CHEM 122 - CHEMICAL KINETICS

CHEM 122 - CHEMICAL KINETICS - T H E R A T E O F A R E A CT...

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THE RATE OF A REACTION chemical kinetics reaction rate The area of chemist ry concerned wit h t he speed, or rat e, at which a chemical react ion occurs change in concent rat ion of a react ant or a product wit h t ime (M/ s) concent rat ion decreases increases is called . Normally the word “kinetic” suggests movement or change. In chemistry, kinetics refers to the rate of a reaction, or the , which is the . Any reaction can be represented by the general equation Reactants Products This equation tells us that during the course of a reaction, reactant molecules are consumed while product molecules are formed. Given any reactant A and any product B, we can write the equation A B. In a solution of one kind or another, we can denote the of either of these entities by using brackets around their symbols; e.g. [A] = concentration of A. Over time, there will be a decrease in the number of A molecules and an increase in the number of B molecules. In general, it is most convenient to express this rate in terms of a change in concentration over time, i.e. Because the concentration of A during the time interval, and because the concentration of B during the time interval, ? A is a negative quantity. This is because the rate of reaction is always a positive quantity. For more complex reactions, we must be careful in writing the rate expression. For example 2A B Means that two moles of A are consumed for every mole of B that is consumed. Therefore, the rate at which B forms is one half the rate at which A disappears. We can write this as either ? ? ?
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And for the reaction aA + bB cC + dD the rate is given by Depending on the nature of the reaction, there are a number of different ways to measure the reaction rate. In the case of molecular bromine, which is brown, reacting with formic acid, which results in clear products, we can use the absorbance of the solution as a standard for reaction in progress. If one of the products or reactants of a reaction is a gas, we can use a manometer to find the reaction rate. ?
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THE RATE LAWS Rate Data for the Reaction One way to study the effect of reactant concentration on reaction rate is to determine how the initial rate depends on the starting concentrations. It is preferable to measure the initial rate because as the reaction proceeds, the concentrations of the reactants decrease and it may become difficult to measure the changes accurately. Also, there may be a reverse reaction such that products reactants, which would introduce error into the rate measurement. Consider the following table: [F 2 ] (M) [ClO 2 ] (M) Initial Rate (M/s) 0.10 0.010 1.2 x 10 -3 0.10 0.040 4.8 x 10 -3 0.20 0.010 2.4 x 10 -3 These represent experimental measurements taken for the reaction F 2 (g) + 2ClO 2 (g) 2FClO 2 (g) Looking at the first and third table entries, we see that doubling the initial concentration of F 2 while leaving ClO 2 constant causes the rate to double.
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This note was uploaded on 02/02/2011 for the course CHEM 122 taught by Professor Bellew during the Fall '07 term at New Mexico.

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CHEM 122 - CHEMICAL KINETICS - T H E R A T E O F A R E A CT...

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