Chemical Reactions

SCH 3U Unit 2

Kinetic Molecular Theory

• Chemical reactions can be explained using the Kinetic Molecular Theory.

Main Ideas of KMT

1) All matter is made up of particles (atoms, ions, or molecules).

2) These particles are in constant motion. (The higher the temperature the faster the motion.)

3) Particles interact with each other.

Molecular Collisions

• As molecules move, they hit or collide with each other. When two molecules collide they can do one of two things…

Ineffective Collisions

• Molecules can bounce off one another and no reaction occurs.

• This occurs when the molecules are moving too slowly or are too stable.

Effective Collisions• Particles can come apart and the

atoms combine to form new particles (the products).

• This occurs when the particles are moving quickly and hit hard enough.

• This energy is called activation energy.

Reaction Rates

• The rate of a chemical reaction is affected by the number of collisions of reactant particles.

• Two ways to increase the rate of reaction

1) increase the number of collisions

2) increase the number of effective collisions

Rate Factors

• There are four factors that affect the rate of chemical reactions.– Temperature– Surface area– Concentration– Catalyst

Lets investigate!

Chemical Equations

• Word Equations – a way to describe chemical reactions using chemical names; it tells you what reacts and what is produced.

iron + oxygen iron (III) oxide

reactants products

Skeleton Equations

• Skeleton Equations – a way to describe chemical reactions using chemical formulas; it tells you what reacts and what is produced.

Fe + O2 Fe2O3

FeO O

Fe

O

O

Fe

O

Balanced Equations

• Balanced Chemical Equations - a chemical equation in which the reactants and products contain the same number of atoms of each element.

4 Fe + 3 O2 2 Fe2O3

Fe

O

O

Fe

O

Fe

O

O

Fe

OO O

O O

O O

Fe Fe

Fe Fe

Why balance an equation?

• Chemical equations must be balanced to obey the Law of Conservation of Mass.

Law of Conservation of Mass

• Law of Conservation of Mass – In a chemical reaction, the total mass of the reactants must equal the total mass of the products.

OR

• Atoms cannot be created or destroyed during a chemical reaction.

Balancing an Equation

• To balance a chemical equation, you must add coefficients in front of each compound in the reaction.

2 K + 2 H2O -----> 2 KOH + H2

Balancing an Equation

Two things you CANNOT do to balance a chemical reaction:

1) You cannot change the subscripts.

H2O not H2O2

2) You cannot place a coefficient in the middle of a formula.

H2O not H22O

Balancing an Equation

Things to remember about balancing chemical equations:

1) If no coefficient appears in front of the symbol/formula, it is assumed that it is 1 (one).

2) The coefficients must be in lowest terms.

4H2 + 2O2 4H2O

2H2 + O2 2H2O

To Balance an Equation

• Balance all metals.• Balance all non-metals (except hydrogen

and oxygen).• Balance hydrogen or oxygen (whichever

is not by itself).• Balance the remaining atoms.• Count the number of atoms of each

element on each side of the equation. THEY SHOULD BE EQUAL.

Practice

1. K + CaCl2 -----> KCl + Ca

2. HCl + Mg(OH)2 H2O + MgCl2

More Practice

3. KNO3 ----> KNO2 + O2

4. C2H2 + O2 -----> CO2 + H2O

5. C4H8 + O2 ----> CO2 + H2O

6. Hg + O2 ----> HgO

7. AgNO3 + NaCl ----> AgCl + NaNO3

Time to get to Work

• Pg 115 #1-10

• Pg 117 #1-6

• Pg 120 #11-20

Classifying Reactions

• Most chemical reactions can be grouped or organized into 5 categories.– Synthesis– Decomposition– Single Displacement– Double Displacement– Combustion

Synthesis Reactions

• A synthesis reaction occurs when two or more small substances combine to produce a larger substance.

A + B AB

Only 1 product!

Types of Synthesis

• Two elements may combine to form a compound:

hydrogen + oxygen water

2H2 + O2 2H2O

Remember to criss-cross new subscripts!

• Two compounds may combine to form a larger compound:

carbon dioxide + water carbonic acid

CO2 + H2O H2CO3

Types of Synthesis

General Synthesis Reactions

• Nonmetal oxide + water oxyacid

• Metal oxide + water metal hydroxide

Na2O + H2O 2NaOH

SO2 + H2O H2SO3

Synthesis of Acids and Bases

Na2O + H2O NaOH

MgO + H2O Mg(OH)2

SO2 + H2O H2SO3

Cl2O5 + H2O HClO3

Applications of Synthesis

• Rusting– Iron combines with oxygen in the

air to form iron (III) oxide or rust. The rust then flakes away, exposing new metal underneath to the oxygen.

