Rate Of Chemical Change (WJEC GCSE Chemistry)

A catalyst is a substance that causes significant changes to the rate of a chemical reaction. Catalysts interact with the reactants in a chemical reaction and eventually release the product allowing the catalyst to be recovered.

Enzymes are biological molecules which catalyse reactions in living organisms. Enzymes tend to have an optimum temperature of 37 °C.

Hydrogen peroxide is a colourless liquid with the chemical formula H₂O₂. It has many uses including as a bleach and an antiseptic. It is also found in all animals and plants. The decomposition of hydrogen peroxide happens very slowly on its own.

2H₂O₂ (aq)  2H₂O (l) + O₂ (g)

The reaction can be catalysed by several substances including iron(III) oxide, Fe₂O₃, manganese dioxide, MnO₂, and the enzyme catalase.

Four catalysts were tested to determine which was the most efficient. An equal mass of each catalyst was added to equal volumes of hydrogen peroxide solution of equal concentration. The volume of oxygen produced in 1 minute was recorded. The experiment was repeated at different temperatures.

Give the number of the statement which is not correct.

1. Catalyst W is more efficient than Y but less efficient than X and Z
2. Catalyst X is the most efficient of the four
3. Catalyst Y is less efficient than all of the others
4. Catalyst Z is more efficient than Y but less efficient than W and X

State which catalyst, W, X, Y or Z, could be catalase. Explain your answer.

Catalyst ........................... 

Three students made statements about the properties of catalysts.

• Katrina said that a catalyst does not get used up in the reaction but makes it happen faster.
• Jo said that the reactants are used up in less time because a catalyst makes a different product.
• Rhys said that adding a catalyst allows the reaction to happen with a lower activation energy, but the same products are made.

Which of the students’ statements are correct?

1. Jo only
2. Rhys and Jo, but not Katrina
3. Katrina and Rhys, but not Jo
4. All three students

The graph shows the volume of gas produced in a reaction between 0.40 g of calcium carbonate powder and excess 1 mol / dm³ hydrochloric acid.

Use the tangent drawn to calculate the rate of the reaction at 20 s.

Rate = ............................. cm³ / s

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A student was investigating how the concentration of sodium thiosulfate solution affects its reaction with hydrochloric acid.

[1]

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

[1]

The results are shown in the table.

Plot the results from the table on the grid. Draw a suitable line.

[3]

[1]

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A second student used a light sensor to investigate the effect of temperature on the rate of this reaction.

He obtained the following results.

[2]

[1] 

Hydrogen peroxide decomposes to give water and oxygen.

Complete the symbol equation to show the reaction taking place.

2H₂O₂    H₂O + ...........................................

The rate of decomposition of hydrogen peroxide can be measured using the following apparatus.

The rate was investigated using three different catalysts. The results are shown in the table.

Dilute hydrochloric acid reacts with sodium thiosulfate to make the products shown in the equation.

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

[1]

 Formula ............................................................

What is meant by this?

[1]

Use information from the equation to explain why the cross disappears.

[2] 

A student studied the effect of temperature on the rate of this reaction. He obtained the following results.

Identify the incorrect mean. Give your reasoning.

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[3]

A student carried out an experiment to investigate the speed of the reaction between a lump of magnesium carbonate of mass 0.32 g and excess dilute hydrochloric acid at 20 °C.

Magnesium carbonate reacts with dilute hydrochloric acid forming carbon dioxide gas. The total volume of carbon dioxide formed was recorded every 5 minutes for 40 minutes.

The results are shown below. The result for 15 minutes is missing.

Use the diagram below to find the volume of carbon dioxide gas formed after 15 minutes.

volume of carbon dioxide = ..................................... cm³

Plot the results from the table, including your answer to part (a), on the grid below.
Draw a suitable line and label this graph X.

Sketch the graph you would expect if the experiment were repeated using 0.32 g of magnesium carbonate powder instead of the lump of magnesium carbonate. Label this graph Y. 

State and explain, using particle theory, the effect of increasing the concentration of the hydrochloric acid. 

The student investigated the same reaction using a different apparatus. A lump of magnesium carbonate was added to excess dilute hydrochloric acid at 20°C.

The change in mass was recorded for 60 minutes and displayed as a graph on a computer screen. The reaction took 40 minutes to complete.

Sketch the graph you would expect to see.

A pupil studied the reaction between calcium carbonate powder and hydrochloric acid.

calcium carbonate + hydrochloric acid → calcium chloride + carbon dioxide + water

State what would be seen during this reaction.

Choose from the box the apparatus that would not be suitable for measuring the volume of gas produced.

The same pupil investigated the reaction at three different temperatures.

The following graphs show the volume of gas produced over time during these three reactions.

Another pupil suggested that the same reaction could be studied using a balance.

The following graphs show the volume of hydrogen produced over time during the reaction between magnesium and hydrochloric acid of two different concentrations. All other factors were kept constant.

State what conclusion can be drawn from the graph and use your understanding of particle theory to explain that conclusion.

Another method of studying this reaction is to use a balance to record the change in mass over time. The data can be recorded directly on a computer.

A_r(H) = 1   A_r(C) = 12   A_r(O) = 16

Zinc reacts with dilute hydrochloric acid to produce hydrogen gas.

The diagram below shows apparatus that can be used to investigate the rate of the reaction between zinc and hydrochloric acid. A small amount of copper sulfate is added because it acts as a catalyst for the reaction.

