Chapter 5Chapter 5

Chemical ReactionsChemical Reactions

Chemical ReactionChemical Reaction Another name for a chemical Another name for a chemical

changechange New properties when you are doneNew properties when you are done No new atoms are madeNo new atoms are made Atoms are rearrangedAtoms are rearranged New compounds can be madeNew compounds can be made Old bonds are brokenOld bonds are broken New bonds are formedNew bonds are formed

Indications of Chemical Indications of Chemical ReactionsReactions

New propertiesNew properties Color changeColor change Odor changeOdor change New phase is madeNew phase is made

• Precipitates- solids in a liquidPrecipitates- solids in a liquid• Gases- bubbles in a liquidGases- bubbles in a liquid

Two parts of reactionTwo parts of reaction• Reactants- the stuff you start Reactants- the stuff you start withwith• Products- the stuff you makeProducts- the stuff you make

Starting a ReactionStarting a Reaction Always takes a little energyAlways takes a little energy Energy goes into breaking bonds in Energy goes into breaking bonds in

the reactantsthe reactants Can use different forms of energyCan use different forms of energy HeatHeat ElectricityElectricity LightLight

Forming Bonds Makes EnergyForming Bonds Makes Energy Releases energyReleases energy Energy is conservedEnergy is conserved Chemical Energy- energy stored in Chemical Energy- energy stored in

the bonds of the chemicals.the bonds of the chemicals. Reactions have an energy changeReactions have an energy change

Exothermic ReactionsExothermic Reactions If breaking bonds takes less energy If breaking bonds takes less energy

than making them- it releases energythan making them- it releases energy ExoExo- outside- outside thermtherm- heat- heat Exothermic reactions release energyExothermic reactions release energy Get hotGet hot Give off lightGive off light Or release electricityOr release electricity

Che

mic

al E

nerg

y

Reactants Products

Change is down

Energy Released

Endothermic ReactionsEndothermic Reactions If breaking bonds takes more energy If breaking bonds takes more energy

than making them- it absorbs energythan making them- it absorbs energy EndoEndo- inside- inside thermtherm- heat- heat Endothermic reactions absorb energyEndothermic reactions absorb energy Get coldGet cold Require heat or energy or they stopRequire heat or energy or they stop

Che

mic

al E

nerg

y

Reactants Products

Change is upHeat is released

Chemical EquationsChemical Equations Copper reacts with chlorine to form Copper reacts with chlorine to form

copper (II) chloride.copper (II) chloride. In a In a word equationword equation Methane + oxygen Methane + oxygen

water +carbon water +carbon dioxidedioxide

Arrow means “yields” or “makes”Arrow means “yields” or “makes” The plus sign means “and”The plus sign means “and” Can use formulasCan use formulas CHCH44 +O +O22 CO CO22 +H +H22OO

Balanced EquationBalanced Equation Atoms can’t be created or Atoms can’t be created or

destroyeddestroyed All the atoms we start with we All the atoms we start with we

must end up withmust end up with A balanced equation has the same A balanced equation has the same

number of each element on both number of each element on both sides of the equation.sides of the equation.

C + OC + O22 CO CO22

This equation is already balancedThis equation is already balanced What if it isn’t already?What if it isn’t already?

C + OO COO

C + OC + O22 CO CO We need one more oxygen in the We need one more oxygen in the

products.products. Can’t change the formula, because Can’t change the formula, because

it describes what isit describes what is

C + O COO

Must have started with two CMust have started with two C 2 C + O2 C + O22 2 CO 2 CO

C

+O

C

OO

OC

C

Rules for balancingRules for balancing Write the correct formulas for all Write the correct formulas for all

the reactants and productsthe reactants and products Count the number of atoms of Count the number of atoms of

each type appearing on both sideseach type appearing on both sides Balance the elements one at a Balance the elements one at a

time by adding coefficients (the time by adding coefficients (the numbers in front)numbers in front)

Check to make sure it is balanced.Check to make sure it is balanced.

