The equilibrium constant, Kc

What is the equilibrium constant?

A number that indicates the proportion of the products compared to the reactants at equilibrium - indictates where the position of equilibrium is

What does K_c mean?

The equilibrium constant that uses the concentrations of chemicals at equilibrium

What does a high value of K_c (significantly greater than 1) tell you?

The products predominate and the equilibrium lies to the right

What does a K_c value of 1 tell you?

The proportions of the reactants and products are equal - the positiion of equilibrium is in the middle

What does a K_c value of less than 1 tell you?

The reactants predominate and equilibrium lies to the right

The K_c of a reaction is specific. When would it only change?

When the temperature of the reaction changes

What is the short-hand way of writing concentrations of chemicals?

By putting square brackets around them

What would be the K_c expression of the reaction aA + bB ⇌ cC + dD?

The product concentrations would be on the top and the reactant concentrations would be on the bottom

What would you do when writing a K_c expression if the reaction contains a solid?

The solid is always ignored in equilibrium expressions

What would be the equilibrium expression for the reaction Ag⁺ (aq) + Fe²⁺ (aq) ⇌ Ag (s) + Fe³⁺ (aq)?

[Ag (s)] is not included in the equilibrium expression as it is a solid

Do you have to include the state symbols in K_c expressions?

No unless the exam question specifically asks for them

What does the units of K_c depend on?

The moles of the products and reactants

Example:

Ethanoic acid and ethanol react according to the following equation:

• CH₃COOH (I) + C₂H₅OH (I) ⇌ CH₃COOC₂H₅ (I) + H₂O (I)

At equilibrium, 500 cm³ of the reaction mixture contained 0.235 mol of ethanoic acid and 0.035 mol of ethanol together with 0.182 mol of ethyl ethanoate and 0.182 mol of water.

Calculate the value of K_c for this reaction.

1. Calculate the concentrations of the reactants and products

500 cm³ = 0.500 dm³

• [CH₃COOH] = 0.235/ 0.500 = 0.470 mol dm^-3

• [C₂H₅OH] = 0.035/ 0.500 = 0.070 mol dm^-3

• [CH₃COOC₂H₅] = 0.182/ 0.500 = 0.364 mol dm^-3

• [H₂O] = 0.182/ 0.500 = 0.364 mol dm^-3

2. Make sure the equation is balanced and calculate the associating concentrations if necessary. Then write the K_c constant for this reaction:

   K_c = [H₂O] [CH₃COOC₂H₅] / [C₂H₅OH] [CH₃COOH]

3. Substitute the equilibrium concentrations into the expression:

   K_c = (0.364) x (0.364) / (0.070) x (0.470)

   K_c = 4.03

4. Deduce the unit for K_c

   K_c = mol dm^-3 x mol dm^-3 / mol dm^-3 x mol dm^-3

   Units cancel out

Therefore, K_c = 4.03 no units

What is stoichiometry? How do you get the stoichiometry of a reaction?

The molar relationship between the relative quantities of substances taking part in a reaction. You get the stoichiometry of a reaction from the balancing numbers in the balanced equation