Reaction RatesReaction Rates

ReviewReview

A chemical equation describes a A chemical equation describes a chemical change (rxn).chemical change (rxn).

2NO(g) + O2(g) 2NO2(g)

BEFOREBEFORE

Reaction!Reaction!Reaction!Reaction!

AfterAfter

Reaction RatesReaction Rates

Reaction rate = “speed” of reaction.Reaction rate = “speed” of reaction.– EX: Burning of hydrogen EX: Burning of hydrogen fast fast– EX: Rusting of iron EX: Rusting of iron slow slow– EX: Conversion of graphite to diamond EX: Conversion of graphite to diamond

extremely slowextremely slow Collision theory: reactions happen when Collision theory: reactions happen when

molecules collide.molecules collide.– Reaction rate depends on two things Reaction rate depends on two things (that (that

we’re going to discuss today)we’re going to discuss today):: How frequently molecules collide.How frequently molecules collide. How fast they are moving when they do collide.How fast they are moving when they do collide.

Activation EnergyActivation Energy

All reactions have an energy profile:All reactions have an energy profile:

Reaction Coordinate Reaction Coordinate

En

erg

y

En

erg

y

Reactants

Products

+H

Activation Energy

-H

React

ants

Products

Activation Energy

Collision TheoryCollision Theory

When reactant molecules collide, When reactant molecules collide, they must collide with they must collide with at leastat least as as much total energy as the activation much total energy as the activation energy.energy.– Any molecules that collide with Any molecules that collide with less thanless than

the activation energy will simply bounce the activation energy will simply bounce apart w/o reacting.apart w/o reacting.

The more molecules there are The more molecules there are colliding with sufficient energy to colliding with sufficient energy to react, the faster the reaction runs.react, the faster the reaction runs.

TemperatureTemperature

In general, rxns run faster at high In general, rxns run faster at high temperatures.temperatures.– Rule of thumb: A rxn’s rate approx. Rule of thumb: A rxn’s rate approx.

doubles when the temp. rises by 10ºC.doubles when the temp. rises by 10ºC.– Not Not alwaysalways true, but generally accurate. true, but generally accurate.

Rise in temp. means molecules move Rise in temp. means molecules move faster.faster.– More collisions per second.More collisions per second.– Collisions w/ greater energy.Collisions w/ greater energy.

ConcentrationConcentration

As solute concentration increases, rxn As solute concentration increases, rxn rate usually increases also.rate usually increases also.– More molecules colliding per unit of time.More molecules colliding per unit of time.

– EX: Zn(s) + 2HCl(aq) EX: Zn(s) + 2HCl(aq) ZnCl ZnCl22(aq) + H(aq) + H22(g)(g)

– 6.00 M HCl soln. will “eat” Zn much faster 6.00 M HCl soln. will “eat” Zn much faster than 1.00 M HCl soln.than 1.00 M HCl soln.

Explains why we usually mix reactants Explains why we usually mix reactants with one reactant in great excess.with one reactant in great excess.– Keeps rxn rate high for entire rxn.Keeps rxn rate high for entire rxn.

ConcentrationConcentrationMg + H2SO4 MgSO4 + H2

Set-Up 1Set-Up 1

Add 1 mole of Add 1 mole of Mg to 1 mole of Mg to 1 mole of

HH22SOSO44..ResultResult

Mg and HMg and H22SOSO44 are are both completely both completely used up, but the used up, but the

reaction gets reaction gets veryvery slow close to the slow close to the

end.end.

Set-Up 2Set-Up 2

Add 1 mole of Mg Add 1 mole of Mg to 5 moles of to 5 moles of

HH22SOSO44..ResultResult

4 moles of H4 moles of H22SOSO44 remain, but the remain, but the

reaction stays fast reaction stays fast all the way through.all the way through.

ConcentrationConcentration

Which bumper car ride will result in Which bumper car ride will result in more collisions per minute for the red more collisions per minute for the red car?car?

Particle SizeParticle Size

The smaller the reactant particles, The smaller the reactant particles, the faster the rxn.the faster the rxn.– Grind up solids.Grind up solids.– Dissolve solids.Dissolve solids.

Breaking up reactant particles Breaking up reactant particles increases:increases:– surface area.surface area.

Particle SizeParticle Size

Aluminum powder bears a warning that it Aluminum powder bears a warning that it is an explosion hazard. Aluminum foil is an explosion hazard. Aluminum foil bears no such warning. Why is Al powder bears no such warning. Why is Al powder so dangerous when Al foil is not?so dangerous when Al foil is not?– Powdered Al has a Powdered Al has a muchmuch greater surface area greater surface area

exposed to oxygen.exposed to oxygen.– The oxidation of aluminum is highly The oxidation of aluminum is highly

exothermic.exothermic.– With powdered aluminum, a highly exothermic With powdered aluminum, a highly exothermic

rxn can occur at a much greater rate.rxn can occur at a much greater rate.

Particle SizeParticle Size

These two blocks have the same total volume, but the one on the right has been broken into 1000 equal pieces. This increases its surface area by 60 times.

1 m

ete

r10 cm

Particle SizeParticle SizeAvailable to react

Not available to react

CatalystCatalyst

CatalystCatalyst – a substance that speeds up a – a substance that speeds up a reaction.reaction.– Not consumed in rxn.Not consumed in rxn.– Lowers activation energy.Lowers activation energy.– Written above the reaction arrow.Written above the reaction arrow.

Example: Platinum metal is often used as Example: Platinum metal is often used as a catalyst to convert methanol vapors into a catalyst to convert methanol vapors into formaldehyde.formaldehyde.

CH4O Pt CH2O + H2

Catalysts and Activation EnergyCatalysts and Activation Energy

Reaction Coordinate

Pote

nti

al En

erg

y

Uncatalyzed

Catalyzed

CatalystsCatalysts

Enzymes are proteins that act as Enzymes are proteins that act as biological catalysts.biological catalysts.– They speed up many biochemical They speed up many biochemical

reactions that would take far too long to reactions that would take far too long to happen naturally.happen naturally.

– Enzymes are essential in just about Enzymes are essential in just about every aspect of cell functioning.every aspect of cell functioning.

– Enzymes function best in specific Enzymes function best in specific temperature ranges.temperature ranges.Human enzymes work best around 37ºC.Human enzymes work best around 37ºC.