NeverNever Change a subscript to balance an Change a subscript to balance an

equation.equation. If you change the formula you are If you change the formula you are

describing a different reaction.describing a different reaction. HH22O is a different compound than HO is a different compound than H22OO22

Never put a coefficient in the middle of Never put a coefficient in the middle of a formulaa formula

2 NaCl is okay, Na2Cl is not.2 NaCl is okay, Na2Cl is not.

ExampleExample

H2 + H2OO2

Make a table to keep track of where you are at

Example

H2 + H2OO2

Need twice as much O in the product

R PH

O

2

2

2

1

Example

H2 + H2OO2

Changes the O

R PH

O

2

2

2

1

2

Example

H2 + H2OO2

Also changes the H

R PH

O

2

2

2

1

2

2

Example

H2 + H2OO2

Need twice as much H in the reactant

R PH

O

2

2

2

1

2

2

4

Example

H2 + H2OO2

Recount

R PH

O

2

2

2

1

2

2

4

2

Example

H2 + H2OO2

The equation is balanced, has the same number of each kind of atom on both sides

R PH

O

2

2

2

1

2

2

4

2

4

Example

H2 + H2OO2

This is the answer

R PH

O

2

2

2

1

2

2

4

2

4

Not this

ExamplesExamples AgNOAgNO33 + Cu + Cu Cu(NO Cu(NO33))22 + Ag + Ag

Mg + NMg + N22 Mg Mg33NN2 2

P + OP + O22 P P44OO1010

Na + HNa + H22O O H H22 + NaOH + NaOH

CHCH44 + O + O22 CO CO22 + H + H22OO

Examples of Balancing EquationsExamples of Balancing Equations

a) Pb(NOa) Pb(NO33))22 + K + K22CrOCrO44 PbCrO PbCrO44 + KNO + KNO33

b) MnOb) MnO22 + HCl + HCl MnCl MnCl22 + H + H22O+ ClO+ Cl22c) Cc) C33HH66 + O + O22 COCO22 +H +H22OO

d) Zn(OH)d) Zn(OH)22 + H + H33POPO44 Zn Zn33(PO(PO44))22

e) CO + Fee) CO + Fe22OO33 Fe + COFe + CO22

f) CSf) CS22 + Cl + Cl22 CClCCl44 +S +S22ClCl22g) CHg) CH44 + Br + Br22 CH CH33Br + HBrBr + HBr

h) Ba(CN)h) Ba(CN)22 + H + H22SOSO44 BaSO BaSO44 + HCN + HCN

Moles and ReactionsMoles and Reactions 2H2H2 2 + O+ O22 2H2H22OO 2 dozen molecules of hydrogen 2 dozen molecules of hydrogen

and 1 dozen molecules of oxygen and 1 dozen molecules of oxygen form 2 dozen molecules of water.form 2 dozen molecules of water.

2 x (6.02 x 102 x (6.02 x 102323) molecules of ) molecules of hydrogen and 1 x (6.02 x 10hydrogen and 1 x (6.02 x 102323) ) molecules of oxygen form 2 x (6.02 molecules of oxygen form 2 x (6.02 x 10x 102323) molecules of water.) molecules of water.

2 moles of hydrogen and 1 mole of 2 moles of hydrogen and 1 mole of oxygen form 2 moles of water.oxygen form 2 moles of water.

Moles and ReactionsMoles and Reactions The coefficients of balanced The coefficients of balanced

equations tell how many particles equations tell how many particles reactreact

And how many moles of particlesAnd how many moles of particles We can make ratios with those molesWe can make ratios with those moles 2 Mg + O2 Mg + O22 2 MgO 2 MgO If 2 moles of Mg react, 1 mole of OIf 2 moles of Mg react, 1 mole of O22

will be requiredwill be required 2 mol Mg 2 mol Mg or or 1 mol O1 mol O22

1 mol O 1 mol O22 2 mol Mg 2 mol Mg

Mole ratiosMole ratios Can be used to figure out how Can be used to figure out how

many moles of products and many moles of products and reactants were used or madereactants were used or made

2H2H2 2 + O+ O22 2H2H22OO If 6 mole of HIf 6 mole of H22 react, how many react, how many

moles of water will form?moles of water will form? How many moles of hydrogen are How many moles of hydrogen are

needed to react with 3.6 mole of needed to react with 3.6 mole of oxygen?oxygen?

