enthalpy

• What is the definition of enthalpy change?

  Heat energy change at constant pressure
• In what units is enthalpy change measured?

  kilojoules per mole (kJ mol-1)
• What are the characteristics of exothermic reactions?

  • Release heat energy to surroundings
  • Have a negative enthalpy change (ΔH is negative)
  • Increase temperature of surroundings
• What is the enthalpy change for the combustion of methane?

  ΔH⦵r = 890 kJ mol-1
• What are the characteristics of endothermic reactions?

  • Absorb heat energy from surroundings
  • Have a positive enthalpy change (ΔH is positive)
  • Decrease temperature of surroundings
• What is the enthalpy change for the thermal decomposition of calcium carbonate?

  ΔH⦵r = +178 kJ mol-1
• What do enthalpy profile diagrams illustrate?

  • Energy levels of reactants and products
  • Activation energy required for reactions
  • Changes in enthalpy during reactions
• What is activation energy?

  Minimum energy needed for a reaction
• In exothermic reactions, how do the energy levels of reactants compare to products?

  Reactants have higher energy than products
• In endothermic reactions, how do the energy levels of reactants compare to products?

  Reactants have lower energy than products
• Why are standard conditions used in measuring enthalpy changes?

  • Actual enthalpy cannot be directly measured
  • Provides consistency in measurements
  • Ensures values are comparable
• What are the standard conditions for measuring enthalpy changes?

  298 K temperature and 101 kPa pressure
• What does the superscript plimsoll (⦵) denote in enthalpy change?

  Refers to standard conditions
• What are the four types of standard enthalpy change?

  1. Standard enthalpy change of reaction (ΔH⦵r)
  2. Standard enthalpy change of formation (ΔH⦵f)
  3. Standard enthalpy change of combustion (ΔH⦵c)
  4. Standard enthalpy change of neutralisation (ΔH⦵neut)
• What is the standard enthalpy change of reaction (ΔH⦵r)?

  Enthalpy change for reaction at standard conditions
• What is the standard enthalpy change of formation (ΔH⦵f)?

  Enthalpy change when 1 mole of compound forms
• What is the standard enthalpy change of combustion (ΔH⦵c)?

  Enthalpy change when 1 mole combusts in oxygen
• What is the standard enthalpy change of neutralisation (ΔH⦵neut)?

  Enthalpy change when acid and alkali neutralise
• What is the enthalpy change for the neutralisation of hydrochloric acid with sodium hydroxide?

  ΔH⦵neut = 57 kJ mol-1
• What is the significance of using standard conditions in enthalpy measurements?

  • Ensures consistency across measurements
  • Allows for comparison of enthalpy changes
  • Standardizes experimental conditions