A level edexcel chemistry specimen
paper 3 Questions With Correct
Update
Solid benzoic acid is usually purified by recrystallisation from hot water. The method
used is:
1 The impure solid is dissolved in the minimum volume of hot water.
2 The solution is quickly filtered.
3 The solution is allowed to cool and crystallise.
4 The crystals are filtered from the remaining solution.
5 The crystals are washed with a little cold water.
6 The crystals are left to dry.
(a) State how steps 2, 4 and 5 remove impurities from the benzoic acid. - Correct
Answer - step 2: insoluble impurities are removed by filtration of the hot solution (1)
• step 4: soluble impurities remain in the solvent left after filtering the cooled mixture (1)
• step 5: the solid product is washed so that no soluble impurities form on the product
as it dries (1)
Describe how impurities affect the melting temperature of an impure solid - Correct
Answer - he melting temperature is over a larger range / is not sharp (1)
• the measured melting temperature is less than for the pure solid
Explain how you can deduce the order of reaction with respect to iodine from this graph
(straight line graph with volume of thiosulfate added on the y axis and time on the x
axis. Gradient is negative) - Correct Answer - the order with respect to iodine is zero (1)
• because the graph is a straight line, showing that the change in iodine concentration
is constant
Continuation from iodine order question.
Two straight line graphs with rate of reaction on the y axis and concentration of
hydrogen ions and propanone on x axis respectively.
paper 3 Questions With Correct
Update
Solid benzoic acid is usually purified by recrystallisation from hot water. The method
used is:
1 The impure solid is dissolved in the minimum volume of hot water.
2 The solution is quickly filtered.
3 The solution is allowed to cool and crystallise.
4 The crystals are filtered from the remaining solution.
5 The crystals are washed with a little cold water.
6 The crystals are left to dry.
(a) State how steps 2, 4 and 5 remove impurities from the benzoic acid. - Correct
Answer - step 2: insoluble impurities are removed by filtration of the hot solution (1)
• step 4: soluble impurities remain in the solvent left after filtering the cooled mixture (1)
• step 5: the solid product is washed so that no soluble impurities form on the product
as it dries (1)
Describe how impurities affect the melting temperature of an impure solid - Correct
Answer - he melting temperature is over a larger range / is not sharp (1)
• the measured melting temperature is less than for the pure solid
Explain how you can deduce the order of reaction with respect to iodine from this graph
(straight line graph with volume of thiosulfate added on the y axis and time on the x
axis. Gradient is negative) - Correct Answer - the order with respect to iodine is zero (1)
• because the graph is a straight line, showing that the change in iodine concentration
is constant
Continuation from iodine order question.
Two straight line graphs with rate of reaction on the y axis and concentration of
hydrogen ions and propanone on x axis respectively.
