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The reaction between sulfuric acid and sodium hydroxide is exothermic.
$ \text{H}_2\text{SO}_4(\text{aq}) + 2\text{NaOH}(\text{aq}) \rightarrow \text{Na}_2\text{SO}_4(\text{aq}) + 2\text{H}_2\text{O}(l) $
By measuring the temperature changes that occur when different volumes of the acid are added to a fixed volume of the alkali, it is possible to determine the neutralisation point. This is the point at which just enough acid has been added to react with all the alkali present. The aim of the investigation is to determine the concentration of the sulfuric acid.
$\text{FA 1 is 2.00 mol dm}^{-3}$ sodium hydroxide, $\text{NaOH}$.
FA 2 is dilute sulfuric acid, $\text{H}_2\text{SO}_4$.
Read through the instructions carefully and prepare a table for your results before starting any practical work.
(a) Method
● Support a plastic cup in a $250\text{cm}^3$ beaker.
● Use a pipette to transfer $25.0\text{cm}^3$ of FA 1 into the plastic cup.
● Record the temperature of FA 1, $T_1$, in the space below.
$$T_1 = \text{………….} \ °C$$
● Fill the burette labelled FA 2 with FA 2.
● Add $5.00\text{cm}^3$ of FA 2 from the burette to the plastic cup.
● Stir the mixture thoroughly and record the temperature of the solution.
● Add a further $5.00\text{cm}^3$ of FA 2 to the plastic cup and again record the temperature.
● Repeat the addition of $5.00\text{cm}^3$ portions of FA 2 until you have added a total of $50.00\text{cm}^3$ of FA 2 to the plastic cup. Measure the temperature after each addition.
● Record in your table below the total volume of FA 2 added and the temperature of the solution after each addition.
(b) After each addition of acid, the temperature rise, $\Delta T$, is given by,
$\Delta T = \text{temperature recorded} - T_1$.
The total volume of solution in the plastic cup, $V_T$ is given by,
$V_T = \text{volume of FA 2} + \text{volume of FA 1}$.
The heat given out by the reaction is proportional to the temperature rise, $\Delta T$, multiplied by the total volume of solution in the plastic cup, $V_T$.
Use your experimental results to complete the following table. You should include:
● the volume of FA 2
● the total volume in the plastic cup, $V_T$
● the temperature of the solution
● the temperature rise, $\Delta T$
● the total volume \times the temperature rise, $(V_T \times \Delta T)$
[Table below]
(c) (i) On the grid below, plot the values of $(V_T \times \Delta T)$ on the y-axis against the volume of FA 2 on the x-axis.
[Graph Grid]
(c) (ii) Draw a straight line of best fit through the points where the values of $(V_T \times \Delta T)$ are increasing. Draw a second straight line of best fit through the points where the values of $(V_T \times \Delta T)$ are decreasing.
(iii) From your graph, determine the volume of FA 2 where the two lines of best fit intersect.
$$\text{volume of FA 2} = \text{…………………………………..} \text{cm}^3$$
(d) The value you recorded in (c)(iii) is the volume of FA 2 which is needed to neutralise $25.0\text{ cm}^3$ of FA 1. In the following calculations you will determine the concentration of FA 2.
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Calculate how many moles of sodium hydroxide are contained in $25.0 \text{cm}^3$ of FA 1.
$$\text{moles of NaOH} = \text{…………………. mol}$$
(ii) Calculate how many moles of sulfuric acid would react with the number of moles of $\text{NaOH}$ in (i).
$$\text{moles of H}_2\text{SO}_4 = \text{…………………. mol}$$
(iii) Calculate the concentration of FA 2.
$$\text{concentration of FA 2} = \text{……………………… mol dm}^{-3}$$
(e) Other than heat losses from the plastic cup to the surroundings, suggest an additional source of error in this experiment and how this error could be reduced.
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523
578
540
A second way to determine the concentration of an acid is by volumetric titration. In this experiment you will first dilute the sample of FA 2 that you used in Question 1 and then titrate this diluted solution using aqueous sodium hydroxide.
$$H_{2}SO_{4} ext{(aq)} + 2NaOH ext{(aq)} \rightarrow Na_{2}SO_{4} ext{(aq)} + 2H_{2}O ext{(l)}$$
FA 2 is dilute sulfuric acid, $H_{2}SO_{4}$.
