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 for temperature change in exothermic reactions

For an exothermic reaction \(CH_{4}(g)+3Cl_{2}(g)\to CHCl_{3}(g)+3HCl(g)\), heat is a product (\(\Delta H<0\)). When the temperature is lowered, according to Le - Chatelier's principle, the system will try to counteract the change. Since heat is a product, the reaction will shift to the right (towards the products) to produce more heat. As the reaction shifts to the right, more \(HCl\) is produced. The pressure of a gas is directly proportional to the amount of gas (assuming volume and temperature are constant, \(P = n\frac{RT}{V}\)). So, the pressure of \(HCl\) will increase.

Step2: Recall Le - Chatelier's principle for temperature change in exothermic reactions (temperature raised)

When the temperature is raised, the system will try to counteract the increase in temperature. For an exothermic reaction, the reverse reaction is endothermic. So, the reaction will shift to the left (towards the reactants). As the reaction shifts to the left, \(Cl_{2}\) (a reactant) is consumed less (or produced more if we consider the reverse reaction). The pressure of \(Cl_{2}\) will increase.

Answer:

• When the temperature is lowered: The pressure of \(HCl\) will increase, and the equilibrium shifts to the right.
• When the temperature is raised: The pressure of \(Cl_{2}\) will increase, and the equilibrium shifts to the left.