what are exothermic reactions?
reactions that transfer energy from the system to the surroundings
temperature increases
what are endothermic reactions?
reactions that take energy into the system from the surroundings
temperature decreases
breaking chemical bonds
requires energy (+ΔH) and therefore is always endothermic
making chemical bonds
releases energy (-ΔH) and therefore is always exothermic
what happens if a reaction is exothermic?
more energy has been released than absorbed
what happens if a reaction is endothermic?
more energy is absorbed than released
what is the standard enthalpy change of reaction (ΔH°r)?
the enthalpy change for a reaction that occurs between the number of moles and reactants specified in the equation
what is the standard enthalpy change of combustion (ΔH°c)?
the enthalpy change when 1 mole of a substance burns completely in oxygen under standard conditions
what is the standard enthalpy change of formation (ΔH°f)?
the enthalpy change when 1 mole of a substance is formed from its constituent elements in their standard states under standard conditions
what is the standard enthalpy change of neutralisation (ΔH°n)?
the enthalpy change when 1 mole of hydrogen ions react with 1 mole of hydroxide ions to form 1 mole of water under standard conditions and in solutions with a concentration of 1moldm-3
what is Hess’ Law?
as long as the starting and finishing points are the same, the enthalpy change for a chemical reaction will always be the same no matter how you got there
what is a hess cycle?
way of calculating ΔH°r, you can use the calculation:
ΔH°r = ΔH°f (products) - ΔH°f (reactants)
what is bond enthalpy?
the energy required to break 1 mole of a particular bond, average over a range of different gaseous compounds
