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AQA A Level Chemistry復習筆記1.8.4 Calculations Involving the Equilibrium Constant

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年6月30日下午2:40

Kc Calculations

Calculations involving?Kc

In the equilibrium expression each figure within a square bracket represents the concentration in?mol dm-3
The?units?of?Kc?therefore depend on the form of the equilibrium expression
Some questions give the?number of moles?of each of the reactants and products at equilibrium together with the volume of the reaction mixture
The concentrations of the reactants and products can then be calculated from the number of moles and total volume

1.7-Equilibria-Equation-for-concentration

Equation to calculate concentration from number of moles and volume

Worked 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?Kc for this reaction.

Answer

Step 1:?Calculate the concentrations of the reactants and products

Equilibrium-Constant-Calculations-WE-Step-1-equation-1

Step 2:?Write out the balanced chemical equation with the concentrations of beneath each substance

Equilibrium-Constant-Calculations-WE-Step-1-equation-2

Step 3:?Write the equilibrium constant for this reaction in terms of concentration

Equilibrium-Constant-Calculations-WE-Step-1-equation-3

Step 4:?Substitute the equilibrium concentrations into the expression

Equilibrium-Constant-Calculations-WE-Step-1-equation-4

Step 5:?Deduce the correct units for?Kc

Equilibrium-Constant-Calculations-WE-Step-1-equation-5

All units cancel out

Therefore,?Kc?=?4.03

Note that the smallest number of significant figures used in the question is 3, so the final answer should also be given to 3 significant figures

Some questions give the?initial and equilibrium concentrations?of the reactants but not the products
An initial, change and equilibrium table should be used to determine the equilibrium concentration of the products?using the molar ratio of reactants and products in the stoichiometric equation

Worked Example

Calculating Kc?of ethyl ethanoate

Ethyl ethanoate is hydrolysed by water:

CH3COOC2H5(I) +?H2O(I)?? CH3COOH(I) + C2H5OH(I)

0.1000 mol of ethyl ethanoate are added to 0.1000 mol of water. A little acid catalyst is added and the mixture made up to 1dm3. At equilibrium 0.0654 mol of water are present. Use this data to calculate a value of Kc?for this reaction.

Answer

Step 1:?Write out the balanced chemical equation with the concentrations of beneath each substance using an initial, change and equilibrium table

1.7-Equilibria-Calculating-Kc-of-ethyl-ethanoate-table