Further Physical Chemistry Practicals (OCR A Level Chemistry)

Exam Questions

3 hours29 questions
1a4 marks

A student was asked to test the pH's of the following solutions using a pH probe.

  • Ethanoic acid
  • Propanoic acid
  • Hydrochloric acid
  • Ammonia
  • Sodium hydroxide 

The pH probe should be calibrated before use. 

Explain how two point calibration of the pH probe should take place.

1b3 marks

Describe how the student could use universal indicator to test the pH values of the solutions.

1c
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1 mark

Using the pH probe, the student records the pH of hydrochloric acid to be 1.04. 

Calculate the concentration of the solution. Give your answer to two decimal places. 

1d1 mark

The student is then asked to make a buffer solution with the ethanoic acid solution.

State the name of a suitable compound which can be added to the ethanoic acid solution which would form a buffer solution. 

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2a2 marks

A pH curve can be drawn from the data recorded from the following procedure:

  1. Placing a fixed volume of acid into a beaker.
  2. Add alkali in known small portions from a burette and stir.
  3. Use pH meter to record pH after every addition of alkali

Sketch a curve on the graph in Figure 1 for a strong acid strong base titration that could be drawn after this procedure is carried out.

Figure 1
1-4_q2a-ocr-a-as--a-level-easy-sq
2b2 marks

The same procedure outlined in part a) was performed but this time ethanoic acid which is a weak acid was used instead of a strong acid. 

Sketch on the graph in Figure 1 another pH curve representing a weak acid strong base titration. 

2c1 mark

State the name of the point at which the concentration of hydrogen ions is equal to the concentration of salt produced during the addition of the acid.

2d3 marks

When 10.00 cm3 of alkali was added to the ethanoic acid, the concentration of H+ ions was equal to the concentration of ethanoate ions. 

Calculate the pH of the solution at this point. 

The acid dissociation point, Ka, of the ethanoic acid is (Ka = 1.78 x 10-5 mol dm-3 at 298 K)

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3a2 marks

Hydrogen peroxide solution decomposes to form water and oxygen.

2H2O2 (aq) → O2 (g) + 2H2O (l)

At room temperature, the rate of this decomposition reaction can be very slow.

Name two possible catalysts for this reaction.

3b3 marks

One method of results collection could be using the downward displacement of water technique.

Describe how to set up the equipment for this technique.

3c1 mark

Name one other piece of equipment that could be used to replace the downward displacement of water technique and still obtain the same results.

3d4 marks

Describe how the results from this experiment would be used to determine the initial rate of reaction.

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4a1 mark

Clock reactions are often used to study reaction kinetics because they show a dramatic colour change at the end of the reaction.

Name a chemical that is typically used as an indicator in an iodine clock reaction.

4b1 mark

One example of the iodine clock reaction uses hydrogen peroxide and potassium iodide. Balance the following equation for the reaction of hydrogen peroxide and iodide.

 
___H2O2 (aq) + ___I- (aq) + ___H+(aq) → ___I2 (aq) + ___H2O (l)

4c2 marks

During the course of the hydrogen peroxide reaction with potassium iodide, there are two chemicals which are in excess. 

Name the two chemicals that are in excess. 

4d3 marks

After completing the iodine clock experiment, a rate-concentration graph is often drawn.

Describe how the rate-concentration graph could be used to identify the order of the reaction with respect to a reactant.

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5a2 marks

A common chemical reaction used in rates experiments is the reaction of sodium thiosulfate and hydrochloric acid.

Na2S2O3 (aq) + 2HCl (aq)  → 2NaCl (aq) + H2O (l) + SO2 (g) + S (s)

The reaction between sodium thiosulfate and hydrochloric acid can be monitored by measuring the time taken for a black cross to disappear.

Suggest why the same size conical flasks should be used when performing this experiment.

5b1 mark
A student collects the following results for the reaction of sodium thiosulfate with hydrochloric acid.
 
Concentration of sodium thiosulfate solution (mol dm–3) Time for cross to disappear (s)
0.05 115.2
0.10 57.6
0.15 30.0
0.20 15.6
0.25 7.2

 

The student used their results to produce a concentration-time graph.

 corrected-rates-graph

State how the student's concentration-time graph shows that the reaction is not zero order with respect to sodium thiosulfate. 

5c3 marks

Describe how the graph could be used to determine if the reaction is first or second order with respect to sodium thiosulfate.

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1a5 marks

The reaction of calcium carbonate with dilute hydrochloric acid results in the formation of a gas.

