Rates of Reaction (Edexcel IGCSE Science (Double Award): Chemistry)

Dilute hydrochloric acid reacts with a solution of sodium thiosulfate (Na₂S₂O₃) to form a precipitate.

The equation for the reaction is

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

State the name of the precipitate that forms.

The reaction is often used to investigate rates of reaction.

The diagram shows the apparatus a student uses to investigate the effect of temperature on the rate of the reaction.

 This is the student’s method.

• Pour 50 cm³ of cold sodium thiosulfate solution into a conical flask and heat it to 20 °C

• Draw a cross (X) on a piece of paper and place it under the flask

• Add 5 cm³ of dilute hydrochloric acid to the flask

• Look at the cross from above and record the time taken until the cross cannot be seen

 The student repeats the experiment four times, using sodium thiosulfate solution at a different temperature each time.

 He keeps the volumes of sodium thiosulfate solution and hydrochloric acid constant in each experiment.

 Give two other factors that the student should keep constant.

 1 ....................................................................................................

 2 ....................................................................................................

The table shows the student’s results.

 The highest temperature the student uses is 60 °C because he thinks the results might not be as accurate at temperatures higher than 60 °C.

 Suggest a reason why the results might not be as accurate at temperatures higher than 60 °C.

The student wants to compare the rates of the reaction at the different temperatures.

 He uses this formula to obtain a value for each rate of reaction

 rate = 

 The table shows the value of the rate of reaction at each temperature. 

 Plot the values of temperature and rate of reaction on the grid.

 Draw a curve of best fit through the points.

i) Use the graph to determine a value for the rate of the reaction at 45 °C.

 Show on the graph how you obtained your answer.

 (2)

 rate of reaction = .............................................................. s⁻¹

 ii) Calculate the time that it would take for the cross not to be seen at 45 °C.

 (2)

 time = .............................................................. s

 iii) Describe the relationship between rate of reaction and temperature shown by the graph.

 (1)

Explain, in terms of particle collision theory, the effect that increasing the temperature has on the rate of a reaction.

Calcium carbonate reacts with hydrochloric acid to produce calcium chloride, water and carbon dioxide:

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

Which statement is not true if the temperature of the reaction decreases?

This question is about reactions that form gases.

Hydrogen peroxide decomposes to form water and oxygen. The equation for the reaction is

2H₂O₂ → 2H₂O + O₂

25.0 cm³ of hydrogen peroxide solution are poured into a conical flask and 1.00 g of solid manganese(IV) oxide is added. Bubbles of oxygen gas are formed.

i) Give the test for oxygen gas.

(1)

ii) Describe a method to show that solid manganese(IV) oxide is a catalyst in this reaction and not a reactant.

(3)

A student uses this apparatus to investigate the rate of the reaction between zinc and an excess of dilute hydrochloric acid.

This is the student’s method.

• pour 50 cm³ of dilute hydrochloric acid into a conical flask

• add about 1.2 g of zinc lumps

• record the volume of hydrogen gas collected every 30 s until no more hydrogen is collected

The graph shows the student’s results.

i) Calculate the mean (average) rate of reaction, in cm³ / s, in the first 120 s.

(2)

mean rate = .............................................................. cm³ / s

ii) The equation for the reaction between zinc and hydrochloric acid is

Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)

Use this equation and the particle collision theory to explain why the rate of reaction is greatest at the start of the reaction.

(3)

iii) The student repeats the experiment at a higher temperature but keeps all other conditions the same. On the grid, draw the curve you would expect to see in this experiment.

(2)

iv) Explain why the rate of reaction is greater if the same mass of zinc powder is used instead of zinc lumps. All other conditions are kept the same.

(2)

In another experiment, the student adds 0.55 g of zinc to a solution containing 2.50 × 10⁻² moles of hydrochloric acid. Use the equation to show that hydrochloric acid is in excess.

Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)

[A_r of Zn = 65]

A student uses this apparatus to investigate the rate of reaction between calcium carbonate chips and dilute hydrochloric acid.

Every 20 seconds the student records the reading on the balance.

Explain why using a cotton wool plug increases the accuracy of the student’s results.

Complete the equation for the reaction by adding the state symbols.

 CaCO₃ (..........) + 2HCl (..........) → CaCl₂ (aq) + H₂O (..........) + CO₂ (..........)

The student uses the balance readings to find the decrease in mass of the flask and contents. 

The graph shows the student’s results.

 i) Give a reason why there are some calcium carbonate chips remaining in the flask when the reaction stops.

(1)

ii) State how the student would know when the reaction has stopped. 

(1)

iii) Use the graph to determine the amount, in moles, of carbon dioxide produced during the reaction. 

[M_r of CO₂ = 44] 

(2) 

amount = .............................. mol

iv) Use the graph to calculate the rate of reaction, in grams per second, at time 60 seconds.  

Show your working on the graph.

(3) 

rate of reaction = .............................. g / s

The student repeats the investigation by diluting the original hydrochloric acid. 

The student then determines the initial rate of reaction at different percentage concentrations of the original hydrochloric acid. 

The graph shows the student’s results.

 i) Describe the relationship between the initial rate of reaction and percentage concentration of the original hydrochloric acid. 

(2)

ii) Explain why changing the concentration of hydrochloric acid has an effect on the initial rate of reaction.

(2)

A student wants to investigate the effect of surface area on the rate of reaction between marble chips and dilute acid.

What pieces of apparatus could be used for this investigation?

A student uses this apparatus to investigate the rate of reaction between marble chips and dilute hydrochloric acid.

i)

Explain how using the cotton wool increases the accuracy of this investigation.

