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CHEMICAL KINETICS INTRODUCTION Chemical thermodynamics predicts stability of a system and the feasibility of a reaction or a process, while chemical kinetics predicts the speed at which such feasible process or reaction would occur. Therefore chemical kinetics is the area of chemistry concerned with the speed or rate at which a chemical reaction occurs. Knowing the speed of a reaction is important in drug design, in pollution control and food processing. The rate of a reaction (reaction rate) is expressed as the change in concentration or product with respect to time. Where A is the reactant and B is the product, such that in the course of the reaction A is consume therefore its concentration decreases while B is produce, consequently increasing in concentration. The negative sign in front of the rate expression in term of concentration of reactant is signifies decrease in the amount of A as it is consumed. Similarly A B CONC (M) t (s) For example in the reaction of bromine with methanoic acid as given below; the change in concentration of the Br2 was measured with respect to time at an interval of 50 s. Time (s) [Br2] (M) Rate (M/s) 0.0 0.0120 4.20 x10-5 50.0 0.0101 3.52 x 10-5 100.0 0.00846 2.96 x 10-5 150.0 0.00710 2.49 x10-5 200.0 0.00596 2.09 x10-5 250.0 0.00500 1.75 x 10-5 300.0 0.00420 1.48 x 10-5 350.0 0.00353 1.23 x 10-5 400.0 0.00296 1.04 x 10-5 (Ensure you understand how to solve this simple question in determining the rate at any give time) Take a look at the concentration of Br2 at t50 and t250 you would observe that the [Br2] at t50 is double that at t250 and also the rate at t50 is double that at t250. This shows that there is rate is directly proportional to the concentration. Recall O’level maths where k is called the rate constant; making k the subject of the formula we have The unit of k in this case is s-1 A plot of rate vs. [Br2] would be a straight line as illustrated in the graph below confirms the direct proportionality between concentration and rate. [Br2] (M) Rate (M/s) 0.0120 4.20 x10-5 3.50 x 10-3 0.0101 3.52 x 10-5 3.49 x 10-3 0.00846 2.96 x 10-5 3.50 x 10-3 0.00710 2.49 x10-5 3.51 x 10-3 0.00596 2.09 x10-5 3.51 x 10-3 0.00500 1.75 x 10-5 3.50 x 10-3 0.00420 1.48 x 10-5 3.52 x 10-3 0.00353 1.23 x 10-5 3.48 x 10-3 0.00296 1.04 x 10-5 3.51 x 10-3 0.00005 0.00004 0.00003 rate (M/s) 0.00002 0.00001 0 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 [Br2] (M) Reaction Rates and Stoichiometry We have considered a case where 1 mole of react A produced 1 mole of product B Now we want to consider a case where the ratio is not 1:1 and derive the rate expression. In the case below 2 moles of A disappear for 1 mole of B to be produced. This means that the rate of consumption or disappearance of A is twice as fast as the rate of appearance or production of B. Therefore in general for any reaction given as The rate would be expressed as Working with Volumes: For example the decomposition of hydrogen peroxide as given below: We can derive an expression relating rate to change in pressure when that is given. P = pressure of gas (in this case oxygen), V = volume of gas, n = no of moles of gas, R = gas constant and T = temperature) Substituting equation 2 in the rate expression in term of O2 (1), then Class Work 1.0 1. Write out the rate expression in terms of each reactant and product (a) (b) 2. Consider the reaction If oxygen is reacting at a rate of 0.0024 M/s (a) At what rate is being formed? (b) At what rate is reacting? 3. Determine the rate of production of oxygen and potassium chloride from the thermal decomposition of potassium trioxochlorate (V) given that the change in pressure is 2 atm and change in time of 2 mins (R= 0.082Latm/Kmol, T = 25 ). Solution 2a Given = 0.024M/s b. Similarly 3a. b.

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posted: | 11/17/2012 |

language: | English |

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