Energitics

• Enthalpy


  The heat change in a reaction at constant pressure
• Delta H


  The symbol for enthalpy change, measured in kilojoules per mole
• Standard conditions


  100 kilopascals and 298 Kelvin (25 degrees Celsius)
• Endothermic reactions


  • Absorb energy from the surroundings
  • Reactants have lower energy than products
  • Delta H is positive
• Example of endothermic reaction


  • Thermal decomposition of calcium carbonate
• Exothermic reactions


  • Release energy to the surroundings
  • Products have lower energy than reactants
  • Delta H is negative
• Example of exothermic reaction

  • Combustion of ethane
• Types of enthalpy change

  • Enthalpy change of reaction
  • Enthalpy of neutralization
  • Enthalpy of combustion
  • Enthalpy of formation
• Enthalpy change of reaction


  The standard enthalpy change of a reaction according to the molar quantities in the equation under standard conditions
• Enthalpy of neutralization


  The standard enthalpy change when 1 mole of water is formed from an acid and an alkali under standard conditions
• Enthalpy of combustion


  The standard enthalpy change when 1 mole of a substance is completely burned in oxygen to form carbon dioxide and water under standard conditions
• Enthalpy of formation


  The standard enthalpy change when 1 mole of a compound is formed from its elements in their standard states
• Bond breaking


  An endothermic process that requires energy to be absorbed by the molecule
• Bond making


  An exothermic process that releases energy
• Bond enthalpy


  The amount of energy needed to break 1 mole of a bond type in a molecule in the gaseous state
• Bonds of the same type can have slightly different bond enthalpies
• Calculating enthalpy change using mean bond enthalpies

  Total energy to break bonds - Total energy released when forming bonds = Enthalpy change
• Calorimetry


  A technique used to measure the enthalpy change of a combustion reaction
• Steps in calorimetry


  1. Weigh fuel before and after burning

  2. Measure temperature rise of water

  3. Calculate energy transferred using Q=mcΔT

  4. Calculate enthalpy change from energy and moles of fuel
• Calorimetry can also be used to measure enthalpy changes of reactions in solutions
• Calorimetryexperiment of solutions


  • Measures energy from solutions
  • Uses a polystyrene cup instead of a copper can
  • Measures temperature change from a neutralization reaction
• Calculating enthalpy of neutralization

  1. Work out Q (energy change)

  2. Calculate moles of HCl

  3. Divide energy by moles to get enthalpy change
• Hess's law


  The total enthalpy change of a reaction is independent of the route taken
• Drawing a formation cycle

  1. Put reactants on one side and products on the other

  2. Elements in standard states at the bottom

  3. Enthalpy of formation values go up the arrows
• Calculating enthalpy of combustion using a formation cycle

  1. Go against the arrow, change the sign

  2. Go with the arrow, keep the sign the same

  3. Add up the values to get the enthalpy of combustion
• Drawing a combustion cycle

  1. Put reactants on the left and products on the right

  2. Combustion products (CO2 and H2O) at the bottom

  3. Enthalpy of combustion values go down the arrows
• Calculating enthalpy of formation using a combustion cycle


  1. Go with the arrow, keep the sign the same

  2. Go against the arrow, change the sign

  3. Add up the values to get the enthalpy of formation