Mole to mole conversionsMole to mole conversions 2 Al2 Al22OO33 AlAl ++ 3O3O22

every time we use 2 moles of Alevery time we use 2 moles of Al22OO33 we make 3 moles of Owe make 3 moles of O22

2 moles Al2O3

3 mole O2

or2 moles Al2O3

3 mole O2

Mole to Mole conversionsMole to Mole conversions How many moles of OHow many moles of O22 are produced when 3.34 are produced when 3.34

moles of Almoles of Al22OO33 decompose? decompose?

2 Al2 Al22OO33 AlAl ++ 3O3O22

3.34 moles Al2O3 2 moles Al2O3

3 mole O2 = 5.01 moles O2

Your TurnYour Turn 2C2C22HH22 + 5 O + 5 O22 4CO 4CO22 + 2 H + 2 H22OO

If 3.84 moles of CIf 3.84 moles of C22HH2 2 are burned, how are burned, how

many moles of Omany moles of O22 are needed? are needed?

How many moles of CHow many moles of C22HH2 2 are needed are needed

to produce 8.95 mole of Hto produce 8.95 mole of H22O?O?

If 2.47 moles of CIf 2.47 moles of C22HH2 2 are burned, how are burned, how

many moles of COmany moles of CO22 are formed? are formed?

Collision TheoryCollision Theory In order to react molecules and In order to react molecules and

atoms must touch each other.atoms must touch each other. They must hit each other hard They must hit each other hard

enough to react.enough to react. Anything that increase these things Anything that increase these things

will make the reaction faster.will make the reaction faster.

Things that Affect RateThings that Affect Rate TemperatureTemperature Higher temperature faster Higher temperature faster

particles.particles. More and harder collisions.More and harder collisions. Faster Reactions.Faster Reactions. ConcentrationConcentration More concentrated closer together More concentrated closer together

the molecules.the molecules. Collide more often.Collide more often. Faster reaction.Faster reaction.

Things that Affect RateThings that Affect Rate Particle sizeParticle size Molecules can only collide at the Molecules can only collide at the

surface.surface. Smaller particles bigger surface area.Smaller particles bigger surface area. Smaller particles faster reaction.Smaller particles faster reaction. Smallest possible is molecules or ions.Smallest possible is molecules or ions. Dissolving speeds up reactions.Dissolving speeds up reactions. Getting two solids to react with each Getting two solids to react with each

other is slow.other is slow.

Things that Affect RateThings that Affect Rate Catalysts- substances that speed Catalysts- substances that speed

up a reaction without being used up a reaction without being used up.up.

Inhibitor- a substance that blocks a Inhibitor- a substance that blocks a catalyst, slowing the reaction downcatalyst, slowing the reaction down

Enzymes are biological catalysts- Enzymes are biological catalysts- made by plants and animals to made by plants and animals to control reactionscontrol reactions

Heat destroys most catalystsHeat destroys most catalysts

ReactionsReactions Some go to completionSome go to completion All the reactants get turned into All the reactants get turned into

productsproducts No reactants leftNo reactants left Some reactions go both directionsSome reactions go both directions They are called reversible They are called reversible

reactionsreactions

Reversible ReactionsReversible Reactions 2H2H22(g) + O(g) + O22(g) (g) 2H 2H22O(g) + O(g) +

energyenergy

2H2H22O(g) + energy O(g) + energy HH22(g) + (g) +

OO22(g)(g)

2H2H22(g) + O(g) + O22(g) (g) 2H2H22O(g) O(g)

+ energy+ energy

EquilibriumEquilibrium

When I first put reactants together When I first put reactants together the forward reaction starts.the forward reaction starts.

Since there are no products there is Since there are no products there is no reverse reaction.no reverse reaction.

As the forward reaction proceeds the As the forward reaction proceeds the reactants are used up so the forward reactants are used up so the forward reaction slows.reaction slows.

