Energetics

• when reaction happens, there is a change in energy
• enthalpy change?


  the heat energy transferred in a reaction at a constant pressure. units: kJ mol-1
• ΔH means substances were in standard states, measurements were made under standard solutions
• standard solutions?

  • 100kPa
  • stated temp
• exothermic reaction energy given out, ΔH is negative
• endothermic reaction energy is taken in, ΔH is positive
• when reaction happens, reactant bonds are broken and product bonds are formed
• bond breaking is endothermic, energy needed to break bonds, ΔH is positive.
• bond making is exothermic, energy released when bonds formed, ΔH is negative.
• enthalpy change?


  overall effect of the exothermic/endothermic changes. if more energy needed to break bonds than released to made bonds, ΔH is positive. if less energy needed to break bonds than released to make bonds, ΔH is negative.
• bond enthalpy?


  energy required to break a covalent bond in gaseous state, under standard conditions
• energy needed to break bond depends on environment it is in
• mean bond enthalpy?


  average energy needed to break a certain type of bond, over a range of compounds
• breaking bonds is endothermic, mean bond enthalpies is positive
• in a reaction, energy is absorbed to break bonds and given out during bond formation, difference is the overall enthalpy change
• enthalpy change = total energy absorbed - total energy released
• standard enthalpy of formation?


  ΔfH, the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
• standard enthalpy of combustion?

  ΔcH, the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions
• Finding enthalpy change using Calorimetry


  Used to find out how much heat is given out by reaction measuring a temperature change
• Finding enthalpy change of combustion of flammable liquid


  1. Burn it inside an apparatus

  2. As fuel burns, water is heated

  3. Energy absorbed is calculated if mass of water, temp change and specific heat capacity (4.18) is known

  4. Heat given out by fuel when burned, would be absorbed by water - so enthalpy change of combustion can be calculated
• Heat can be lost to surroundings - hard to get accurate results
• When fuel is burned, some combustion can be incomplete, so less energy given out
• Flammable liquids are often volatile to some fuel and may be lost to evaporation
• moles = concentration x volume - used to find energy change per mole of reactant
• to find temperature change: starting temperature - highest temperature
• q = mcΔT
• Hess's Law?


  • the total enthalpy change of a reaction is independent of the route taken
• chemical system?


  all of the chemical reactants present in the reaction
• enthalpy change = products - reactants
• activation energy?


  The minimum energy required for a chemical reaction to occur.
• standard temperature?


  298K and 25'C
• standard pressure?


  100kPa
• standard concentration?


  1 mol dm-3
• complete combustion occurs when products are CO2 + H2O
• standard enthalpy change of neutralisation?

  the enthalpy change when one mole of water is formed in a neutralisation reaction under standard conditions
• standard enthalpy change of formation?

  the enthalpy change when one mole of a compound is formed from its elements under standard conditions and all of the chemicals are in their standard states.
• standard enthalpy change of reaction?

  enthalpy change when a reaction takes place in the molar quantities shown by the balanced chemical equation under standard conditions
• Determining the standard enthalpy change of combustion of a liquid fuel

  1. Combust fuel and use thermal energy released to heat a known mass of water

  2. Measure temp change of water, standard enthalpy change of combustion can be determined
• Experimental procedure


  1. Use a balance to measure starting mass of spirit burner + fuel

  2. Use thermometer to measure start temperature of water

  3. Remove cap from spirit burner and light the wick - fuel will evaporate

  4. Thermal energy released causing temperature of water to increase

  5. Stir water with thermometer - thermal energy is distributed

  6. Place cap wick on spirit burner and read temp of water

  7. Use balance to find final mass of fuel + spirit burner
• reasons why results may not be accurate:

  • unlit spirit burner left open, fuel will evaporate - appear that more fuel was burned and appear less exothermic
  • a lot of heat energy released by fuel foes not pass into water
  • not all fuel underwent complete combustion
  • not carried in standard conditions