Enthalpy Changes 1

• What is enthalpy change?

  A heat change measured at constant pressure
• What are the standard conditions?

  100 kPa or 1 atm. pressure and a temperature of 298 K or 25°C
• Why are standard conditions required?

  To enable comparison
• In and exothermic reaction is enthalpy change a positive or negative value?

  negative
• In an exothermic reaction:

  • The chemical system loses energy

  • The surroundings gains energy

  • The temperature of the surroundings increase
• In an endothermic reaction:

  • The chemical system gains energy

  • The surroundings loses energy

  • The temperature of the surroundings decrease.
• Definition of activation energy.

  is the minimum energy needed to start a reaction by the breaking of bonds
• How is activation often provided?

  heating or a spark
• Definition of standard enthalpy of formation.

  Enthalpy change of formation (ΔfH) - The enthalpy change that takes place when one mole of a compound in its standard state is formed from its constituent elements in their standard states under standard conditions.
• Definition of standard enthalpy of combustion.

  Enthalpy change of combustion (ΔcH) - The enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions.
• Definition of standard enthalpy change of reaction.

  Standard Enthalpy Change of Reaction (ΔrH) - The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions.
• Definition of the enthalpy change of neutralisation.

  Enthalpy change of neutralisation (ΔnH) - The change in enthalpy that occurs when one equivalent of an acid and one equivalent of a base undergo a neutralisation reaction to form water and a salt. It is defined as the energy released with the formation of 1 mole of water.
• What apparatus is used to measure ethalpy change?

  calorimeter
• What 3 measurements are taken when carrying out an experiment for enthalpy change.

  
  

  • Initial and final temperature to workout the change
  • Volume of solution or amount of water where the temperature change occurs
  • number of moles of reactants
• What is the equation for energy change?


  Energy change = m c ∆T
• What is the equation for the enthalpy change per mole.

  energy change/moles of substance used
• Simple experiments to measure enthalpy of combustion often produce results that are less than the accepted (literature) values.

  Suggest 3 reasons why this may be the case.

  • heat loss to environment

  • Incomplete combustion of fuel

  • Evaporation of fuel from wick

  • Non standard conditions
• List 3 reasons that make the bomb calorimeter an improvement over the calorimeter we would use in our laboratory.

  • No heat loss

  • No evaporation of fuel

  • Plenty of O2 supplied so incomplete combustion less likely.

  • Standard conditions can be ensured