Exothermic and Endothermic Reactions, Calorimetry

Exothermic and Endothermic Reactions

• Exothermic Reactions:
  • Transfer thermal energy (heat) to surroundings.
  • Increase the temperature of surroundings.
  • Examples: combustion, oxidation, neutralization.
  • Everyday examples: self-heating cans, hand warmers.
• Endothermic Reactions:
  • Take in thermal energy (heat) from surroundings.
  • Decrease the temperature of surroundings.
  • Examples: thermal decomposition, reaction of citric acid and sodium hydrogen carbonate.
  • Sports injury packs.
• Mnemonic:
  • EXOthermic: Energy EXits.
  • ENdothermic: Energy ENters.

Reaction Pathway Diagrams

• Show exothermic or endothermic nature of a reaction.
• Start at reactants, end at products.
• Arrows indicate activation energy ($E_a$) and enthalpy change ($\Delta H$).
• Exothermic Diagram:
  • Reactants at higher energy level than products.
  • Energy released into surroundings.
  • Energy change is negative.
• Endothermic Diagram:
  • Reactants at lower energy level than products.
  • Energy enters the system.
  • Energy change is positive.

Enthalpy Change ($\Delta H$)

• Transfer of thermal energy during a reaction.
• Exothermic: $\Delta H$ is negative.
• Endothermic: $\Delta H$ is positive.

Activation Energy ($E_a$)

• Minimum energy colliding particles need to react.
• Particles need sufficient energy for successful collisions.

Reaction Pathway Diagrams (Extended)

• Include reactants, products, enthalpy change ($\Delta H$), and activation energy ($E_a$).
• $E_a$: Arrow from reactants' energy to maximum energy (peak).
• $\Delta H$: Arrow from reactants' energy to products' energy.
• Y-axis: Energy, X-axis: Progress of reaction.
• Curved line connects reactants and products.
• Exothermic:
  • Reactants higher than products.
  • $\Delta H$ negative (arrow downwards).
• Endothermic:
  • Reactants lower than products.
  • $\Delta H$ positive (arrow upwards).

Bond Breaking and Bond Making (Extended)

• Bond Breaking:
  • Endothermic (energy taken in).
• Bond Making:
  • Exothermic (energy released).
• Enthalpy Change:
  • Exothermic: Energy released making bonds > Energy taken in breaking bonds.
  • Endothermic: Energy taken in breaking bonds > Energy released making bonds.

Calculating Enthalpy Change Using Bond Energies (Extended)

1. Sum of bond energies in reactants = 'energy in'.
2. Sum of bond energies in products = 'energy out'.
3. $\Delta H$ = energy in – energy out

Calorimetry Calculations

• Calorimetry measures enthalpy changes of reactions.
• Simple calorimeter: polystyrene cup, vacuum flask, or metal can.
• Specific heat capacity (c): energy to raise the temperature of 1g of a substance by 1K.
• Water's specific heat capacity: $4.18 J g^{-1} K^{-1}$
• Energy transferred: $q = m \times c \times \Delta T$
  • q = energy transferred (J)
  • m = mass (g)
  • c = specific heat capacity (J g-1 K-1)
  • $\Delta T$ = temperature change (K)
• Enthalpy change calculation: $\Delta H = q \div n$
• Units: kJ/mol

Enthalpy of Combustion Experiments

• Heat from combustion increases water's heat content.
• Simple calorimeter measures water temperature changes.
• Heat loss is a source of error, minimize by:
  • Placing calorimeter close to flame.
  • Using a lid.
  • Shielding from draughts.