Chemical EquilibriumCollision theoryRates of reactionsCatalystsReversible reactionsChemical equilibriumLe Chatelier’s Principle

ConcentrationTemperatureVolumeCatalysts

A. Collision Theory

Reaction rate depends on the collisions between reacting particles.

Successful collisions occur if the particles...

collide with each other

have the correct orientation

have enough kinetic energy to break bonds

A. Collision TheoryParticle Orientation

Required Orientation

Successful CollisionUnsuccessful

Collisions

Exothermic Endothermic

Time

En

erg

y

Activation energy

Energy of reaction

Time

En

erg

y

Activation energy: minimum energy required for a reaction to occur

Chemical kinetics: the study of the rate (the speed) of a reaction

Rate of a chemical reaction depends on:

16.2: Rates of Reactions

3. TEMPERATURE (T) of reactants

2. CONCENTRATION of reactants

4. Presence/absence of a CATALYST

1. SURFACE AREA

SURFACE AREA

Surface Areahigh SA = fast rxn ratemore opportunities for collisionsIncrease surface area by…• using smaller particles• dissolving in water

Concentration:increasing concentration of reactants results in more collisions.

Effect of Concentration on Rate

More collisions = increased rate of reaction

Temperature:Increasing T increases particle speed.

Effect of Temperature on Rate

Faster reactants means more collisions have the activation energy, which increases the rate of the reaction.

TemperatureAnalogy: 2-car collision

5 mph “fender bender”

50 mph “high-speed crash”

A catalyst:A chemical that influences a reaction, but is not consumed in the reaction. (It can be recovered unchanged at the end of the reaction.)Lowers the activation energy of the reaction.

Effect of Catalysts on Rate

Activation energy

Time

En

erg

y

Activation energy with catalyst

CatalystsEnzyme Catalysis

16.1: Reversible Reactions* Thus far, we have considered only one-way reactions: A + B → C + D

Some reactions are reversible:They go forward (“to the right”) : A + B → C + D

and backwards (“to the left”) : A + B ← C + D

Written with a two-way arrow:A + B ↔ C + D

Boiling & condensingFreezing & melting

Examples:

Chemical Equilibrium

o At chemical equilibrium there is At chemical equilibrium there is no no net changenet change in the actual amounts in the actual amounts of the components of the system.of the components of the system.

o And although the And although the ratesrates of the of the forward & reverse rxns forward & reverse rxns are equalare equal at chemical equilibrium, the at chemical equilibrium, the concentrationsconcentrations of the components of the components on both sides of the chem-ical eqn on both sides of the chem-ical eqn are not necessarily the sameare not necessarily the same..

*In fact they can be dramatically *In fact they can be dramatically different.different.

Reversible ReactionsReversible Reactions

o Consider a set of escalators as being like the double arrows in a dynamic equilibrium.

o The # of people using the up escalator must be the same as the # of people using the down escalator for the # of people on each floor to remain at equilibrium

• However, the # of people upstairs do not have to equal the # of people downstairs

• Just the transfer between floors must be consistent

Examples of irreversible reactions:

Striking a match / burning paperDropping an eggCooking (destroys proteins)

For a reversible reaction, when the forward rate equals the backward rate, a chemical equilibrium has been established.

Both the forward and backward reactions continue, but there is a balance of products “un-reacting” and reactants reacting.

A + B ↔ C + D

A B+ C D+A B+A B+ C D+C D+

16.3: Chemical Equilibrium

Equilibrium ExpressionChemist’s generally express the position of equilibrium in terms of numerical values

These values relate the amounts of reactants to products at equilibrium

Consider this hypothetical rxn…

wA + xB yC + zD

• Where “w” mols of reactant A and “x” mols of reactant B react to give “y” mols of product C and “z” mols of product D at equil.

We can write a mathematical expression to We can write a mathematical expression to show the ratio of product concentrations to show the ratio of product concentrations to reactant concentrations called anreactant concentrations called an equilibrium equilibrium expressionexpression

Equilibrium Equilibrium ExpressionExpression

[C][C]yy[D][D]zz

[A][A]ww[B][B]xx

o The The concentrationconcentration or or pressurepressure of each of each substance is raised to a power equal substance is raised to a power equal to the # of mols of that substance in to the # of mols of that substance in the balanced rxn eqn.the balanced rxn eqn.

o The square brackets indicate The square brackets indicate concentration in Molarity (mol/Lconcentration in Molarity (mol/L))

K =

The resulting ratio of the The resulting ratio of the equilibrium is called theequilibrium is called the equilibrium constant or Keqequilibrium constant or KeqThe Keq is dependent on the tempThe Keq is dependent on the temp

If the temp changes so does the KeqIf the temp changes so does the Keq

Equilibrium Equilibrium ExpressionExpression

Keq=Keq= [C][C]yy[D][D]zz

[A][A]ww[B][B]xx

NOTE: pure NOTE: pure solidssolids and and liquidsliquids cannot cannot change concentration, therefore they change concentration, therefore they ARE NOT ARE NOT included in the equilibrium included in the equilibrium

expressionexpression

Equilibrium constants provide Equilibrium constants provide valuable chemical informationvaluable chemical informationThey show whether products or They show whether products or reactantsreactants are favored in a rxnare favored in a rxn

always written as a ratio of products always written as a ratio of products over reactants over reactants a value of a value of Keq > 1 Keq > 1 means that means that products are favored products are favored Keq < 1 Keq < 1 than than reactants are favoredreactants are favored

