Question
How to calculate enthalpy change for a reversed reaction
Answers
GPT-4o mini
To calculate the enthalpy change (\( \Delta H \)) for a reversed reaction, you need to follow these steps:
1. **Identify the Original Reaction**: Start with the balanced chemical equation for the original reaction and its associated enthalpy change (\( \Delta H \)).
For example, consider a reaction:
\[
A + B \rightarrow C + D \quad (\Delta H = x \, \text{kJ})
\]
2. **Reverse the Reaction**: The reversed reaction would be:
\[
C + D \rightarrow A + B
\]
3. **Change in Enthalpy**: The enthalpy change for the reversed reaction is equal in magnitude but opposite in sign to that of the forward reaction. Thus, if the forward reaction has an enthalpy change of \( x \) kJ, the reversed reaction will have:
\[
\Delta H_{\text{reversed}} = -x \, \text{kJ}
\]
### Example:
Suppose you have the following reaction:
\[
\text{H}_2(g) + \frac{1}{2}\text{O}_2(g) \rightarrow \text{H}_2\text{O}(l) \quad (\Delta H = -286 \, \text{kJ})
\]
To find the enthalpy change for the reversed reaction:
1. The reversed reaction is:
\[
\text{H}_2\text{O}(l) \rightarrow \text{H}_2(g) + \frac{1}{2}\text{O}_2(g)
\]
2. The enthalpy change is calculated as:
\[
\Delta H_{\text{reversed}} = -(-286 \, \text{kJ}) = +286 \, \text{kJ}
\]
### Conclusion:
In summary, to find the enthalpy change for a reversed reaction, simply take the negative of the enthalpy change associated with the forward reaction.
1. **Identify the Original Reaction**: Start with the balanced chemical equation for the original reaction and its associated enthalpy change (\( \Delta H \)).
For example, consider a reaction:
\[
A + B \rightarrow C + D \quad (\Delta H = x \, \text{kJ})
\]
2. **Reverse the Reaction**: The reversed reaction would be:
\[
C + D \rightarrow A + B
\]
3. **Change in Enthalpy**: The enthalpy change for the reversed reaction is equal in magnitude but opposite in sign to that of the forward reaction. Thus, if the forward reaction has an enthalpy change of \( x \) kJ, the reversed reaction will have:
\[
\Delta H_{\text{reversed}} = -x \, \text{kJ}
\]
### Example:
Suppose you have the following reaction:
\[
\text{H}_2(g) + \frac{1}{2}\text{O}_2(g) \rightarrow \text{H}_2\text{O}(l) \quad (\Delta H = -286 \, \text{kJ})
\]
To find the enthalpy change for the reversed reaction:
1. The reversed reaction is:
\[
\text{H}_2\text{O}(l) \rightarrow \text{H}_2(g) + \frac{1}{2}\text{O}_2(g)
\]
2. The enthalpy change is calculated as:
\[
\Delta H_{\text{reversed}} = -(-286 \, \text{kJ}) = +286 \, \text{kJ}
\]
### Conclusion:
In summary, to find the enthalpy change for a reversed reaction, simply take the negative of the enthalpy change associated with the forward reaction.