H2 Chemistry 9 - Chemical Equilibria: Kc Calculation and Le Chatelier's Principle

Step 1 - State the problem

Consider the esterification reaction: ethanol reacts with ethanoic acid to form ethyl ethanoate and water.

Step 1 - State the problem

At equilibrium in a one cubic decimetre container, the mixture contains zero point three three moles of ethanol, zero point three three moles of ethanoic acid, zero point six seven moles of ethyl ethanoate, and zero point six seven moles of water.

Step 1 - State the problem

Calculate K c and predict the effect of adding more ethanol.

Step 2 - Write the Kc expression

K c equals the product of the concentrations of the products, each raised to their stoichiometric coefficients, divided by the product of the concentrations of the reactants.

Step 2 - Write the Kc expression

For this reaction, all coefficients are one, so K c equals the concentration of ethyl ethanoate times the concentration of water, divided by the concentration of ethanol times the concentration of ethanoic acid.

Step 2 - Write the Kc expression

Note that water is included because all species are in the same liquid phase.

Step 3 - Substitute and calculate Kc

Substitute the equilibrium concentrations.

Step 3 - Substitute and calculate Kc

K c equals zero point six seven times zero point six seven, divided by zero point three three times zero point three three.

Step 3 - Substitute and calculate Kc

The numerator is zero point four four eight nine. The denominator is zero point one zero eight nine.

Step 3 - Substitute and calculate Kc

K c equals four point one two, which we round to four point one.

Step 4 - Apply Le Chatelier's Principle: adding ethanol

If we add more ethanol to the equilibrium mixture, the system will respond to oppose the change.

Step 4 - Apply Le Chatelier's Principle: adding ethanol

The position of equilibrium shifts to the right, towards the products, to consume the extra ethanol.

Step 4 - Apply Le Chatelier's Principle: adding ethanol

This increases the yield of ethyl ethanoate.

Step 4 - Apply Le Chatelier's Principle: adding ethanol

Crucially, the value of K c does not change because K c depends only on temperature.

Step 5 - Effect of temperature and common pitfall

Esterification is slightly exothermic.

Step 5 - Effect of temperature and common pitfall

If the temperature increases, the equilibrium shifts to the left, the endothermic direction, and K c decreases.

Step 5 - Effect of temperature and common pitfall

If the temperature decreases, the equilibrium shifts to the right, and K c increases.

Step 5 - Effect of temperature and common pitfall

Common mistake: saying that adding a catalyst changes K c or the position of equilibrium. A catalyst speeds up both forward and reverse reactions equally, so neither K c nor the equilibrium position changes.