Enthalpy and Enthalpy Change

Introduction

Enthalpy (H): Total thermal content of a system. Being a state function, it depends on pressure, temperature, and composition. Enthalpy is crucial for understanding heat transfers in reactions occurring at constant pressure.

Enthalpy Change (ΔH): Represents the change in heat at constant pressure. It is calculated as:

$\Delta H = H_{\text{products}} - H_{\text{reactants}}$

This concept is essential for comprehending how reactions progress under constant pressure, similar to the role of heat in cooking.

The Relationship between ΔH and qp

• At constant pressure, the enthalpy change ΔH is equivalent to the heat transfer qp.

$\Delta H = q_p$

• Constant Pressure: Ensures that the heat transfer equals the enthalpy change without any change in volume.

Calorimetry

• Calorimetry: A technique for measuring heat exchange during chemical reactions. It determines whether a reaction absorbs or releases energy, providing insight into reaction enthalpy.
• Formula: $q = mc\Delta T$ (where m is mass, c is specific heat capacity, and $\Delta T$ is the change in temperature).

Enthalpy Diagrams

Enthalpy Diagrams visually depict energy changes between reactants and products, displaying activation energy and $\Delta H$.

Exothermic Reactions

• Definition: Reactions that release heat, resulting in a negative ΔH.

• Examples:
  • Combustion of fuels.
  • Respiration.

Endothermic Reactions

• Definition: Reactions that absorb heat, resulting in a positive ΔH.

• Examples:
  • Photosynthesis.
  • Ice melting.

Hess's Law

Hess's Law asserts that the total enthalpy change of a reaction is independent of the route by which the reaction occurs.

• Reversibility: Reversing a reaction changes the sign of ΔH.

  • Example: An exothermic reaction (ΔH negative) becomes endothermic (ΔH positive).

• Stoichiometric Adjustments:

  • Adjusting ΔH proportionally when reaction coefficients are altered.

Worked Example

Let's calculate the enthalpy change for a multi-step reaction using Hess's Law:

Given:

• Reaction 1: A → B, ΔH = -50 kJ/mol
• Reaction 2: B → C, ΔH = +30 kJ/mol

To find ΔH for A → C:

1. Write out the pathway: A → B → C
2. Add the enthalpy changes: ΔH = (-50) + (+30) = -20 kJ/mol

Therefore, A → C is exothermic with ΔH = -20 kJ/mol.

Example Problems with Solutions

1. Problem: Calculate the enthalpy change for the combustion of octane.

   Solution: The combustion of octane is exothermic with ΔH = -5510 kJ/mol.

2. Problem: Determine the enthalpy change when ice melts.

   Solution: The melting of ice is endothermic with ΔH = +6 kJ/mol.

3. Problem: Is a reaction with ΔH = -245 kJ/mol exothermic or endothermic?

   Solution: The reaction is exothermic because the ΔH value is negative, indicating heat is released to the surroundings.

Common Misconceptions

• Remember, exothermic reactions release heat, and endothermic reactions absorb it, regardless of the perception of temperature change.
• The activation energy is different from the enthalpy change. Even exothermic reactions require activation energy to begin.