Important questions in chemistry

How far?

How fast?

Learning objectives

► Describe fundamental principles behind reaction kinetics

► Describe factors that affect reaction rates

► Define rate of reaction

► Describe concept of equilibrium in terms of forward and reverse reactions

► Write equilibrium constant expressions

► Predict concentrations of reactants and products using equilibrium constant expressions

► Describe physical basis of Le Chatelier’s principle

► Predict responses of systems to changes in conditions using Le Chatelier’s principle

Equilibrium

►Not all reactions go to completion – equilibrium

► State of equilibrium is when system has reached lowest energy state

Not a static event but a dynamic one

Rate of formation of products = rate of formation of reactants

► Le Chatelier’s principle predicts how systems at equilibrium respond to changes in conditions

Underlying principles behind reactions: echoes of kinetic theory

►Molecules are in motion

►Molecules undergo collisions

►Only some collisions result in products

►Temperature increases motion of molecules

Reaction pathway in energy

► In order for a reaction to proceed reacting molecules must leap over energy barrier; molecules with insufficient energy don’t make it

Bond breaking

Bond making

Requires 100 kJ

Returns 100 kJ

Kinetics deals with things in the box

Reactants at

equilibrium

Province of kinetics

Products at

equilibrium

Exo-thermic and endo-thermic

►H2 + O2 gives out energy – exothermic

►N2 + O2 absorbs energy - endothermic

The pathway of a reaction

Chance meetings

►On the molecular level reactions occur via collisions

Big But: Not all collisions result in reaction

►Two considerations

►Energy of the molecules

Overcoming the barrier

►Orientation of the molecules

Getting them lined up

Very significant for larger molecules

Insufficient energy

Wrong orientation

Factors that affect reaction rate

►Concentration of REACTANTS

►Temperature (kinetic energy of molecules)

►Presence of a catalyst

►Physical state of reactants (surface area)

Higher concentration = more collisions

Ways over (or around) the barrier

►Temperature increases reaction rate by increasing fraction of molecules with enough energy to jump barrier

►A catalyst is a way to lower the barrier. A catalyst increases the reaction rate, but is not consumed itself during the reaction

Catalysts modify the pathway

►Addition of chlorine catalyst increases rate of decomposition of O3

to O2 – reason for ozone hole (CFCs)

► Pathway is modified: two barriers smaller than one without catalyst

Clean air and catalysis

►The metal surface catalyzes oxidation of unwanted exhaust to CO2, H2O and N2

Without catalysts, there would be no life at all, from microbes to humans

► ENZYMES are biological catalysts

►Most enzymes are proteins – large molecules

►Have correct shape to bring reactant molecules together in correct orientation

Enzyme

Equilibrium: a rate of reaction perspective

►Forward reaction

A + B → C + D

►Backward reaction

A + B ← C + D

►Equilibrium results: Rate of forward reaction = rate of backward reaction

A + B ↔ C + D

Equilibrium constant expression

aA + bB ↔ cC + dD

Always Products overReactants

ba

dc

eqBA

DCK

][][

][][

ReactantsProducts

Coefficient

Not all products and reactants are included

► Ignore all pure solids and liquids – they do not change concentrations during reactions

►Consider

MnO2(s) + 4HCl(aq) = MnCl2(aq) + Cl2(g) + 2H2O(l)

Not all products and reactants are included

► Ignore all pure solids and liquids – they do not change concentrations during reactions

MnO2(s) + 4HCl(aq) = MnCl2(aq) + Cl2(g) + 2H2O(l)

4

22

][

]][[

HCl

ClMnClKeq

Significance of large Keq

► The finishing point is the same coming from either direction

Significance of small Keq

Calculations – putting numbers in

►Consider the reaction

2HI(g) ↔ H2(g) + I2(g)

What is the value of Keq if [HI] = 0.54 M, [H2] = [I2] = 1.72 M?

2

22

][

]][[

HI

IHKeq

2.10]54.0[

]72.1][72.1[2eqK

Solving problems with K

► Equilibrium constant for reaction:

N2O4(g) = 2NO2(g) is 4.6 x 10-3

► If [NO2] = 0.050 M, what is [N2O4]?

Solving problems with K

► Equilibrium constant for reaction:

2NOBr(g) = 2NO(g) + Br2(g) is 2.0

► If [NO] = 2.0 M and [Br2] = 1.0 M, what is [NOBr]?

Upsetting the applecart

►What happens to the equilibrium when changes are made?

►Le Chatelier’s Principle

If a stress is placed on a system at equilibrium, the system will respond by changing its position to minimize the stress

Changes in composition

►Consider the reaction at equilibrium

2HI(g) ↔ H2(g) + I2(g)

►What happens if additional H2(g) is added?

The system responds by trying to reduce the amount of added material; H2 is converted into HI – the equilibrium shifts away from the point of change

2HI(g) ↔ H2(g) + I2(g)

In general:

Add products: products → reactants

aA + bB ↔ cC + dD

Add reactants: reactants → products

aA + bB ↔ cC + dD

►Other effects;

Temperature

Pressure

Specific example

Temperature and equilibrium

N2(g) + 3H2(g) = 2NH3(g) + heat

Exothermic Reaction:

Supply heat: equilibrium adjusts to disperse heat: shifts towards reactants

►Less NH3 is made

Endothermic reactions show opposite response

Heat shifts towards products

►Why we heat endothermic reactions

Pressure and equilibrium

2HI(g) ↔ H2(g) + I2(g)

2 moles reactants → 2 moles products

No overall pressure change

N2(g) + 3H2(g) = 2NH3(g)

4 moles reactants → 2 moles products

Increase pressure drives reactants →

products

Summary of Le Chatelier