Chemical Equilibrium

Chapter 15

Introduction

Lets consider the catalytic methanation reaction

The Equilibrium Constant

Some equilibrium compositions for the methanation reaction

Example 

The Law of Mass Action

The manipulation rules of equilibrium constants

Heterogeneous Equilibria

Calculating Equilibrium Constants

Example

USING THE EQUILIBRIUM CONSTANT

we described how a chemical reaction reaches equilibrium.

how this equilibrium can be characterised by the equilibrium constant.

Applications of Equilibrium Constants

Predicting the direction of Reaction.

Calculating Equilibrium Concentrations

If Qc > Kc, the reaction will go left

If Qc < Kc, the reaction will go right

If Qc = Kc, the reaction is at equilibrium

Example

Three steps in solving equilibrium concentrations:

Set up a table of concentrations.

Substitute the expressions in x for equilibrium concentrations into the equilibrium constant expression.

Solve the equilibrium constant expression for the values of the equilibrium concentrations.

Changing the Reaction Conditions:

Le Chatelier’s Principle

Change in Reactant or Product Concentrations

Effects of Volume and Pressure Changes

Example

The Effect of Catalyst

…

• Le Chatelier’s Principle

• AP Chemistry

• Le Chatelier’s Principle

• Factors that Affect Equilibrium

• Concentration

• Temperature

• Pressure

–For gaseous systems only!

• The presence of a catalyst

• Concentration Changes

• Add more reactant è Shift to products

• Remove reactants è Shift to reactants

• Reaction Quotient

• The reaction quotient for an equilibrium system is the same as the equilibrium expression, but the concentrations are NOT at equilibrium!

• Changes in Concentration

Changes in concentration are best understood in terms of what would happen to “Q” if the concentrations were changed.

N2O4(g)  ó  2NO2(g)

• Temperature Changes

  Exothermic Reactions

• Consider heat as a product in exothermic reactions.

–Add heat è

  Shift to reactants

–Remove heat è

   Shift to products

• Temperature Changes

  Endothermic Reactions

• Consider heat as a reactant in endothermic reactions.

–Add heat è

  Shift to products

–Remove heat è

  Shift to reactants

• Pressure Changes

• Only affects equilibrium systems with unequal moles of gaseous reactants and products.

• N2(g) + 3H2(g) = 2NH3(g)

• Increase Pressure

–Stress of pressure is reduced by reducing the number of gas molecules in the container . . . . . .

• N2(g) + 3H2(g) = 2NH3(g)

• There are 4 molecules of reactants vs. 2 molecules of products.

–Thus, the reaction shifts to the product ammonia.

• PCl5(g) = PCl3(g) + Cl2(g)

• Decrease Pressure

–Stress of decreased pressure is reduced by increasing the number of gas molecules in the container.

• PCl5(g) = PCl3(g) + Cl2(g)

• There are two product gas molecules vs. one reactant gas molecule.

• Thus, the reaction shifts to the products.

• Presence of a Catalyst

• A Catalyst lowers the activation energy and increases the reaction rate.

• It will lower the forward and reverse reaction rates,

• Therefore, a catalyst has NO EFFECT on a system at equilibrium!

• It just gets you to equilibrium faster!

• Presence of an Inert Substance

• An inert substance is a substance that is not- reactive with any species in the equilibrium system.

• These will not affect the equilibrium system.

• If the substance does react with a species at equilibrium, then there will be a shift!

• Given:

• S8(g) + 12O2(g)  ó  8 SO3(g)   +  808 kcals

• What will happen when ……

• Oxygen gas is added?

• The reaction vessel is cooled?

• The size of the container is increased?

• Sulfur trioxide is removed?

• A catalyst is added to make it faster?

Given

  2NaHCO3(s)  ó Na2CO3 (s) +  H2O (g)   +  CO2(g)

• What will happen when . . . . . . .

• Carbon dioxide was removed?

• Sodium carbonate was added?

• Sodium bicarbonate was removed?

• Given

Ca5(PO4)3OH(s)  ó  5Ca2+(aq)  + 3PO43-(aq) +  OH- (aq)

• What will happen when. . . . . .

• Calcium ions are added?

• NaOH is added?

• 1 M HCl is added?

• Na3PO4(aq) is added?

…