Which of the following expressions is correct to calculate enthalpy of a reaction ?

Options

A. `DeltaH_("reaction")=SigmaDelta_(f)H_("reactions")-SigmaDelta_(f)H_("products")`
B. `DeltaH_("reaction")=SigmaB.E._("products")-Sigma B.E._("reactants")`
C. `DeltaH_("reaction")=SigmaB.E._("reactants")-SigmaB.E._("products")`
D. `DeltaH_("reaction")=DeltaH_(1)xxDeltaH_(2)xxDeltaH_(3)...`

Answer

To determine the correct expression for calculating the enthalpy of a reaction, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Enthalpy of Reaction**:
- The enthalpy change (ΔH) for a reaction can be defined as the difference in enthalpy between the products and the reactants.

2. **Using Standard Enthalpy of Formation**:
- One way to calculate the enthalpy of a reaction is by using the standard enthalpy of formation (ΔH_f) of the reactants and products.
- The formula is:
\[ \Delta H_{\text{reaction}} = \sum \Delta H_f \text{(products)} - \sum \Delta H_f \text{(reactants)} \]

3. **Using Bond Energies**:
- Another method to calculate the enthalpy change is by using bond energies.
- The formula is:
\[ \Delta H_{\text{reaction}} = \sum \text{Bond Energies (reactants)} - \sum \text{Bond Energies (products)} \]

4. **Evaluating the Given Options**:
- **Option 1**: States that ΔG (Gibbs Free Energy) is equal to the summation of ΔH_f of reactions. This is incorrect.
- **Option 2**: States that ΔH reaction is equal to the summation of bond energy of products. This is also incorrect.
- **Option 3**: States that ΔH reaction is equal to the summation of bond energy of reactants minus the summation of bond energy of products. This is correct.

5. **Conclusion**:
- The correct expression to calculate the enthalpy of a reaction is given by Option 3:
\[ \Delta H_{\text{reaction}} = \sum \text{Bond Energies (reactants)} - \sum \text{Bond Energies (products)} \]