chapter 9

• enthalpy is the measure of heat energy in a chemical reaction
• enthaply change=H(products)-H(reactants)
• exothermic is negative as the energy required to break bonds is lower than the energy released when new bonds are formed
• endothermic is positive as there is more energy required to break bonds than released when making them
• activation energy is the minimum energy required for a reaction to take place
• standard conditions

  pressure= 1 atm

  temp= 25 c

  conc= 1 mol/dm3
• enthalpy change of reaction is the enthalpy change of a chemical equation under standard conditions
• enthalpy change of formation is the enthalpy change when one mole of a substance is formed from its elements under standard conditions
• enthalpy change of formation of all elements is always 0
• the enthalpy change of combustion is the enthalpy change when one mole of a compound reacts completely with oxygen under standard conditions
• enthalpy change of neutralisation is the enthalpy change when an acid and base react to form one mole of water under standard conditions
• to change from degree celsius to kelvin, add 273
• q=mc(delta)T

  q= energy released

  m= mass inside the cup(g)

  c= shc of water(unless other wise)= 4.183

  delta T= change in temperature
• enthalpy change= -q/1000xmoles of limiting reactant
• why can the value not be accurate?

  • heat loss to surroundings
  • incomplete combustion of reactant
  • evaporation of products
  • non standard conditions
• average bond enthalpy is the energy required to break one moles of bonds in a gaseous molecule
• enthalpy change using bond enthalpys=

  enthalpy change of reaction= sum of reactant - sum of products
• hess' law states if a reaction can take place by two routes, and the start and finish conditions are the same, the total enthalpy change is the same for each route
• hess law arrow

  elements= arrows up

  combustion= arrows down