group 7 contains very reactive non-metals such as chlorine - Cl.

Fowling groups also named as:

Group 1 Alkali metals Group 2 Alkaline Earth metals Group 7 Halogens Group 0 (8) Noble gases Transition metals (Elements between group 2 and group 3)

Note that you will never find a compound in the periodic table, because these consist of two or more different elements joined together by chemical bonds.

Semi metals (metalloids) Elements that have properties of metals and non-metals.

Semi-metals

Common properties of the alkali metals (Group 1)

The alkali metals have the following properties in common:

they have low melting and boiling points compared to most other metals

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May/June 2016 they are very soft and can be cut easily with a knife they have low densities (lithium, sodium and potassium will float on water) they react quickly with water, producing hydroxides and hydrogen gas their hydroxides and oxides dissolve in water to form alkaline solutions.

Common properties of the Halogens (Group 7)

The halogens have the following properties in common:

they are non-metals they have low melting and boiling points they are brittle when solid they are poor conductors of heat and electricity they have coloured vapours their molecules each contain two atoms (they are diatomic)

The noble gases - Group 0

Common propertiesThe noble gases have the following properties in common:

they are non-metals they are very unreactive gases they are colourless they exist as single atoms (they are monatomic)

Sub-atomic particles

Each atom consists of a nucleus containing protons and neutrons, with electrons arranged around it in energy levels. (shells). Protons, electrons and neutrons are called sub-atomic particles.

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Mass number and atomic number

The mass number of an atom is the total number of protons and neutrons it contains The atomic number of an atom is the number of protons it contains

Notice that most of the mass of an atom is found in the nucleus.For example

This symbol tells you that the chlorine atom has 17 protons.

It will also have 17 electrons, because the number of protons and electrons in an atom is the same. The symbol also tells you that the total number of protons and neutrons in the chlorine atom is 35.

Working out number of neutrons

Note that you can work out the number of neutrons from the mass number and atomic number. In this example, it is 35 – 17 = 18 neutrons.

Electrons are arranged in shells at different distances around the nucleus.

Maximum number of electrons

1stshell 22nd shell 83rd shell 8Then the fourth shell begins to fill.

Isotopes

Isotopes are atoms of an element with the same number of protons and electrons, but different numbers of neutrons. Isotopes have the same atomic number, but different mass numbers.

The different isotopes of an element have identical chemical properties. However, some isotopes are radioactive.

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ExamplesIsotopes of hydrogenMost hydrogen atoms consist of just one proton and one electron, but some also have one or two neutrons.

Radioactive Isotopes

Radioactive isotope means its nucleus is unstable. Sooner or later the atom breaks down naturally or decays, giving out radiation in the form of rays and particles, plus a large amount of energy.

Uses of radioactivity

Tracers

Radioisotopes are used as tracers in industry and hospitals. They're used to find out what is happening inside objects without the need to break into the object.

In industry they can be used to:

find leaks or blockages in underground pipes find the route of underground pipes track the dispersal of waste

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May/June 2016To locate a leak in an underground pipe a very small amount of radioactive material that gives off gamma rays is put into the pipe. A detector is moved along the ground above the pipe. The leak is located where there's an increase in activity and little or no activity after that point.

Diatomic Elements:All these elements exist as diatomic molecules:I2, H2, N2, Cl2, O2, Br2, F2

I Have No Clever Or Bright Friends

Relative Atomic Mass Symbol Ar

The mass of an atom compared with the carbon-12 atom is called its relative atomic mass. The smaller r stands for relative to the mass of a carbon-12 atom.For example: Ar of hydrogen is 1 and the Ar of magnesium is 24.

The relative atomic mass calculationRelative atomic mass (Ar) of an element = % abundance x atomic mass for the first isotope + % abudance x atomic mass of second isotope and so on, for all its natural isotopes ÷ 100. For example chlorine has two isotopes 35Cl and 37Cl% abundance

75 35Cl25 37Cl

Relative atomic mass of chlorine = 0.75 x 35 + 0.25 x 37 = 35.5

Relative atomic masses of some elements are given below.

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May/June 2016The abundance of isotopes calculations

The atomic weight of Thallium is 204.3833 amu. The masses for the two stable isotopes are 202.9723amu for thallium-203 and 204.9744amu for thallium-205. Calculate the percent abundance of each isotope.

