Lab_7-S08-bleach_kinetics - Lab 7 CHEMICAL KINETICS TO DYE...

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Lab 7: CHEMICAL KINETICS TO DYE FOR Laboratory Goals In this week’s lab you will: • Determine concentrations via spectroscopy using Beer’s Law • Determine the rate law for the reaction between bleach and two dyes • Determine the rate constant for the same reaction. Introduction In this lab, you will be determining how quickly household bleach reacts with a couple of different dyes. In order to be able to understand this, we have to first visit the world of kinetics. Kinetics Chemical Kinetics is the branch of chemistry which is concerned with the study of the rate of chemical reactions. The rate of a reaction is a measure of how quickly reactants are turned into products. This area of study directly complements the study of thermodynamics which focuses exclusively upon the energetic favorability of reactions. Consider the hypothetical reaction A + 2B 2C + D (1) The rate of formation of C is: t C t t C C Rate i f i f c Δ Δ = = ] [ ] [ ] [ (2) where [C] f and [C] i are the concentrations of C at times t f and t i , respectively. The symbol Δ stands for “change.” The rate of formation of C is the change in the concentration of C over the time interval Δ t (t f -t i ). Similarly, the rate of formation of D is t D Rate D Δ Δ = ] [ (3) The rates of consumption of A and B are t A Rate E Δ Δ = ] [ (4) t B Rate B Δ Δ = ] [ (5) The negative signs in equations (4) and (5) arise from the fact that although the rates are positive numbers, the concentrations of the reactants decrease with time so their changes are negative. From the stoichiometry of reaction (1) we see that the consumption of 1 mole of A results in the consumption of 2 moles of B and the formation of 2 moles of C and 1 mole of D. B is consumed twice as fast as A, and C is produced twice as fast as D. Thus, the relationships between the rate expressions in equations (2)-(5) is 7-1
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B A C D t t t t Δ Δ = Δ Δ = Δ Δ 2 2 (6) se Δ = Δ ] [ 1 ] [ ] [ 1 ] [ The rate of the reaction, Rate RXN , can be expres d either in terms of the rate of disappearance of reactants or the rate of appearance of products: t t t t Rate RXN B A C D Δ Δ = Δ Δ = Δ Δ = Δ = 2 2 (7) Δ ] [ 1 ] [ ] [ 1 ] [ general, the rate of a reaction depends on the concentration of the reactant as follows: and ely. These exponents are usually the integers 0, 1, 2, or 3, but in some reactions are MUST be determined by experiment and CAN NOT reaction. Rate Laws involving only one reactant: E products the rate law is: In Rate RXN = k[A] x [B] y (8) Equation (8) is the rate law for the reaction. In the generic rate law the concentration of each of the reactants raised to a power to give the overall rate law. This makes intuitive sense because one would anticipate that as the reactant concentration was increased that the rate of the reaction would also increase due to a greater number of molecular collisions between the reactants. In this case it shows that the rate is proportional to the product of the concentrations of the
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This note was uploaded on 04/15/2009 for the course LB 171L taught by Professor All during the Spring '08 term at Michigan State University.

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Lab_7-S08-bleach_kinetics - Lab 7 CHEMICAL KINETICS TO DYE...

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