Thermochemistry

Thermochemistry studies energy changes in chemical reactions, crucial for understanding heat flow in processes like combustion or metabolism. Energy, often as heat, is absorbed or released, affecting reaction feasibility. This article covers energy types, enthalpy, Hess’s Law, and applications.

Energy Changes

Reactions are classified by heat:

  • Exothermic: Releases heat (\( q < 0 \)), e.g., \( \ce{CH4 + 2O2 -> CO2 + 2H2O} \), \( \Delta H = -890 \, \text{kJ/mol} \).
  • Endothermic: Absorbs heat (\( q > 0 \)), e.g., \( \ce{N2 + O2 -> 2NO} \), \( \Delta H = +180 \, \text{kJ/mol} \).

Heat (\( q \)) relates to specific heat (\( c \)): \( q = m c \Delta T \).

Enthalpy

Enthalpy (\( H \)) measures heat content at constant pressure:

\[ \Delta H = H_{\text{products}} - H_{\text{reactants}} \]

Negative \( \Delta H \): exothermic; positive: endothermic. Example: \( \ce{H2 + 1/2 O2 -> H2O} \), \( \Delta H = -286 \, \text{kJ/mol} \).

Hess’s Law

Hess’s Law states \( \Delta H \) is path-independent. For \( \ce{C + O2 -> CO2} \):

  1. \( \ce{C + 1/2 O2 -> CO} \), \( \Delta H_1 = -111 \, \text{kJ} \).
  2. \( \ce{CO + 1/2 O2 -> CO2} \), \( \Delta H_2 = -283 \, \text{kJ} \).
\[ \Delta H = \Delta H_1 + \Delta H_2 = -394 \, \text{kJ} \]

Matches direct reaction.

Applications

Thermochemistry drives:

  • Energy: Fuel efficiency (e.g., gasoline combustion).
  • Biology: Caloric content of food.
  • Industry: Heat management in reactors.

It’s essential for sustainable design.