Bond Enthalpy Flashcards
Mean bond enthalpy
Enthalpy required to break a covalent bond averaged over a range of different compounds
Calculating ΔH for a reaction
ΔH = Σ (bonds broken) - Σ (bonds formed)
Why are they different?
- values are averaged from a range of different compounds
* bond enthalpies ignore intermolecular forces
Standard enthalpy change of formation ΔHƟf
The enthalpy change when one mole of a compound is formed from its elements under standard conditions, all reactants and products in their standard states
Standard enthalpy change of combustion ΔHƟc
The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, all reactants and products in their standard states
Measuring enthalpy changes experimentally
Experiment: substances mixed in an insulated container and temperature rise measured
Thermometer
Polystyrene cup:
- good insulator
- low heat capacity
Sources of error:
- Heat loss to surroundings
- Reaction or dissolving may be incomplete or slow
- Density of solution taken to be the same as water
- Assumption that all solutions have the same specific heat capacity as water
Calculating the enthalpy change of reaction from experimental data
q = mcΔT
q = heat gained or lost in J
m = mass in g
ΔT = change in temperature
c = specific heat capacity of water (4.18 Jg-1K-1)
ΔHӨ = q/n
Enthalpy changes in solution (neutralisation)
Simple calorimeter
Assumes all heat is transferred to the water
Sources of error:
- Flame affected by draughts
- Heat lost by convection from top of calorimeter
- Heat lost by conduction through calorimeter -Incomplete combustion of fuel
- Evaporation of fuel
Flame calorimeter
Heat loss reduced even further
• Spiral chimney made of copper
• The flame is enclosed
• The fuel burns in pure oxygen, rather than air
Bomb calorimeter
• No heat loss
• Accurate values
for enthalpies of combustion
