Analysis, Conclusions and Evaluation

Complete Table 2.1. Record your answers to the appropriate number of decimal places.

Use the relationship $q = mc\Delta T$ to calculate the energy, $q$, in J, gained by the water.

$1.00\text{ cm}^3$ of water has a mass of $1.00\text{ g}$.

Calculate the enthalpy change of combustion, $\Delta H_c$, of butane, in $\text{kJ mol}^{-1}$.

Give your answer to three significant figures.

Without changing the apparatus, suggest what should be done in step 6 before recording the maximum temperature reached to improve the experimental procedure.

The $500\text{ cm}^3$ measuring cylinder has graduations every $5\text{ cm}^3$.

Calculate the percentage error in the measurement of the volume of water.

Show your working.

A student suggests that the value calculated in (c) is different from the actual value of $\Delta H_c$ of butane because of heat lost during the experiment. Suggest one change to the apparatus that would reduce the heat lost.

Suggest why this might contribute to a reduction in the accuracy of $\Delta H_c$ of butane.

Suggest why this might contribute to an increase in the accuracy of $\Delta H_c$ of butane.

State which step is used to determine the concentration of $\text{H}^+(\text{aq})$ ions from the catalyst in the mixture.

The iced water in conical flask A is used to significantly reduce the rate of reaction.

Suggest two reasons why the rate of reaction is significantly reduced when the reaction mixture is transferred to conical flask A.

Complete Table 2.1.

Identify the independent variable.

Identify one variable that needs to be controlled, apart from concentrations and volumes of solutions.

Reading 2 should have been taken at 10 minutes and not at 13 minutes.

State whether this result should have been included or not. Explain your answer.

Plot a graph on the grid in Fig. 2.1 to show the relationship between $(V_{\infty} - V_t)$ and time.

Use a cross ($\times$) to plot each data point. Draw a line of best fit.

Reading 5 was not taken. Use the graph to predict the total volume of $\text{NaOH}(\text{aq})$ needed to neutralise the total amount of $\text{H}^+(\text{aq})$ at 40 minutes.

It is not possible to repeat the experiment.

State whether the data from the experiment is reliable. Justify your answer.

Use Fig. 2.1 to calculate the hydrogen ion concentration of pure water at $45\text{ }^\circ\text{C}$.

Calculate the value of $K_{\text{w}}$ for pure water at $45\text{ }^\circ\text{C}$.

State the relationship between $K_{\text{w}}$ and temperature.

Complete Table 2.1.

Record $\frac{1}{T}$ to three significant figures using standard form. Record $\log K_{\text{w}}$ to two decimal places.

Plot a graph on the grid to show the relationship between $\log K_{\text{w}}$ and $\frac{1}{T}$. Use a cross ($\times$) to plot each data point.

Draw a line of best fit.

Circle the one point on the graph that you consider to be most anomalous.

Suggest one reason to explain the anomalous point you have circled. Assume there was no error in determining $K_{\text{w}}$.

Use your graph to determine the gradient of the line of best fit.

State the coordinates of both points you used in your calculation. These must be selected from your line of best fit.

The relationship between $\log K_{\text{w}}$ and $\frac{1}{T}$ is given by the equation shown.

Use the gradient determined in (b)(v) to calculate a value for the enthalpy change, in $\text{kJ mol}^{-1}$, for the dissociation of water, $\Delta H$.

If you were unable to determine a value for the gradient in (b)(v), use the value $-2550$. This is not the correct value.

Suggest why each sample is applied to the chromatography paper using a thin capillary tube rather than a dropping pipette.

Suggest why it is necessary to spray a developing agent over the chromatography paper before the chromatogram can be analysed.

Table 2.1 shows $R_f$ values for some amino acids in the solvent used in Fig. 2.1.

Table 2.1

Use the data in Table 2.1 to identify the amino acids in tripeptide A.

Suggest why the hydrolysed sample of B produces only two spots.

State the reason for the overlap.

Suggest an improvement to the method that would allow the overlapping spots to be distinguished clearly.

Complete Table 1.1 and determine the mean titre to be used in calculating the concentration of dissolved oxygen.

Table 1.1

mean titre = .............................. $\text{cm}^3$

Calculate the concentration of dissolved oxygen in the $25.0\text{ cm}^3$ of solution. Show your working.

concentration of dissolved oxygen in $25.0\text{ cm}^3$ of solution = .............................. $\text{mol dm}^{-3}$

Suggest a suitable piece of apparatus for the transfer of $25.0\text{ cm}^3$ of the solution containing aqueous iodine.

