Rate of Reaction

1. Give a formula to measure the mean rate of reaction.

Answer: rate = (qty. of reactant used) / (time taken)  OR  rate = (qty. of product formed) / (time taken)


2. List five factors that can affect the rate of a reaction.

Answer: concentrations of reactants in solution, the pressure of reacting gases, the surface area of solid reactants, the temperature and the presence of catalysts


3. Define the term "activation energy".

Answer: The minimum amount of energy that particles must have to react.


4. Give the name of a catalyst used in a biological system.

Answer: enzyme


5. Describe how catalysts increase the rate of reaction.

Answer: Provide an alternative reaction pathway with a lower activation energy.


6. Give two units for measuring rate of reaction.

Answer: g/s or cm3/s


7. Use collision theory to explain how a chemical reaction happens.

Answer: Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.


8. Describe how increasing the concentration of a solution affects the rate of a reaction.

Answer: Increases the frequency of collisions and so increases the rate of reaction.


9. Magnesium reacts with hydrochloric acid to make hydrogen and magnesium chloride. A student investigated how changing the concentration affected the rate of reaction by collecting the gas that was produced. They plotted their results on a graph, shown in the figure below.


a) Write a word equation for the reaction between magnesium and hydrochloric acid.

Answer: magnesium + hydrochloric acid  →  hydrogen + magnesium chloride


b) Describe the chemical test that can be used to show that the gas produced was hydrogen.

Answer: Use a lighted splint and you will hear a (squeaky) pop if hydrogen is present.


c) Calculate the mean rate of reaction in the first 4 minutes for the 2.0 mol/dm3 concentration of acid.

Answer: rate = product formed / time  ;  rate = 60 ÷ 4.0 = 15 cm3/min


10. Calcium carbonate is a white basic solid. Hydrochloric acid reacts with calcium carbonate to make water, calcium chloride and a gas. A student investigated how the rate of reaction was affected by the surface area of the calcium carbonate. They used lumps of calcium carbonate and powdered calcium carbonate.

a) Name the gas produced in this reaction.

Answer: carbon dioxide


b) Suggest how the mass might change as the reaction progressed.

Answer: The mass would decrease as carbon dioxide is lost to the air.


c) Use collision theory to predict how the rate of reaction would be different when using lumps of calcium carbonate compared with using powder.

Answer: Powdered calcium carbonate will have a faster rate of reaction than lumps. Powdered calcium carbonate has a higher surface area to volume ratio than lumps. In powdered calcium carbonate there are more particles exposed at any time, so there is more chance of a collision in the same amount of time. There would be more successful collisions in the same time for the powder than the lumps and so a faster rate of reaction.


11. Sodium thiosulfate reacts with hydrochloric acid: Na2S2O3 + 2HCl  →  2NaCl + H2O + SO2 + S

a) Name the chemical in this reaction that is a solid.

Answer: sulfur


b) Describe a method to investigate how changing the concentration could affect the rate of reaction. Explain how you will make this a valid investigation.

Answer: Use a measuring cylinder (or volumetric pipette) to measure out  25 cm3 of sodium thiosulfate solution into a conical. Add 10cm3 of hydrochloric acid.
Method 1: Put the conical flask on top of a cross and start the stopwatch. Time how long it takes until the cross can no longer be seen through the solution. Repeat with different concentrations of sodium thiosulfate.
Method 2: Use a light meter to monitor the amount of light transmitted through the sample.
To make the investigation valid, you should use the same volume of acid and thiosulfate, the same conical flask (or same depth of solution), complete the investigation at the same temperature and the same cross.


12. Hydrogen peroxide (H2O2 ) decomposes to form water and oxygen. This can be represented on an energy level diagram shown in the figure below.


a) Write the symbol equation for the reaction.

Answer: 2H2O2  →  O2 + 2H2O


b) Give the letter on the diagram that represents the activation energy of the reaction without a catalyst.

Answer: D


c) Give the letter on the diagram that represents the activation energy of the reaction with a catalyst.

Answer: E


13. Lithium reacts with water to make lithium hydroxide [Li(OH)2 ] and hydrogen.

a) Write the symbol equation for the reaction.

Answer: 2Li(s) + 2H2O(l)  →  2Li(OH)2(aq) + H2(g)


b) A student is investigating how the surface area of the lithium metal affected the rate of reaction. Explain why a method involving a change in colour or turbidity would not give valid results.

Answer: no turbidity / no solid made / no cloudiness so cross would always be visible.


c) The student collected the gas produced in a measuring cylinder and recorded the volume after 1 minute. Explain why the results would be more accurate if a gas syringe rather than a measuring cylinder was used to collect and measure the volume of gas.

Answer: Gas syringe gives more precise measurements than a measuring cylinder. Less gas is likely to escape using a gas syringe.


d) Suggest how the student could improve the reliability of their results.

Answer: repeat three times, use only similar results, calculate a mean


14. Sodium thiosulfate reacts with hydrochloric acid to make a sulfur suspension. A student investigated how the rate of reaction was affected by changing the concentration of sodium thiosulfate. The table below shows the student’s results.


a) Complete the table by calculating the rate of reaction for a concentration of 1.2 units.

Answer: 1 ÷ 54 = 0.0185


b) Plot a graph of concentration against time for the data in the table. Label the axes.

Answer: 


c) Why is it not appropriate to draw a line of best fit on this graph?

Answer: There needs to be at least five data points for a trend, therefore, you cannot be sure of the pattern in the data and so a line of best fit cannot be drawn with any certainty.


15. A student investigated the volume of carbon dioxide gas produced when 7.3 g/dm3 solution of hydrochloric acid reacted with excess powdered calcium carbonate. The figure below shows the student’s results.


a) Identify the independent variable.

Answer: time


b) Give the units of the dependent variable.

Answer: volume of CO2 in cm3


c) Calculate the rate of reaction at 18 seconds.

Answer: draw a tangent to the curve at 18s  ;  calculate change in volume 55 - 5 = 50 cm3  ;  calculate change in time 45 - 0 = 45s  ;  calculate the gradient (rate) = 50 ÷ 45 = 1.11 cm3/s


d) Describe and explain how the graph would look different if a concentration of 3.65 g/dm3 of hydrochloric acid was used.

Answer: The line would be less steep as the rate of reaction would be slower as there are less acid particles available for a collision at any one time. They would have the same shape as the original line as the rate of reaction will slow as time goes on. This is due to the reactant particles being used up during the reaction and so reducing the number of successful collisions in any one time. The reaction would finish (shown as a horizontal line) with half the volume of gas compared to the original. This is because there are half as many acid particles available for collision (calcium carbonate is in excess so does not limit the amount of gas made).


16. Zinc metal reacts with hydrochloric acid to make zinc chloride and hydrogen.

a) Write an ionic equation for this reaction.

Answer: Zn + 2H+  →  Zn2+ + H2


b) Define a redox reaction and then justify how this reaction can be classified as a redox reaction.

Answer: Zinc is losing electrons and is being oxidised. The acid is gaining electrons and being reduced. Both oxidation and reduction are happening in the same chemical reaction. As zinc is being oxidised while the acid is being reduced in the same reaction, it must be an example of a redox reaction.


c) How will the surface area of the zinc affect the rate of the reaction? Use collision theory to explain your answer.

Answer: Increasing the surface area (surface area to volume ratio) increases the rate of reaction. There are more reactant particles available for a collision at any one time. There will be more collisions and therefore more successful collisions in the same time.