Question

methane and chlorine react to form chloroform and hydrogen chloride, like this:

ch₄(g)+3cl₂(g)→chcl₃(g)+3hcl(g)

the reaction is exothermic. suppose a mixture of ch₄, cl₂, chcl₃ and hcl has come to equilibrium in a closed reaction vessel. predict what change, if any, the perturbations in the table below will cause in the composition of the mixture in the vessel. also decide whether the equilibrium shifts to the right or left.

Explanation:

Step1: Recall Le - Chatelier's principle

Le - Chatelier's principle states that if a system at equilibrium is subjected to a change (perturbation), the system will adjust itself to counteract that change. For an exothermic reaction \(CH_{4}(g)+3Cl_{2}(g)
ightleftharpoons CHCl_{3}(g)+3HCl(g)\), heat is a product (\(\Delta H<0\)).

Step2: Analyze the effect of lowering temperature

When the temperature is lowered, the system will try to increase the temperature. Since the forward reaction is exothermic (produces heat), the equilibrium will shift to the right. As the equilibrium shifts to the right, more \(CHCl_{3}\) is produced. According to the ideal gas law \(PV = nRT\) (at constant \(V\) and \(T\), \(P\propto n\)), the number of moles of \(CHCl_{3}\) increases, so the pressure of \(CHCl_{3}\) will increase.

Step3: Analyze the effect of raising temperature

When the temperature is raised, the system will try to decrease the temperature. Since the reverse reaction is endothermic (absorbs heat), the equilibrium will shift to the left. As the equilibrium shifts to the left, more \(CH_{4}\) is produced. According to the ideal gas law \(PV=nRT\) (at constant \(V\) and \(T\), \(P\propto n\)), the number of moles of \(CH_{4}\) increases, so the pressure of \(CH_{4}\) will increase.

Answer:

• When the temperature is lowered: The pressure of \(CHCl_{3}\) will increase and the equilibrium shifts to the right.
• When the temperature is raised: The pressure of \(CH_{4}\) will increase and the equilibrium shifts to the left.