Question

1.   If a reactant is added to a reaction at equilibrium, which way will the reaction shift to re-establish equilibrium (use forward or backward)? Why?

2.   If a reaction is endothermic, is heat a reactant or a product?

3.   For an endothermic reaction, if heat is added, which way will the reaction shift to re-establish equilibrium (use forward or backward)? Explain your reasoning.

4.   A reaction has initial concentration of 0.330 M Fe3+ and 0.210 M SCN- At equilibrium, the concentration of FeSCN2+ is 0.150 M.
A) What is the concentration of the Fe3+ at equilibrium?
B) What is the concentration of the SCN- at equilibrium?

5.   For the equilibrium in question 4, what is the value of the equilibrium constant?

Answer 1

1.

When a reactant is added, the reactant concentration is increased in the reaction mixture. According to Le Chatelier's principle, the reaction will try to minimize the concentration of the added reactant species by converting it to product. Hence, the reaction will shift in the forward direction to re-establish equilibrium.

2.

Heat is a reactant as heat is not produced but absorbed in the reaction.

3.

In endothermic reaction, heat is absorbed in the reaction. Hence, if heat is added then the reaction will move in the forward direction as the forward direction minimizes the stress due to added heat by absorbing it. Hence, the reaction will shift in the forward direction to re-establish equilibrium.

4.

Fe³⁺ + SCN^- FeSCN^2-

From the balanced equation, it is clear that 0.150 M of FeSCN^2- is formed from the reaction of 0.150 M of Fe³⁺ and 0.150 M of SCN^-.

A)

Initial concentration of Fe³⁺ = 0.330 M

Concentration of Fe³⁺ at equilibrium = (0.330 - 0.150) M = 0.180 M

B)

Initial concentration of SCN^- = 0.210 M

Concentration of SCN^- at equilibrium = (0.210 - 0.150) M = 0.0600 M

5.

Equilibrium constant,

K = [FeSCN^2-]/[Fe³⁺][SCN^-]

= (0.150)/(0.180)(0.0600)

= 13.9