Edexcel IAL Chemistry AS 3.P5 Measurement uncertainty and error

Pearson Edexcel IAL Chemistry syllabusChemistry YCH11First assessment 2019

Combine uncertainties for repeated readings where required, divide by the measured change or quantity and explain why a smaller signal gives a larger percentage value.

How this is tested

Calculate a measurement range from instrument uncertainty and the number of readings.
Calculate percentage uncertainty for mass, temperature change or another measured quantity.
Explain reliability differences through signal size, instrument resolution and error direction.

[Maximum number: 1]

An experiment was carried out to determine the enthalpy change of neutralisation for the reaction between sulfuric acid and sodium hydroxide.

\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})+2 \mathrm{NaOH}(\mathrm{aq}) \rightarrow \mathrm{Na}_{2} \mathrm{SO}_{4}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

Procedure

Step $125.0 \mathrm{~cm}^{3}$ of $1.25 \mathrm{~mol} \mathrm{dm}^{-3}$ sulfuric acid was placed in a polystyrene cup.
The polystyrene cup was then placed in a beaker. A thermometer was used to measure the temperature of the acid and a clock was started.

Step 2 The temperature of the sulfuric acid was measured every 30 seconds for $2 \frac{1}{2}$ minutes.

Step $350.0 \mathrm{~cm}^{3}$ of $1.25 \mathrm{~mol} \mathrm{dm}^{-3}$ sodium hydroxide solution was added to the acid in the polystyrene cup at 3 minutes and the mixture was constantly stirred.

Step 4 The temperature of the mixture in the polystyrene cup was measured at $3 \frac{1}{2}$ minutes and then every 30 seconds until the clock reached 10 minutes.

The data from the experiment are shown on the graph.

Figure for Question 4(d)(iii) — Edexcel A-Level Chemistry AS

Give a possible reason why the temperature does not appear to change at various times, such as $5 \frac{1}{2}$ to $6 \frac{1}{2}$ minutes.
(1)

How this is tested

Question 4(d)(iii)