For a reaction at equilibrium which of the following is correct ?

Options

A. Concentration of reactant = concentration of product
B. Concentration of reactant is always greater than product
C. Rate of forward reaction = rate of backward reaction
D. `Q_(c) = k`

Explanation

To determine which statements are correct for a reaction at equilibrium, let’s analyze the options provided step by step.

### Step 1: Understanding Equilibrium
At equilibrium, the rates of the forward and backward reactions are equal. This means that the concentration of reactants and products remains constant over time, but it does not imply that their concentrations are equal.

### Step 2: Analyzing the Options
1. **Concentration of reactant equals concentration of product**: This statement is not necessarily true. At equilibrium, the concentrations of reactants and products can be equal, greater, or lesser depending on the reaction and the conditions. Therefore, this option is incorrect.

2. **Concentration of reactant is always greater than that of product**: This statement is also not true. The concentration of reactants can be greater than, less than, or equal to that of products at equilibrium. Thus, this option is incorrect.

3. **Rate of forward reaction equals rate of backward reaction**: This statement is true. At equilibrium, the rate of the forward reaction is equal to the rate of the backward reaction. This is a fundamental principle of chemical equilibrium. Therefore, this option is correct.

4. **Qc equals k**: Qc (reaction quotient) is defined in the same way as the equilibrium constant (K) but can be calculated at any point in the reaction. At equilibrium, Qc is equal to K. Therefore, this option is also correct.

### Conclusion
The correct statements for a reaction at equilibrium are:
- The rate of forward reaction equals the rate of backward reaction (Option 3).
- Qc equals K (Option 4).

### Final Answer
The correct options are:
- Rate of forward reaction equals rate of backward reaction.
- Qc equals k.

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