CHemical Reactions and Stoichiometry

EQUATIONS & REACTIONS

A. Chemical Changes and Reactions1. New substances are producedproduced.2. Chemical reaction – chemical bonds between atoms or ions

breakbreak, and new bondsnew bonds form between atoms or ions.B. Evidence of a Chemical Reaction

1. color changecolor change2. formation of a precipitateformation of a precipitate3. temperature changetemperature change4. formation of a gasformation of a gas

1. Describing Reactions

Describing Chemical ReactionsDescribing Chemical Reactions• Cellular phone messages

make use of symbols and abbreviations to express ideas in shorter form. Similarly, chemists often use chemical equations in place of words.

C. Mechanics of a Chemical Reaction1. Starting Materials – reactantsreactants2. Ending Materials - productsproducts3. reactantsreactants → productsproductsArrow = yieldsyields or producesproduces

4. If there are multiple products or reactants, they are connected with a + symbol.

5. Symbols above the yield sign represent conditionsconditions necessary for a reaction to proceed. Ex) = delta = heatheat

= electrolysiselectrolysis

Δ

elec

5. Some reactions occur spontaneouslyspontaneously.6. Symbols represent the statestate of the reactants and products.

Liquid = ll Gas = gg Solid = ss Crystal = crcrAqueous = aqaq (solids in water solutionwater solution)

DEMO or video https://www.youtube.com/watch?v=Hv15HxOVpzE

Ex) 2Al(s) + 3CuCl2(aq) → 2AlCl3(aq) + 3Cu(s)

silver blue gray red

7. Complete chemical equations include the subscript to indicate the physical statestate of each substance.

8. Diatomic molecules – certain elements exist in nature as diatomic molecules (X2)

List them: N2 O2 F2 Cl2 Br2 I2 H2

Natural States of the Elements■ Diatomic Molecules

Nitrogen gas containsN2 molecules.

Oxygen gas containsO2 molecules.

Write a Word Equation of Chemical Reacting

• Put the name of the reactants on the left, separating with a +.

• Use an arrow to replace the verb for reacting.

• Put the names of the products on the right, separated by + signs

Solid calcium and water react to yield calcium hydroxide and hydrogen gas.

Solid calcium and water react to yield calcium hydroxide and hydrogen gas.

Ca(s) + H2O -> Ca(OH) 2 + H2

Write a Skeleton Equation of Chemical

Reactions• The symbols for reactants on the left,

separating with a +. • An arrow to replace the verb for reacting.• Products formulas are on the right, seperated

by + signs.

• If You are starting with a word equations, you are just replacing the names with the formulas.

Sodium hydroxide and hydrochloric acid produce sodium chloride and water.

Sodium hydroxide and hydrochloric acid produce sodium chloride and water.

Is it Balanced ?

■ A balanced equation has the same number of each type of molecules in the reactants and products.

Balancing Chemical Equations

1. Conservation of massmass leads to balancing equations – the number of atoms of each element must be the same before & after the reactionbefore & after the reaction.

2. The Law of Conservation of Mass also states that the total massmass before and after the reaction must be the samesame. You cannot lose or gain mass.

3. Therefore the MASS OF THE PRODUCTS = MASS OF REACTANTSMASS OF REACTANTS

4. Subscript – indicates number of atomsatoms of an element present in a compound.

5. Coefficient – indicates the number of atomsatoms or moleculesmolecules involved in the reaction.

6. Steps to Balance Equations:A. Write equation with symbols.B. Count # of atoms on each side of the

reaction.C. Balance atoms using coefficients. D. General Rule: Balance all elements first.

Then, balance C, H, and O. E. NEVER CHANGE

SUBSCRIPTS!!!!

