Chemistry - Enthalpy

Overview

A thorough understanding of heat of combustion and enthalpy of formation is fundamental in chemistry. These concepts are pivotal for enhancing energy efficiency and supporting sustainable practices, contributing to more effective energy production and promoting environmentally friendly processes.

Definition of Key Terms

• Heat of Combustion: The thermal energy released when one mole of a substance combusts completely in the presence of excess oxygen under standard conditions.
• Standard Enthalpy of Formation (ΔH$_f$): The change in energy during the formation of one mole of a compound from its constituent elements in their standard states.

Measurement Techniques

• Calorimetry: The primary technique for measuring heat of combustion.
  • Bomb Calorimeter:
    • Key Components: Ignition coil, oxygen bomb, water jacket, and thermometer.
    • Steps:
      1. Insert the sample into the oxygen bomb.
      2. Secure the bomb within the water jacket.
      3. Initiate combustion using the ignition coil.
      4. Record the temperature change with the thermometer.

Temperature Measurement Details

• Formula: $q = mc\Delta T$
  • m: mass
  • c: specific heat capacity
  • ΔT: temperature change

Hess's Law

Hess's Law states that the total enthalpy change of a chemical reaction is independent of the pathway taken, consistent with the law of conservation of energy.

• Formula: $\Delta H = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$

Worked Examples

Example 1: Methane Combustion

The combustion of methane is represented by the equation: $\text{CH}_4 + 2\text{O}_2 \to \text{CO}_2 + 2\text{H}_2\text{O}$

• Calculation of $\Delta H$:
  • Apply the formula:
    $\Delta H = \sum H_f(\text{products}) - \sum H_f(\text{reactants})$

Example 2: Calculating Enthalpy of Formation of CO₂

Given Data:

• Heat of combustion of carbon: $-393.5 \, \text{kJ/mol}$
• Heat of combustion of hydrogen: $-285.8 \, \text{kJ/mol}$

Objective: Determine the enthalpy of formation for carbon dioxide using Hess's Law.

Solution:

1. Write the combustion equation for carbon: $C(s) + O_2(g) \rightarrow CO_2(g)$ This directly gives us the enthalpy of formation of CO₂: $-393.5 \, \text{kJ/mol}$

2. The enthalpy of formation of CO₂ is equal to its heat of combustion because carbon and oxygen are both in their standard states.

Assumptions and Limitations

Recognising the limitations and assumptions is essential when using heats of combustion to evaluate enthalpies of formation.

• Complete Combustion: Assumes all fuel is entirely converted to CO₂ and H₂O, which might not always be achievable in practice.
• Constant Conditions: Assumes standard conditions of 1 atm and 25°C, which may not always be relevant in different contexts.
• Perfect Insulation: Assumes no heat is lost, though slight heat losses can occur.

Practice Problem

Problem: Determine the enthalpy of formation of compound Z from substances X and Y using combustion data.

• Steps:
  1. Decompose into individual reactions.
  2. Apply Hess's Law to perform the calculations.

Solution: Given the combustion data for substances X, Y, and Z:

• Heat of combustion of X: -200 kJ/mol
• Heat of combustion of Y: -300 kJ/mol
• Heat of combustion of Z: -400 kJ/mol

Using Hess's Law:

1. Write all combustion reactions
2. Manipulate equations to isolate formation of Z
3. Calculate: ΔH(formation Z) = ΔH(combustion X) + ΔH(combustion Y) - ΔH(combustion Z)
4. ΔH(formation Z) = -200 + (-300) - (-400) = -100 kJ/mol

Correction Methods

• Improved Insulation: Utilise superior materials to minimise heat loss.
• Precise Calorimeter Calibration: Ensure precise settings and regular calibration checks for accuracy.

Exam Tips

• Sign Conventions are essential when reversing reactions.
• Verify that each intermediate step correctly leads to the final equation.

Visual Learning

Engaging with diagrams can significantly enhance comprehension.