Topics
Equilibrium constant expression
Solving problems
a) Calculating Keq given concentrations of reactant and products

i) Calculations given equilibrium concentrations
ii) Calculations using ICE charts
iii) Calculations using Algebra

b) Calculating concentrations using Keq values

Equilibrium Constant Expression (Kc or Keq)

Chemists have described a mathematical expression of Le Chatelier's Principle that allows them to predict the exact concentrations of substances in the equilibrium when shifts in concentration, temperature and pressure are made. This allows chemists to use chemical equilibrium in large scale settings (industrial factors) and predicts the expected yield of products for certain equilibrium processes.

This mathematical expression is termed an equilibrium constant expression (Law of chemical equilibrium) and was first described by two Norwegian chemists in 1864 called Cato Guldberg and Peter Waage. The value that is produced by this expression is given the symbol Keq or Kc . The expression produces a constant value for any equilibrium at a specified temperature and pressure.

The following flash presentation illustrates how the expression can be written for the general equation

aA

+

bB

cC

+

dD

 

Key Notes relating to Kc or Keq

a) Products are placed over reactants
b) The balances or coefficients in the balanced chemical equation become the exponents used in the expression for each substance.
c) Substances in a solid or liquid state are not included because their concentration can't change to any great degree.
d) The size of Kc or Keq gives chemists a quick idea of the position of equilibrium.

• Large Keq indicates many products at equilibrium.
• Keq around 1 indicates about equal amounts of reactant and product.
• Keq that is very small indicates few products at equilibrium.

e) The exact value of Keq is dependent on temperature. Since heat can shift equilibrium concentrations, the Keq is listed at a certain temperature. The value of Kc is independent of concentration however.
f) The units for Kc or Keq will vary from reaction to reaction, and are often not listed with units. In this course the Kc or Keq will be listed without the units as just a constant number

 

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Writing Kc Expressions

Writing the Kc expressions Using the procedure shown in the flash presentation above write the Kc expression for the following reaction 1) 2 SO2(g) + O2 (g) 2 SO3 (g) Answer; a) Place products over reactant b) Raise balances to exponents c) Multiply concentration of substances d) Remove any solids or liquids from the expression [ SO3(g) ] 2 Kc = [ SO2 (g) ] 2 X [ O2 (g) ]   2) CaCO3(s) CaO(s) + O2 (g) Answer; a) Place products over reactant b) Raise balances to exponents c) Multiply concentration of substances d) Remove any solids or liquids from expression O2 (g) Kc = 1

 

Check your understanding Write the Kc expression for the following reaction N2 (g) + 3 Cl2 (g) 2 NCl3 (g) click on the eye to check the answer

Activities and Assignment

Assignment : Writing Kc or Keq expressions click on the notepad and complete the questions