chapter 9 - enthalpy

• enthalpy is a measure of the heat energy in a chemical system
• enthalpy change = enthalpy of products - enthalpy of reactants
• the conservation of energy is the law that energy cannot be created or destroyed
• from the system to the surrounding - exothermic
• from the surroundings to the system - endothermic
• exothermic enthalpy profile

  products lower than reactants

  enthalpy change is negative
• endothermic enthalpy profile

  reactants below products

  enthalpy change is positive
• the activation energy is the energy input required to break bonds in the reactants
• standard conditions

  • pressure - 100kPa
  • temperature - 298K
  • concentration - 1 mol dm-3
  • state - physical state of a substance under standard conditions
• standard enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states
• standard enthalpy change of formation is the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
• all elements have a enthalpy change of formation of 0
• standard enthalpy change of combustion is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states
• standard enthalpy change of neutralisation is the energy change that accompanies the reaction of an acid by a base to form one mole of H2O, under standard conditions, with all reactants and products in their standard states
• to convert from degrees to Kelvin, +273
• the specific heat capacity is the energy required to raise the temperature of 1g of a substance by 1K
• to calculate energy change use this equation:

  q = mcΔT
• factors affecting the accuracy of enthalpy change of combustion experiment

  • heat loss to the surroundings
  • incomplete combustion of methanol
  • evaporation of methanol from the wick
  • non-standard conditions
• experiment to determine enthalpy change of combustion

  1. using a measuring cylinder, measure out 150cm3 of water. pour the water into the beaker. record the initial temperature of the water to the nearest 0.5 degrees
  2. add methanol to the spirit burner. weigh down the spirit burner containing methanol
  3. place the spirit burner under the beaker. light the burner and burn the methanol whilst stirring the water
  4. after 3 mins, extinguish the flame and record the maximum temperature reached by the water
  5. reweigh the spirit burner containing methanol, find the change in mass
• to calculate enthalpy change of combustion

  use q = mcΔT

  then divide this value by 1000

  then divide it by the moles
• average bond enthalpy is the energy required to break one mole of a specified type of bond in a gaseous molecule
• bond enthalpies are always endothermic
• actual bond enthalpy can vary depending on the chemical environment of the bond, so an average is calculated
• enthalpy change of a rectaion = bond enthalpies in reactants - bond enthalpies in products
• hess' law states that, if a reaction can take place by 2 routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route