Solution:Let x = % of thallium-203 1-x = % of thallium-205

204.3833 = [(1-x)( 204.9744 ) + (x)( 202.9723 )]

204.3833 = 204.9744 - 204.9744x + 202.9723x-204.9744 = -204.9744

-0.5911 = -2.0021x -2.0021 -2.0021

0.2952 = x

X = .2952 x 100 = 29.52% for thallium-2031-x = 100 – 29.52 = 70.476 % for thallium-205

Working out Molecular mass (Mr)

To find the relative molecular mass of a substance, you just add together the relative atomic mass values for all the atoms in its formula. Here are three examples:

Example 1

Find the Mr of carbon monoxide, CO Mr = 12 + 16 = 28

Example 2

Find the Mr of sodium oxide, Na2O Mr = (23 × 2) + 16 = 46 + 16 = 62

Example 3

Find the Mr of magnesium hydroxide, Mg(OH)2

Mr = 24 + 2 × (16+1) = 24 + 34 = 58 (Remember that there are two of each atom inside the brackets)

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May/June 20163. MOLES:

1. Moles and Particles:

a. Avogadro's number is a very important relationship to remember: 1 mole = atoms, molecules, protons, etc.

b. To convert from moles to atoms, multiply the molar amount by Avogadro's number.c. To convert from atoms to moles, divide the atom amount by Avogadro's number (or multiply

by its reciprocal

1 mole contains Avogadro’s number of particles which is 6.02 x 1023

N=Lxn

Number of particles, N, equals the number of moles, n, times Avogadro’s number, L.

Similarly, n=N/L

Example:

How many atoms of silver are found in 0.1 mole?

Answer:

N= Lxn

N=6.02 x 1023x0.1= 6.02 x 1022 atoms

Example:

How many moles of gold contain 3.01x1023 atoms?

Answer:

n=N/L

n=3.01x1023 / (6.02 x 1023) = 0.5 mole

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May/June 20162. Moles and Mass

Molar mass

Relative formula mass, Mr

To find the relative formula mass (or Mr) of a substance, you add together the relative atomic mass 

Example 1

What is the relative formula mass of water, H2O?

(Ar of H = 1, Ar of O = 16)

Mr of H2O = 1 + 1 + 16 = 18

Example 2

What is the relative formula mass of calcium hydroxide, Ca(OH)2?

(Ar of Ca = 40, Ar of O = 16, Ar of H = 1)

Mr of Ca(OH)2 = 40 + 16 + 1 + 16 + 1 = 74

The mole

The unit for amount of substance is called the mole, shown as mol.

Conservation of mass

Mass is never lost or gained in chemical reactions. We say that mass is always conserved. In other words, the total mass of products at the end of the reaction is equal to the total mass of the reactants   at the beginning.

One mole of any element weighs the equivalent of the atomic mass but in grams. Example: if the relative atomic mass of sodium is 23 then the mass of 1 mole of sodium atoms is 23g. So the molar mass of sodium is 23g/mol. Similarly for compounds, the molar mass of water is 18g/mol.

m=nxM which means that the mass, m, is equal to the number of moles, n, times the molar mass, M.

Example:

The mass of 2 moles of sodium equals:

Answer:

m=nxM

m=2x23=46g

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May/June 2016The number of moles can be calculated as follows:

n=m/M

Example:

How many moles of water weigh 180g?

Answer:

n=m/M

n=180/18=10 moles

To change from mass to number of particles or vice versa, you have to convert to moles first.

Example:

How many atoms of sodium are found in 2.3g of sodium?

Answer:

n=m/M

n=2.3/23=0.1mole

N=nxL

N=0.1x6.02 x 1023 = 6.02 x 1022 atoms

3. Moles, Mass and Particles

Example:

What is the mass of 1.806x1022 molecules of H2 ?

Answer:

n=N/L

n= 1.806x1022 / (6.02 x 1023 )= 0.03 mole

m=nxM

m=0.03x2=0.06g

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May/June 2016Word and balanced equations

Chemical equations show what happens in a reaction. In general, we write:

reactants → products

The reactants are the substances that react together. The products are the substances produced in the reaction. Individual substances are separated by a plus sign.

Word equations

A word equation gives the names of the substances involved in a reaction. For example:

copper + oxygen → copper(II) oxide

Copper and oxygen are the reactants, and copper(II) oxide is the product.

