Enthalpy changes

• What does "system" mean in a chemical reaction?

  The atoms and bonds involved
• What does the law of conservation state?

  Energy cannot be created or destroyed
• What energy change is associated with breaking bonds?

  Energy is taken in → endothermic reaction

  
• What energy change is associated with making bonds?

  Energy is released → exothermic reaction

  
• What is an endothermic reaction?

  Overall positive enthalpy change (+ΔH)
• What is an exothermic reaction?

  Overall negative enthalpy change (-ΔH)
• What does activation energy mean?

  Minimum energy required for a reaction
• Which way does the arrow for activation energy point on an enthalpy profile diagram?

  Always points upwards
• What are the standard conditions in chemistry?

  100 kPa and 298 K
• What does "in standard state" mean?

  State at standard conditions (100 kPa, 298 K)
• Define enthalpy change of formation.

  Energy change when 1 mole forms from elements
• Give an example of an equation representing standard enthalpy of formation.

  H2 (g) + ½ O2 (g) → H2O (l)
• Define enthalpy change of combustion.

  Energy change when 1 mole is completely combusted
• Give an example of an equation representing standard enthalpy of combustion.

  C (s) + O2 (g) → CO2 (g)
• Define enthalpy change of neutralisation.

  Energy change when 1 mole of water forms
• What does enthalpy change of reaction mean?

  Energy change associated with a given reaction
• How can you calculate enthalpy change from experimental data?

  Use Q = mcΔT equation
• What is the specific heat capacity of water?

  4.18 gJ^-1K^-1
• What are the advantages of using a bomb calorimeter?

  • Minimises heat loss
  • Uses pure oxygen for complete combustion
• Why might experimental methods for enthalpy determination not be accurate?

  Heat loss to surroundings and incomplete reactions
• What does average bond enthalpy mean?

  Mean energy to break 1 mole of bonds
• Why are bond enthalpies less accurate than standard enthalpy values?

  Bond enthalpies are averages across molecules
• How to calculate enthalpy change of reaction using average bond enthalpies?

  ΔH = Σ(bond enthalpies of reactants) - Σ(products)