A few pieces of zinc were placed in excess dilute hydrochloric acid and the volume of hydrogen produced was recorded every 10 seconds. The experiment was carried out at room temperature. The results obtained are shown below.

A chemical reaction takes twice as long if the temperature is decreased by 10 °C.

At 30 °C, milk undergoes a chemical reaction that makes it go sour in 1 day.

Calculate how long it will take milk to go sour at 10 °C.

One of the main dangers in the coal mining industry is that coal dust can form an explosive mixture with air.

Explain why an explosion is more likely to occur with coal dust than with lumps of coal.

A chemical reaction goes twice as fast if the temperature is increased by 10 °C.

At 5 °C, milk undergoes a chemical reaction that makes it go sour in 8 days.

Calculate how long it will take milk to go sour at 35 °C.

A pupil investigated the effect of temperature on the rate of fermentation using the apparatus shown below.

The experiment was carried out three times at five different temperatures. The volume of gas collected after 10 minutes was recorded each time. The results are shown below.

Suggest why the highlighted value is considered to be anomalous.

Plot a graph of the mean volume of gas collected against temperature on the grid.

State what conclusions can be drawn from the graph.

Write a word equation for the reaction taking place.

........................................ → ........................................ + ........................................

Yeast produces a catalyst that allows this reaction to take place. Name the type of catalyst produced by yeast.

The following apparatus can be used to investigate the rate of the reaction between magnesium and excess dilute hydrochloric acid.

The results obtained at two different temperatures are shown below.

Explain the results obtained at different temperatures in terms of particle theory.

Explain the change in rate over time.

State one way of improving the validity of the results obtained. Explain your answer.

Higher tier only

The equation for the reaction taking place is as follows.

Mg + 2HCl  MgCl₂ + H₂

[3]

A_r(Mg) = 24                        A_r(H) = 1

Mass = ………............................................... g

[2]

This question is about rates of reaction.

Hydrogen peroxide, H₂O₂ decomposes in the following reaction:

2H₂O₂   →   2H₂O   +   O₂ 

The catalyst for this reaction is manganese dioxide.

A student investigated the effect of manganese dioxide particle size on the rate of reaction using the following method:

1. Measure 30 cm³ of 0.3 mol/dm³ hydrogen peroxide solution into a conical flask 
2. Add a spatula of coarse manganese dioxide powder to the conical flask 
3. Measure the volume of gas made every minute for ten minutes. 
4. Repeat steps 1-3 with some fine manganese dioxide lumps.

This method did not give the student valid results. 

Which two improvements would need to be made that would produce valid results? 

Tick (✓) two boxes. 

Describe and give the result of a test that could be done to identify the gas made during the decomposition of hydrogen peroxide.

Another student followed a method that produced valid results.

The results are shown in Figure 1. 

Calculate the mean rate of reaction, in cm³/s, between 1 and 3 minutes for coarse manganese lumps.

Give your answer to 2 significant figures.

Use data from the graph. 

Explain why the rate of reaction is different when manganese dioxide is used as coarse lumps rather than fine powder.

You should answer in terms of collision theory.

A solution of sodium thiosulfate reacts with dilute hydrochloric acid producing a cloudy solution as the reaction progresses.

The reaction equation is:

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

Explain why the solution turns cloudy.

The rate of reaction is affected by the concentration of the sodium thiosulfate solution. 

Plan an investigation to show this relationship.

Your plan should allow the collection of valid results.

Another student investigated the effect of increasing temperature of the sodium thiosulfate solution on the rate of reaction. 

State what the effect would be and explain this effect in terms of particles and collisions.

A student investigated the rate of reaction between hydrochloric acid and marble chips using the apparatus shown in Figure 3. 

Figure 3

Table 2 below shows the student’s results.

Table 2

On Figure 4:

• Plot the results from the investigation on the grid.
• Draw a line of best fit on the grid.

Figure 4

Sketch a line on the grid in Figure 4 to show the results you would expect if the experiment was repeated but instead using 20 g of marble chips of smaller size.

Explain, in terms of particles, how and why the rate of reaction changes during the reaction.

A different student investigated the rate of reaction for the same reaction by measuring the change in mass.

Figure 5 shows the graph plotted from the results obtained in this investigation.

Figure 5

Use Figure 5 to calculate the mean rate of the reaction up to the time when the reaction has gone to completion. 

Give your answer correct to three significant figures.

Mean rate of reaction = ______________________ g / s

1. Put 50 cm³ of hydrochloric acid into a conical flask 
2. Add 12 g of large calcium carbonate lumps into the flask
3. Attach the gas syringe
4. Measure the volume of gas produced every 30 seconds for 180 seconds 
5. Repeat steps 1 - 4 using 12 g of small calcium carbonate lumps.
6. The number of moles of gas for each volume was calculated.

The results for large calcium carbonate lumps are shown below.

Table 1. 

The student had already plotted the data for small calcium carbonate lumps.

Plot the data for the large calcium carbonate lumps and draw a line of best fit.

Determine the mean rate of reaction for small calcium carbonate lumps between 35 seconds and 90 seconds.

Give the unit.

Use the graph in part (a)

What conclusion can be made about the rate of reaction of small calcium carbonate lumps compared to large calcium carbonate lumps?

Give one reason for your answer.

Complete and balance the equation for the reaction between calcium carbonate and hydrochloric acid.

__________   +   __________     →     CaCl₂   +   __________   +    __________