Water samples are collected in full sealed flasks.

Explain why the sealed flask must be completely full.

Plot a graph of concentration of oxygen ($y$-axis) against temperature ($x$-axis) on the grid. Use a cross ($\times$) to plot each data point. Draw a smooth curve of best fit.

Use the graph to deduce the concentration of oxygen at $25\,^\circ\text{C}$.

concentration of oxygen at $25\,^\circ\text{C}$ = .............................. $\text{mol dm}^{-3}$

Circle the most anomalous point on the graph.

Suggest an explanation for this anomaly. Assume that there was no error in measuring oxygen concentration.

Complete the table. Record your answers to the correct number of decimal places.

Calculate the number of moles of ethanol burned. Give your answer to three significant figures.

$[A_r: \text{C}, 12.0; \text{H}, 1.0; \text{O}, 16.0]$

Use the formula $q = mc\Delta T$ to determine the energy change, $q$, that took place during the experiment. Use $q$ and your answer to (b) to calculate the enthalpy change of combustion of ethanol, $\Delta H_c$, in $\text{kJ mol}^{-1}$.

Include a sign in your answer.

$1.00\text{ cm}^3$ of water has a mass of $1.00\text{ g}$
$c = 4.18\text{ J g}^{-1}\text{ K}^{-1}$

Calculate the percentage error of the temperature change recorded in the table in (a).

Show your working.

State the effect, if any at all, on the accuracy of the experiment if the spirit burner was allowed to burn for longer. Explain your answer.

The flame was extinguished, but the lid of the spirit burner was not replaced immediately.

Predict how this would affect the value of $\Delta H_c$. Explain your answer.

Other than the reaction not being carried out under standard conditions, suggest two reasons why the value the student obtained in (c) is different from the actual value.

It is possible to calculate $\Delta H_c$ of ethanol using average bond enthalpies and the chemical equation for the reaction.

Using average bond enthalpies, $\Delta H_c$ of ethanol is $-1297\text{ kJ mol}^{-1}$.

Explain why this value is different from the actual value for $\Delta H_c$ of ethanol under standard conditions.

State the two additional measurement steps that each student must perform in order to find the formula of the chloride of iron.

1

2

The flow of dry chlorine gas must start before the iron wire is heated.

Explain why.

State an assumption that has to be made for the measurements made in this experiment to be valid.

Calculate the mass of chlorine that reacts with the iron wire in each experiment. Record each mass to two decimal places.

Calculate the amount of iron, in mol, and amount of chlorine atoms, in mol, that reacts in each experiment. Record the number of moles to three significant figures.

[$A_r$: Fe, 55.8; Cl, 35.5]

Plot a graph on the grid of the amount of chlorine atoms against the amount of iron.

Use a cross ($\times$) to plot each data point.

Draw a line of best fit through the plotted points. You should consider whether (0,0) should be on the line of best fit.

Circle the point on the graph you consider to be most anomalous.

Suggest one reason why this anomaly may have occurred during this experimental procedure.

Use the graph to determine the gradient of the line of best fit.

State the coordinates of both points you used in your calculation. These must be on your line of best fit.

Give your answer to three significant figures.

coordinates 1 ................................................. coordinates 2 ...................................................

gradient = ..............................

The formula of the chloride of iron produced in this experiment is $\text{FeCl}_3$.

State how the results student 4 obtains could be used to determine this formula.

Student 3 weighs the conical flask using a balance accurate to two decimal places and records its mass. After the chloride of iron is produced the mass increases by $4.03\text{ g}$.

Calculate the percentage error in measuring the mass of this chloride of iron.

Student 8 follows the same procedure as student 3.

State whether the results from student 8 will have more or less percentage error than those from student 3.

Explain your answer.

A sample of a chloride of iron prepared in this way contains 44% iron by mass.

Show that the formula of this chloride of iron is $\text{FeCl}_2$.

[$A_r$: Fe, 55.8; Cl, 35.5]

Explain, using the electrode potential values in the table, why the methods in (a) and (f) do not produce the same chlorides of iron.

A series of experiments is performed using the same amount, $0.100\text{ mol}$, of naphthalene each time.

Calculate the mass of naphthalene, $\text{C}_{10}\text{H}_8$, that should be used for each of these experiments.