The effect of increasing the concentration of the acid on the rate of reaction can be determined by measuring the amount of gas produced at regular intervals for several minutes.

i)
Give the equation for the reaction of calcium carbonate with dilute hydrochloric acid. Include state symbols.

ii)
State two experimental conditions that should remain constant during this experiment.

iii)
Explain what effect an increase in either one of the two conditions stated in (ii) would have on the rate of the reaction.
1b6 marks

The following procedure was used to measure the rate of the reaction of CaCO3 with HCl:

Procedure

    1. Support a gas syringe with a stand, boss and clamp
    2. Measure 50 cm3 of 0.050 mol dm-3 dilute HCl in a beaker and then add it to a conical flask
    3. Add about 2.5 g of CaCO3 to the flask using a spatula, immediately connect the gas syringe and start counting.
    4. Record the volume of gas produced every 20 seconds for 2 minutes.
    5. Repeat Steps 1-4 for different concentrations of HCl.
i)
Suggest two improvements to the method used.
ii)
Complete the missing hazards, risk and precautions in the table below.
iii)
How can you tell that the reaction is complete?

Hazard

Risk

Precaution

 

Causes skin and eye irritation

 

Effervescence in the reaction mixture

 

Use a large conical flask;

Do not look over the top when adding the CaCO3; use eye protection; wear gloves

1c4 marks

The results for the reaction of 2.5 g of CaCO3 with 50 cm3 of 0.050 mol dm-3 dilute HCl were recorded in the table below.

Time (s)

 0.00

 10.0

  20.0

30.0

  40.0

  50.0

  60.0

  70.0

  80.0

  90.0

 100.0

 110.0

120.0

Volume of CO2 (cm3)

 0.00

 4.80

  9.10

12.3

  16.0

  20.2

  23.7

  26.4

  28.2

  28.9

  29.1

  29.1

  29.2


Plot a graph of time (s) on the x-axis and volume of CO2 (cm3) on the y-axis. Draw a line of best fit on the graph.

graph-paper

1d
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3 marks

Use the results and your graph to calculate the initial rate of reaction. Give your answer to 2 decimal places and state the correct units.

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2a4 marks

A student investigates the rate of reaction between copper sulfate solution and dilute sodium hydroxide solution.

75 cm3 of copper sulfate solution was poured into a 250 cm3 conical flask using a measuring cylinder.

The conical flask was then placed on a cross drawn on a piece of paper. 15 cm3 of dilute sodium hydroxide was added to the conical flask as shown.

The timer was started, and then was stopped when the cross could not be seen anymore. The student then recorded the time. 

i)
Write down the equation for the reaction between copper sulfate and dilute sodium hydroxide. Include state symbols.
ii)
Explain why the cross on the paper disappears.
iii)
Give the name of the product formed in the reaction that causes the cross on the paper to disappear.
2b4 marks

The student investigates the effect of temperature on the rate of the reaction, described in part (a), using the following procedure.

Procedure

    1. Measure out copper sulfate using a beaker and pour it into a conical flask.
    2. Record the initial temperature of the copper sulfate in the conical flask using a thermometer.
    3. Place the flask with the solution onto a cross drawn on a piece of paper.
    4. Heat the sodium hydroxide to a specific temperature.
    5. Start the stopwatch and then add the dilute sodium hydroxide.
    6. Stop the stopwatch when the cross can no longer be seen.
    7. Repeat the experiment at different temperatures.
i)
Identify one key mistake and one improvement to the students method. 

ii)
Suggest two control variables which should remain constant during this experiment.
2c3 marks

The student concludes that the rate of reaction increases with increasing temperature.

Explain whether the student’s conclusion is correct using the collision theory.

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3a6 marks

Peroxodisulafte(VI) ions, S2O82-, reacts with iodide ions, I, in solution to form sulfate ions, SO42-. and iodine, I2. 

S2O82- (aq) + 2I (aq) → 2SO42- (aq) + I2 (aq)

If the reaction is carried out in the presence of a fixed amount of aqueous thiosulfate ions, S2O32- (aq), the iodine produced is reduced back to iodine.

2S2O32- (aq) + I2 (aq) → S4O62- (aq) + 2I (aq)

When all of the thiosulfate ions have been used up, the iodine remaining will react with starch solution to produce a blue black colour.

Using appropriate apparatus and the chemicals listed below, plan an experiment to find the order of this reaction with respect to iodide, I (aq). Your answer should quote suitable volumes of solutions for the experiment

Chemicals

    • 1.00 mol dm-3 aqueous potassium iodide, KI (aq)
    • 0.040 mol dm-3 aqueous dipotassium peroxodisulfate(VI), K2S2O8 (aq)
    • 0.010 mol dm-3 aqueous sodium thiosulfate, Na2S2O3 (aq)
    • 1% aqueous starch solution
    • Distilled water

3b4 marks

Explain how the results can be used to show that the reaction is first order with respect to iodide ions, I (aq).