Balanced equations

Balanced equations give the symbols and formulas of the substances involved in a reaction. In the example above, if we just replace the words shown above with the correct chemical formulas. This is called formula equation or symbol equation. We will get an unbalanced equation, as shown here:

Cu + O2    →    CuO

To make things equal, we need to adjust the number of units of some of the substances until we get equal numbers of each type of atom on both sides of the arrow.

Here is the balanced symbol equation:

2Cu + O2    →    2CuO

You can see that we now have two copper atoms and two oxygen atoms on each side. This matches what happens in the reaction.

Two atoms of copper react with two atoms of oxygen to form two molecules of copper oxide

Here are some other examples of balanced equations. Check that you understand why they are balanced.

Mg + Cl2    →    MgCl2

2Na + Cl2    →    2NaCl 4Fe + 3O2    →    2Fe2O3

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May/June 2016 4Na + O2    →    2Na2O 2Na + 2H2O    →    2NaOH + H2.

2Al + 6HNO3 → 2Al (NO3)3 + 3H2

4. CHEMICAL BONDING

a. Ionic Bonding:

It is defined as the electrostatic attraction between oppositely charged ions.

Positive and negative ions

Ions are electrically charged particles formed when atoms lose or gain electrons. They have the same electronic structures as noble gases.

Metal atoms form positive ions, while non-metal atoms form negative ions. The strong electrostatic forces of attraction between oppositely charged ions are called ionic bonds.

How ions form

Ions are electrically charged particles formed when atoms lose or gain electrons. This loss or gain leaves a complete highest energy level, so the electronic structure of an ion is the same as that of a noble gas - such as a helium, neon or argon.

Metal atoms and non-metal atoms go in opposite directions when they ionise:

Metal atoms lose the electron, or electrons, in their highest energy level and become positively charged ions ( For example, sodium ion Na+)

Non-metal atoms gain an electron, or electrons, from another atom to become negatively charged ions ( CHLORIDE ION, Cl-)

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May/June 2016Representing positive ions

You need to be able to show the electronic structure of some common metal ions, using diagrams like these:

Lithium, Li

Lithium is in Group 1. It has one electron in its highest energy level. When this electron is lost, a lithium ion Li+ is formed.

Sodium, Na

Sodium is also in Group 1. It has one electron in its highest energy level. When this electron is lost, a sodium ion Na+ is formed.

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May/June 2016Magnesium, Mg

Magnesium is in Group 2. It has two electrons in its highest energy level. When these electrons are lost, a magnesium ion Mg2+ is formed.

A magnesium ion has the same electronic structure as a neon atom (Ne).

Representing negative ions

You need to be able to show the electronic structure of some common non-metal ions, using diagrams like these:

Fluorine, F

Fluorine is in Group 7. It has seven electrons in its highest energy level. It gains an electron from another atom in reactions, forming a fluoride ion, F-.

Note that the atom is called fluorine, but the ion is called fluoride.

Neon atom

Note that a fluoride ion has the same electronic structure as a neon atom (Ne).

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Oxygen, O

Oxygen is in Group 6. It has six electrons in its highest energy level. It gains two electrons from one or two other atoms in reactions, forming an oxide ion, O2-.

How many charges?

There is a quick way to work out what the charge on an ion should be:

The number of charges on an ion formed by a metal is equal to the group number of the metal

The number of charges on an ion formed by a non-metal is equal to the group number minus eight

Hydrogen forms H+ ions.

When metals react with non-metals, electrons are transferred from the metal atoms to the non-metal atoms, forming ions. The resulting compound is called an ionic compound.

Consider reactions between metals and non-metals, for example:

sodium + chlorine → sodium chloride magnesium + oxygen → magnesium oxide calcium + chlorine → calcium chloride

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For example; Ionic bonding in sodium chloride

Ionic bonding in magnesium oxide

Magnesium ions have the formula Mg2+, while oxide ions have the formula O2-. You need to show one magnesium ion and one oxide ion.

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May/June 2016Ionic bonding in calcium chloride

Calcium ions have the formula Ca2+. Chloride ions have the formula Cl-.

You need to show two chloride ions, because two chloride ions are needed to balance the charge on a calcium ion.

Polyatomic ions

Name FormulaHydroxide OH-

Nitrate NO3-

Hydrogencarbonate HCO3-

Carbonate CO32-

Sulphate SO42-

Phosphate PO43-

Ammonium NH4+

Example:

What is the formula of magnesium phosphate?

