Topic 6: Energetics - Edexcel IAL Chemistry A-Level

Enthalpy Change (Ho)

• Enthalpy change is heat energy change, represented by Ho.
• Measured under standard conditions: 100 kPa and 298 K.
• Reaction processes:
  • Bonds breaking: energy absorbed from surroundings.
  • Bonds forming: energy released to surroundings.
• Overall enthalpy change:
  • Positive Ho: endothermic (energy absorbed).
  • Negative Ho: exothermic (energy released).

Energy Level Diagrams

• Illustrate if a reaction is endothermic or exothermic.
• Endothermic diagram:
  • More energy required to break bonds than to form new ones.
  • Overall positive Ho.
• Exothermic diagram:
  • More energy required to form new bonds than to break existing ones.
  • Overall negative Ho.

Measuring Enthalpy Change

Types of Enthalpy Changes

• Reaction (rHo): Enthalpy change for substances reacting in standard states.
• Formation (fHo): Enthalpy change when one mole of a substance is formed from elements.
• Combustion (cHo): Enthalpy change when one mole of a substance is burned in oxygen.
• Neutralisation (neutHo): Enthalpy change when acid and alkali solutions react to form water.
• Atomisation (atHo): Enthalpy change when one mole of gaseous atoms is formed.

Calorimetry

• Experimental method to find enthalpy change by measuring temperature change.
• Energy change formula:
  • q = mcΔT
  • q: energy change (J), m: mass (g), c: specific heat capacity (J g⁻¹ K⁻¹), ΔT: temperature change.
• Specific heat capacity: Energy to raise 1g of a substance by 1K without state change.
• Sources of error:
  • Potential heat loss via conduction, convection, or measurement inaccuracies.
  • Mitigated by insulating calorimeter and using lids.

Hess's Law

• States energy conservation: enthalpy change is route-independent.
• Used when enthalpy changes cannot be directly measured.
• Involves a triangular cycle method with intermediate products.

Bond Enthalpies

• Definition: Energy to break one mole of bonds in gaseous state.
• Values vary per covalent bond type; experimentally determined.
• Mean bond enthalpy: Average energy across compounds.
  • Used to infer bond strength and potential breakage in reactions.
• Calculations:
  • Overall reaction enthalpy = sum of bond enthalpies (bonds broken) - sum of bond enthalpies (bonds formed).

Example: Methane Combustion

• Chemical equation: CH₄ + 2O₂ → CO₂ + 2H₂O
• Bonds formed and broken are calculated using given enthalpies:
  • C=O, H-O formed.
  • C-H, O=O broken.
• Enthalpy change calculation:
  • (1652 + 996) - (1856 + 1598) = -806 kJ mol⁻¹

This document provides a structured and detailed overview of energetics, focusing on enthalpy changes, their measurement and calculation, and principles like Hess's Law. It includes practical examples and potential sources of error in calorimetry.

Note: These notes are based on detailed educational materials provided by PMT Education.