FA 3 is 0.150mol dm$^{-3}$ sodium hydroxide, NaOH.
distilled water
(a) Method
Dilution of FA 2
• Use the burette labelled FA 2 to transfer 25.00cm$^{3}$ of FA 2 into the 250cm$^{3}$ graduated (volumetric) flask, labelled FA 4.
• Make up the contents of the flask to the 250cm$^{3}$ mark with distilled water.
• Stopper the flask and mix the contents thoroughly. This is solution FA 4.
Titration
• Fill the burette labelled FA 3 with FA 3.
• Use a clean pipette to transfer 25.0cm$^{3}$ of FA 4 into a conical flask.
• Add to the flask a few drops of the acid-base indicator provided.
• Titrate the acid in the flask with the alkali, FA 3.
You should perform a rough titration.
In the space below record your burette readings for this rough titration.
$$\begin{align*}
\text{The rough titre is .................... cm}^{3}.
\end{align*}$$
• Carry out as many accurate titrations as you think necessary to obtain consistent results.
• Record, in a suitable form below, all of your burette readings and the volume of FA 3 added in each accurate titration. Make certain that any recorded results show the precision of your practical work.
(b) From your titration results obtain a suitable value to be used in your calculation. Show clearly how you have obtained this value.
25.0 cm$^{3}$ of FA 4 required .............. cm$^{3}$ of FA 3.
[1]
(c) (i) Calculate how many moles of NaOH are contained in the volume recorded in (b).
moles of NaOH = .................... mol
(ii) Hence, calculate how many moles of $H_{2}SO_{4}$ are contained in 25.0cm$^{3}$ of FA 4.
moles of $H_{2}SO_{4}$ = .................... mol
(iii) Calculate the concentration of the sulfuric acid, FA 2.
concentration of FA 2 = .................... mol dm$^{-3}$
[3]
(d) You have used two methods to determine the concentration of the sulfuric acid in FA 2. Use your answers to 1(d)(iii) and 2(c)(iii) to calculate the difference in these values as a percentage of the value found by the volumetric titration method.
percentage difference = .................... %
[1]
560
509
535
Qualitative Analysis
At each stage of any test you are to record details of the following.
- colour changes seen
- the formation of any precipitate
- the solubility of such precipitates in an excess of the reagent added
Where gases are released they should be identified by a test, described in the appropriate place in your observations.
You should indicate clearly at what stage in a test a change occurs.
Marks are not given for chemical equations.
No additional tests for ions present should be attempted.
If any solution is warmed, a boiling tube MUST be used.
Rinse and reuse test-tubes and boiling tubes where possible.
(a) FA 5, FA 6, FA 7 and FA 8 are aqueous solutions each of which contains a single cation and a single anion. Some of the ions present are listed below.
Pb^{2+}, Cl^−, CO_3^{2−}, CrO_4^{2−}
By observing the reactions that occur when pairs of the solutions are mixed together, you will be able to identify which solution contains which of these ions.
Use a 1 cm depth of each solution in a test-tube and record your observations in the following table.
Marks: 8
(b) From your observations deduce which solution contains each of the following ions.
| ion | Pb^{2+} | Cl^− | CO_3^{2−} | CrO_4^{2−} |
|---|---|---|---|---|
| solution |
Marks: 2
(c) Identify another ion that is present in one of the solutions. Explain your reasoning.
ion ...................
explanation ..........................................................................................................................
.................................................................................................................................
Marks: 1
(d) (i) If chloride ions, Cl^{-}, were to be replaced with bromide ions, Br^{-}, in one of the solutions, would it make any difference to the observations you made in (a)? Explain your answer.
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Marks: 2
(ii) FA 9 is an aqueous solution containing either chloride ions or bromide ions. Select a pair of reagents to identify which anion is present.
reagents ........................................................
Carry out this test and record your observations and conclusion.
observations ...............................................................
.................................................................................................................................
The anion in FA 9 is ........................................... .
Marks: 4
523
582
556
