International A LevelChemistryEdexcelExam Questions6. Practical Skills in Chemistry II6.1 Advanced Physical Chemistry Core Practicals

Advanced Physical Chemistry Core Practicals (Edexcel International A Level Chemistry)

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6.1 Advanced Physical Chemistry Core Practicals

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6.1 Advanced Physical Chemistry Core Practicals

1a1 mark

A student investigated the kinetics of the reaction between bromide ions, Br– , and bromate(V) ions, begin mathsize 16px style Br O subscript 3 to the power of minus sign end style , in aqueous acid.

5Br–(aq) + Brbegin mathsize 16px style O subscript 3 to the power of minus sign end style(aq) + 6H+(aq) → 3Br2(aq) + 3H2O(l)

To determine the rate equation for the reaction, the student varied the concentration of bromide ions, bromate(V) ions and acid in turn.

The effect of the concentration of bromide ions was investigated first.

Procedure

Step 1 Add 10.0 cm3 of 0.0050 mol dm–3 potassium bromate(V), 15.0 cm3 of acidified methyl orange indicator solution and 5.0 cm3 of 0.00010 mol dm–3 aqueous phenol to a beaker labelled P.
Step 2 Prepare the contents of beaker Q for Run 1 as specified in the table.
Step 3 Pour the contents of beaker Q into beaker P and start a timer.
Pour the contents of beaker P back into beaker Q and place beaker Q on a white tile.
Step 4 Stop the timer as soon as the mixture turns colourless.
Record the time, along with the temperature of the solution.
Step 5 Repeat Steps 1 to 4 for the remaining runs.

Contents of beaker Q
Run Volume of 0.01 mol dm–3 KBr / cm3 Volume of H2O / cm3
1 10.0 0
2 8.0 2.0
3 6.0 4.0
4 5.0 5.0
5 4.0 6.0
6 3.0 7.0

Give one reason why the contents of beaker P are poured back into beaker Q in Step 3.

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1b4 marks

Methyl orange indicator is bleached colourless by bromine.
Under the experimental conditions, the reciprocal of the time taken for the methyl orange to be bleached (1/t) is proportional to the initial rate.

i)
Explain why phenol is added to the reaction mixture.

(2)

ii)
Give the colour of the reaction mixture before it turns colourless.

(1)

iii)
Give the reason why the temperature is measured in Step 4.

(1)

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1c6 marks

The student’s results are shown.

   Volume of KBr / cm3 10.0 8.0 6.0 5.0 4.0 3.0
   Time, t / s 23 24 32 39 48 64
   (1/t) / s–1 0.043 0.042 0.031 0.026    
   Temperature / °C 18 22 22 22 22 22

i)
Complete the table by calculating the remaining values of 1/t.

(1)

ii)
Plot a graph of 1/t against the volume of KBr. Include a line of best fit.
(3)
q3c-ii-paper-6-jan-2022-edexcel-ial-chemistry

iii)
State why the volume of KBr is proportional to the concentration of bromide ions in the reaction mixture.
(1)

iv)
State the order of reaction with respect to bromide ions, using your graph.
(1)

How did you do?

1d1 mark

After investigating the effect on the rate of the concentration of bromate(V) ions and the concentration of hydrogen ions, the student obtained the graphs shown.

q3d-paper-6-jan-2022-edexcel-ial-chemistryDeduce the rate equation for the reaction, using these data and your answer to (c)(iv).

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2a1 mark

A student carried out an experiment to determine the enthalpy change when solid lithium chloride, LiCl, dissolved in water to form a solution.

Procedure
Step 1 Use a pipette to place 25.0 cm3 of distilled water into a polystyrene cup.
Step 2 Measure and record the initial temperature of the water.
Step 3 Add 2.12 g of lithium chloride to the water.
Step 4 Stir the mixture and record the highest temperature reached.

Give a reason why a polystyrene cup was used instead of a glass beaker in Step 1.

How did you do?

2b
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The temperature rise was 12.5°C.

Calculate the enthalpy change for the formation of this solution of lithium chloride.
Include a sign and units in your answer.

[Assume: specific heat capacity of the solution = 4.18 J g−1 °C−1
density of the solution = 1.00 g cm–3]

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2c
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The thermometer used to measure the temperature change had an uncertainty of ±0.25 °C for each measurement.

Calculate the percentage uncertainty in the temperature change in this experiment.

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2d5 marks

The temperature rise in this experiment was lower than expected, due to heat loss to the surroundings.

Describe changes to the procedure that would give a more accurate temperature rise.

Include the use of a stopwatch and details of a graph you would plot.

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