Reaction Rates | OCR AS Chemistry Structured Questions 2017

1a2 marks

This question is about how successful reactions occur.

According to collision theory, what two things do particles need for a successful collision to occur?

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1b1 mark

An increase in concentration means that there are more particles per unit volume.

In terms of collisions, how does this increase the rate of reaction?

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1c2 marks

Propane, C₃H₈, undergoes complete combustion to form carbon dioxide and water.

Write a balanced symbol equation, including state symbols, for this combustion reaction.

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1d4 marks

The enthalpy of combustion for butane is around -2878 kJ mol^-1.

Identify, with reasons, two errors in the following enthalpy profile diagram for the combustion of butane.

Error 1: ..............................................................................................................................

Reason: ............................................................................................................................

Error 2: ..............................................................................................................................

Reason: ............................................................................................................................

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1a4 marks

This question is about the reaction between sodium thiosulfate, Na₂S₂O₃, and dilute hydrochloric acid.

When a solution containing sodium thiosulfate ions is mixed with dilute acid, sodium chloride, water, sulfur dioxide, which is readily soluble in water, and a precipitate of sulfur are formed. The rate of reaction can be investigated using a colorimeter which measures the amount of light absorbed by the solution.

i)

Write the balanced ionic equation, including state symbols, for the reaction between a solution of sodium thiosulfate and dilute hydrochloric acid.

ii)

Suggest why colorimetry is a suitable method to study the rate of this reaction.

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1b

5 marks

A student used a colorimeter to investigate how the rate of reaction varies with the concentration of thiosulfate ions. Different volumes of a solution containing thiosulfate ions were each made up to 10 cm³ with distilled water. Each mixture was placed into the colorimeter and 5 cm³ of dilute acid was added. The time taken for the intensity of the light passing through the reaction mixture to fall to 75% of the original intensity was measured for each mixture.

The results of the experiment are given in the table below:

Volume of thiosulfate ions in solution (cm³)	10	9	8	7	6	5	4	3
Time for light intensity to fall to 75% (s)	9.3	10.3	11.5	13.2	15.4	18.7	23.1	30.8

The rate of reaction can be calculated by $\frac{1}{time}$ .

i)

What are the units of rate calculated using this equation?

ii)

For each reaction mixture, calculate the rate of reaction. Give your answers to 3 decimal places.

iii)

Plot a graph of the rate against the volume of thiosulfate ions in the solution used. Include a line of best fit.

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1c

5 marks

Use the graph drawn in part (b) to:

i)

Determine how long it would take for the light intensity of a solution containing 4.6 cm³ of thiosulfate ions diluted to 10 cm³ with distilled water to fall to 75% of its original value when reacted with 5 cm³ of the same acid used in part (b).

ii)

Describe the relationship between the rate of reaction and the concentration of thiosulfate ions (the volume of thiosulfate is directly proportional to its concentration).

iii)

Explain the shape of the graph using collision theory.

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1d3 marks

For the solution containing 3 cm³ of thiosulfate ions, the student accidentally used an acid that was half the concentration than the acid used in the rest of the investigation.

What effect would this have on the rate of reaction?

Explain your answer.

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4a4 marks

This question is about changes in rate of reaction and catalysts.

Reaction rates can be affected by factors, such as changes in pressure and temperature.

i)

On the graph below, sketch two Boltzmann distributions with one at a higher temperature. Label the Boltzmann distribution at the lower temperature T₁ and the Boltzmann distribution at the higher temperature T₂.

ii)

How would the mean energy of the molecules differ at T₂ compared with T₁?

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4b4 marks

Hydrogen iodide is used in the manufacturing of pharmaceuticals and can be broken down back into iodine and hydrogen.

2HI (g) $⇌$ H₂(g) + I₂(g) ΔH = - 52 kJ mol^-1

The activation energy when uncatalysed is +183 kJ mol^-1

i)

Sketch an energy profile diagram for the reaction. Including the

reactants and products
curves for both the catalysed and uncatalysed reactions
activation energy for the uncatalysed forward reaction and the catalysed forward reaction
enthalpy change of reaction, ΔH.

ii)

Calculate the activation energy for the reverse reaction in both the uncatalysed reaction.

iii)

The forward reaction can be catalysed using gold.

Suggest a possible value for the activation energy of the catalysed forward reaction.

