Core & Extension Chemical Reactions-Speed II Speed of Rxn 1.pptx

IGCSE　

1. © 2004-09 Dorje Gurung Core & Extension Chemical Reactions: Speed of Reaction (Part I)

2. Core & Extension Chemical Rxns: Speed of Rxn Slide 2 of 28 Learning Objectives Concepts: speed (rate) of reaction, concentration, catalyst, surface area, temperature, activation energy Skills: Core Describe a practical method for investigating the speed of a reaction involving gas evolution by collecting and recording volume of gas data over time and graphing the result. Extension Analyze and interpret data obtained from experiments on speed of reaction Devise experiments to test the effect of any four of the factors on speed of reaction

3. Core & Extension Chemical Rxns: Speed of Rxn Slide 3 of 28 Preamble You have learned that concentration of a reactant particles, temperature, surface area of the reactants, and catalyst affect the speed of reaction. But how can we actually follow the progress of a chemical reaction to confirm this? Whereas speed of a vehicle is change of distance per unit time, speed of a chemical reaction is a measure of change of a measurable quantity per unit time. Speed of reaction is a measure of how fast a reactant is being used up or how fast a product is being produced. We must therefore be able to follow the progress of a reaction by looking at a quantity that changes over time.

4. Core & Extension Chemical Rxns: Speed of Rxn Slide 4 of 28 Following the Progress of a Chemical Reaction We have two choices: 1.Record the change in quantity of a product as the reaction progresses. The quantity measured could be volume or mass or concentration of the product (We will be looking at methods involving collection and recording volume of gas produced first and then mass of reaction vessel.) 2.Record the change in quantity of a reactant as the reaction progresses. Again, the quantity measured could be mass or volume or concentration of the reactant. (We will not be looking at any example of this kind because in IGCSE you are not likely to encounter it.) The method chosen depends on the chemical reaction being studied. Reactions that produce a gas for example can be followed by measuring the change in volume of gas evolved or mass of the reaction vessel. To be sure, there are other ways of following the progress of a reaction.

5. Core & Extension Chemical Rxns: Speed of Rxn Slide 5 of 28 1. Progress of Chemical Reaction: Volume change of a gas As an example consider the reaction between zinc and hydrochloric acid. Zn (s) + 2HCl (aq)  ZnCl 2 (aq) + H 2 (g) We can follow this reaction by collecting the gas given off in one of two ways. Dilute acid Lumps of zinc Gas being collected The first method of collecting the gas is by displacement of water. The progress of the reaction can be followed by measuring the volume of the gas given off.

6. Core & Extension Chemical Rxns: Speed of Rxn Slide 6 of 28 1. Progress of Chemical Reaction: Volume change of a gas The second method of collecting and measuring the gas is in a gas syringe. For a reaction that produce a water soluble gas, this setup would have to be employed. (How would one initiate the above reaction?)

7. Core & Extension Chemical Rxns: Speed of Rxn Slide 7 of 28 1. Progress of Chemical Reaction: Volume change of a gas A reaction that does produce water soluble gas is that between calcium carbonate (marble chips) and hydrochloric acid. CaCO 3 (s) + 2HCl (aq)  CaCl 2 (aq) + H 2 O (l) + CO 2 (g) Marble chips (Carbon dioxide)

8. Core & Extension Chemical Rxns: Speed of Rxn Slide 8 of 28 Data Collection and Analysis Method I Either way, to get information about the speed of the reaction, volume of gas given off at regular interval of time (such as every 30 seconds) can be recorded until the reaction stops—when the volume doesn’t change—or exceeds the capacity of the measuring cylinder. Speed of reaction can be derived from the data collected by plotting a graph of volume against time. Dilute acid Marble chips Gas being collected

9. Core & Extension Chemical Rxns: Speed of Rxn Slide 9 of 28 Method I Analysis: Volume-time Graph Plotting the volume of gas collected over time would give you a graph similar to the one shown below. Reaction fastest at the beginning. Slowing down. Reaction finished. Time/sec. 60 120 180 240 Volume of gas /cm 3 200 100 300 All very well, but what does the graph tell you? A couple of things: 1. The gradient or slope of the graph shows the rate of the reaction. Steeper slope = faster reaction. 2. The time when the curve starts leveling off indicates when the reaction ended.

10. Core & Extension Chemical Rxns: Speed of Rxn Slide 10 of 28 Method I Analysis: Slope and Speed Slope is high in the beginning (solid line tangent). Time/sec. 60 120 180 140 Volume of gas /cm 3 200 100 300 After about 60 seconds, the speed is much lower (broken line tangent). 300 cm 3 of gas is produced every 60 seconds (speed is 5 cm 3 /s) Only 250 cm 3 (350  100) is produced every 150 seconds (speed is 1.6 cm 3 /s). Speed decreases over time because the concentration of the reactant (acid) decreases (and therefore speed of reaction also decreases).

11. Core & Extension Chemical Rxns: Speed of Rxn Slide 11 of 28 Another Perspective It may help to remember displacement-time graph for deceleration that you learned in (form 3) physics. Time/sec. 60 120 180 140 Displacement /m 200 100 300 Distance covered in the same period of time (30 seconds) steadily decreases as time goes by. In other word, speed is steadily decreasing! Change displacement to volume of gas or mass of reactant or mass of vessel and you get the idea!

12. Core & Extension Chemical Rxns: Speed of Rxn Slide 12 of 28 Method I Analysis: Volume-time Graph Comparison of speed of reaction between two concentrations of acid could then be carried out by graphing the results of the two experiments using different concentrations of the acid. Volume of gas /cm 3 Time/sec. The black curve is steeper, and the reaction ends earlier (the graph levels off earlier). Therefore, the speed of reaction is higher at higher concentration of the acid. 2M HCl 1M HCl (It must be noted that the above curves would be obtained only if CaCO 3 were the limiting reagent.)