Answer:

Mg PO43-

2 3

Crisscross: Mg3(PO4)2

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May/June 2016Naming:

The cation is named first then the anion. The anions made of one atom only lose the last syllable and get an –ide instead.

So:

Oxygen becomes oxide

Nitrogen becomes nitride

Phosphorus becomes phosphide

Chlorine become chloride

Sulphur becomes sulphide

And so on.

Example: AlP is called sodium phosphide

If the compound contains a transition metal, you have to specify the charge of the transition metal and you can get it like this:

CuCl

Cl has a charge of – 1 since it is in group 7 so this means that the charge on the Cu is +1 since the whole compound must be neutral. So this compound is called copper (I) chloride.

CuCl2

In this case, the Cu has a charge of 2+ since there are 2 Cl ions each with a charge of -1 so this compound is called copper (II) chloride.

You need to know the charges of these common transition metals:

Zn2+

Ag+

Fe2+

Fe3+

Cu2+

Cu+

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May/June 2016Lattice structure

Example: Sodium chloride

Lattice structure of sodium chloride

Sodium chloride, NaCl, forms when sodium and chlorine react together. It contains oppositely charged ions held together by strong electrostatic forces of attraction – the ionic bonds. The ions form a regular lattice in which the ionic bonds act in all directions. Ionic compounds are giant.

b. Covalent BondingA covalent bond is a strong bond between two non-metal atoms. It consists of a shared pair of electrons. A covalent bond can be represented by a straight line or dot-and-cross diagram. It is defined as the electrostatic attraction between the 2 nuclei and the shared electrons.

Hydrogen and chlorine can each form one covalent bond, oxygen two bonds, nitrogen three, while carbon can form four bonds.

For example; Single covalent bondCovalent bonding between two hydrogen atoms to form a molecule of hydrogen gas, H2.

One electron pair (two electrons) share between tow hydrogen atoms.

Double covalent bondCovalent bonding between two oxygen atoms to form a molecule of oxygen gas, O2.

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Two electron pairs (four electrons) shared between tow oxygen atoms.

Triple covalent bond

Three electron pairs (six electrons) shared between two nitrogen atoms.

Lewis Structures:

Rules:1. Count the number of valence electrons found in each atom and add them up.2. The total number of valence electrons must be an even number.3. The odd atom in the formula is always in the middle.4. Hydrogen is never in the middle since it has 1 electron only so it can form one single bond only.5. Distribute the electrons around the atoms in such a way that if an atom needs 2 electrons to

have a complete octet then it shares 2 electrons to get 2 more. If it needs 3 electrons then it shares 3 and so on.

Examples:

H2O: 1+1+6=8e

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Naming of covalent compounds: We used the following prefixes for covalent compounds only:

NO is nitrogen monoxide. Note that when the first element is only one, we do not say “mono”. It is understood by convention.P4O6 is called tertaphosphorus hexoxide (we drop the “a” at the end of hexa since it is followed by another vowel).

MacromoleculesMacromolecules have giant covalent structures. They contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds. Their atoms are arranged into giant lattices, which are strong structures because of the many bonds involved.

Substances with giant covalent structures have very high melting points, because a lot of strong covalent bonds must be broken. Graphite, for example, has a melting point of more than 3,600°C.

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1. Diamond

Diamond

Diamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure in a tetrahedral arrangement where the bond angle is 109.5o. As a result,

Diamond is very hard Has a high melting point. It does not conduct electricity. Diamond is 3-D Used in jewellery and in making the blades of cutting tools

2. Graphite

Graphite is a form of carbon in which the carbon atoms form layers. Each carbon atom in a layer is joined to only three other carbon atoms. The layers can slide over each other because there are no covalent bonds between them. The layers are heald together by weak London Dispersion forces. This makes graphite much softer than diamond. It is used in pencils and as a lubricant. Graphite conducts electricity. It is 2-D. The bond angle is 120o.

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3. Graphene:

It is one layer of graphite. It is 2D. The bond angle is 120o . It is used in coating mobile phone screens.

All the previous molecules are giant since there is not specific number of atoms that exists in a molecule. It depends how big the piece is.

