This question is about enthalpy changes.
An experiment was carried out to determine the enthalpy change of combustion for ethanol.
C2H5OH (l) + 3O2 (g) → 2CO2 (g) + 3H2O (l)
1.19 g of ethanol was burned in a spirit burner. The heat energy from this combustion raised the temperature of 100 g of water from 21.6°C to 63.9 °C.
Calculate the number of moles of ethanol in 1.19 g.
[Molar mass of ethanol = 46.0 g mol−1 ]
(1)
Calculate the heat energy required to raise the temperature of 100g of water from 21.6 °C to 63.9 °C.
[Specific heat capacity of water = 4.18 J g−1 °C−1]
(2)
Give your answer to an appropriate number of significant figures and include a sign and units.
(3)
The value of the enthalpy change of combustion from this experiment was very inaccurate.
Give two reasons why this value was so inaccurate, apart from heat loss.
(2)
Mean bond enthalpies can be used to calculate a value for the enthalpy change of combustion of a compound.
(2)
(3)
CH3OH + 1½O2 → CO2 + 2H2O
C–H | C–O | O–H | O=O | C=O | |
Mean bond enthalpy / kJ mol−1 | 413 | 358 | 464 | 498 | 805 |
Enthalpy changes of combustion can be used to calculate the enthalpy change of formation of a compound.
Substance | Standard enthalpy change of combustion, ∆c/ kJ mol−1 |
C (s,graphite) | – 394 |
H2 (g) | – 286 |
CH3CHO (g) | – 1167 |
Complete the Hess cycle and use it to calculate the standard enthalpy change of formation for ethanal, CH3CHO.
Did this page help you?