Question

(a) How is a reaction quotient used to determine whether a system is at equilibrium? (b) If \(Q_{c}>K_{c}\), how must the reaction proceed to reach equilibrium? (c) At the start of a certain reaction, only reactants are present; no products have been formed. What is the value of \(Q_{c}\) at this point in the reaction?

Short answer

(a) A reaction quotient (Qc) is used to determine whether a system is at equilibrium by comparing its value to the equilibrium constant (Kc). If Qc = Kc, the system is at equilibrium. If Qc ≠ Kc, the system is not at equilibrium and will proceed to reach equilibrium. (b) If \(Q_{c}>K_{c}\), the reaction must proceed in the reverse direction, converting some products back into reactants, to reach equilibrium. (c) The value of Qc at the starting point of the reaction, when only reactants are present and no products have been formed, is 0.

Step-by-step solution

1. Define the terms, Reaction Quotient (Qc) and Equilibrium Constant (Kc)

Reaction quotient (Qc) is a measure of the relative amounts of reactants and products in a chemical reaction at any particular point in time. It is calculated using the concentrations of the products and reactants raised to their stoichiometric coefficients in the balanced reaction equation. The equilibrium constant (Kc) is the value of the reaction quotient (Qc) when the reaction reaches equilibrium at a given temperature. It is calculated in the same way as Qc, but at equilibrium conditions.

2. Explain how the reaction quotient is used to determine if a system is at equilibrium

We can use the relationship between Qc and Kc to determine whether a system is at equilibrium. (a) If Qc = Kc, the system is at equilibrium, meaning that the forward and reverse reactions are occurring at the same rate and no net change in reactants and products concentration is observed. (b) If Qc ≠ Kc, the system is not at equilibrium, and the reaction will proceed in such a way that the reaction quotient approaches the value of the equilibrium constant.

3. Understand the relationship between Qc and Kc to determine how the reaction must proceed to reach equilibrium

We can use the comparison of Qc and Kc to find the direction a reaction will proceed: 1. If Qc > Kc, this means that there are too many products relative to reactants in the current system and the reaction will shift towards the reactants' side (proceeds in the reverse direction) to reach equilibrium. Answer to part (b): If \(Q_{c}>K_{c}\), the reaction must proceed in the reverse direction, converting some products back into reactants, to reach equilibrium.

4. Calculate the reaction quotient Qc when only reactants are present

When only reactants are present in the system (and no products have formed), the concentration of the products is zero. Therefore, the reaction quotient Qc at this point can be calculated by dividing the concentration of the products raised to their stoichiometric coefficients by the concentration of the reactants raised to their stoichiometric coefficients. Since the concentration of products is zero, the entire expression will be equal to zero. Answer to part (c): The value of Qc at the starting point of the reaction, when only reactants are present and no products have been formed, is 0.