[$A_r$: C, 12.0; H, 1.0]

The melting point and freezing point of a substance are the same. The melting point, $T_m$, of a substance can be found by recording the temperature at which the substance freezes, measured when crystals first start to appear on cooling.

The results of a series of experiments using $0.100\text{ mol}$ of naphthalene and different masses of diphenylamine are shown.

Process the results to complete the table.

Record all your data to three significant figures.

The mole fraction of naphthalene, $Y$, is calculated as shown.

$n_N = \text{amount in moles of naphthalene, } \text{C}_{10}\text{H}_8, = 0.100$
$n_D = \text{amount in moles of diphenylamine, } (\text{C}_6\text{H}_5)_2\text{NH}$

Plot a graph on the grid to show the relationship between $\frac{1}{T_m}$ and $\log Y$.

Use a cross (×) to plot each data point. Draw the straight line of best fit.

Use the graph to determine the gradient of the line of best fit. State the co-ordinates of both points you used in your calculation.

co-ordinates 1 .............................................. co-ordinates 2 .............................................

Use your answer to (c)(i) to determine the value of the enthalpy change of fusion of naphthalene, $\Delta H_{\text{fusion}}$, in $\text{kJ}\text{ mol}^{-1}$.

Do you consider the results obtained to be reliable? Explain your answer.

Different literature values for the enthalpy change of fusion of naphthalene suggest that $10.00\text{ g}$ of naphthalene require between $1.45\text{ kJ}$ and $1.47\text{ kJ}$ to melt.

Use this information to calculate the range of $\Delta H_{\text{fusion}}$ values of naphthalene, $\text{C}_{10}\text{H}_8$, given in literature.

Use your values to comment on the accuracy of the experimental procedure.
[$A_r$: C, 12.0; H, 1.0]

If you were not able to calculate $\Delta H_{\text{fusion}}$ in (c)(ii), you may use $18.4\text{ kJ}\text{ mol}^{-1}$, but this may not be the correct answer.

range ..................................................................................................................... $\text{kJ}\text{ mol}^{-1}$

comment .............................................................................................................................

The enthalpy change calculated in this reaction is actually $\Delta H_1$, shown in the Hess' cycle.

It is assumed that the enthalpy change when $\text{C}_{10}\text{H}_8(\text{l})$ and diphenylamine are mixed, $\Delta H_{\text{mixing}}$, is zero, and therefore $\Delta H_1 = \Delta H_{\text{fusion}}$.

State how the value of $\Delta H_1$ compares to the value of $\Delta H_{\text{fusion}}$ if the mixing of naphthalene and diphenylamine is endothermic.

Explain your answer.

Predict the effect this will have on the calculated values of $Y$.

Explain your answer.

$n_N = \text{amount in moles of naphthalene}$
$n_D = \text{amount in moles of diphenylamine}$

The student uses the incorrectly calculated value of $Y$ from (f)(i) in the determination of $\Delta H_{\text{fusion}}$.

Predict how the student's calculated value of $\Delta H_{\text{fusion}}$ is different from the actual value.

Explain your answer.

Use the information above to explain why lowering the vapour pressure of a liquid increases the temperature at which it boils.

A student carries out an experiment to determine the boiling point constant, $K_b$, for water. The student uses anhydrous glucose, $\text{C}_6\text{H}_{12}\text{O}_6$, as the solute because it is non-volatile and very soluble in water.

The experimental set-up the student uses is shown.

Show, using a labelled arrow, where the cooling water enters the reflux condenser.

A digital probe thermometer is used as shown in the diagram.

Explain why a normal laboratory glass thermometer would not be suitable.

The student follows this procedure.

1. Transfer $75.00\text{ g}$ of distilled water to the round-bottomed flask.
2. Add anti-bumping granules to the distilled water to prevent violent, uneven boiling.
3. Heat the distilled water until it boils and record the highest stable temperature.
4. Stop heating and allow the distilled water to cool to room temperature.
5. Remove the reflux condenser and add about $1\text{ g}$ of anhydrous glucose, measured accurately.
6. Replace the reflux condenser and heat the solution until it boils, noting the highest stable temperature.
7. Repeat steps 4 to 6, each time adding approximately $1\text{ g}$ more of anhydrous glucose, accurately weighed, until sufficient readings are taken.

In step 4, the heating is stopped and the distilled water allowed to cool from its boiling point, before removing the reflux condenser.

Apart from for safety reasons, explain why this is essential.