3c
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3 marks

In a separate experiment, a student reacts together:

    • 7.2 x 10-3 mol dm-3 I (aq)
    • 3.8 x 10-3 mol dm-3 S2O82- (aq)

The initial rate is 8.95 x 10-3 mol dm-3 s-1. The reaction is first order with respect to both I and S2O82-.

Calculate the rate constant k for this reaction. State the units, if any.

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4a5 marks

The following pH curve was produced when an acid and an alkali were reacted together.

ph-curve-sq-

i)
This pH curve shown is typical of a particular combination of acid and alkali reacting together. Suggest the possible formulae for the alkali and acid that would have reacted together to produce the pH curve shown.

ii)
Outline a brief practical procedure that a student could use to obtain results which would produce the pH curve shown.
4b8 marks

In order to obtain a pH curve, a student is provided with a conical flask containing 50.0 cm3 of a 0.500 mol dm–3 ethanoic acid solution and a burette filled with 0.250 mol dm–3 potassium hydroxide solution. They are also provided with a pH meter. 

i)
Briefly describe how the student would ensure that the reading on the pH meter was accurate.

ii)
Describe how the student would carry out the titration for this reaction in order to obtain results to plot a pH curve.
4c4 marks

A student performs a titration with a weak acid and weak base to produce the following pH curve by titration.

ph-curve-wa-wb

i)
Explain the difference between the terms equivalence point and end point.

ii)
Explain why the student would not be able to use an indicator solution to show the end point of their titration. Refer to the pH change in your answer.

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1a2 marks

A student is asked to investigate the effect of changing the concentration of iodide ions on the iodine clock reaction between hydrogen peroxide and iodide ions.

Hydrogen peroxide reacts with iodide ions to form iodine which then immediately reacts with the limited amount of thiosulfate ion as shown.

H2O2 (aq) + 2H+ (aq) + 2I (aq) → I2 (aq) + 2H2O (l)

2S2O32– (aq) + I2 (aq) → 2I (aq) + S4O62– (aq)

Explain how the colour change in the indicator is caused.

1b5 marks

The student is given 100 cm3 of 0.10 mol dm-3 potassium iodide solution.  

Describe how they could produce a minimum of five different concentrations of potassium iodide solution. Your answer should consider possible control variables.

1c2 marks

A second student completes the same experiment. Using their results, the second student plots a rate-concentration graph. 

Describe how the second student calculates the rate for their graph.

1d4 marks

The graph of the second student is shown.

 
4b1
 

From the graph, the second student concludes that the reaction is first order with respect to iodide ions. 

Evaluate the second student’s conclusion.

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2a2 marks

The reaction between methanoic acid and bromine water, in the presence of an acid catalyst, can be monitored using the following equipment.

Rate Gas Syringe

Write a balanced chemical equation for the reaction between methanoic acid and bromine water.

2b4 marks

Using the equipment shown in part (a), describe the steps required to collect the results and determine the order of reaction with respect to bromine.

2c2 marks

Another method to determine the order of reaction with respect to bromine involves changing the concentration of the bromine solution and using a white tile.

State the dependent variable in this experiment and suggest how the use of a white tile may improve the reliability of the results.

2d3 marks

The order of reaction with respect to bromine can be investigated experimentally by using a colorimeter to measure the change in concentration of the bromine solution over time. 

The results of such an investigation are shown.

 
Time / s [Br2 (aq)] / mol dm-3 
0 0.0200
30 0.0180
60 0.0162
90 0.0146
120 0.0132
180 0.0106
240 0.0088
360 0.0056
480 0.0040
600 0.0026
 

Plot a graph of the results.

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3a1 mark

The reaction between sodium thiosulfate solution, Na2S2O3, and hydrochloric acid is commonly investigated by measuring the time taken for a cross placed underneath the reaction mixture to disappear, as shown.

Thiosulfate reaction IB chem 1.2 SQ H Q2a

Write a balanced symbol equation, including state symbols, for the reaction between sodium thiosulfate and hydrochloric acid.

3b2 marks

A group of students are asked to investigate the effect of hydrochloric acid concentration on the reaction between sodium thiosulfate solution and hydrochloric acid.

One student identifies the following control variables:

  • Volume of chemicals used
  • Size of conical flask
  • Person judging the end of the reaction 

State the assumption that links the size of the conical flask to the amount of sulfur produced.

3c2 marks

Another student identifies the production of sulfur dioxide as a potential chemical hazard in this reaction.

Explain two associated precautions that can be implemented with respect to the sulfur dioxide produced. You do not need to discuss the use of personal protective equipment or fume cupboards.

3d
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2 marks

A third student suggests that the initial rate of the reaction between sodium thiosulfate and hydrochloric acid could be measured by collecting the sulfur dioxide produced in a 100 cm3 gas syringe. 

