Energetics

• State the meaning of the term standard enthalpy of combustion:

  The enthalpy change when 1 mole of a substance burns completely in oxygen with all substances in their standard states
• Enthalpy of combustion determined experimentally is less exothermic than calculated enthalpies of formation.

  Give one reason for this other than heat loss:

  Incomplete combustion
• Define the term enthalpy change:

  Heat energy change at constant pressure
• Suggest two disadvantages of using a glass beaker on a tripod and gauze

  Glass is a poorer conductor than copper

  Tripod and gauze would reduce heat transfer
• Suggest two reasons why the value of enthalpy of combustion is less exothermic than a data book value

  Heat loss to surroundings

  Incomplete combustion

  Use a wind shield to reduce heat loss
• State the meaning of the term enthalpy change as applied to a chemical reaction

  Heat energy change at constant pressure
• Enthalpy of hydration is:

  Enthalpy change when 1 mole of gaseous ions forms 1 mole aqueous ions under standard conditions
• What is 'mean bond enthalpy'?

  Enthalpy change to dissociate a specified covalent bond, averaged over different molecules
• What does Hess' Law state?

  The enthalpy change for a chemical reaction is the same, regardless of the route taken from reactants to products
• Explain why the difference in enthalpy of lattice formation of silver iodide based on a perfect ionic model gives a smaller value than a calculated value


  AgI has covalent character therefore bonds are stronger than predicted
• Identify a reagent that could be used to indicate the presence of iodide ions in an aqueous solution and describe the observation made


  Reagent: AgNO3

  Observation: Yellow ppt