Thermochemistry Guide

Thermochemistry studies energy changes in chemical reactions and physical transformations, bridging chemistry and thermodynamics. This guide explores energy flow, enthalpy, Hess’s Law, and practical applications with visualizations.

Energy Changes

Energy changes determine whether reactions are exothermic (heat-releasing) or endothermic (heat-absorbing).

Exothermic Reactions

Release heat (\( q < 0 \)). Example: propane combustion:

\[ \ce{C3H8 + 5O2 -> 3CO2 + 4H2O} \] \[ \Delta H = -2220 \, \text{kJ/mol} \]

Heat flow: \( q = m c \Delta T \).

Endothermic Reactions

Absorb heat (\( q > 0 \)). Example: calcium carbonate decomposition:

\[ \ce{CaCO3 -> CaO + CO2} \] \[ \Delta H = +178 \, \text{kJ/mol} \]

Heat Capacity

For 50 g water heated from 25°C to 75°C (\( c = 4.18 \, \text{J/g°C} \)):

\[ q = 50 \times 4.18 \times 50 = 10450 \, \text{J} = 10.45 \, \text{kJ} \]

Internal Energy

\( \Delta E = q + w \), where \( w = -P \Delta V \). At constant volume, \( \Delta E = q_v \).

Enthalpy

Enthalpy (\( H = E + PV \)) measures heat at constant pressure:

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

Standard Enthalpy of Formation

For \( \ce{2H2 + O2 -> 2H2O} \), \( \Delta H_f^\circ(\ce{H2O}) = -285.8 \, \text{kJ/mol} \):

\[ \Delta H = 2 \times (-285.8) = -571.6 \, \text{kJ} \]

Calorimetry

Bomb calorimeter: \( q_{\text{rxn}} = -C_{\text{cal}} \Delta T \). For 1 g glucose (\( \Delta T = 2.8°C \), \( C_{\text{cal}} = 10 \, \text{kJ/°C} \)):

\[ q = -10 \times 2.8 = -28 \, \text{kJ} \]

Phase Change

For 36 g water vaporization (\( \Delta H_{\text{vap}} = 40.7 \, \text{kJ/mol} \)):

\[ q = 2 \times 40.7 = 81.4 \, \text{kJ} \]

Hess’s Law

\( \Delta H_{\text{total}} = \sum \Delta H_{\text{steps}} \). Example: \( \ce{C + O2 -> CO2} \):

\[ \Delta H = -110.5 + (-283.0) = -393.5 \, \text{kJ} \]

Visualizations

Heat changes in exothermic vs. endothermic reactions.

Applications

Energy

Octane combustion: \( \Delta H = -5470 \, \text{kJ/mol} \).

Biology

Glucose oxidation: \( \Delta H = -2803 \, \text{kJ/mol} \).

Industry

Cement production: \( q = 178 \, \text{kJ} \) for 100 g \( \ce{CaCO3} \).

Environment

Methane oxidation: \( \Delta H = -890 \, \text{kJ/mol} \).