(2)

ii)

Why is mass lost from the flask?

(1)

This is the equation for the reaction between marble chips and dilute hydrochloric acid. Complete the equation by adding the state symbols.

CaCO₃ (.........) + 2HCl (.........) → CaCl₂ (.........) + H₂O (.........) + CO₂ (.........)

The student uses large marble chips in the investigation. This is a graph of his results.

The student repeats the experiment using the same total mass of smaller marble chips. On the graph, draw the curve that would be obtained. [assume the marble chips are in excess]

The rate of this reaction can be altered by increasing the temperature or by increasing the concentration of the hydrochloric acid.

i) Explain, using the particle collision theory, how increasing the concentration of the hydrochloric acid would affect the rate of this reaction.

(3)

ii) Explain, using the particle collision theory, how increasing the temperature would affect the rate of this reaction.

(3)

A solution of hydrogen peroxide decomposes when a catalyst of manganese(IV) oxide is added.

The products of the reaction are water and oxygen.

Complete the chemical equation for this reaction.

............ H₂O₂ → ........... H₂O + ........... O₂

Give a test for oxygen.

State the reason for adding a catalyst.

A student investigates how changing the concentration of the hydrogen peroxide solution affects the rate of this reaction.

She uses this apparatus.

The student records the volume of oxygen that collects every 2 minutes for 16 minutes.

The table shows her results.

i) Plot the student’s results on the grid.

(1)

ii) Draw a circle on the grid around the anomalous result.

(1)

iii) Draw a curve of best fit through the points, ignoring the anomalous result.

(1)

iv) Suggest a mistake that the student might have made to cause the anomalous result.

(1)

v) Determine the volume of oxygen collected during the first 3 minutes. Show on your graph how you obtain your answer.

(2)

volume of oxygen = ............................................... cm³

The student repeats the experiment using hydrogen peroxide solution of half the concentration of the original solution. She keeps the volume of the hydrogen peroxide solution and all other conditions the same.

i) Draw on the grid the curve you would expect the student to obtain.

(2)

ii) Explain how using hydrogen peroxide solution of half the concentration affects the rate of the reaction.

Refer to particle collision theory in your answer.

(3)

Which of these statements about catalysts is not correct?

Zinc reacts with dilute hydrochloric acid to form hydrogen.

 i) Give a chemical equation for this reaction.

(2)

ii) Give a test for hydrogen gas.

(1)

A student investigates the reaction between pieces of zinc and dilute hydrochloric acid.

In each experiment, he uses the same mass of zinc but a different volume of the acid.

He collects the hydrogen and measures its volume in each experiment.

The graph shows the student’s results.

i) Use the graph to find the minimum volume of acid needed to react with all of the zinc.

(1)

ii) The student repeats the investigation, using hydrochloric acid of double the original concentration.

Determine the volume of hydrogen that would be collected using 15 cm³ of this acid.

Show your working on the graph.

(2)

volume = ............................................... cm³

Explain how increasing the concentration of the hydrochloric acid affects the rate of reaction.

The rate of reaction could also be affected by changing the temperature of the hydrochloric acid, or by using a catalyst. Explain one other way in which the rate of reaction between zinc and hydrochloric acid can be affected.

A student uses this apparatus in an experiment to study the rate of the reaction between zinc and dilute sulfuric acid.

This is the student’s method.

• Add a few zinc granules to a conical flask on a balance

• Add 100 cm³ of dilute sulfuric acid to the flask, start a timer and immediately record the mass of the flask and contents

• Record the mass of the flask and contents every minute until the mass remains constant

The mass of the flask and contents decreases because hydrogen gas is produced and leaves the flask.

The student uses the mass readings to calculate the total mass of hydrogen produced.

Complete the equation for the reaction by adding the state symbols. 

 Zn (................) + H₂SO₄ (................) → ZnSO₄ (................) + H₂ (................)

The table shows the student’s results.

 i) Plot the student’s results. The first three have been done for you.

 (1)

 ii) Draw a circle around the anomalous result.

 (1)

 iii) Draw a curve of best fit.

 (1)

 iv) Give a possible reason for the anomalous result.

 (1)

 v) Determine a more likely value for this result.

 (1)

i) Explain how the shape of the curve shows how the rate of the reaction changes as time increases.

 (2)

 ii) At the end of the experiment there is no zinc left in the flask.

 Give a conclusion the student could make from this observation.

 (1)

The student does another experiment using

• the same amount of similarly sized magnesium granules instead of zinc

• the same volume of sulfuric acid, but of a lower concentration

Explain why it is difficult to predict how the rate of reaction in this experiment compares with the rate of reaction in the first experiment.

Explain, in terms of particle collision theory, how increasing the temperature affects the rate of a reaction.

A student investigates the rate of reaction between an excess of hydrochloric acid and a limiting mass of magnesium using the apparatus shown:

Which of the graphs could represent her results?

Some students investigate the rate of reaction between marble chips and hydrochloric acid.

 The diagram shows the apparatus they use.

What is the purpose of the cotton wool?

Hydrogen peroxide, H₂O₂, quickly decomposes to produce water and oxygen when a catalyst is added to the solution:

2H₂O₂ (aq) → 2H₂O (l)  + O₂ (g)

The volume of gas produced was measured and is shown in the graph below:

Which statement is not true for this reaction?

Magnesium metal reacts with an excess of hydrochloric acid solution to form magnesium chloride and hydrogen:

Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)

Which of the following will not increase the rate of this reaction?