Enthalpy of Formation

Introduction

The standard enthalpy of formation ($\Delta H^\circ_f$) is a fundamental concept in chemistry. It forecasts reaction dynamics and finds significant applications in fields such as environmental chemistry and industrial synthesis. A firm grasp of $\Delta H^\circ_f$ aids in the design of chemical processes and assessment of energy transformations.

Definition and Explanation

$\Delta H^\circ_f$: The enthalpy change when one mole of a compound is created from its constituent elements in their standard states.

• Enthalpy Change: This term refers to the thermal energy absorbed or released during a reaction at constant pressure.
• Reactions can be exothermic (release heat) or endothermic (absorb heat).
  • Exothermic example: Combustion of methane.
  • Endothermic example: Melting of ice.

Clarification of Standard States

Standard State: A vital reference point in thermodynamics ensuring consistency in measurements.

• The most stable form at 1 bar (100.0 kPa) pressure and 298 K.
• Examples:
  • Oxygen as $\mathrm{O}_2(\mathrm{g})$.
  • Nitrogen as $\mathrm{N}_2(\mathrm{g})$.
  • Water as liquid at 25°C and 1 bar.

Illustrative Examples

Formation of Water

• Balanced Equation: $2\mathrm{H}_2(\mathrm{g}) + \mathrm{O}_2(\mathrm{g}) \rightarrow 2\mathrm{H}_2\mathrm{O}(\mathrm{l})$
• $\Delta H^\circ_f$ provides insights into reaction feasibility, which is essential for industrial applications.

Formation of Ammonia

• Balanced Equation: $\mathrm{N}_2(\mathrm{g}) + 3\mathrm{H}_2(\mathrm{g}) \rightarrow 2\mathrm{NH}_3(\mathrm{g})$
• Demonstrates $\Delta H^\circ_f$'s importance in anticipating reaction behaviour, a key factor in industrial ammonia production.

Introduction to the Formula

• Reaction enthalpies: Represent the thermal energy change during a chemical reaction.
• Apply the formula:

$\Delta H^{\circ}_{\mathrm{rxn}} = \sum \Delta H^{\circ}_{\mathrm{f}} (\text{products}) - \sum \Delta H^{\circ}_{\mathrm{f}} (\text{reactants})$

Step-by-Step Example Calculations

Combustion of Propane ($\mathrm{C}_3\mathrm{H}_8$)

• Balanced Chemical Equation:

  • $\mathrm{C}_3\mathrm{H}_8 (\mathrm{g}) + 5\mathrm{O}_2 (\mathrm{g}) \rightarrow 3\mathrm{CO}_2 (\mathrm{g}) + 4\mathrm{H}_2\mathrm{O} (\mathrm{l})$

• $\Delta H^{\circ}_{\mathrm{f}}$ Values:

  • Propane $(\mathrm{C}_3\mathrm{H}_8)$: $-104$ kJ/mol
  • Oxygen $(\mathrm{O}_2)$: $0$ kJ/mol
  • Carbon dioxide $(\mathrm{CO}_2)$: $-393.5$ kJ/mol
  • Water $(\mathrm{H}_2\mathrm{O})$: $-285.8$ kJ/mol

• Calculate Net Enthalpy Change:

  • Products Enthalpy: $(3 \times -393.5) + (4 \times -285.8) = -2350.7$ kJ/mol
  • Reactants Enthalpy: $-104$ kJ/mol
  • Net Change: $\Delta H^{\circ}_{\mathrm{rxn}} = -2350.7 - (-104) = -2246.7$ kJ/mol

Industrial Applications

• Combustion Processes:
  • The automotive industry utilises enthalpy to enhance fuel efficiency, influencing costs and sustainability.

• Haber Process:
  • Crucial for producing fertilisers, essential for global food production.

Environmental Applications

• Greenhouse Gas Emissions:

  • Informs the development of alternative fuels to reduce emissions.

• Acid Rain Formation:

  • Insights from enthalpy help mitigate the precursors leading to acid rain.

Introduction to Hess's Law

Hess's Law:

• The enthalpy change of a reaction is invariant regardless of the path taken.
• It is employed in calculating enthalpies for reactions that do not proceed directly, adhering to energy conservation.

Explanation of Hess's Law

• The total enthalpy change during a chemical reaction is unaltered by the reaction path.
• Calculations:

$$\Delta H_{\text{reaction}} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$$

Fun Fact Box