energetics

• Bond energy

  -enthalpy changes take place because bonds are being broken and formed

  -energy is needed to overcome attractive forces

  -bond breaking is ENDOTHERMIC

  -bond making is EXOTHERMIC

  -sum of energy in and out determines overall reaction is exo or endo
• transition state

  -stage during reaction at which chemical bonds are partially broken and formed

  -very unstable - a molecule in state cannot be isolated and is higher in energy than reactants and products
• activation energy

  -the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction

  -energy needed to react the transition state
• energy level diagrams - exothermic

  -reactants higher in energy than products

  -reactants are closer in energy to transition state

  -exothermic reactions have a lower activation energy compared to endo
• energy level diagrams - endothermic

  -reactants are lower in energy than products

  -reactants are further away in energy to transition state

  -endothermic reactions have higher activation energy compared to exo
• enthalpy change

  -the amount of energy absorbed by a system as heat during a process at constant pressure

  -delta H
• enthalpy changes - exothermic reactions

  -products have less energy than reactants

  -heat energy is given off to surroundings

  -energy of system decreases

  -have -ve enthalpy

  -thermodynamically possible

  -if rate is too slow reaction may not occur - in this case creation is kinetically controlled
• enthalpy changes - endothermic reactions

  -products have more energy than reactants

  -heat energy absorbed from surroundings

  -energy of system increases

  -+ve delta H
• standard enthalpy changes

  -to be able to compare changes in enthalpy all thermodynamic measurement are carried out under standard conditions

  -100kPa

  -298K (25C)

  -delta H underground sign = standard enthalpy change
• standard enthalpy change of reaction

  The enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions

  -delta H underground r

  -both exo and endo
• standard enthalpy change of formation

  -The enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions

  -delta H underground f

  -both exo and endo
• standard enthalpy change of combustion

  -the enthalpy change when one mole of a substance is burnt in excess oxygen in its standard state under standard conditions

  -delta H underground c

  -exo
• standard enthalpy change of neutralisation

  -the enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions

  -delta H underground neut

  -exo
• calorimetry

  -measurement enthalpy changes in chemical reactions

  -made from polystyrene cup, metal can or vacuum flask
• specific heat capacity (c)

  -the energy required to raise the temperature of one gram of a substance by one degree Celsius

  -4.18Jg-1 degreec-1

  -q=mc deltaT
• Hess' Law

  -If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.
• bond dissociation energy (E)

  -energy required to break one moles of a specific covalent bond

  -aka bond energy of bond enthalpy
• average bond energy

  -bond energies are affected by other atoms in the molecule

  -average of the same type of bond but in different environments is calculated