Evaluative Task
Preparation
Enthalpy change of combustion of propan-1-ol
Jack, Harry and Nigel carried out a practical task to determine the enthalpy change of combustion of propan-1-ol.
Each student was provided with the same equipment and chemicals and they were given the following method.
- Using a 500cm3 measuring cylinder, measure 300cm3 of water.
Pour the water into the beaker.
Record the initial temperature of the water to the nearest 0.5°C.
- Fill the spirit burner with propan-1-ol.
Weigh the spirit burner containing propan-1-ol and record its mass.
- Place the spirit burner under the beaker as shown in the diagram.
Light the burner and burn the propan-1-ol whilst stirring the water with a thermometer.
- After about three minutes, extinguish the flame.
Immediately record the maximum temperature reached by the water.
- Re-weigh the spirit burner containing the methanol.
Assume the wick has not been burned.
(a)Complete the table to show how the mass of fuel burnt and the temperature change.
(b)Describe and give the short hand notation of standard enthalpy change of combustion.
(c)State the conditions required to be considered as standard conditions.
(d)Write an equation that represents the standard enthalpy change of combustion of propan-1-ol, CH3CH2CH2OH (l).
Include state symbols.
(e)Calculate the energy, in J, absorbed by the 300 cm3 in Jack’s experiment.
Give your answer to the nearest whole number.
The specific heat capacity of water = 4.18 J g-1 K-1.
The density of the water = 1.0 g cm-3.
(f)Calculate the amount, in moles, of propan-1-ol that was burnt in the experiment.
(g)Calculate the enthalpy change of combustion of propan-1-ol, in kJ mol -1.
Give your answer to three significant figures.
(h)Jack compared his experimental value for the enthalpy change of combustion of methanol with the standard value from the reference book.
He found that his experimental value was less exothermic, he then evaluated the measurements in his experiment for accuracy.
He used a 500 cm3 measuring cylinder to measure 300 cm3 of water. The maximum error in this volume is ± 1.5 cm3.
Calculate the percentage error in the volume of water measured in this experiment.
(i)Harry told Jack this error could be reduced by repeatedly using a 50 cm3 measuring cylinder to measure 300 cm3 of water.
The 50 cm3 measuring cylinder had a maximum error of ± 0.5 cm3 in each reading.
State and explain whether using this method would reduce the error in measuring 300 cm3 of water.
(j)Nigel helped Jack to calculate the errors in the remaining measurements. They realised that the measurement errors could not account for the difference between the experimental data Jack had obtained and the standard value in the reference book.
Suggest various reasons for this, others than measurement errors.
(k)Harry completed the same experiment; however he used the warm water in the beaker from Jacks experiment. He re-weighed the spirit burner and the initial temperature was 70.0 °C.
He burned exactly the same mass of fuel as in Jacks experiment, but his calculated enthalpy change was less exothermic than Jacks.
Explain why.
(l)Nigel followed the original instructions (the same as Jack) however he left his spirit burner heating the water for 45 seconds longer.
Nigel’s calculated enthalpy change of combustion of methanol was very similar to that obtained by Jack.
Explain why.
(m)When Nigel was completing his calculations, he incorrectly calculated the molar mass of propan-1-ol to be 32.0 g mol -1.
State and explain how this error would affect the calculated enthalpy change of combustion of propan-1-ol.
(n) Jack repeated the experiment using 400 cm3 of water rather than 300 cm3. He burned the same mass of propan-1-ol.
What effect, if any, would this have on the value of enthalpy change of combustion.