22711679-Ch-13-Rate-of-Reaction-t

22711679-Ch-13-Rate-of-Reaction-t - HOMEWORK GUIDE CHAPTER...

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HOMEWORK GUIDE CHAPTER 13: Rate of reaction Summary Reaction rate Rate of reaction is a measure of how fast a chemical reaction takes place. In a fast reaction, the reactants change quickly into products. The rate of reaction is high. Examples of fast reactions: -- Explosions. -- The reaction of potassium with water. In a slow reaction, the reactants take a long time to change into products. The rate of reaction is low. Examples of slow reactions: -- Rusting. -- The action of acid rain on limestone. Measuring reaction rate 1. Measuring the time for a reaction to be completed. -- Useful when comparing rates of two or more reactions. The reaction taking the longest time is the slowest. -- Example : To compare the reaction of magnesium ribbon in hydrochloric acid solutions of differing concentration. 1

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2. Measuring changes that occur in a period of time during a reaction (e.g. every minute of half-minute). Example : The reaction between magnesium and dilute hydrochloric acid. The rate can be measured three ways: (a) By measuring the volume of H 2 produced at regular intervals (b) By measuring the mass of the reaction flask and its contents. (The mass decreases as the gas produced is given off.) (c) My measuring the change in pressure of the gas. (The pressure increases as the volume of gas increases.) 2
The measurements are then plotted against time: volume of mass of pressure H 2 (cm 3 ) contents (g) time (min) time (min) time (min) Interpretation of reaction rate graphs The rate of the reaction at any time is shown by the gradient of the curve. The steeper the gradient, the faster the reaction. The rate is greatest at the start of a reaction and decreases over time. The curve becomes horizontal when the reaction is completed. (a) Initial gradient greatest; reaction is fastest. (b) The curve is less steep, indicating that the reaction is slower. (c) The curve is horizontal, indicating that the reaction is completed. Factors affecting rate of reaction A reaction occurs because reacting particles collide. But particles only react if they have sufficient energy. The rate of a reaction is affected by: (a) the surface area / particle size of a reactant. (b) the concentration of the reactants (especially solutions). (b) the temperature at which the reaction takes place. (d) the presence of a catalyst . 3 (c) (b) (a) time amount of product

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1. Surface area : Small particles react faster than larger particles as they have a larger surface area for reaction to take place. Example : Reaction between calcium carbonate of different particle size and hydrochloric acid. At time = 0, the gradient of graph A is steeper than that of graph B. Therefore, reaction A (powdered calcium carbonate) is faster than Reaction B (marble chips).
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This note was uploaded on 12/04/2009 for the course CHEM 120 taught by Professor Johnmichael during the Spring '09 term at Tennessee Martin.

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22711679-Ch-13-Rate-of-Reaction-t - HOMEWORK GUIDE CHAPTER...

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