13. Core & Extension Chemical Rxns: Speed of Rxn Slide 13 of 28 Method I Analysis: Volume-time Graph If on the other hand, the same volume of acid and mass of CaCO 3 were used and the acid were the limiting reagent, then the following curves would be obtained. 1M HCl 2M HCl Volume of gas /cm 3 Time/sec. 100 50

14. Core & Extension Chemical Rxns: Speed of Rxn Slide 14 of 28 Data Collection and Analysis Method II Instead of following the change in volume of gas at regular intervals of time, if the purpose is to compare speed of reaction at different concentrations (or temperature) for example, then time taken to produce a fixed volume of gas can be recorded instead . It’s like determining the relative speeds of different cars by determining how long they take to travel a fixed distance (from Lilongwe for example to Blantyre). The car that takes the shortest time will, on average, have travelled at the highest speed (fastest), and the one that takes the longest time will have travelled at the lowest speed (slowest). Same idea with speed of reaction. One that takes the shortest time to produce the same volume of gas will have had reacted at the fastest speed.

15. Core & Extension Chemical Rxns: Speed of Rxn Slide 15 of 28 Data Collection and Analysis Method II The result can be analyzed graphically too of course. Taking the same reaction as before (the one between CaCO 3 and HCl), if the rates between 4 concentrations of acid were compared by timing the production of say 200 cm 3 of carbon dioxide then data such as the one below may be obtained. The time taken for reaction to come to completion increases as concentration decreases. Therefore, speed must be higher at higher concentration . Concentration of acid /mol dm -3 Time/sec. 1.0 2.0 3.0 30 60 90 120 150

16. Core & Extension Chemical Rxns: Speed of Rxn Slide 16 of 28 Practice Questions: Q1 1. J05/6/3. In a set of experiments zinc was reacted with sulfuric acid to form hydrogen. The apparatus below was used. The same mass of zinc was used each time. The volume of acid used was different each time.

17. Core & Extension Chemical Rxns: Speed of Rxn Slide 17 of 28 Practice Questions: Q1 Table of results Use the syringe diagrams to record the volume of hydrogen produced each time in the table.

18. Core & Extension Chemical Rxns: Speed of Rxn Slide 18 of 28 Practice Questions: Q1 (a) Plot the results on the grid. Draw a smooth line graph. [4]

19. Core & Extension Chemical Rxns: Speed of Rxn Slide 19 of 28 Practice Questions: Q1 (b) (i) Use the graph to find the volume of sulfuric acid that will produce 33 cm 3 of gas. [1] (ii) Show on the graph how you obtained the answer for ‘i’ above. [1] (c) What volume of gas is produced if 10 cm 3 of sulfuric acid is used?[1]

20. Core & Extension Chemical Rxns: Speed of Rxn Slide 20 of 28 Practice Questions: Q2 2. N04/6/2. The addition of calcium carbonate chips to excess (more than enough) dilute nitric acid produces carbon dioxide. A student collects the gas, records the volume given off at 20 second intervals and has plotted it on grid to the right.

21. Core & Extension Chemical Rxns: Speed of Rxn Slide 21 of 28 Practice Questions: Q2 (a) Draw a smooth line graph on the grid. [1] (b) (i) Circle the result which appears to be incorrect? Why have you selected this result? [2] (ii) Suggest a source of experimental error which may be responsible for that result. [1] (iii) How long did the experiment last? [1] (iv) What volume of carbon dioxide was produced by the reaction? [1]

22. Core & Extension Chemical Rxns: Speed of Rxn Slide 22 of 28 Practice Questions: Q2 (c) Explain why the curve get less and less steep over time? [1] (d) On the same grid draw a curve that would be obtained using higher concentration of nitric acid. [2]

23. Core & Extension Chemical Rxns: Speed of Rxn Slide 23 of 28 Practice Questions: Q2 (e) (i) Describe the shape of the curve that would be obtained if the same mass but a lot fewer number of chips were used. [2] (ii) Explain the shape of the curve (therefore the effect on speed of the reaction) using collision theory. [2]

24. Core & Extension Chemical Rxns: Speed of Rxn Slide 24 of 28 Practice Questions: Q2 (f) Describe the method the student followed. You must describe the different pieces of apparatus used and the stepwise procedure followed to conduct and collect the necessary data. You may support your description with a labeled diagram of the setup.

25. Core & Extension Chemical Rxns: Speed of Rxn Slide 25 of 28 Practice Questions: Q3 3. N04/3/1(d)(ii). The rate of photosynthesis of pond weed can be measured using the following experiment. What measurements are needed to calculate the rate of this reaction? (There are two different measurements you can make. Can you think of both?) [2]

26. Core & Extension Chemical Rxns: Speed of Rxn Slide 26 of 28 Practice Questions: Q4 4. N03/3/2bi. Sodium chlorate(I) decomposes to form oxygen and sodium chloride. 2NaClO(aq) → 2NaCl(aq) + O 2 (g) Describe how the rate of this reaction could be measured. [2]

27. Core & Extension Chemical Rxns: Speed of Rxn Slide 27 of 28 Practice Questions: Q5 5. J02/2/3b. (a) The apparatus below can be used to compare the reactivity of the alkali metals with water. (i) State the name of the gas formed when the alkali metals react with water. [1] (ii) Describe a test and the result that you would get which would positively identify this gas. [2]

28. Core & Extension Chemical Rxns: Speed of Rxn Slide 28 of 28 Practice Questions: Q5 (ii) Explain how this apparatus could be used to compare the rates of reaction of the alkali metals. [3] (iii) How does the rate of reaction of the alkali metals with water change as you go down Group I. [1]