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(a) (i) Predict how the solubility of potassium nitrate will change if the solution temperature is increased.
Explain your prediction using the fact that dissolving potassium nitrate is endothermic.
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(a) (ii) Display your prediction in the form of a sketch graph, labelling clearly the axes.
(b) In the experiment you are about to plan, identify the following.
(i) the independent variable ......................................................................................................................
(ii) the dependent variable ........................................................................................................................
(c) Design a laboratory experiment to test your prediction in (a).
In addition to the standard apparatus present in a laboratory you are provided with the following materials,
a boiling tube,
a looped wire stirrer,
a thermometer covering the temperature range 0°C to 100°C.
Describe how you would carry out the experiment. You should
• ensure a wide range of results suitable for analysis by graph,
• decide on the amounts of water and potassium nitrate to use,
• measure the amounts of the two reagents,
• heat the apparatus,
• decide at what point the temperature of the solution is to be taken.
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(d) State a hazard that must be considered when planning the experiment and describe precautions that should be taken to keep risks to a minimum.
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(e) Draw a table with appropriate headings to show the data you would record when carrying out your experiments and the values you would calculate in order to construct a graph to support or reject your prediction in (a). The headings must include the appropriate units.
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Chemical reactions occur more rapidly as the temperature of the reaction mixture increases. The mathematical relationship that summarises this is $$\log_{10} (\text{rate of reaction}) = \frac{-E_A}{19T}$$ where $E_A$ is the activation energy of the reaction and $T$ is the absolute temperature in Kelvin and the rate of reaction can be taken as the reciprocal of the time taken in seconds (1/time).
An experiment was carried out to investigate this relationship using dilute hydrochloric acid and aqueous sodium thiosulfate.
• 20cm3 of dilute hydrochloric acid was placed in a boiling tube contained in a water bath.
• 20cm3 of aqueous sodium thiosulfate was added to the dilute hydrochloric acid, while stirring and a stopwatch started.
• The temperature of the water bath was recorded.
• After a period of time the liquid became cloudy (opaque) due to the formation of a precipitate of sulfur.
• As soon as this cloudiness (opacity) appeared the time was recorded.
• The temperature of the water bath was raised and the whole experiment repeated.
(a) The results of several such experiments are recorded below.
Process the results in the table to calculate $\log_{10} (\text{rate of reaction})$, the reciprocal of the absolute temperature ($1/T$) and the 'rate of reaction' ($1/\text{time}$). You should expect the values of $\log_{10} (\text{rate of reaction})$ to be negative.
Record these values to three significant figures in the additional columns of the table.
Label the columns you use. For each column you use include units where appropriate and an expression to show how your values are calculated.
You may use the column headings A to F for these expressions (e.g. A–B). [3]
[Table_1: The table below includes columns A, B, C, where A: temperature / °C, B: absolute temperature / K, C: time / s. The rest of the columns D, E, F are to be filled based on calculations.]
(b) Plot a graph to show the relationship between $\log_{10} (\text{rate of reaction})$ and the reciprocal of the absolute temperature. You are reminded that the values for $\log_{10} (\text{rate of reaction})$ are negative. Draw the line of best fit.
[Graph paper provided]
(c) Circle and label on the graph any point(s) you consider to be anomalous. For each anomalous point give a different reason why it is anomalous, clearly stating which point you are describing.
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[3]
(d) Comment on whether the results obtained can be considered as reliable.
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(e) Determine the slope of the graph. Mark clearly on the graph any construction lines and show clearly in your calculation how the values from the intercepts were used in the calculation of the slope.
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(f) Using the value of the slope of your graph calculated in (f) calculate a value for the activation energy, $E_A$. Correct use of the equation will produce an answer in kJ mol-1.
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(g) By considering the movement of particles in the reaction explain why the rate of reaction increases with increasing temperature.
[2]
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