At $101\text{ kPa}$ ($1\text{ atm}$), distilled water is known to boil at $100.00\text{ }^{\circ}\text{C}$.

Suggest why the boiling point of distilled water in this experiment was found to be $99.48\text{ }^{\circ}\text{C}$.

Assume that the digital probe thermometer was reading correctly.

The student constructed the table shown to record the results for this experiment.

Complete columns C and D to three significant figures and column E to two decimal places.
[The $M_r$ of glucose is 180.]

You may use the space below for any working.

Plot a graph on the grid to show the relationship between $\Delta T$ and the amount of glucose in $1\text{ kg}$ of water, Z.
Use a cross (×) to plot each data point. Draw a line of best fit.

Circle the most anomalous point on your graph.

Use the graph and the equation to determine the boiling point constant, $K_b$, for water. Give this value to three significant figures and state the units.

State the co-ordinates of both points you used in your calculation.

co-ordinates 1 ................................................. co-ordinates 2 ................................................

$K_b$ =

units =

The experiment was repeated using solid from a bottle that was labelled glucose, $\text{C}_6\text{H}_{12}\text{O}_6$, but actually contained sucrose, $\text{C}_{12}\text{H}_{22}\text{O}_{11}$. Sucrose is non-volatile and very soluble in water.

What would be the effect of this on the value the student obtained for $K_b$?

Explain your answer.

The student used distilled water to dissolve the glucose.

Suggest why the student did not use tap water.

The student repeated this experiment using sodium chloride as the solute. The student found their calculated value of $K_b$ was twice the calculated value of that obtained with glucose.

Suggest a reason for this.

Plot a graph on the grid on page 9 to show the relationship between concentration of sucrose, $c$, and observed angle of rotation, $\alpha_{\text{obs}}$.

Use a cross ($\times$) to plot each data point. Draw a line of best fit.

Circle the most anomalous point on your graph.

Use the graph to determine the specific rotation, $[\alpha]$, of sucrose.

Give this value to two decimal places.

State the co-ordinates of both points you used in your calculation.

co-ordinates 1 ............................................. co-ordinates 2 .............................................

specific rotation of sucrose, $[\alpha]$ = ..............................

Calculate the mass, in g, of sucrose the student would need to use.

mass of sucrose = .............................. g

Describe how the student should accurately prepare the standard solution using a sample of sucrose of mass calculated in (i).

The student used the standard solution prepared in (b) to prepare the solutions in the table on page 8.

Calculate the volume of standard solution of concentration $0.0750\text{ g cm}^{-3}$ and the volume of distilled water needed to prepare $15.00\text{ cm}^3$ of sucrose solution of concentration $0.0350\text{ g cm}^{-3}$.

Give your answers to two decimal places.

volume of standard solution = .............................. $\text{cm}^3$

volume of distilled water = .............................. $\text{cm}^3$

The volumes of the two solutions given in (c)(i) could be measured using the same type of apparatus.

Name a suitable piece of apparatus which could be used to measure these volumes.

In (a)(ii) you circled an anomalous point. This was caused by the student incorrectly making one of the sucrose solutions.

Suggest the error made by the student that caused this anomaly.

The student recorded the observed angle of rotation, $\alpha_{\text{obs}}$, for a sucrose solution of unknown concentration as +3.75.

Determine the concentration of this sucrose solution in $\text{mol dm}^{-3}$.

[$M_r$ sucrose: 342]

concentration of sucrose = .............................. $\text{mol dm}^{-3}$

The glass cell of $10\text{ cm}$ length is expensive, so one cell is used for all the solutions that are placed in the polarimeter.

Suggest how you would ensure that the concentration of solution in the cell is accurate each time the cell is used for the different sucrose solutions.

Concentration of sucrose is the independent variable in this polarimeter experiment.

The glass cell of $10\text{ cm}$ length is replaced by a glass cell of $20\text{ cm}$ length. The $20\text{ cm}$ glass cell is filled with $0.0750\text{ g cm}^{-3}$ sucrose solution.

Predict the value for the observed angle of rotation, $\alpha_{\text{obs}}$, for the sucrose solution of concentration $0.0750\text{ g cm}^{-3}$ when the $20\text{ cm}$ cell is used. Explain your answer.

predicted value = ..............................

explanation ................................................................................................................................

Before the angles of rotation of the sucrose solutions are measured, the glass cell is first filled with distilled water and the angle of rotation measured.

Explain why this measurement is taken.