4. Buckminsterfullerene (NOT A MACROMOLECULE-NOT GIANT)

It is not giant since it has 60 carbons per molecule! It is 3-D. The bond angle is between 109.5-120o. It has the shape of a truncated icosahedron with 20 hexagons and 12 pentagons. Bucky balls are used in introducing medicines inside the human body.

These 4 structures are the allotropes of carbon.

There are other giant structures like:

Silicon

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May/June 2016Each silicon atom is bonded to 4 other atoms in a tetrahedral arrangement where the bond angle is 109.5o

And silicon dioxide which is the main component of sand

where each silicon atom (grey) is covalently bonded to 4 oxygen atoms (red) and each oxygen atom is bonded to 2 silicon atoms.

3. Metallic Bonding:

Metals structure

Metals form giant structures in which electrons in the outer shells of the metal atoms are free to move. The metallic bond is the force of attraction between these free delocalized electrons and the lattice of positive metal ions.

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May/June 2016Metallic bonds are strong, so metals can maintain a regular structure and usually have high melting and boiling points. Layers of atoms slide over each other when metals are bent or stretched

Metals are malleable - they can be bent and shaped. This is because they consist of layers of atoms. These layers can slide over one another when the metal is bent, hammered or pressed without breaking the bonds.

Metals are ductile – they can be drawn out into wires. This is because the layers can slide over each other. The layers can slide over without the metallic bond breaking, because the electrons are free to move.

Metals are good conductors of heat

This is because the free electrons can move throughout the metal. They can take in heat energy, which makes them move faster. They quickly transfer the heat through the metal structure.

Metals are good conductors of electricity

Electrons in the metal lattice are free to move and can carry charge across the metal lattice. Silver is the best conductor of all the metals.

The reactivity series

Some metals are very unreactive. This means they do not easily take part in chemical reactions. For example, platinum does not react with oxygen in the air, even if it is heated in a Bunsen burner flame.

Some metals are very reactive. They easily take part in chemical reactions to make new substances.

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Magnesium is very reactive. It ignites when heated and burns with a brilliant white flame.

Other metals may be more reactive than magnesium, or in between magnesium and platinum. If we put the metals in order of their reactivity, from the most reactive down to the least reactive, we get a list called the reactivity series.

The reactivity series for some common metals

Reactions of acids with metals

Acids react with most metals and, when they do, a salt is produced. But unlike the reaction between acids and bases, we do not get water. Instead we get hydrogen gas.

This is the general word equation for the reaction:

metal + acid → salt + hydrogen

For example, magnesium reacts with hydrochloric acid to produce magnesium chloride:

magnesium + hydrochloric acid → magnesium chloride + hydrogen

Mg + 2HCl → MgCl2 + H2

It doesn't matter which metal or which acid is used, if there is a reaction we always get hydrogen gas as well as the salt. However, how quickly the reaction goes depends on the metal used and how high up in the reactivity series it is.

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The reaction of a metal with acids gets faster the more reactive it is. Group one metals are too reactive so we should not use them. Copper, silver, gold and platinum are unreactive so they will not react in the first place.

The lab test for hydrogen

There is a simple laboratory test to see if a gas is hydrogen. A burning wooden splint goes pop if it is put into a test tube of hydrogen. This is because the flame ignites the hydrogen, which burns explosively to make a loud sound.

Bonding and Physical Properties:

Ionic compounds conduct electricity only in molten or aqueous states because then they will have free ions to carry the charges. They do not conduct electricity in the solid state because the ions are not free to carry the charge.

Metals conduct electricity in the solid and molten states due to the sea of delocalized electrons.

Covalent substances do not conduct electricity since they do not have mobile ions or electrons to carry the charge.

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May/June 2016Physical changes

This type of change means that no new substances are made, but there is a change in the appearance of a chemical. Examples of physical change include state changes and dissolving.

State changes

All substances can be solid, liquid or gas.

In this process, water (hydrogen oxide) has not been chemically changed, but by cooling or heating it, you can change its state.

Chemical changes

Chemical reactions usually involve a change in appearance (e.g. colour) or a detectable energy change (e.g. involving heat, light or sound).

All chemical reactions involve the formation of one or more new substances.

Change in appearance

Rusting involves shiny, grey coloured iron becoming red and crumbly iron oxide

Another example of change in appearance is seen when adding iodine solution to starch. This will cause a reaction which changes the iodine solution from orange-brown to blue-black.

