Lesmahagow High School

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1 Lesmahagow High School Higher Chemistry Controlling the Rate - Tutorial Questions

2 Rates of Reaction Tutorial Questions 1 1. The graph shows the variation of concentration of a reactant with time as a reaction proceeds. What is the average reaction rate during the first 20 s? A moll -1 s -1 B moll -1 s -1 C moll -1 s -1 D moll -1 s In an experiment, ethanal vapour (CH 3 CHO) decomposes to produce methane and carbon monoxide as the only products. The concentration of ethanal is plotted against time [ethanal] mol/l Time (s) a. Use the graph to determine the average rate for the first 10 seconds. b. Use the graph to determine the average rate for the last 10 seconds. c. Explain the difference in these two rates.

3 Rates of Reaction Tutorial Questions 2 1. Which of the following is the correct interpretation of the above energy distribution diagram for a reaction as the temperature decreases from T 2 to T I? 2. The effect of temperature changes on reaction rate can be studied using the reaction between an organic acid solution and acidified potassium permanganate solution. 5(COOH) 2(aq) + 6H + (aq) + 2MnO 4- (aq) 2 Mn 2+ (aq) + 10CO 2(g) + 8H 2 O (l) The apparatus required is shown in the diagram: a. Name the organic acid. b. Describe how the reaction time can be measured. c. The headings for a set of results are shown below. Complete the headings by entering the correct unit in the third column.

4 Rates of Reaction Tutorial Questions 3 1. Excess zinc was added to 100cm 3 of hydrochloric acid, concentration 1 moll -1. Graph I refers to this reaction. Graph II could be for A excess zinc reacting with 100cm 3 of hydrochloric acid, concentration 2 moll -1. B excess zinc reacting with 100cm 3 of sulphuric acid, concentration 1 moll -1. C excess zinc reacting with 100cm 3 of ethanoic acid, concentration 1 moll -1. D excess magnesium reacting with 100cm 3 of hydrochloric acid, concentration 1 moll A student was asked to write a plan of the procedure for an investigation. The entry made in her laboratory note book is shown. a. Why is instruction 4 not the best way of altering the concentration of the potassium iodide solution? b. State two other ways of improving the student's plan of this investigation procedure.

Rates of Reaction Tutorial Questions 4 3. Excess marble chips (calcium carbonate) were added to 25 cm3 of hydrochloric acid, concentration 2 moll-i.

5 Rates of Reaction Tutorial Questions 4 3. Excess marble chips (calcium carbonate) were added to 25 cm3 of hydrochloric acid, concentration 2 moll-i. Which measurement, taken at regular intervals and plotted against time, would give the graph shown below? A B C D Temperature Volume of gas produced ph of solution Mass of the beaker and contents 2. The effect of temperature on reaction rate can be studied using the reaction between oxalic acid and acidified potassium permanganate solutions. (a) What colour change would indicate that the reaction was complete? (b) A student's results are shown on the graph below. i. Use the graph to calculate the reaction time, in s, at 40 c. ii. Why is it difficult to obtain an accurate reaction time when the reaction is carried out at room temperature?

6 Rates of Reaction Tutorial Questions 5 1. When copper carbonate is reacted with excess acid, carbon dioxide is produced. The curves shown were obtained under different conditions. The change from P to Q could be brought about by: A B C D increasing the concentration of the acid decreasing the mass of copper carbonate decreasing the particle size of the copper carbonate adding a catalyst. 2. The following reaction takes place: 2HCl (aq) + Na 2 S 2 O 3(aq) 2NaCl (aq) + SO 2(g) + S (s) + H 2 O (l) Solid sulphur forms in the solution. In one set of experiments the effect of varying the concentration of sodium thiosulphate was studied. Some of the volumes of solutions used are shown: a. Complete the table to show the volumes of water and acid that would have been used. b. Describe how the reaction time could have been measured. c. Describe how the relative rate of reaction would be obtained from each of the results.

7 Rates of Reaction Tutorial Questions 6 1. The same reaction was carried out at four different temperatures. The table shows the times taken for the reaction to occur. A B C D a small rise in temperature results in a large increase in reaction rate. the activation energy increases with increasing temperature. the rate of the reaction is directly proportional to the temperature. the reaction is endothermic. 2. A simplified diagram of a portable hair curler is shown. The catalyst is heated electrically. Butane gas reacts with oxygen from the air. The tongs remain hot until the gas is switched off. a. Although the catalyst has to be kept hot off after the reaction has started. Suggest why this is possible. b. Name the type of catalysis taking place. c. Why is the catalyst spread out?

Rates of Reaction Tutorial Questions 7 1. A pupil is asked to react chalk with acid and measure accurately the volume of carbon dioxide produced every minute.

8 Rates of Reaction Tutorial Questions 7 1. A pupil is asked to react chalk with acid and measure accurately the volume of carbon dioxide produced every minute. P From the diagrams above, which of the following combinations is the best one to use? A B C D P and Q P and R S and Q S and R 2. Dilute hydrochloric acid reacts with sodium thiosulphate solution producing a precipitate of sulphur, which will slowly obscure a cross marked on a piece of paper below the reaction beaker. a. Describe fully how this reaction can be used to investigate the effect of temperature on reaction rate. The graph shows the effect of temperature on the rate of reaction when 5 cm 3 of 0.1 moll -l hydrochloric acid is added to 200 cm 3 of 0.2 moll -l sodium thiosulphate solution. b. For how long was the reaction at 37 C timed? (Show your working clearly.) c. Continue the graph to predict the rate at 52 C.

9 Rates of Reaction Tutorial Questions 8 1. The graph shows the volume of hydrogen given off against time when an excess of magnesium ribbon is added to 100 cm 3 of hydrochloric acid, concentration 1 moll -1, at 20 C. Which graph would show the volume of hydrogen given off when an excess of magnesium ribbon is added to 50 cm3 of hydrochloric acid of the same concentration at 30 C? (The axes all have the same scales as the above graph.) 2. Ozone, O 3, is attacked by chlorine radicals in a series of processes that result in the depletion in concentration of ozone in the upper atmosphere. The chlorine radicals come from CFC s being broken down by the effect of UV light. Cl * (g) + O 3(g) ClO * (g) + O 2(g) ClO * (g) + O 3(g) Cl * (g) + 2O 2(g) a. Write an overall balanced equation for the depletion of ozone. b. Explain why a single chlorine radical can have such a massive on the ozone in the upper atmosphere. c. State whether the chlorine radical is acting as a homogeneous or heterogeneous catalyst and how you came to this conclusion. d. With reference to an industrial process explain how a surface active catalyst works.

REACTION RATES AND REVERSIBLE REACTIONS

NAME SCHOOL INDEX NUMBER DATE REACTION RATES AND REVERSIBLE REACTIONS 1. 1989 Q 4 P1 The graph shows the loss in total mass of a mixture of marble chips and dilute hydrochloric acid with time at 250C Loss