• Acid Rain– Non-metal oxides found in the air due

to pollution combine with water to form acids. These acids fall to the ground as acid rain.

Applications of Synthesis

• Polymerization – Small molecules are linked together to

form long-chain molecules called polymers.

Applications of Synthesis

Decomposition Reactions

• A decomposition reaction occurs when one larger substance breaks up into two or more smaller substances.

AB A + B

Only 1 reactant!

Decomposition reactions are the opposite to synthesis!!

Types of Decomposition

• A compound may decompose to produce two elements

water hydrogen + oxygen

2H2O 2H2 + O2

Remember to make the seven sisters diatomic!

• A large compound may decompose to produce two smaller compounds

Types of Decomposition

potassium potassium + oxygen chlorate oxide

KClO3 KCl + O2

General Decomposition Reactions

• Metal carbonates decompose to produce a metal oxide and carbon dioxide

CaCO3 CaO + CO2

calcium calcium + carboncarbonate oxide dioxide

Applications of Decomposition

• Production of Hydrogen– The decomposition of water is used in

the production of hydrogen for hydrogen fuel cells.

• Return of atmospheric carbon– Decomposition reactions release

carbon dioxide back into the atmosphere to re-enter the carbon cycle.

Applications of Decomposition

• Pg 125 # 7-10

• Pg 127 # 21-30

• Pg 132 #13, 15, 16

• Pg 134 # 31-40

• Pg 136 # 2, 4, 7, 8, 15

Time to get to Work

Single Displacement

• In a single displacement reaction an element takes the place of another element within a compound to form new products.

A + CD AD + C

Or

A + CD D + CA

Metal Replacement

• Metals switch places with other metals

Mg + 2AgNO3 2Ag + Mg(NO3)2

• The metal element will only replace the metal ion in the compound if the ion is less reactive than the metal itself.

Mg is more reactive than Ag, therefore it will replace it in the compound.

Activity Series

• Whether or not one element can replace another element in a compound depends on the relative reactivity of the two elements.

• The element doing the replacing must be MORE reactive than the one being replaced.

Nonmetal Replacement

• Non-metals switch places with other non-metals

Cl2 + 2NaBr 2NaCl + Br2

Similarly to metal replacement, the element doing the replacing must be MORE reactive than the one being replaced.

Examples

• Zn + CuSO4 →

• Fe + 3AgNO3 →

• Ni + CaCO3

• F2 + NaCl

Brainiac - Alkali Metal Reaction With Water

Applications of Single Displacement

• Sacrificial anodeConcrete pillars have iron rebar in them for strength. However, salt water can quickly react with the iron to form iron (II) chloride. To prevent this, a metal like zinc or magnesium is attached to the rebar and will protect the iron. Here's the reaction:Zn + FeCl2 -> Fe + ZnCl2

Applications of Single Displacement

• Isolating precious or harmful elements– Single displacement reactions are

carried out to isolate metals from their respective ore

– They are also used to isolate harmful ions from a solution such as barium

Time to get to Work

• Page 165 # 1, 2, 4, 5, 6

• Page 169 # 1-10

Double Displacement

• In a double displacement reaction the cations within two ionic compounds exchange partners to form new products.

AB + CD AD + CB

Identifying a Double Displacement

• Double displacement reactions tend to occur in aqueous solutions.

• They are identified by:• The formation of a precipitate• The production of a gas• The production of water

Predicting Products• For the majority of double displacement reactions, the products are

predicted by switching the cations to form new products.

• If the products are insoluble in water, a precipitate is formed.

CaCl2 (aq) + AgNO3 (aq)

•Some double displacement reactions result in the formation of water. These are typically neutralization reactions between an acid and a base.

Predicting Products

NaOH(aq) + HCl(aq)

Predicting Products

• On occasion one of the products formed is unstable and further decomposes to produce water and a gas.

Na2CO3 + 2 HCl 2 NaCl + H2CO3

H2O + CO2

Typical compounds that decompose:

•H2CO3

•NH4OH

•H2SO3

Time to Practice

• K2S + CuSO4

• KOH + NH4Cl

• (NH4)2SO4 + NaCl

• CoCl3 + NaOH

• Na2CO3 + HNO3

Time to get to Work

• Pg 173 # 7-9, 12

• Pg 175 # 11-20

• Pg 177 #16, 18

• Pg 179 #21-30