AS & A Level Chemistry - 9701 Paper 3 2019 Summer Zone 5

The reaction between acids and alkalis is exothermic. You will find the concentration of a monoprotic acid, HZ, by a thermometric method using a solution of sodium hydroxide of known concentration. HZ(aq) + NaOH(aq) → NaZ(aq) + H₂O(l) FA 1 is a solution of acid HZ. FA 2 is 2.00 mol⋅dm⁻³ sodium hydroxide, NaOH. (a) Method - Place the thermometer into FA 1. Record the temperature of FA 1 in the table. This is the temperature when the volume of FA 2 is 0.0. - Rinse and dry the thermometer. - Place the thermometer into FA 2. Record the temperature of FA 2 in the table. This is the temperature when the volume of FA 1 is 0.0. - Fill a burette with FA 1. - Support the plastic cup in the 250 cm³ beaker. - From the burette transfer 35.0 cm³ of FA 1 into the plastic cup. - Use the 50 cm³ measuring cylinder to measure 5.0 cm³ of FA 2. - Transfer the 5.0 cm³ of FA 2 into the plastic cup. Stir the mixture and record the highest temperature. - Tip out the solution, rinse the plastic cup with water, shake it to remove excess water and replace the cup in the beaker. - Rinse and dry the thermometer. - Use the burette to transfer 30.0 cm³ of FA 1 into the plastic cup. - Use the measuring cylinder to transfer 10.0 cm³ of FA 2 into the plastic cup. - Stir the mixture and record the highest temperature. - Tip out the solution, rinse the plastic cup with water, shake it to remove excess water and replace the cup in the beaker. - Rinse and dry the thermometer. - Continue the experiment using the volumes of FA 1 and FA 2 given in the table and record the maximum temperature of each mixture. [Table_1: volume FA 1 / cm³, volume FA 2 / cm³, temperature / °C]

(b) (i) Plot a graph of temperature of solution (y-axis) against volume of FA 2 added (x-axis) on the grid. Select a scale on the y-axis to include a temperature of 2°C above your maximum thermometer reading. Label any points you consider anomalous. Draw two lines of best fit through the points on your graph, the first for the increase in temperature and the second for the decrease in temperature of the mixtures. Extrapolate the two lines so they intersect.

(b) (ii) The intersection on your graph occurs at the volume of FA 2 that reacted to form a neutral solution. Determine the volumes of FA 1 and FA 2 required to form a neutral solution. ............................. cm³ of FA 1 neutralises ............................. cm³ of FA 2.

(c) (i) Calculate the number of moles of sodium hydroxide, FA 2, required to obtain a neutral solution in this experiment. moles of NaOH = ............................. mol

(c) (ii) Hence calculate the concentration of HZ in FA 1. concentration of HZ = ............................. mol⋅dm⁻³

(d) Explain how you would use the data obtained to calculate the enthalpy change of neutralisation of HZ. You do not need to carry out the calculation.

You will now determine the concentration of HZ in FA 1 by titration using aqueous sodium carbonate of known concentration.
$$2H^+(aq) + CO_3^{2-}(aq) \rightarrow H_2O(l) + CO_2(g)$$
FA 3 is 0.0353 mol dm^-3 aqueous sodium carbonate, Na₂CO₃.
methyl orange indicator

(a) Dilution of FA 1
• Use the 10.0 cm³ pipette to transfer 10.0 cm³ of FA 1 into the 250 cm³ volumetric flask.
• Add distilled water to the mark.
• Shake the flask to mix the solution thoroughly and label it FA 4.

Titration
• Fill the second burette with FA 4.
• Pipette 25.0 cm³ of FA 3 into a conical flask.
• Add approximately 10 drops of methyl orange.
• Perform a rough titration and record your burette readings in the space below.

The rough titre is .............................. cm³.
• Carry out as many accurate titrations as you think necessary to obtain consistent results.
• Make certain that any recorded results show the precision of your practical work.
• Record all of your burette readings and the volume of FA 4 added in each accurate titration.

(b) From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value.
25.0 cm³ of FA 3 required .............................. cm³ of FA 4.

(c) (i) Give your answers to (ii), (iii) and (iv) to an appropriate number of significant figures. [1]
(ii) Calculate the number of moles of sodium carbonate in the FA 3 pipetted into the conical flask.

moles of Na₂CO₃ = .............................. mol [1]
(iii) Deduce the number of moles of HZ present in the volume of FA 4 recorded in (b).

moles of HZ = .............................. mol [1]
(iv) Calculate the concentration of HZ present in FA 1.

concentration of HZ in FA 1 = .............................. mol dm^-3

(d) In Question 1 you determined the concentration of HZ in FA 1 by a thermometric method.
In Question 2 you determined the concentration of HZ in FA 1 by titration.
Tick which one of the following statements you believe to be true.

• The method in Question 1 is more accurate than the method in Question 2.
• The method in Question 2 is more accurate than the method in Question 1.
• The two methods are of equal accuracy.

(e) A teacher informed a class that 112.3 g of pure HZ had been dissolved in distilled water to make 1 dm³ of FA 1. A student in the class suggested that HZ could be ethanoic acid.

Using your answer to (c)(iv) show, by calculation, whether the student was correct.
(If you were unable to complete the calculation in (c)(iv) you may assume the concentration was 2.08 mol dm^-3. This is not the correct value.)

The student was correct/incorrect because ....................................................

(a) FA 5, FA 6 and FA 7 are solutions each containing one cation and one anion. One of the cations and all of the anions are listed in the Qualitative Analysis Notes. You will carry out a series of tests on FA 5, FA 6 and FA 7 and draw conclusions from your observations. Use a separate 1 cm depth of each solution in a test-tube for the following tests.

[Table_1]

(b) Use your observations from (a) to identify as many ions as possible. Give the formula of each ion present. Write ‘unknown’ if you were unable to make a positive identification of an ion.

[Table_2]

(c) Give the ionic equation for any precipitation reaction involving FA 5 that you observed in (a). Include state symbols.
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