Consider the reaction `HCN_((aq))hArrH_((aq))^(+) + CN_((aq))^(-)` . At equilibrium, the addition of `CN_((aq))^(-)` would

Options

A. Reduce `HCN_((aq))` concentration
B. Decrease the `H_((aq))^(+)` ion consideration
C. Increase the equiliberium constant
D. Decrease the equilibrium constant

Explanation

To solve the problem regarding the effect of adding \( CN^- \) on the equilibrium of the reaction \( HCN_{(aq)} \rightleftharpoons H^+_{(aq)} + CN^-_{(aq)} \), we can follow these steps:

### Step 1: Understand the Reaction
The given reaction is an equilibrium reaction where hydrogen cyanide (HCN) dissociates into hydrogen ions (\( H^+ \)) and cyanide ions (\( CN^- \)). At equilibrium, the concentrations of the reactants and products remain constant.

### Step 2: Identify the Effect of Adding \( CN^- \)
When we add \( CN^- \) to the system, we are increasing the concentration of one of the products. According to Le Chatelier's Principle, if a system at equilibrium is disturbed by changing the concentration of one of the components, the system will shift in a direction that counteracts the change.

### Step 3: Determine the Direction of Shift
Since we are increasing the concentration of \( CN^- \), the equilibrium will shift to the left (toward the reactants) to reduce the concentration of \( CN^- \). This means that more \( HCN \) will form as the system tries to counteract the increase in \( CN^- \).

### Step 4: Analyze the Consequences of the Shift
As the equilibrium shifts to the left:
- The concentration of \( HCN \) will increase.
- The concentration of \( H^+ \) will decrease because the forward reaction (producing \( H^+ \)) will slow down.

### Step 5: Evaluate the Options
Now, let's evaluate the provided options based on our analysis:
1. **Reduce HCN concentration** - This is incorrect because the concentration of \( HCN \) will actually increase.
2. **Decrease the \( H^+ \) ion concentration** - This is correct because as the equilibrium shifts left, the concentration of \( H^+ \) will decrease.
3. **Increase the equilibrium constant** - This is incorrect because the equilibrium constant remains unchanged unless the temperature changes.
4. **Decrease the equilibrium constant** - This is also incorrect for the same reason as above.

### Conclusion
The correct answer is that adding \( CN^- \) will **decrease the \( H^+ \) ion concentration**.

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