6.1 Rate of Reaction

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 9.0 g/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. 

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. 

The student repeated the experiment with coarse lumps of manganese dioxide. 

They used the same volume of hydrogen peroxide but used a concentration of 3.0 g/dm³.

Sketch on the graph the curve you would expect to see. 

Assume that the reaction is complete after 9 minutes. 

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

Use Figure 5 to determine the rate of reaction at 150 seconds.

Show your working clearly on Figure 5, giving your answer in standard form.

A student investigated the effect of the size of calcium carbonate lumps, CaCO₃, on the rate of reaction with hydrochloric acid, HCl, using the following method:

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   +   __________   +    __________

This question is about the rate of reaction between sodium thiosulfate solution and dilute hydrochloric acid:

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

The reaction between these two substances produces a precipitate which makes the mixture turn cloudy. 

A student timed how long it took until the cross could no longer be seen through the precipitate, shown in Figure 1.

They then calculated the rate of reaction. 

Figure 1 

The student investigated the effect of temperature on the rate of the reaction in part (a).

Their results are plotted in Figure 2. 

 Describe the trends shown in Figure 2.

Suggest two control variables for the investigation that would ensure valid results are obtained.

They came to the following conclusion:

Explain why the student is correct. 

Answer the question in terms of particles. 

This question is about the effect of temperature on rate of reacton.

A student was investigating the effect of temperature on the the rate of reaction between calcium carbonate and hydrochloric acid. 

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)

They used the apparatus in Figure 1 to record the volume of gas collected every five seconds and repeated their experiment at different temperatures. 

Figure 1

Their results are shown in Figure 2. 

What conclusion could the student make about the effect of temperature on the rate of the reaction? 

Suggest one reason for your answer. 

Identify the following variables in this investigation:

      Independent: _________________________________________

      Dependent: __________________________________________

The student had collected 40 cm³ after 20 seconds for the hydrochloric acid at 60 °C.

Calculate the average rate of reaction from 0-20 seconds.

Give units for your answer. 

The student then investigated the effect of the surface area of the calcium carbonate on the rate of reaction.

Explain, in terms of particles and collisions, the effect of surface area on rate of reaction. 

Calcium carbonate can be used as a catalyst for the industrial production of biodiesel.

How does a catalyst increase the rate of reaction? 

A student was investigating the rate of reaction between dilute hydrochloric acid and an excess of calcium carbonate. 

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

They measured the volume of gas every minute and drew a graph of their data shown in Figure 1. 

Figure 1

What is the limiting reagent in this reaction?

After how many minutes did the reaction finish?

Use the graph to explain what happens to the rate of reaction from 0 - 8 minutes.

The student carried out the experiment at a higher temperature, but kept all other variables the same.

Draw a line on the curve you would expect to see for this set of results. 

Another student investigated the effect of increasing the concentration of hydrochloric acid on the rate of reaction.

Explain why increasing concentration increases the rate of reaction.