Chemical Reactions

Chemical EquationDescribes what you had before

and then after the changeReactants: substances that

undergo changeProducts: new substances

formed

Reactants Products

Chemical EquationChemical equation:

representation of a chemical reaction in which the reactants and products are expressed as formulas

Examples:

C + O2 CO2

Conservation of MassAntoine Lavoisier 1743-1794

◦Law of conservation of mass: mass is neither created nor destroyed in a chemical reaction

Mass of the products is always equal to the mass of the reactants

Balancing Equations

H2 + O2 H2O

NOT BALANCED!

Number of atoms on the reactant side MUST equal the number of atoms on the product side!!!

Balancing Equations

H2 + O2 H2O

Use Coefficients to balance equations

2H2 + O2 2H2O

ExampleCu + O2 CuO

Balanced or unbalanced???

ExamplesNa + Cl2 NaCl

H2 + Cl2 HCl

AgNO3 + Cu Cu(NO3)2 + Ag

BaS + PtF2 BaF2 + PtS

Types of Reactions

Classifying reactionsReactions are often classified by

the type of reactant or the number of reactants and products

General types include:◦Synthesis reaction◦Decomposition reaction◦Single-replacement◦Double-replacement◦Combustion

SynthesisTwo or more substances react to

form a single substance

A + B AB

Example: 2Cu + O2 2CuO

DecompositionCompound breaks down into two

or more simpler substances

AB A + B

Example:CaCO3 CaO +

CO2

Single ReplacementOne element takes the place of

another element in a compound

A + BC B + AC

Example:Cu + 2AgNO3 2Ag +

Cu(NO3)2

Double ReplacementTwo different compounds exchange

positive ions and form two new compounds

AB + CD AD + CB

Example:CaCO3 + 2HCl CaCl2 + H2CO3

CombustionSubstance reacts rapidly with

oxygen, often producing heat and light

Breaking and Forming BondsBreaking bonds:

◦Requires energy Example: Propane grills have a starter

that produces a spark

Forming bonds:◦Releases energy

Example: heat and light are given off by a propane grill

Exothermic ReactionChemical reaction that releases

energy to its surroundings

Endothermic ReactionChemical reaction that absorbs

energy from its surroundings

Exothermic vs EndothermicExo:

◦Reactants have more energy than products AB+ CD+ 450 J AD + CB

Endo: ◦Products have more energy than

reactants AB + CD AD + CB + 450 J

Reaction Rates

Reactions over timeAny change that happens over a

period of time can be expressed as a rate

Reaction rate: rate at which reactants change into products over time◦Tell us how fast a reaction is going

How fast reactants are being consumed How fast the products are being formed How fast energy is being absorbed or

released

Factors Affecting Reaction RatesReaction rate depends on how

often particles collide◦More frequent = increase reaction rate◦Less frequent = decrease reaction rate

TemperatureSurface areaConcentrationStirringCatalysts

TemperatureIncrease in temperature =

increase in reaction rateDecrease in temperature =

decrease in reaction rate

Surface AreaAn increase in surface area

increases the exposure of reactants to one another

An increase in surface area = increase in reaction rate

StirringStirring causes collisions to occur

more rapidlyStirring = increase in reaction

rate

ConcentrationConcentration = number of

particles in a given volumeGreater the concentration =

increase in reaction rate

CatalystsCatalyst: substance that affects

the reaction rate without being used up in the reaction◦Used to speed up reactions or used

to enable a reaction at a lower temperature

◦Written over the arrow because it is not used up

2SO2 + O2 2SO3

V2O3

CatalystsIn order for a reaction to occur,

particles must collide with enough energy to break the bond◦Catalyst lowers this energy barrier◦Provides a surface area for reacting

particles to come together