Edexcel IAL Chemistry 6.8 Evaluating calorimetry experiments

Extrapolate the cooling line to mixing time, compare measured and accepted enthalpies and link the direction of error to a named heat-transfer or reaction limitation.

Exam points

Extrapolate a cooling curve to determine the corrected maximum temperature change.
Explain why heat loss or incomplete combustion makes measured values less exothermic.
Evaluate insulation, apparatus, uncertainty and assumptions with directional effects.

[Maximum number: 2]

Fuels

Fuels burn in oxygen to release a lot of energy.
Many hydrocarbons and alcohols are used as fuels. During complete combustion, they produce carbon dioxide and water.

Petrol contains 2,2,4-trimethylpentane, an isomer of octane, that promotes smooth combustion.

Figure for Question 6.8—Evaluate the results obtained from experiments and comment on sources of error and uncertainty and any assumptions made question 1 — Edexcel A-Level Chemistry AS

2,2,4-trimethylpentane

Alcohols, such as methanol and ethanol, can be used as fuels either on their own or as additives in petrol.

In an experiment to determine the enthalpy change of combustion of ethanol, $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}$ , a student used the apparatus shown.

Results:
Mass of water $=100.0 \mathrm{~g}$
Mass of ethanol used $=0.305 \mathrm{~g}$
Temperature rise of water $=13.2^{\circ} \mathrm{C}$

The student looked in a data book and found the actual value for the standard enthalpy change of combustion of ethanol was more exothermic than the experimental value obtained.

Give two reasons for the difference between the data book value and the experimental value, other than referring to standard conditions.

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6.8—Evaluate the results obtained from experiments and comment on sources of error and uncertainty and any assumptions made question 1