Measuring enthalpy changes

difference between heat and temperature

• temperature = measure of average kinetic energy of the particles

• heat = measure of energy content of a substance

• chemical reactions involve a transfer of energy between the system and the surroundings, while total energy is conserved

enthalpy

• total chemical energy inside a substance measured at constant pressure

• when reactions happen, enthalpy changes

exothermic reactions

• when products have less enthalpy than reactants

• heat is given off by the system to the surroundings

  • temp of surroundings increase, temp of system decrease

• ΔH is negative

endothermic reactions

• when products have more enthalpy than reactants

• heat is taken in by the system from the surroundings

  • temp of surroundings decrease, temp of system increase

• ΔH is positive

energy profiles

• shows energies of reactants, transition state and products over time

  • transition state is stage during reaction when chemical bonds are partially broken and formed, very unstable

• activation energy needed to reach transition state

  • minimum amt of energy needed for reactant molecules to successfully collide and start reaction

standard enthalpy change

• ΔHꝊ refers to heat transferred at constant pressure under standard conditions

• calculated in kJ mol–1

standard enthalpies

energy transferred as heat equation

no need to convert C into K if given in C

• ΔH = - heat energy Q / n

specific heat capacity

energy needed to raise the temp of 1g of substance by 1K

enthalpy changes for reactions in solution

• reaction with excess of one reagent, temp is measured over time

• assumptions made:

  • specific heat capacity is same as water

  • density of solution is same as water (1 gcm3)

  • negligible heat losses

  • reaction goes to completion

  • specific heat capacity of container is ignored

temperature correction graphs

• reactions aren’t instantaneous, so there is delay before max temp is reached, during delay substances may be losing heat to surroundings, so true max temp is never reached

• graphs are extrapolated to calculate the max temp that couldve been reached

enthalpy of combustion experiments

• heat released from combustion reaction used to increase temp of water

• to minimise heat loss calorimeter should be placed close to the flame and lid placed over calorimeter

• main sources of error

  • heat losses

  • incomplete combustion