Reaction Rates and Reaction Rates and EquilibriumEquilibrium

Chapter 17Chapter 17

17.1 Expressing Reaction Rates17.1 Expressing Reaction Rates

rates are expressed as a change in rates are expressed as a change in quantity (concentration) over a quantity (concentration) over a change in timechange in time– the rate of a chemical reaction is the rate of a chemical reaction is

defined as the change in concentration defined as the change in concentration of a reactant or product per unit time, of a reactant or product per unit time, expressed as mol/(Ls)expressed as mol/(Ls)

Expressing Reaction RatesExpressing Reaction Rates

Reaction rates must always be Reaction rates must always be positive!positive!– If you’re measuring the disappearance If you’re measuring the disappearance

of a reactant then you use a negative of a reactant then you use a negative sign to show that the concentration is sign to show that the concentration is decreasing.decreasing.

The Collision TheoryThe Collision Theory

The collision theory states that The collision theory states that atoms, ions, and molecules must atoms, ions, and molecules must collide in order to react.collide in order to react.– Only a small fraction of collisions Only a small fraction of collisions

product reactions! How could this be?product reactions! How could this be?– Particles must have the right orientation Particles must have the right orientation

and the right energy for a reaction to and the right energy for a reaction to happen!happen!

Collision TheoryCollision Theory

When the orientation of two colliding When the orientation of two colliding molecules is correct, a reaction molecules is correct, a reaction occurs in which an intermediate occurs in which an intermediate substance is formed. substance is formed. – activated complex or transition state activated complex or transition state

It is subsequently consumed and It is subsequently consumed and does not show up in the overall does not show up in the overall reaction.reaction.

Collision TheoryCollision Theory

Particles must have a minimum Particles must have a minimum amount of energy to form the amount of energy to form the activated complex and to lead to a activated complex and to lead to a reaction.reaction.– A high activation energy leads to a low A high activation energy leads to a low

reaction rate.reaction rate.– A low activation energy means that A low activation energy means that

more particles will have enough energy more particles will have enough energy to react, so reaction rate is high (fast).to react, so reaction rate is high (fast).

Summary of Collision TheorySummary of Collision Theory

Reacting substances (atoms, ions, or Reacting substances (atoms, ions, or molecules) must collide. molecules) must collide.

Reacting substances must collide Reacting substances must collide with the correct orientation.with the correct orientation.

Reacting substances must collide Reacting substances must collide with sufficient energy to form the with sufficient energy to form the activated complex.activated complex.

17.2 Factors affecting rates17.2 Factors affecting ratesNature of Reactants - reactivityNature of Reactants - reactivityTemperature-Kinetic energy is Temperature-Kinetic energy is affectedaffectedConcentration-More frequent Concentration-More frequent collisionscollisionsSurface Area – size of particlesSurface Area – size of particlesParticle size- Greater surface areaParticle size- Greater surface areaCatalysts- Lower the activation Catalysts- Lower the activation energyenergy

Catalysts and InhibitorsCatalysts and InhibitorsInhibitors slow down Inhibitors slow down the reaction rate or the reaction rate or prevent it from prevent it from happening at all!happening at all!Heterogeneous Heterogeneous catalysts are in a catalysts are in a different physical different physical state than the state than the reactions they reactions they catalyze.catalyze.Homogeneous Homogeneous catalysts are in the catalysts are in the same state.same state.

17.3 Reaction Rate Laws17.3 Reaction Rate Laws

The equation that expresses the The equation that expresses the relationship between the rate of a relationship between the rate of a reaction and the concentration of reaction and the concentration of reactants is called a reactants is called a rate lawrate law..– For the reaction A For the reaction A B the rate law is B the rate law is

rate = k[A]rate = k[A]where k is the specific rate constantwhere k is the specific rate constant

Rate Law ConstantRate Law Constant

Units for the rate constant may be:Units for the rate constant may be:– L/(mol s), LL/(mol s), L22/(mol/(mol22 s), or s s), or s-1-1

– k is unique for every reaction and k is unique for every reaction and depends on temperature!depends on temperature!

– k does not change with concentrationk does not change with concentration

Reaction OrderReaction Order

The reaction order for a reactant The reaction order for a reactant defines how the rate is affected by defines how the rate is affected by the concentration of that reactant.the concentration of that reactant.– First order: doubling the reactant First order: doubling the reactant

concentration doubles the rateconcentration doubles the rate– Second order: doubling the reactant Second order: doubling the reactant

concentration quadruples the rateconcentration quadruples the rate

Reaction OrderReaction Order

For the reaction: aA + bB For the reaction: aA + bB cC cC – The general rate law is:The general rate law is:

rate = k[A]rate = k[A]mm[B][B]nn

where m and n are the orders for A and where m and n are the orders for A and BB

– For example: 2NO + 2HFor example: 2NO + 2H22 N N22 + 2H + 2H22O O has the rate law: rate = k[NO]has the rate law: rate = k[NO]22[H[H22] ]

Which reactant is first order? Second order?Which reactant is first order? Second order?

What is the overall reaction order?What is the overall reaction order?

Determining Reaction OrderDetermining Reaction Order

Look at Trials 1 & 2: What happens when Look at Trials 1 & 2: What happens when [A] is doubled?[A] is doubled?Look at Trials 2 & 3: What happens when Look at Trials 2 & 3: What happens when [B] is doubled? [B] is doubled? What would be the rate law for this What would be the rate law for this reaction?reaction?

Reversible ReactionsReversible Reactions

These are reactions that can run These are reactions that can run forward and backward at the same forward and backward at the same time. time.

A limited amount of materials will A limited amount of materials will determine the ratedetermine the rate

A balance will occur between the two A balance will occur between the two reactions at equilibrium.reactions at equilibrium.

Won’t always be 50% each directionWon’t always be 50% each direction

Factors affecting EquilibriumFactors affecting Equilibrium

ConcentrationConcentration

TemperatureTemperature

PressurePressure

It will rebalance according to Le It will rebalance according to Le Chatelier’s principleChatelier’s principle

Free EnergyFree Energy

Spontaneous reactions will occur Spontaneous reactions will occur without help and release energywithout help and release energy

Nonspontaneous don’t occur readily Nonspontaneous don’t occur readily or favor productsor favor products

Entropy- amount of disorder, an Entropy- amount of disorder, an increase in entropy will drive a increase in entropy will drive a reaction (Law of disorder)reaction (Law of disorder)

Reactions and possibilitiesReactions and possibilities

Heat, Free Energy and Entropy all Heat, Free Energy and Entropy all combine to determine if a reaction combine to determine if a reaction will occurwill occur

Exothermic-Entropy Increase (Yes)Exothermic-Entropy Increase (Yes)

Endothermic-Entropy decrease (No)Endothermic-Entropy decrease (No)

Other combinations are a maybeOther combinations are a maybe

Entropy CalculationsEntropy Calculations

ΔΔS˚ = S˚(Products)- S˚(Reactants)S˚ = S˚(Products)- S˚(Reactants)

S˚= standard entropy, page 558 S˚= standard entropy, page 558

Free Energy CalculationsFree Energy Calculations

ΔΔG = G = ΔΔH – TH – TΔΔSSΔΔ G˚ = G˚ = ΔΔ G G˚f ˚f (Products)- (Products)- ΔΔ G˚ G˚ff (Reactants) (Reactants)