Data analysis and evaluation

Processing results, plotting graphs, drawing conclusions and suggesting improvements.

To find the enthalpy change of neutralisation, a student measures the temperature of $25.0\,\text{cm}^3$ of sodium hydroxide solution in a polystyrene cup every 30 seconds for 3 minutes. At 3 minutes, $25.0\,\text{cm}^3$ of hydrochloric acid (measured with a measuring cylinder) is added, and the temperature is recorded every 30 seconds for a further 5 minutes.

(a) Explain why the temperature change is found by extrapolating a graph rather than from the highest temperature recorded, and describe how the extrapolation is done.

(b) Suggest two improvements to the procedure.

(a) Heat is lost to the surroundings while the mixture warms up, so the highest recorded temperature is lower than the true maximum (1)
(a) Draw a line of best fit through the cooling points after mixing and extrapolate it back to the time of mixing (3 minutes); read the temperature change from the line before mixing to this point (1)
(b) Use a lid on the polystyrene cup to reduce heat loss (1)
(b) Measure the volumes with a pipette or burette rather than a measuring cylinder (1)

A student's experimental value for the enthalpy change of neutralisation is $-48.2\,\text{kJ mol}^{-1}$ . The data-book value is $-57.1\,\text{kJ mol}^{-1}$ . The student calculated that the total measurement uncertainty in the experiment was .

Percentage difference $= (57.1 - 48.2) \div 57.1 \times 100 = 15.6\%$ (1)
This is larger than the $4.5\%$ measurement uncertainty, so measurement uncertainty cannot explain the difference (1)
So there is a systematic error in the procedure, such as heat loss to the surroundings (1)

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