The student’s suggested method is as follows:

  1. Place 100 cm3 of 0.075 mol dm-3 of sodium thiosulfate solution into a conical flask
  2. Add 10 cm3 of 3.0 mol dm-3 hydrochloric acid into the conical flask
  3. Connect the conical flask to the gas syringe with a delivery tube and start the timer
  4. Record the volume of gas produced every 30 seconds until the volume of gas has remained constant for 2 minutes
  5. Plot and use a graph of the results to estimate the initial rate of reaction

 Using your equation from part (a), justify whether or not this method would be effective in producing valid results in a school laboratory.

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4a3 marks

The reaction between potassium manganate(VII) solution and ethanedioic acid, in the presence of sulfuric acid, can be used to determine the activation energy of a reaction.

2KMnO4 (aq) + 5H2C2O4 (aq) + 3H2SO4 (aq) → 2MnSO4 (aq) + K2SO4 (aq) + 8H2O (l)
                        + 10CO2 (g)

Identify any spectator ion(s) and the reducing agent in this reaction, showing all your reasoning.

4b
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4 marks

The following table provides information about the three reactants in this experiment.

 
  Concentration
/ mol dm-3 
Density
/ g cm-3 
Volume used
/ cm3
Ethanedioic acid 0.25 1.90 1.0
Potassium manganate(VII) 0.1 2.70 0.5
Sulfuric acid 2.0 1.83 0.5
 
i)
Prove, by calculation, that potassium manganate(VII) is the limiting reagent. 
 
[2]
ii)
Explain why potassium manganate(VII) has to be the limiting reagent in this experiment.
 
[2]
4c2 marks

The method for this experiment is as follows. 

  1. Measure 10.0 cm3 of each of the following into a separate boiling tube:
    • 0.1 mol dm-3 potassium manganate(VII) solution, KMnO4 (aq)
    • 2.0 mol dm-3 sulfuric acid, H2SO4 (aq)
    • 0.25 mol dm-3 ethanedioic acid, H2C2O4 (aq)
  2. Heat all three boiling tubes in a water bath
  3. Place a test tube in a test tube rack
  4. Measure and record the temperature of all three solutions
  5. Add 0.5 cm3 of the potassium manganate(VII) solution to the test tube
  6. Add 0.5 cm3 of the sulfuric acid to the test tube
  7. Then add 1.0 cm3 of the ethanedioic acid to the test tube
  8. Immediately start the stopwatch and swirl the mixture
 

Explain why a measuring cylinder can be used, in step 1, to measure the reagents into the boiling tubes but a pipette should be used, in steps 5, 6 and 7, to measure the reagents into the test tube.

4d1 mark

The temperature of each reagent is recorded before the experiment is performed.  

Explain why three thermometers may be required, in step 4, to measure the temperature of each reagent.

4e2 marks

The purpose of heating the ethanedioic acid, potassium manganate(VII) solution and sulfuric acid in a water bath is to provide the variety of temperatures required. The results from the various temperatures are used to calculate the activation energy of the reaction.

Explain why the temperature recorded, in step 4, for each experiment, is only an approximation.

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5a2 marks

A student performed an experiment to determine the acid dissociation constant by reacting a strong base with a weak acid and drawing a pH curve from their results. 

In order to do this the student wrote down their method before beginning the experiment.

Step 1 Pour a fixed volume of acid in a beaker

Step 2 Add alkali in small portions from a burette 

Step 3 Use a pH meter to record the pH after each portion of alkali was added 

Suggest two improvements to the student’s method.

5b4 marks

The student’s data was recorded in Table 1

Table 1

Volume of NaOH / cm3 0.00 5.00 15.00 18.00 20.00 22.00
pH 5.10 5.40 6.90 6.40 11.40 12.40

Volume of NaOH / cm3 23.00 24.00 25.00 28.00 30.00 35.00
pH 12.70 12.98 13.40 13.46 13.48 13.55


Figure 1

q1-1

Plot the graphs on the graph paper shown in Figure 1. You should start the y-axis at pH 4.00. 

5c
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3 marks

Using your graph and the data in Table 1 in part (b), calculate the value for Ka of this acid.

5d
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6 marks

The student repeated the experiment using a pH probe. The pH probe was calibrated using buffer solutions of pH 5.00 and 9.00. 

Explain how a buffer solution of pH 5.00 and a volume of 100 cm3 could be made from 10 cm3 of 0.25 mol dm-3 ethanoic acid and sodium ethanoate. Your answer should Include relevant calculations to determine the mass of sodium ethanoate required. 


(Ka ethanoic acid = 1.78 x 10-5Mr CH3COOH = 60.0, Mr CH3COONa = 82.0)

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