AQA Chemistry - Energetics Revision

Introduction

• Presenter: Chris Harris from alerts.com
• Topic: Energetics revision for AQA Chemistry
• Purpose: Provide an overview and revision aid
• Additional Material: PowerPoints available for purchase (link in the video description)

Enthalpy (ΔH)

• Definition: Change in heat during a chemical reaction at constant pressure
• Symbol: ΔH
• Units: kJ per mole
• Standard Conditions: 100 kPa pressure, 298 K (25°C)
• Notation: ΔH°

Types of Reactions

• Endothermic: Absorbs energy (feels colder)

  • Energy Profile: Reactants lower in energy than products
  • ΔH: Positive
  • Example: Decomposition of CaCO₃ (+178 kJ/mol)

• Exothermic: Releases energy (feels warmer)

  • Energy Profile: Reactants higher in energy than products
  • ΔH: Negative
  • Example: Combustion of ethane (-1560 kJ/mol)

Bond Breaking and Making

• Bond Breaking: Requires energy (endothermic, ΔH positive)
• Bond Making: Releases energy (exothermic, ΔH negative)
• Mean Bond Enthalpy: Average energy for breaking/making specific bonds
  • Example: 4 CH bonds in methane, mean bond enthalpy by averaging

Calculations Using Mean Bond Enthalpies

• Formula: ΔH = Σ(Bond energies of reactants) - Σ(Bond energies of products)
• Example: Combustion of methane
  • Breaking Bonds: Total energy = 2736 kJ/mol
  • Forming Bonds: Total energy = 3466 kJ/mol
  • Calculation: ΔH = 2736 - 3466 = -730 kJ/mol (exothermic)

Calorimetry

• Purpose: Measure enthalpy changes of combustion or reactions in solution
• Method: Burn fuel or mix reactants in a polystyrene cup and measure temperature change
• Formula: Q = mcΔT
  • Q: Energy (J)
  • m: Mass of water or solution (g)
  • c: Specific heat capacity (4.18 J/g·K)
  • ΔT: Temperature change (K)

Example Calculations

• Combustion of ethanol

  • Energy Calculation (Q): Q = mcΔT = 14212 J = 14.212 kJ
  • Moles of Fuel: moles = mass/Mr = 0.039 mol
  • Enthalpy Change (ΔH): ΔH = Q/moles = -364.5 kJ/mol

• Neutralization Reaction

  • Setup: Polystyrene cup, measure T before and after adding reactants
  • Example: HCl and NaOH
    • Energy Calculation (Q): Q = 1.254 kJ
    • Moles of HCl: 0.025 mol
    • Enthalpy Change (ΔH): ΔH = -50.16 kJ/mol

Hess's Law

• Definition: Total enthalpy change is independent of the path taken
• Cycles
  • Formation Cycle: Elements -> Reactants & Products (arrows up)
  • Combustion Cycle: Reactants & Products -> Combustion Products (arrows down)
  • Rule: Keep signs for arrows in same direction, flip signs for arrows against the direction

Example Cycles

• Formation Cycle Example

  • Reaction: Methanol combustion
  • Calculation: Use formation data, balance elements and apply Hess's Law
  • Result: ΔH combustion = -732 kJ/mol

• Combustion Cycle Example

  • Reaction: Formation of pentane
  • Calculation: Use combustion data, balance elements and apply Hess's Law
  • Result: ΔH formation = -177 kJ/mol

Summary

• Importance: Understand and apply concepts of ΔH, endothermic/exothermic reactions, bond enthalpies, calorimetry, and Hess's Law
• Additional Material: PowerPoints available via link for further revision

[Music] Thank you for watching and good luck with your studies!