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May/June 20165. TYPES OF CHEMICAL REACTIONS (see video on myHomeworK)

6. ACIDS AND BASES:

Acids, bases, alkalis and metals are found in the laboratory and at home. They can be irritant or corrosive and must be handled carefully.

Acids in the laboratory

Dilute acids

You will have used some dilute acids at school, such as hydrochloric acid, sulphuric acid and nitric acid. Their bottles are labelled with the warning symbol for 'irritant'.

This means that if any of them makes contact with your skin, it will become red or blistered. You must wash off any spills with plenty of water, otherwise your skin will soon feel as if it is burning.

Concentrated acids

You are unlikely to have used concentrated acids but your teacher might have shown you some experiments with them. This is because concentrated acids are corrosive. They can attack metals and destroy skin if spilled.

Acids at home

Laboratory acids are far too dangerous to taste, but you will have swallowed some dilute weak acids. Acids have a sour taste, like vinegar, which contains ethanoic acid, and lemons, which contain citric acid. These are safe to use in food, but they can still hurt if they get into a cut or into your eyes.

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May/June 2016Other acids you will find at home are carbonic acid in fizzy drinks, tannic acid in tea and ascorbic acid which is vitamin C, found in fruit and vegetables.

Bases v alkalis

Bases are substances that react with acids and neutralise them. They are usually metal oxides, metal hydroxides, metal carbonates or metal hydrogen carbonates. Many bases are insoluble - they do not dissolve in water.

If a base does dissolve in water, we call it an alkali.

Here are two examples:

Copper oxide is a base because it will react with acids and neutralise them, but it is not an alkali because it does not dissolve in water.

Sodium hydroxide is a base because it will react with acids and neutralise them. It's also an alkali because it dissolves in water.

Bases at home

Bases react with oils and fats, so they are often used in strong household cleaners. Drain cleaners and oven cleaners usually contain sodium hydroxide for example. And ammonia is also commonly used in cleaners. Ammonia can be recognised by its choking smell.

Indicators and the pH scale

When an acid is dissolved in water we get an acidic solution, and alkalis make alkaline solutions. If a solution is neither acidic nor alkaline we call it neutral. Pure water is neutral, and so is paraffin.

Indicators are substances that change colour when they are added to acidic or alkaline solutions. You can prepare homemade indicators from red cabbage or beetroot juice - these will help you see if a solution is acidic or alkaline.

Litmus and universal indicator are two indicators that are commonly used in the laboratory.

Litmus

Litmus indicator solution turns red in acidic solutions and blue in alkaline solutions - and it turns purple in neutral solutions.

Litmus paper is usually more reliable, and comes as red litmus paper and blue litmus paper. The table shows the colour changes it can make.

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Notice how we say 'stays red'. This is better than saying 'nothing' or 'stayed the same', because it tells us the colour we actually see.

Alkalis turn red litmus paper blue Acids turn blue litmus paper red

The pH scale

It is possible to tell if a solution is acidic or alkaline by using an indicator. An indicator is a substance which has different colours when it is in acidic or alkaline conditions. Litmus is probably the most well-known indicator. This is red in acids and blue in alkalis. Litmus can be used as a liquid, or as litmus paper.

Solutions of acids and alkalis can vary widely in their acidity and alkalinity. It is useful to know not just whether a solution is an acid or an alkali, but how acidic or how alkaline it is. To measure acidity and alkalinity, we can use the pH scale.

The easiest way to do this is to use Universal indicator. This is a mixture of several different indicators, and can be used as a liquid or paper. It has many different colour changes.

The colour of the Universal indicator shows the pH value of the solution. The pH scale runs from pH 0 to pH 14.

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Diagram of pH scale and universal indicator colours

To get a more accurate measurement of the pH of a solution, we can use a pH meter. This device can measure the pH of a solution to 0.01 of a pH unit.

These are the important points about the pH scale:

neutral solutions are pH 7 exactly acidic solutions have pH values less than 7 alkaline solutions have pH values more than 7 the closer to pH 0 you go, the more strongly acidic a solution is the closer to pH 14 you go, the more strongly alkaline a solution is

Neutralisation:

A chemical reaction happens if you mix together an acid and a base. The reaction is called neutralisation. A neutral solution is made if you add just the right amount of acid and base together. Neutralisation is an exothermic reaction, so the reaction mixture warms up during the reaction.

