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Topic 3 – Covalent Compounds Topic 3 – Covalent Compounds and Separation Techniquesand Separation Techniques

Starter: What is ionic bonding?Starter: What is ionic bonding?

• Water• Methane• Carbon dioxide• Oxygen • Hydrogen chloride

EXT: How do you know these?

What are the formulae for What are the formulae for the following molecules?the following molecules?

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Introduction to Bonding Introduction to Bonding

Hi. My name’s Johnny Chlorine. I’m in Group 7, so I have 7 electrons in my outer shell

I’d quite like to have a full outer shell. To do this I need to GAIN an electron. Who can help me?

Cl

Cl

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Covalent BondingCovalent BondingHere comes another one of my

friends, Harry Hydrogen

Hey Johnny. I’ve only got one electron but it’s really close to my nucleus so I don’t want to

lose it. Fancy sharing?

Cl

H

Cl

H

Now we’re both really stable. We’ve formed a covalent bond.

The “valence” electrons in an atom are the ones on the outer shell.

“Co” is a prefix that means joint, with, accompanying.

Covalent Bonds are those that involve the sharing of outer electrons. It occurs when non-metals bond with other non-metals only.

OxygenHydrogen

The “valence” electrons in an atom are the ones on the outer shell.

“Co” is a prefix that means joint, with, accompanying.

Covalent Bonds are those that involve the sharing of outer electrons. It occurs when non-metals bond with other non-metals only.

OxygenHydrogen

Covalent Bond

Hydrogen

H2O

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Covalent bondingCovalent bondingConsider an atom of hydrogen:

Notice that hydrogen has just __ electron in its outer shell. A full (inner) shell would have __ electrons, so two hydrogen atoms get together and “_____” their electrons:

Now they both have a ____ outer shell and are more _____. The formula for this molecule is H2.

When two or more atoms bond by sharing electrons we call it ____________ BONDING. This type of bonding normally occurs between _______ atoms. It causes the atoms in a molecule to be held together very strongly but there are ____ forces between individual molecules. This is why covalently-bonded molecules have low melting and boiling points (i.e. they are usually ____ or ______).

Words – gas, covalent, non-metal, 1, 2, liquid, share, full, weak, stable

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Dot and Cross DiagramsDot and Cross Diagrams

HOH

Water, H2O:

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Dot and Cross DiagramsDot and Cross Diagrams

Oxygen, O2:

O O

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Dot and cross diagramsDot and cross diagramsWater, H2O:

Oxygen, O2:

OH H

O O

H

H

O

O O

Step 1: Draw the atoms with their outer shell:

Step 2: Put the atoms together and check they all have a full outer shell:

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Dot and cross diagramsDot and cross diagramsNitrogen, N2:

Carbon dioxide, CO2:Ammonia NH3:

Methane CH4:

H HN

H

HH

H

H

CN N

O OC

21/04/23Other ways of drawing covalent Other ways of drawing covalent bondsbonds

Consider ammonia (NH3):

H HN

H H HN

H H HN

H

Bonds formed between non-metals are usually covalent. Common examples are NH3, CO2, CH4, H2O etc.

Covalent bonding in Covalent bonding in methanemethane

How do carbon and hydrogen atoms form covalent bonds in a molecule of methane?

CH4 or H C H

H

H

HH

H

H

C

Covalent bonding in carbon Covalent bonding in carbon dioxidedioxide

How do carbon and oxygen atoms form covalent bonds in a molecule of carbon dioxide?

CO2 or O C O

double bonds

A double bond is when two pairs of electrons are shared. In carbon dioxide there are two double bonds – one between each oxygen atom and the carbon atom.

O OC

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What am I learning?What am I learning?Today I am learning about the Today I am learning about the

properties of covalent substancesproperties of covalent substances

What am I learning?What am I learning?Today I am learning about the Today I am learning about the

properties of covalent substancesproperties of covalent substances

How will I know I’ve learnt it?How will I know I’ve learnt it?State some general properties of State some general properties of covalent substances (D)covalent substances (D)Describe the properties of covalent Describe the properties of covalent substances (C)substances (C)Explain the properties of covalent Explain the properties of covalent substances (B)substances (B)

How will I know I’ve learnt it?How will I know I’ve learnt it?State some general properties of State some general properties of covalent substances (D)covalent substances (D)Describe the properties of covalent Describe the properties of covalent substances (C)substances (C)Explain the properties of covalent Explain the properties of covalent substances (B)substances (B)

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Dot and cross diagramsDot and cross diagramsWater, H2O:

Oxygen, O2:

OH H

O O

H

H

O

O O

Step 1: Draw the atoms with their outer shell:

Step 2: Put the atoms together and check they all have a full outer shell:

21/04/23Properties of covalent Properties of covalent moleculesmolecules

HH

Recall our model of a simple covalent compound like hydrogen, H2:

Hydrogen has a very low melting point and a very low boiling point. Why?

1) The intermolecular forces are very weak so each one of these H2 molecules doesn’t really care about the others – it’s very easy to pull them apart.

2) When a substance is heated it is the intermolecular forces that are overcome, NOT the covalent bond in each molecule, which is much stronger!

Also, the molecules do not carry a charge so covalent compounds usually do not conduct electricity.

Simple molecular covalent compounds

These include gases such as hydrogen, methane, oxygen, carbon dioxide and liquids such as water.

They have low melting points and low boiling points because there are only weak forces between nearby molecules.

Exam questionsExam questions

Describe the properties of covalent compounds (3 marks)

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Simple molecular covalent compounds

They have low melting points and low boiling points because there are only weak forces between nearby molecules meaning less energy is required to break the bonds.

