Bond Enthalpy and Enthalpy of Reaction Formulas in GOB Chemistry

Bond Enthalpy and Enthalpy of Reaction

Introduction to Bond Enthalpy

Bond enthalpy (or bond energy, ΔHbond) is the amount of energy stored in a bond between atoms in a molecule. It represents the energy required to break one mole of a specific type of bond in a gaseous substance. Bond energy values are essential for calculating the enthalpy of reaction (ΔHrxn).

• Endothermic Process: Energy is absorbed to break a bond; ΔH has a positive sign.

• Exothermic Process: Energy is released to form a bond; ΔH has a negative sign.

Enthalpy of Reaction Formulas

Enthalpy of reaction can be calculated using bond energies or the enthalpy of formation for a compound.

• Using Bond Energies: $\Delta H_{rxn} = \sum \text{Bond Energies of Reactants} - \sum \text{Bond Energies of Products}$

• Using Enthalpy of Formation: $\Delta H_{rxn} = \sum \Delta H_f^{\circ} (\text{Products}) - \sum \Delta H_f^{\circ} (\text{Reactants})$

Worked Example: Formation of Ammonia

The formation of ammonia is accomplished by the reaction between hydrogen and nitrogen gas:

N≡N + 3 H–H → 2 N–H

Given bond enthalpies: N≡N = 945 kJ/mol, H–H = 432 kJ/mol, N–H = 391 kJ/mol.

1. Multiply the coefficients of each bond type by its bond enthalpy value: Reactants: N≡N (1 × 945), H–H (3 × 432) Products: N–H (6 × 391)

2. Calculate the enthalpy of reaction: $\Delta H_{rxn} = [1 \times 945 + 3 \times 432] - [6 \times 391]$

Practice Problems

Practice 1: Enthalpy of Reaction for H2S and F2

Equation: H2S (g) + 3 F2 (g) → SF4 (g) + 2 HF (g)

Given bond energies:

Bonds	H (kJ/mol)
S–H	347
F–H	565
F–F	159

Determine the bond enthalpy value for the F–S bond (not provided; may require inference or lookup).

Practice 2: Combustion of Acetylene

Equation: 2 C2H2 (g) + 5 O2 (g) → 4 CO2 (g) + 2 H2O (g)

Standard bond energies:

Bonds	kJ/mol	Bonds	kJ/mol
C–C	347	C=O	745
C≡C	614	O=O	1070
C–H	839	O–H	204
O–H	413	C–O	358
			467

Use these values to estimate the enthalpy of reaction for the combustion process.

Summary Table: Standard Bond Energies

Bond	Bond Energy (kJ/mol)
S–H	347
F–H	565
F–F	159
C–C	347
C≡C	614
C–H	839
O–H	413
C=O	745
O=O	1070
C–O	358
O–H	467

Key Steps for Calculating Enthalpy of Reaction

1. Check if the chemical equation is balanced. Balance if necessary.

2. Multiply the coefficients of each bond type by its bond enthalpy value.

3. Sum the total bond energies for reactants and products.

4. Apply the formula: $\Delta H_{rxn} = \sum \text{Bond Energies of Reactants} - \sum \text{Bond Energies of Products}$

Additional info:

• Bond enthalpy calculations are an approximation; actual enthalpy changes may differ due to molecular environment and phase.

• These concepts are foundational for understanding energy changes in chemical reactions, especially in GOB Chemistry courses.