Practise Edexcel IAL Chemistry 3.P5 by calculating percentage uncertainties, comparing ranges with accepted values and explaining reliability through error direction and instrument limits.
In an experiment to measure the enthalpy change of combustion of ethanol, a student uses the apparatus shown.
A student devised the procedure shown to determine the enthalpy change of combustion for ethanol.
- of water is placed in the beaker and its temperature recorded
- the burner containing ethanol is weighed, placed under the beaker of water and then lit
- after 5 minutes the burner is removed
- the temperature of the water in the beaker is recorded again
- the flame is extinguished, and the burner and its contents are reweighed.
The uncertainty in each reading of the thermometer was .
Calculate the percentage uncertainty in the temperature change in the student's experiment.
The student repeated the experiment using the same apparatus, in an attempt to improve its accuracy. The water was heated for 15 minutes before measuring the final temperature and reweighing the burner.
The data book value for the standard enthalpy change of combustion of ethanol is .
The teacher said that the changes would improve the percentage uncertainty in the measurement of mass and temperature but have little effect on the accuracy of the value obtained.
State how the changes in the experiment improve the percentage uncertainties in the measurement of mass and temperature. No calculation is required.