_____ Na + ____ H2O ____ NaOH + _____ H2

2 Na + 2 H2O 2 NaOH + H2

_____ NH3 + ____ NO ___ N2 + ___ H2O

4 NH3 + 6 NO 5 N2 + 6 H2O

3 H2 + N2 → 2 NH3

3 Na2SO4 + Ca3(PO4)2 → 2 CaSO4 + 2 Na3PO4

2 NaNO3 → 2 NaNO2 + O2

2 C8H18 + 25 O2 → 16 CO2 + 18 H2O

DO NOW: DemonstrationWhen electricity is run through a container of distilled water, bubbles form at both electrodes. The negative end makes lots of tiny bubbles really quickly. If the gases are captured in vials, there is twice the amount of gas at the negative end, than at the positive end

A. Synthesis Reactions (direct direct combinationcombination)1. Two or more elements or elements or compoundscompounds combine

to form a more complexcomplex product. A + B → AB

2. Ex. Fe + S → FeS CaO + H2O → Ca(OH)2

2. Classifying Chemical Reactions

B. Decomposition Reactions (analysisanalysis)

1. A single reactant single reactant breaks down into simpler simpler compounds or elements.

AB → A + B2. The oppositeopposite of a synthesis reaction.3. Ex. 2 HgO → 2 Hg + O2

CaCO3 → CaO + CO2

C. Single Replacement Reactions

1. Atoms of an uncombined element replacereplace atoms of another element in a compound.

A + BX → AX + B2. A moremore active element will replace a less

active element. 3. Ex: CuCl2 + Zn ZnCl2 + Cu

Br2 +2 KI 2KBr + I2

D. Double-Replacement Reactions

1. Atoms or ions from two differentdifferent compounds replace each other.

AX + BY → AY + BX

2. These types of reactions will (A) form precipitatesprecipitates (↓)(B) form gasesgases (↑)(C) are acid-baseacid-base neutralizations (produce water and salt)

Double Replacement Examples:A. Pb(NO3)2 + 2 KI → PbI2 ↓ + 2 KNO3

B. CaCO3 + 2 HCl → CaCl2 + H2CO3

C. NaOH + HCl → NaCl + HOH

3. In letter B above, carbonic acid, H2CO3, is unstable and will immediately decompose into carbon dioxidecarbon dioxide and waterwater. CaCO3 + 2 HCl → CaCl2 + CO2↑ + H2O

E. Combustion Reactions1. One substance reacts with oxygen, oxygen,

OO22 to produce oxide compounds.2. Occurs during burningburning or

oxidationoxidation (rusting.)3. The reactions that only add oxygen

are classified as synthesissynthesis reactions.

Ex) S + O2 → SO2

4. Combustion reactions are exothermicexothermic, releasing a large amount of energy as light, heat, or sound.

5. A true Combustion reaction occurs when a hydrocarbonhydrocarbon (compound containing HH & C C ) reacts to form carbon dioxide and waterwater that are always the products.CxHx + O2 → CO2 + H2O

6. Ex. __CH4 + __O2 → __CO2 + __H2O + 803 kJ

C6H12 O6 + O2 → CO2 + H2O + heat

22

666

Men with Hats

What kind of Reaction?

What kind of reaction?

CuCO3(s) CO2(g) + CuO(s)

copper (II) carbonate

metal oxide

flame goes out

What is one of the products?

mercury (II) oxide

mercury (II) oxide

2HgO → 2Hg + O2

3CuCl2 + 2Al 2AlCl3 + 3Cu

Sodium Metal plus Chlorine Gas Video

2 Na + Cl2 2 NaCl https://youtu.be/VBReOjo3ri8

3. The Mole

Percent Composition is the percent of any element contributes to the mass of a compound.

4. Chemical Composition

Steps to Calculate % Comp

1. Write formula2. Find mass of individual elements3. Total for molecular (formula) mass.4. Part/whole x 100 = %

• Can be used to find the mass of an element in compound or solution.

Potassium Permanganate

Empirical Formula

• Empirical- Relying on or derived from observation or experiment. Verifiable or provable by means of observation or experiment.

Empirical Formula

• The empirical formula is the simplest formula for a compound. A molecular formula is the same as or a multiple of the empirical formula, and is based on the actual number of atoms of each type in the compound. For example, if the empirical formula of a compound is C3H8 , its molecular formula may be C3H8 , C6H16 , etc.

Empirical?

• H2O

• H2O2

• Mg(OH) 2

• C10H14O

• C6H12O6

• Hg2 (CN) 2

Steps to find Empirical Formula

1. Find grams of each element (assume 100g)

2. Change grams to moles3. Look for smallest whole number ratio

(divide by smallest)

• Compare to mass molecular mass to find molecular formula