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Let’s explore the chemical Kinetics of our favorite
Chapter , Problem 1DE(choose chapter or problem)
Let’s explore the chemical Kinetics of our favorite hypothetical reaction:
\(\mathrm{aA}+\mathrm{bB} \rightarrow \mathrm{cC}+\mathrm{dD}\).
We shall assume that all the substances are soluble in water and that we carry out the reaction in aqueous solution. Substances A and C both absorb visible light, and the absorption maxima are 510 nm for A and for 640 nm for C. Substances B and D are colorless. You are provided with pure samples of all four substances, and you know their chemical formulas. You are also provided appropriate instrumentation to obtain visible absorption spectra (see the Closer Look box on using spectroscopic methods in Section 14.3).
Let’s design an experiment to ascertain the kinetics of our reaction,
(a) What experiments could you design to determine the rate law and the rate constant for the reaction at room temperature? Would you need to know the values of the stoichiometric constants a and c in order to find the rate law?
(b) Design an experiment to determine the activation energy for the reaction. What challenges might you face in actually carrying out this experiment?
(c) You now want to test whether a particular water-soluble substance Q is a homogeneous catalyst for the reaction. What experiments can you carry out to test this notion?
(d) If Q does indeed catalyze the reaction, what follow-up experiments might you undertake to learn more about the reaction profile for the reaction?
Questions & Answers
QUESTION:
Let’s explore the chemical Kinetics of our favorite hypothetical reaction:
\(\mathrm{aA}+\mathrm{bB} \rightarrow \mathrm{cC}+\mathrm{dD}\).
We shall assume that all the substances are soluble in water and that we carry out the reaction in aqueous solution. Substances A and C both absorb visible light, and the absorption maxima are 510 nm for A and for 640 nm for C. Substances B and D are colorless. You are provided with pure samples of all four substances, and you know their chemical formulas. You are also provided appropriate instrumentation to obtain visible absorption spectra (see the Closer Look box on using spectroscopic methods in Section 14.3).
Let’s design an experiment to ascertain the kinetics of our reaction,
(a) What experiments could you design to determine the rate law and the rate constant for the reaction at room temperature? Would you need to know the values of the stoichiometric constants a and c in order to find the rate law?
(b) Design an experiment to determine the activation energy for the reaction. What challenges might you face in actually carrying out this experiment?
(c) You now want to test whether a particular water-soluble substance Q is a homogeneous catalyst for the reaction. What experiments can you carry out to test this notion?
(d) If Q does indeed catalyze the reaction, what follow-up experiments might you undertake to learn more about the reaction profile for the reaction?
ANSWER:Step 1 of 5
Rate of a reaction
The rate of a chemical reaction is called the change in concentration of reactants or products in a given time. It gives the speed at which a chemical reaction reaches its completion or attains equilibrium.
For a reaction given \(\mathrm{aA}+\mathrm{bB} \rightarrow \mathrm{cC}+\mathrm{dD}, the rate of the reaction can be given as the decrease in the concentration of reactants:
\(\begin{aligned} \text { Rate } & =-\frac{1}{\mathrm{a}} \frac{\mathrm{d}[\mathrm{A}]}{\mathrm{dt}} \\ & =-\frac{1}{\mathrm{~b}} \frac{\mathrm{d}[\mathrm{B}]}{\mathrm{dt}} \end{aligned}\)
The rate is also the increase in the concentration of reactants with time given by:
\(\begin{aligned} \text { Rate } & =\frac{1}{\mathrm{c}} \frac{\mathrm{d}[\mathrm{C}]}{\mathrm{dt}} \\ & =\frac{1}{\mathrm{~d}} \frac{\mathrm{d}[\mathrm{D}]}{\mathrm{dt}} \end{aligned}\)
The rate of a reaction also depends on the initial concentration of the reactant and is given by the rate law equation.
\(\text { Rate }=\mathrm{k}[\mathrm{A}]^{\mathrm{x}}[\mathrm{B}]^{\mathrm{y}}\)
Where k is the rate constant of the reaction. The sum of the powers of reactants is called the order of reaction and it is an experimental quantity.
\(\text { Reaction order }=\mathrm{x}+\mathrm{y}\)