Chemistry_Grade_10-12 (1).pdf

Of the reactants increases there is also a greater

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of the reactants increases, there is also a greater chance that successful collisions will occur. Definition: Collision theory Collision theory is a theory that explains how chemical reactions occur and why reaction rates differ for different reactions. The theory assumes that for a reaction to occur the reactant particles must collide, but that only a certain fraction of the total collisions, the effective collisions , actually cause the reactant molecules to change into products. This is because only a small number of the molecules have enough energy and the right orientation at the moment of impact to break the existing bonds and form new bonds. When the temperature of the reaction increases, the average kinetic energy of the reactant particles increases and they will move around much more actively. They are therefore more likely to collide with one another (Figure 16.1). Increasing the temperature also increases the number of particles whose energy will be greater than the activation energy for the reaction (refer section 16.5). 293
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16.3 CHAPTER 16. REACTION RATES - GRADE 12 B A B A B A B B A A A B Low Temperature B A B A B A B B A A A B High Temperature Figure 16.1: An increase in the temperature of a reaction increases the chances that the reactant particles (A and B) will collide because the particles have more energy and move around more. Exercise: Rates of reaction Hydrochloric acid and calcium carbonate react according to the following equa- tion: CaCO 3 + 2 HCl CaCl 2 + H 2 O + CO 2 The volume of carbon dioxide that is produced during the reaction is measured at different times. The results are shown in the table below. Time (mins) Volume of CO 2 produced (cm 3 ) 1 14 2 26 3 36 4 44 5 50 6 58 7 65 8 70 9 74 10 77 Note: On a graph of production against time, it is the gradient of the graph that shows the rate of the reaction. Questions: 1. Use the data in the table to draw a graph showing the volume of gas that is produced in the reaction, over a period of 10 minutes. 2. At which of the following times is the reaction fastest ? Time = 1 minute; time = 6 minutes or time = 8 minutes. 3. Suggest a reason why the reaction slows down over time. 4. Use the graph to estimate the volume of gas that will have been produced after 11 minutes. 5. After what time do you think the reaction will stop? 6. If the experiment was repeated using a more concentrated hydrochloric acid solution... (a) would the rate of the reaction increase or decrease from the one shown in the graph? (b) draw a rough line on the graph to show how you would expect the reaction to proceed with a more concentrated HCl solution. 294
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CHAPTER 16. REACTION RATES - GRADE 12 16.4 16.4 Measuring Rates of Reaction How the rate of a reaction is measured will depend on what the reaction is, and what product forms. Look back to the reactions that have been discussed so far. In each case, how was the rate of the reaction measured? The following examples will give you some ideas about other ways to measure the rate of a reaction: Reactions that produce hydrogen gas: When a metal dissolves in an acid, hydrogen gas is produced.
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