1. ACID+BASESALT + WATER

Metal oxides

Metal oxides act as bases. Here is the general word equation for what happens in their neutralisation reactions with acids:

metal oxide + acid → a salt + water

The salt made depends on the metal oxide and the acid used. For example, copper chloride is made if copper oxide and hydrochloric acid are used:

copper oxide + hydrochloric acid → copper chloride + water

CuO + 2HCl → CuCl2 + H2O

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May/June 2016Metal hydroxides

Metal hydroxides act as bases. Some of them dissolve in water, so they form alkaline solutions. Here is the general word equation for what happens in their neutralisation reactions with acids:

metal hydroxide + acid → a salt + water

As with metal oxides, the salt made depends on the metal hydroxide and the acid used. For example, sodium sulfate is made if sodium hydroxide and sulfuric acid are used:

sodium hydroxide + sulfuric acid → sodium sulfate + water

2NaOH + H2SO4 → Na2SO4 + 2H2O

Notice that a salt plus water are always produced when metal oxides or metal hydroxides react with acids.

2. ACID + CARBONATE SALT + WATER + CARBON DIOXIDE

Metal carbonates

Most carbonates are usually insoluble (they do not dissolve in water). They also neutralise acids, making a salt and water, but this time we get carbon dioxide gas too.

Here is the general word equation for what happens:

metal carbonate + acid → a salt + water + carbon dioxide

The reaction fizzes as bubbles of carbon dioxide are given off. This is easy to remember because we see the word 'carbonate' in the chemical names. For example, copper carbonate reacts with nitric acid:

copper carbonate + nitric acid → copper nitrate + water + carbon dioxide

CuCO3 + 2HNO3 → Cu(NO3)2 + H2O + CO2

Using neutralisation

Here are some ways neutralisation is used:

Farmers use lime (calcium oxide) to neutralise acid soils. Your stomach contains hydrochloric acid, and too much of this causes indigestion. Antacid

tablets contain bases such as magnesium hydroxide and magnesium carbonate to neutralise the extra acid.

Bee stings are acidic. They can be neutralised using baking powder, which contains sodium hydrogen carbonate.

3. ACID + METAL SALT + HYDROGEN

Mg + 2HCl → MgCl2 + H2

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May/June 2016Acid/Base strength:

Strong acids and bases fully dissociate:

Strong acid: HCl H++Cl-

Weak acid: CH3COOH CH3COO- + H+

Strong base: NaOH Na+ + OH-

Weak base: NH3 + H2O NH4+ + OH-

Strong acids: nitric HNO3, hydrochloric HCl and sulphuric H2SO4

Weak acid: ethanoic CH3COOH

Strong bases: All group 1 hydroxides plus barium and strontium hydroxides

Weak base: ammonis, NH3

Indicators that can distinguish between strong and weak acids and bases: universal indicator and red cabbage

Indicators that distinguish between acids and bases only: litmus

Acids turn blue litmus red and bases turn red litmus blue.

Acids rain:

Rain is naturally acidic with a pH around 5 because carbon dioxide dissolves in rain water forming a weak acid known as carbonic acid.

Acid rain has a pH of less than 5.

For sources of acid rain, see the 2 videos on myHomework.

Acid rain damages the waxy layer on the leaves of trees. This makes it more difficult for trees to absorb the minerals they need for healthy growth and they may die. Acid rain also makes rivers and lakes too acidic for some aquatic life to survive.

To minimize the effects of acid rain, we should shift to greener sources of energy like solar, wind and tidal energies. Also, catalytic converters are used to treat the nitrogen dioxide that causes acid rain before it is released to the atmosphere-see video on myHomework. We can also add weak bases to neutralize soil acidity.

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May/June 2016

Pollution from Burning fossil fuels

Sulfur dioxide and carbon monoxide

Sulfur dioxide and carbon monoxide are two of the gases produced when fossil fuels burn.

Carbon monoxide

Carbon monoxide, CO, is produced when fuels burn in a limited amount of air. It is a colourless, odourless and tasteless gas. Carbon monoxide passes into the red blood cells after breathing it in. It binds more strongly to haemoglobin than oxygen does, so the blood will be able to carry less oxygen than it should. This can cause tiredness, unconsciousness and even death.