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4c4 marks

Catalysts are often used in industrial processes.

i)

Explain why it is likely that the solid gold catalyst was used in powder form to catalyse the reaction mentioned in part (c).

ii)

State whether gold, when used in manufacturing hydrogen iodide, is acting as a homogeneous or heterogeneous catalyst. Explain your answer.

iii)

State how, if at all, the area under the curve of a Boltzmann distribution curve, changes as a catalyst is introduced. The temperature and the total number of molecules remains the same.

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4d3 marks

The Contact process is an important industrial multi-staged process, used in the production of sulfuric acid. During the second stage, solid vanadium (V) oxide, a heterogeneous catalyst, is used to make sulfur trioxide from sulfur dioxide and oxygen. This process is reversible.

i)

Write a balanced symbol equation for this reaction. Include state symbols in your answer.

ii)

Explain why the use of the catalyst in the Contact process has benefits for increased sustainability.

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5a4 marks

Methanol, for use as a fuel, can be produced by the reaction of carbon monoxide with hydrogen. The reaction is typically carried out at 310 °C and 3.25 × 10⁷ Pa with a catalyst which is a mixture of zinc oxide and chromium(VI) oxide. The Boltzmann distribution for 1 dm³ of the reaction mixture is shown by sample 1 in the diagram below.

CO (g) + 2H₂ (g) → CH₃OH (g) ΔH = – 90 kJ mol^–1

A change was made to the reaction conditions as shown by sample 2 in diagram above.

i)

State the change that was made to the reaction conditions.

ii)

Explain the effect this change has had on the rate of the reaction.

iii)

State the effect, if any, this change has had on the most probable value for the energy of the molecules.

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5b6 marks

Consider the Boltzmann distribution curve below.

3-5_q5b-ocr-a-as--a-level-hard-sq

For each of the changes in parts (i), (ii) and (iii) below, state and explain the effect that the change would have on:

The area under the curve
The position of the peak of the curve
The proportion of molecules with energy greater than or equal to E_a

i)

The temperature of the original reaction is increased, but no other changes are made.

ii)

The number of molecules in the original reaction mixture is increased, but no other changes are made.

iii)

A catalyst is added to the original reaction mixture, but no other changes are made.

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5c4 marks

A chemist performed a reaction at three different temperatures, 100K, 300K and 700K as shown by the Boltzmann distribution graph below.

q5c-boltzmann-temp-2

i)

Label each curve with the correct temperature values, 100K, 300K and 700K.

ii)

Consider the following statement, ‘All reacting molecules have higher kinetic energy at 700K than they do at 300K’. State whether you agree this statement is correct and justify your reasons.

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5d5 marks

Hydrogen will react with chlorine to form the hydrogen halide, hydrogen chloride, a colourless gas.

H₂(g) + Cl₂(g) → 2HCl (g)

i)

Give one reason why most collisions between hydrogen and chlorine molecules do not lead to the formation of hydrogen chloride.

ii)

Apart from changing the temperature, state and explain two ways of speeding up the formation of hydrogen chloride.

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1a

5 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.

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1b

6 marks

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

Step 1 - Support a gas syringe with a stand, boss and clamp

Step 2 - Measure 50 cm³ of 0.050 mol dm^-3 dilute HCl in a beaker and then add it to a conical flask

Step 3 - Add about 2.5 g of CaCO₃ to the flask using a spatula, immediately connect the gas syringe and start counting.

Step 4 - Record the volume of gas produced every 20 seconds for 2 minutes.

Step 5 - Repeat Steps 1-4 for different concentrations of HCl.

i)

Suggest two improvements to the method used.

ii)

How can you tell that the reaction is complete?

iii)

Complete the missing hazards, risk and precautions below.

Hazard

Risk

Precaution(s)

Causes skin and eye irritation

Effervescence in the reaction mixture

Use a large conical flask;

Do not look directly into the reaction vessel when adding the CaCO₃;

Wear eye protection

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1c

4 marks

The results for the reaction of 2.5 g of CaCO₃ with 50 cm³ of 0.050 mol dm^-3 dilute HCl are shown below.

Using the results given in the table, 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

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1d

3 marks

Use the results and your graph from part (c) to calculate the initial rate of reaction.

Give your answer to 2 decimal places and state the correct units.

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Time / s	0	100	200	300	400	500	600	700
Volume of O₂ / cm³	0.0	7.0	13.5	18.0	21.0	22.5	23.0	23.0

OCR AS Chemistry

Topic Questions

3.5 Reaction Rates