The products build up, and the The products build up, and the reverse reaction speeds up.reverse reaction speeds up.

EquilibriumEquilibrium Eventually you reach a point where the Eventually you reach a point where the

reverse reaction is going as fast as the reverse reaction is going as fast as the forward reaction.forward reaction.

This is dynamic equilibrium.This is dynamic equilibrium. The rate of the forward reaction is equal The rate of the forward reaction is equal

to the rate of the reverse reaction.to the rate of the reverse reaction. The concentration of products and The concentration of products and

reactants stays the same, but the reactants stays the same, but the reactions are still running. reactions are still running.

EquilibriumEquilibrium

Equilibrium position- how much Equilibrium position- how much product and reactant there are at product and reactant there are at equilibrium.equilibrium.

Shown with the double arrow.Shown with the double arrow. Reactants are favoredReactants are favored Products are favoredProducts are favored Catalysts speed up both the forward Catalysts speed up both the forward

and reverse reactions so don’t affect and reverse reactions so don’t affect equilibrium position.equilibrium position.

LeChâtelier’s PrincipleLeChâtelier’s Principle

Regaining EquilibriumRegaining Equilibrium

LeChâtelier’s PrincipleLeChâtelier’s Principle If something is changed in a If something is changed in a

system at equilibrium, the system system at equilibrium, the system will respond to undo that change.will respond to undo that change.

Three types of changes are Three types of changes are described.described.

Changing ConcentrationChanging Concentration If you add reactants (or increase If you add reactants (or increase

their concentration).their concentration). The forward reaction will speed up.The forward reaction will speed up. More product will form.More product will form. Equilibrium “Equilibrium “Shifts to the rightShifts to the right”” Reactants Reactants products products

Changing ConcentrationChanging Concentration If you add products (or increase If you add products (or increase

their concentration).their concentration). The reverse reaction will speed up.The reverse reaction will speed up. More reactant will form.More reactant will form. Equilibrium “Equilibrium “Shifts to the leftShifts to the left”” Reactants Reactants products products

Changing ConcentrationChanging Concentration If you remove products (or If you remove products (or

decrease their concentration).decrease their concentration). The reverse reaction will slow down The reverse reaction will slow down More product will form.More product will form. Equilibrium reverse“Equilibrium reverse“Shifts to the Shifts to the

rightright”” Reactants Reactants products products

Changing ConcentrationChanging Concentration If you remove reactants (or If you remove reactants (or

decrease their concentration).decrease their concentration). The forward reaction will slow The forward reaction will slow

down.down. More reactant will form.More reactant will form. Equilibrium “Equilibrium “Shifts to the leftShifts to the left”.”. Reactants Reactants products products Used to control how much yield Used to control how much yield

you get from a chemical reaction.you get from a chemical reaction.

Changing TemperatureChanging Temperature Reactions either require or release Reactions either require or release

heat.heat. Endothermic reactions go faster at Endothermic reactions go faster at

higher temperature.higher temperature. Exothermic go faster at lower Exothermic go faster at lower

temperatures.temperatures. All reversible reactions will be All reversible reactions will be

exothermic one way and exothermic one way and endothermic the other.endothermic the other.

Changing TemperatureChanging Temperature As you raise the temperature the As you raise the temperature the

reaction proceeds in the reaction proceeds in the endothermic direction.endothermic direction.

As you lower the temperature the As you lower the temperature the reaction proceeds in the exothermic reaction proceeds in the exothermic direction.direction.

Reactants + heat Reactants + heat Products Products at high at high TT

Reactants + heat Reactants + heat Products Products at low at low TT

Changes in PressureChanges in Pressure As the pressure increases the As the pressure increases the

reaction will shift in the direction reaction will shift in the direction of the least gases.of the least gases.

At high pressureAt high pressure2H2H22(g) + O(g) + O22(g) (g) 2 H 2 H22O(g)O(g)

At low pressureAt low pressure 2H 2H22(g) + O(g) + O22(g) (g) 2 2

HH22O(g)O(g)