5.3 b/c: Keq Calculations

Keq Calculations + Effect of Temperature

Y12 Chem

​Syllabus dot points:

b	perform calculations to find the value of Keq and concentrations of substances within an equilibrium system, and use these values to make predictions on the direction in which a reaction may proceed
c	qualitatively analyse the effect of temperature on the value of Keq

​What we will be learning about

• ​review: equilibrium expressions

• ​how to calculate Keq

  • ​using initial concentrations

  • ​using concentration + stoichiometry

  • ​using equilibrium concentrations

• ​using calculations to use Keq (and Q) values

​Equilibrium expression

• ​Keq = [products]^coefficient/[reactants]^coefficient

• ​Example: 2NO(g) + Cl₂(g) --> 2NOCl(g)

​Reminder!

​Pure solids and liquids are not included in the equilibrium expression

​Calculating Keq

• ​can calculate and calculate around Keq a number of different ways

  • ​using concentrations at equilibrium

  • ​using Keq to calculate an unknown equilibrium concentration

  • ​using stoichiometry to determine equilibrium concentrations from initial concentrations (usually to calculate Keq)

    • ​This is RICE tables

​Calculating Keq: Concentrations at equilibrium

Open Ended

Keq = ?

​Calculating concentrations at equilibrium:

Open Ended

Concentration = ?

​Calculating Keq using stoichiometry:

Open Ended

Keq = ?

Open Ended

Keq = ?

What if x is very small?

• is x = <10^-3 then we can assume that it is negligible in terms of numerical impact, and can disregard it

• More on this later!

​Effect of Temperature on the equilibrium constant

• The equilibrium constant provides information about the ratio of products to reactants - at a specific temperature

• Adjusts like Le Chateliers Principle - to counteract the change to the reaction

• The equilibrium constant will change as the temperature changes

• If the reaction shifts to the right, the product concentrations will increase and therefore K will increase

• If the reaction shifts to the left, the reactant concentrations will increase and therefore K will decrease