UNIT 2.1 2025

Overview of Thermochemistry

  • Definition: Thermochemistry focuses on energy changes during chemical and physical processes (exothermic vs endothermic).

  • Mathematical Skills:

    • Change the subject of an equation and solve complex equations related to enthalpy.

    • Skills in data manipulation (graphical and numerical).

Core Concepts

1. Key Terms

  • Exothermic Reaction: A process that releases heat energy, resulting in a negative enthalpy change (ΔH < 0).

  • Endothermic Reaction: A process that absorbs heat, resulting in a positive enthalpy change (ΔH > 0).

  • Enthalpy (): The total heat content of a system at constant pressure.

  • Enthalpy Change (): The heat change associated with a reaction under constant conditions.

  • Law of Conservation of Energy: Energy cannot be created or destroyed, only transformed.

  • Hess's Law: The total enthalpy change for a reaction is the same, regardless of the path taken.

  • Standard Conditions: Defined conditions for measuring thermochemical properties (25ºC, 1 atm, 1 mol dm−3).

  • Bond Enthalpy: The energy required to break one mole of a bond in a gaseous substance.

  • Mean/Average Bond Enthalpy: Average energy required to break bonds, varies across different compounds.

2. Energy Changes in Chemical Reactions

  • Most reactions involve either the release (exothermic) or absorption (endothermic) of energy.

  • Measured in kJ mol−1 (kiloJoules per mole).

  • For example:

    • Burning of carbon: C(s) + O₂(g) → CO₂(g) releases 394 kJ/mol.

    • Neutralization: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) releases 57 kJ/mol.

3. Enthalpy Change ()

  • Enthalpy change can be expressed as the heat gained or lost in a system when the pressure is constant:

    • Exothermic reactions: negative ΔH.

    • Endothermic reactions: positive ΔH.

4. Hess's Law and Energy Cycles

  • Energy changes depend only on the initial and final states, not the reaction pathway.

  •  totals can be calculated using various routes (energy cycle diagrams).

5. Standard Conditions

  • Essential for accurate comparisons of reaction enthalpies:

    • Temperature: 298 K (25ºC)

    • Pressure: 1 atm (101.3 kPa)

    • Concentration: 1 mol/dm³

Practical Work

Determining Enthalpy Changes

  • Coffee Cup Calorimeter:

    • Uses insulated cups to minimize heat loss during reactions in solution.

    • Variables measured: temperature change, initial temperature, mass of reactants.

    • Heat exchange calculated as q = mcΔT, where c = specific heat capacity (for water, 4.18 J/g·K).

    • Calculating Enthalpy Change: ΔH = -q/n (q = heat transferred, n = moles of limiting reagent).

Indirect Determination Examples

  • Example with magnesium oxide and the reaction with hydrochloric acid, where the enthalpy change is calculated using reaction pathways.

Bond Energy and Calculations

  • Bond Entropy: Energy required to break bonds; essential for calculating reaction enthalpies using bond energies (adjusted for formation and dissociation).

  • Example Calculation: Reaction of diatomic gases containing individual covalent bonds.

Summary Checklist

  • Understand enthalpy changes of reactions (formation and combustion).

  • Apply Hess’s Law to energy cycles, calculate average bond enthalpies, and determine enthalpy changes using verified procedures.