Sulfur dioxide

Fossil fuels naturally contain sulfur compounds. These produce sulfur dioxide, a gas with a sharp, choking smell, when the fuel is burned. When sulfur dioxide dissolves in water droplets in clouds, it makes the rain more acidic than normal. This is called acid rain. Power stations give out sulfur dioxide which is thought to be a cause of acid rain.

7. ORGANIC CHEMISTRY: ORGANIC means coming from a living organism or a dead organism.

Hydrocarbons are compounds made from carbon and hydrogen atoms joined by covalent bonds. They include ALKANES, ALKENES and ALKYNES. Alkanes are saturated - they have only single bonds. Alkenes have a double bond and alkynes have a triple bond - they are both unsaturated.

1. Alkanes

General formula: CnH2n+2

They do not have a functional group.

The number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, the molecular formula of methane is CH4. For ethane, it is C2H6.

Alkane molecules can be represented by displayed formulae in which each atom is shown as its symbol (C or H), and the covalent bonds between them by a straight line.

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Naming Alkanes:

1. The substituents are named alphabetically: bromo then chloro then fluoro then iodo then methyl

2. Always start numbering from the side that will give you a lower combination of numbers.3. Numbers must be separated by a comma and numbers and words are separated by a

hyphen.4. The prefixes di- and tri- indicate 2 and 3 respectively.

Eaxmple:

1,3-dibromo-3-fluoro butane

This is a halogenoalkane.

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This is the condensed formula of 1-bromo-2-methylpropane:

CH3CH(CH3)CH2Br

Note that the branching is indicated by a bracket.

The above compounds are halogenoalkanes.

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May/June 2016Alkanes burn in air to produce carbon dioxide and water.

Incomplete combustion:

CH4+ 1.5 O2 CO + 2H2O + energy

CH4+ O2 C + 2H2O + energy

2. Alkenes

General formula: CnH2n

Functional group: C=C

Alkenes are hydrocarbons that contain a carbon-carbon double bond. The number of hydrogen atoms in an alkene is double the number of carbon atoms. For example, the

molecular formula of ethene is C2H4, while for propene it is C3H6.

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3. Alkynes:

General formula: CnH2n-2

Functional group:

Alkenes are hydrocarbons that contain a carbon-carbon triple bond.

4. Alcohols

General formula: CnH2n+1OHThe alcohols are a homologous series or family of organic compounds. They all contain the functional group –OH. This group is responsible for the properties of alcohols.

The names of alcohols end with ‘-ol’ – eg ethanol.

The first three alcohols in the homologous series are methanol, ethanol and propanol. These alcohols are highly flammable, making them useful as fuels. They are also used as solvents in marker pens, medicines, and cosmetics (such as deodorants and perfumes).

Ethanol is the alcohol found in alcoholic drinks such as wine and beer. Ethanol is usually mixed with petrol for use as a fuel

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They burn in the air (complete combustion), releasing energy and producing carbon dioxide and water.

methanol + oxygen → carbon dioxide + water

2CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(l)+energy

Like alkanes, incomplete combustion of alcohols produces carbon monoxide or carbon plus water.

5. Carboxylic acids

General formula: CnH2n+1COOH

Functional group: COOHNote that “n”can be zero here which gives us the first member which is methanoic acid: HCOOH

This is the full structural formula of butanoic acid. This is its condensed formula:

CH3CH2CH2COOH or CH3(CH2)2COOH.

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May/June 20168. RATES OF REACTIONS

Rate means speed. Atoms and molecules react by colliding. There’s a minimum amount of energy needed for each reaction. This is called the activation energy, which is the minimum energy needed for a reaction to occur. Not all collisions will lead to a reaction. Only effective collisions will. The effective collisions which will bring about a reaction must:

1. Provide enough energy (energy equal to or greater than the activation energy)2. Be in correct orientation3. Be frequent enough

The factors which affect the rate of reaction include:

1. concentration (solutions only) 2. temperature3. pressure (gases only)4. surface area or particle size (solids only)5. catalyst (a substance that speeds up the reaction by lowering the activation energy. The

catalyst doesn’t take part in the reaction. It is not consumed in the reaction).

See videos on myHomework for the explanation of the effects of concentration, temperature, catalyst and pressure.

Particle size:

A crushed sugar cube will dissolve faster in water than a whole sugar cube since surface area increases (particle size decreases) when it is crushed and this will lead to a higher frequency of effective collisions.

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