3.2.1 enthalpy changes Flashcards
define activation energy
the minimum energy which particles need to collide and start a reaction
define enthalpy change
the heat energy exchange between the reactants and the surroundings at a constant pressure
define average bond enthalpy
the average enthalpy change requires to break 1 mole of bonds in gaseous molecules to form individual gaseous atoms averaged over different molecules
define standard enthalpy change of combustion
the enthalpy change that takes place when one mole of it
a substance is combusted completely in O2
under standard conditions (298k, 100kPa) with all reactants and products in their standard states
define standard enthalpy change of formation
the enthalpy change that takes place when one mole of it
a compound is formed from its elements
under standard conditions (298k, 100kPa) with all reactants and products in their standard states
define standard enthalpy change of neutralisation
the enthalpy change that takes place when one mole of it
water is formed from neutralisation
under standard conditions (298k, 100kPa) with all reactants and products in their standard states
define the mean bond enthalpy
the enthalpy change when one mole of bonds of (gaseous covalent) bonds is broken (averaged over different molecules)
what goes on the vertical and horizontal axis for an enthalpy profile diagram
vertical- enthalpy
horizontal- progress of reaction
describe an exothermic change
- energy is transferred from the chemicals to the surroundings
- products have less energy than the reactants as heat energy is lost to the surroundings
describe an endothermic change
- energy is transferred from the surroundings to the chemicals
- products have more energy than reactants as heat energy is taken in
working out enthalpy change
q=mcdeltaT /1000
n=m/mr
q/n
why may experimental values differ from actual ones
- heat loss to surroundings
- heat loss due to evaporation
- specific heat capacity of apparatus is different to water
- conditions are not standard
what are some errors of combustion enthalpy of combustion using calorimetry
- heat loss from calorimeter
- incomplete combustion of fuel
- heat capacity of calorimeter not included
- evaporation of fuel after weighing
why is enthalpy of combustion much lower than the enthalpy of formation
in combustion heat is lost to the surroundings so there is less energy