Equilibrium ConstantEquilibrium Constant

Sample Problem 1Sample Problem 1Dinitrogen tetroxide (NDinitrogen tetroxide (N22OO44), a colorless ), a colorless

gas, and nitrogen dioxide (NOgas, and nitrogen dioxide (NO22), a ), a brown gas, exist in equilibrium with brown gas, exist in equilibrium with

each other according to the following each other according to the following eqn:eqn:

NN22OO44(g)(g) 2NO 2NO22(g)(g)

A liter of gas mixture at 10A liter of gas mixture at 10C at C at equilibrium contains 0.0045mol equilibrium contains 0.0045mol NN22OO44 & 0.030 mol NO & 0.030 mol NO22. Write the . Write the

Keq expression and calculate Keq Keq expression and calculate Keq for the reaction.for the reaction.

Known:Known:[N[N22OO44] =] = .0045mol/L.0045mol/L

[NO[NO22] =] = .030mol/L.030mol/L

UnknownUnknown::Keq expression = ?Keq expression = ?

Keq = ?Keq = ?

Analyze: list what we Analyze: list what we knowknow

o At equilibrium, there is no net At equilibrium, there is no net change in the amount of Nchange in the amount of N22OO44 or or NONO22 at any given instant at any given instant

The only product of the rxn is NOThe only product of the rxn is NO22, , which has a coefficient of 2 in the which has a coefficient of 2 in the balanced eqnbalanced eqnThe only reactant NThe only reactant N22OO44 has a has a coefficient of 1 in the balanced eqncoefficient of 1 in the balanced eqn

The equilibrium expression is:The equilibrium expression is:

Calculate: solve for Calculate: solve for unknownsunknowns

[.030M][.030M]22

[.0045M][.0045M]11Keq=Keq=

[NO[NO22]]22

[N[N22OO44]]11Keq=Keq=

o Keq is equal to: Keq= 0.20Keq is equal to: Keq= 0.20o Keq < 1, therefore rxn doesn’t Keq < 1, therefore rxn doesn’t

favor productsfavor products

1.1. A mixture at equilibrium at 827°C A mixture at equilibrium at 827°C contains 0.552 M COcontains 0.552 M CO22, 0.552 M H, 0.552 M H22, , 0.448 M CO, and 0.448 M H0.448 M CO, and 0.448 M H22O. O.

COCO22(g)+ H(g)+ H22(g)<==> CO(g) + H(g)<==> CO(g) + H22O(g)O(g)

a.a. Write the equilibrium Write the equilibrium expression for the above rxn.expression for the above rxn.

b.b. Calculate KCalculate Keqeq at this temp? at this temp?

c.c. More COMore CO22 is added to the is added to the system, which direction will the system, which direction will the reaction shift?reaction shift?

d.d. Are the reactants or products Are the reactants or products favored in this reaction?favored in this reaction?

Classwork:Classwork:

* Le Chatelier’s Principle is about reducing stress – a stress applied to a chemical equilibrium

(1850 – 1936)

Relax! Reduce stress brought on by chemical equilibrium

with me, Henri Le Chatelier!

Le Chatelier’s Principle:When a stress is applied to a system (i.e. reaction) at equilibrium, the system responds to relieve the stress.A stress is a change in:

ConcentrationTemperatureVolume

16.4: Le Chatelier’s Principle

Ex: Co(H2O)62+ + 4 Cl- ↔ CoCl42- + 6

H2O

(pink) (blue)

Stress Result

16.5: Stress: Change Concentration

Add Cl1- Forward rxn favored

Shifts forward to use up extra Cl-

More CoCl42- will formAdd H2O Backward rxn favored

Shifts backward to use up extra H2O

More Co(H2O)62+ will form

Ex: heat + Co(H2O)62+ + 4 Cl1- ↔ CoCl42- + 6 H2O

(pink) (blue)

This reaction is endothermic. For Le Chatelier’s principle, consider “heat” as a chemical.

Stress Result

16.7: Stress: Change Temperature

Decrease T Backward rxn favored; shifts backward to replace “lost” heat

More Co(H2O)62+ will form

Increase T Forward rxn favored; shifts forward to reduce extra heat

More CoCl42- will form

Ex: 1 N2 (g) + 3 H2(g) ↔ 2 NH3(g)

(1 + 3 = 4 moles of gas) ↔ (2 moles of gas)

Stress Result

16.6: Stress: Change Volume

Increase V Backward rxn favored; shifts backward to side with more moles of gas (to fill the larger volume with more molecules)

Decrease VForward rxn favored; shifts forward to side with fewer moles of gas (reduces # of

molecules packed into this smaller volume)

Ex: 2 H2O2 (aq) ↔ 2 H2O (l) + O2 (g)

Since a catalyst increases the forward and backward rates equally, it will not shift the equilibrium.

16.7: Catalysts & Equilibrium

MnO2