When two reactants A and B are mixed to give products C and D, the reaction quotient (Q) at the initial stages of the reaction

Options

A. Is zero
B. Decreases with time
C. Is independent of time
D. Increases with time

Explanation

To solve the question regarding the reaction quotient (Q) when two reactants A and B are mixed to give products C and D, we can follow these steps:

### Step-by-Step Solution:

1. **Write the Balanced Chemical Equation**:
The first step is to write the balanced equation for the reaction. For the given reactants A and B producing products C and D, the reaction can be represented as:
\[ aA + bB \rightleftharpoons cC + dD \]
where \(a\), \(b\), \(c\), and \(d\) are the stoichiometric coefficients.

2. **Define the Reaction Quotient (Q)**:
The reaction quotient \(Q\) is defined as:
\[ Q = \frac{[C]^c[D]^d}{[A]^a[B]^b} \]
where \([C]\), \([D]\), \([A]\), and \([B]\) are the molar concentrations of the respective species.

3. **Analyze Initial Conditions**:
At the initial stages of the reaction (when \(t = 0\)), the concentrations of products C and D are zero because the reaction has just begun. Therefore, we can express:
\[ [C] = 0 \quad \text{and} \quad [D] = 0 \]

4. **Calculate Q at Initial Stage**:
Substituting the initial concentrations into the expression for \(Q\):
\[ Q = \frac{[0]^c[0]^d}{[A]^a[B]^b} = \frac{0}{[A]^a[B]^b} = 0 \]
Thus, at the initial stage, \(Q = 0\).

5. **Consider Changes Over Time**:
As the reaction progresses, the concentrations of products C and D will increase while the concentrations of reactants A and B will decrease. Therefore, over time, \(Q\) will change.

6. **Determine the Trend of Q**:
Since \(Q\) starts at 0 and increases as the concentrations of products increase, we can conclude that:
- \(Q\) increases with time until it reaches a value that corresponds to the equilibrium constant \(K\) when the system reaches equilibrium.

### Conclusion:
Based on the analysis, the correct answer to the question is that the reaction quotient \(Q\) **increases with time**.