Simple molecular covalent compounds

An electric current is a flow of charged particles. The atoms in simple molecular covalent molecules have not lost or gained electrons so there are no charged particles that can move around. Simple molecular covalent compounds are therefore poor conductors of electricity.

Exam questionsExam questions

Describe the electrical conductivity of covalent compounds (3 marks)

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Simple molecular covalent compounds

An electric current is a flow of charged particles. The atoms in simple molecular covalent molecules have not lost or gained electrons so there are no charged particles that can move around. Simple molecular covalent compounds are therefore poor conductors of electricity.

21/04/23Giant Covalent structures Giant Covalent structures (“lattices”)(“lattices”)

1. Diamond – a giant covalent structure with a very ____ melting point due to ______ bonds between carbon atoms

2. Graphite – carbon atoms arranged in a layered structure, with free _______ in between each layer enabling carbon to conduct _________ (like metals)

Words – melting, high, electrons, bonds, strong, electricity

3. Silicon dioxide (sand) – a giant covalent structure of silicon and oxygen atoms with strong _____ causing a high ______ point and it’s a good insulator as it has no free electrons

O OO

O

Si

O O

OSi

OO

O

Si

Notice that giant covalent structures have very different properties to individual covalent molecules:

Giant molecular covalent compounds

These consist of billion of atoms all joined together by covalent bonds. These have high melting points and high boiling points because all the atoms are joined together by these strong covalent bonds and so a lot of energy is needed to break these bonds.

Giant molecular covalent compounds

Carbon can form two different types of giant molecular covalent compounds.1)Diamond which is very hard and so is used in cutting tools.2)Graphite which is very soft and is used to make electrodes or used as a lubricant.

Giant molecular covalent compounds

Diamond

All atoms are joined with strong covalent bonds.

Giant molecular covalent compounds

Graphite

Easily rubs away in layers because although the covalent bonds within the layers are strong, there are only weak forces between the layers.

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Using Covalent StructuresUsing Covalent StructuresForm of carbon

Property Uses Why?

Carbon – diamond

Very hard Drill tips Extremely strong covalent structure

Graphite Soft, conducts electricity

Lubricants and making electrodes

Graphite is arranged in layers that can slide over each other and it contains free electrons.

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What am I learning?What am I learning?Today I am learning about Today I am learning about

separation techniquesseparation techniques

What am I learning?What am I learning?Today I am learning about Today I am learning about

separation techniquesseparation techniques

How will I know I’ve learnt it?How will I know I’ve learnt it?Describe some techniques for separating Describe some techniques for separating miscible and immiscible liquids (C) miscible and immiscible liquids (C) Explain the science behind the separation Explain the science behind the separation techniques (B)techniques (B)

How will I know I’ve learnt it?How will I know I’ve learnt it?Describe some techniques for separating Describe some techniques for separating miscible and immiscible liquids (C) miscible and immiscible liquids (C) Explain the science behind the separation Explain the science behind the separation techniques (B)techniques (B)

How do you separate oil and water?

ImmiscibleImmiscible

21/04/23Separating Immiscible Separating Immiscible LiquidsLiquids

“Immiscible” means “two liquids that can’t be dissolved”, e.g. oil and water:

Separating these liquids is fairly easy – you simply allow them to settle and then “tap off” the heavier liquid at the bottom using a separating funnel.

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Separating miscible liquidsSeparating miscible liquidsMiscible liquids are liquids that have dissolved together, so separating them is much harder. Here’s an example – distillation:

This apparatus can be used to separate water and ethanol because they have different _____ ______. The ______ will evaporate first, turn back into a _______ in the condenser and collect in the _______. The water remains in the round flask, as long as the _______ does not exceed 100OC.

Words – temperature, boiling points, ethanol, beaker, liquid

Fractional distillationFractional distillation

Remember from core…

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Fractional Distillation of AirFractional Distillation of AirAir can be distilled in the same way – you simply have to get it cold enough first:

Liquefied air at -200OC

Gaseous nitrogen out

at -190OC

Liquid oxygen out at -185OC

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What am I learning?What am I learning?I am learning about chromatography

as a separation technique

What am I learning?What am I learning?I am learning about chromatography

as a separation technique

How will I know I’ve learnt it?How will I know I’ve learnt it?Describe the method of chromatography as a separation technique (C)Carry out the method for producing a chromatogram (C)Calculate the Rf values using a chromatogram (B)

How will I know I’ve learnt it?How will I know I’ve learnt it?Describe the method of chromatography as a separation technique (C)Carry out the method for producing a chromatogram (C)Calculate the Rf values using a chromatogram (B)

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R G B X

ChromatographyChromatographyChromatography can be used to separate a mixture of different inks. Some example questions…

1 2 3 Z

1) Ink X contains two different colours. What are they?

2) Which ink is ink Z made out of?

21/04/23RRff value valueThe Rf value is a way of measuring how far a substance has moved:

R G B

This line marks the distance travelled by the

solvent

Rf value =

Distance travelled by substance

Distance travelled by solvent

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Example questionsExample questionsCalculate the Rf values of the following:

R G B

10cm

2cm

8cm

5cm

359 Foods Containing Colouring Sudan 1 Recalled he Food Standards Agency has announced the recallof another 100 foods, bringingthe total to 359, found to contain the illegal colouring Sudan 1, which is linked to cancer.Sudan 1 (bright red dye), also used in shoe and floor polish, has been banned in foods since 1995 in the UK.In this recent scare, Sudan 1inadvertently got intothe foods...

25th May 2006

T

How could we test this?

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Using ChromatographyUsing ChromatographyChromatography can be used to test which foods contain which ingredients. For example, consider the dye Sudan 1, which was found in 450 foods in 2005. Which dye contains Sudan 1?

Sudan 1

Dye 1

Dye 2

Dye 3