Chemical Equilibrium

Dynamic Equilibrium

Under certain conditions – the rate of the reverse reaction increases as the rate of the forward reaction decreases

Eventually the rate of the forward reaction equals the rate of the reverse reaction

Occurs when opposing changes occur at the same time and the same rate (reactants are being formed as fast as they are being consumed)

A B

Three physical processes that reach equilibrium are:

A solid in contact with a solution that contains this solid (saturated solution)

The vapor above a pure liquid

The vapor above a pure solid

Two chemical processes that reach equilibrium are:

Homogeneous equilibrium

Heterogeneous equilibrium

The 4 conditionsEquilibrium in achieved in a reversible process when the rates of opposing changes are equal.

The macroscopic properties of a system at equilibrium are constant (ex: color, pressure, concentration, and pH)

Equilibrium can only be reached in a closed system

Equilibrium can be approached from either direction

Video: http://www.youtube.com/watch?v=LMIbJ-B92Ho

Review Quiz – Dynamic

Equilibriumhttp://www.mcgrawhill.ca/school/learningcentres/mod/quiz/attempt.php?id=14338

The Equilibrium Constant

The Law of Chemical Equilibrium: At equilibrium, there is a constant ratio between the concentrations of the products and reactants in any change

The constant ratio is called the equilibrium constant Kc

The subscript “c” is used to show that the equilibrium constant is expressed in terms of molar concentration

Some textbooks will use Keq which is the same as Kc

The Effect of Temperature

For any given system at equilibrium, the value of Kc depends only on temperature

Adding a chemical involved in the reaction will not effect the overall equilibrium constant.

Equilibrium Constants

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aA( )phase + bB( )phase ⇔ cC( )phase + dD( )phase

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aA( )phase + bB( )phase ⇔ cC( )phase + dD( )phase

K = [C]c × [D]d

[A]a × [B]b K is the equilibrium constant (unitless)

Only reactants and products that are in the gaseous or aqueous state are written in the KC expression.

If K > 1 then products are favoredIf K< 1 then reactants are favored

ExamplesWrite the equilibrium expression for each

homogenous reactions

The reaction between propane and oxygen to form carbon dioxide and water vapor:

C3H8 + 5H2(g)3CO2(g) + 4H2O(g) Kc =

The reaction between nitrogen gas and oxygen gas at high temperatures:

N2(g) + O2(g)2NO(g) Kc =

The reaction between hydrogen gas and oxygen gas to form water vapor:

2H2(g) + O2(g)2H2O(g) Kc =

The oxidation of ammonia:

4NH3(g) + 5o2(g)4NO(g) + 6H2O(g) Kc =

Example CalculationA mixture of nitrogen and chlorine gases

was kept at a certain temperature in a 5.0L reaction flask. The reaction is as follows:

N2(g)+3Cl2(g) 2NCl3(g)

When the equilibrium mixture was analyzed, it was found to contain 0.0070mol of N2(g), 0.0022mol of Cl2, and 0.95mol of NCl3.

Calculate Kc

Practice ProblemsPurple Book

Page – 499 #6-#10

Solutions

#6) 1.9x10-2

#7) 1.2x102

#8) 0.013

#9) 0.046mol/L

#10) 0.15

HAHA!!!