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ลำดับตอนที่ #12 : Rate and Order of Reactions

Rate and Order of Reactions

 

Skills to develop

• Derive the integrated rate laws from differential rate laws.
• Describe the variation of concentration vs. time for 1st order reactions.
• Describe the variation of concentration vs. time for 2nd order reactions.
• Figure out order of reaction from concentration vs. time plots.

 

Rate and Order of Reactions

The rate of a chemical reaction is the amount of substance reacted or produced per unit time. The rate law is an expression indicating how the rate depends on the concentrations of the reactants and catalysts. The power of the concentration in the rate law expression is called the order with respect to the reactant or catalyst.

This page deals specifically with first- and second-order reaction kinetics, but you should know that other orders such as zeroth-order and 3rd order may also be involved in chemical kinetics.

 

 

Reaction Rates and Stoichiometry

In acidic solutions, hydrogen peroxide and iodide ion react according to the equation:

H₂O₂ + 2H⁺ + 3I^- = 2 H₂O + I₃^-.

In this reaction, the reaction Rate can be expressed as

decreasing Rate of H₂O₂, - d[H₂O₂]/dt
decreasing Rate of H⁺, - d[H⁺]/dt
decreasing Rate of I^-, - d[I^-]/dt
increasing Rate of H₂O, + d[H₂O] /dt
increasing Rate of I₃^-, d[I₃^-]/dt

 

However, from the stoichiometry, you can easily see the following relationship:

 

   d[H₂O₂]     1 d[H⁺]       1 d[I^-]     1 d[H₂O]   d[I₃^-]

 - ------- = - - ------ = - - ------- = - ------ = --------

    dt         2 dt        3 dt       2   dt      dt

In reality, if the concentration of H₂O₂ is low, the changes in concentrations of H⁺ and H₂O are very difficult to detect because their quantities are so large in the solution. The merit in this equation is to show you that the rates of decreasing of reactant concentrations are governed by the stoichiometry. So do the rates of increasing of products concentrations.

There are many ways to express the reaction Rates, for example

 

   d[H₂O₂]     1 d[H⁺]       1 d[I^-]     1 d[H₂O]   d[I₃^-]

 - ------- = - - ------ = - - ------- = - ------ = -------- = Rate

    dt         2 dt        3 dt       2   dt      dt

or

 

    d[I^-]

 - ------- = Rate'

     dt

Obviously,

Rate = 3 Rate'

But both Rate and Rate' are reasonable expressions.

To generalize it, let the chemical reaction be represented by,

a A + b B -> c C + d D

then the rate is represented by any one of the following

 

          1 d[A]      1 d[B]    1 d[C]    1 d[D]

rate = - --- ---- = - --- ---- = --- ---- = --- ----

          a   dt       b   dt     c   dt     d   dt

Unless the rate expression is specified.

 

 

 

Differential Rate Laws and Integrated Rate Laws

For simplicity, let us consider the reactions:

                           k

     A + other reactants -----> products

where A is one of the reactants, and k is the rate constant.

For most experiments in chemical kinetics, the concentration of one reactant or product is monitored. If the concentrations of other reactants are high, they are not greatly changed. Thus, we have a pseudo decomposition reaction.

For the decomposition of A with a rate constant k,

A -> products

The concentration [A] can be monitored. Let the order of the reaction be n, then the expression

 

    d[A]

 - ---- = k [A]ⁿ

     dt

is called the differential rate law.

The differential expressions can be integrated to give an explicit relation of [A] with respect to time t. These explicit relations are called integrated rate laws.

Depends on the value of n, the integrated equations are different. If the reaction is first order with respect to [A], integration with respect to time, t, gives: 

[A] = [A]_o exp(-k t) - - - - (1)

where [A]_o is the concentration of A at t = 0, and [A] is the concentration at time t.

For a second order reaction, the integrated rate law is:

 

    1       1

   --- = ---- + k t   - - - - (2)

   [A]     [A]_o

Derive the above equations yourself.

Determination of Rate Constants Using the Integrated Rate Laws

The usual approach to calculate the rate constant k makes use of the differential rate law. A series of experiments are performed with various initial concentrations, and their rates measured. The rate constants are calculated from the initial concentration and time of measurement. Often, you should construct a graph for their evaluation. However, results so obtained contain errors due to the approximation whereas values for k calculated using the integrated rate laws (1) and (2) are more accurate. Today, calculations can easily be performed with the aid of calculators and computers.

Thus, we emphasize that you use the integrated rate laws whenever possible. Whether you use the differential rate laws or the integrated laws, you have to evaluate the order first.

Equation (1) may be rewritten as

 

        ln [A]_o - ln [A]

   k = ----------------- - - - - - - - (1')

               t

 

In a real experiment, you should plot - ln [A] vs. t and find a line to best fit your data. The slope of the line is k.

Similarly, equation (2) may be rewritten as:

 

         1       1

        --- - ----

        [A]     [A]_o

   k = --------------    - - - - - - - (2')

             t

 

Thus, plot of 1/[A] vs. t should yield a straight line, and the slope is k. 

 

 

Half Lives of First and Second Order Reactions

The half-life (hl) of a reaction is the time period required to reduce the reactant to half of its original value. The half life of first order reaction is a constant, independent of the initial concentration. The rate constant and half-life has the relationship: 

hl = ln(2) / k
hl * k = ln(2) 

For 2nd order reactions, the half life depends on the initial concentration, [A]_o, and the rate constant k: 

hl = 1 / (k [A]_o)
hl * k [A]_o = 1 

Since the concentration is reduced to half of its original value at the end of its first half-life, the second half-life is twice as long as the first half life. Thus, a plot of [A] vs. t easily reveal the order of the reaction by tracking its half life.

Either plot [A] vs. t or plot the appropriate linear relationship will reveal the order of the reaction. Both types are available in the simulation. Unfortunately, when we switched to the present format, the simulation cannot be implemented here.

A summary of the relationships for first and second order reactions are given below: 

\	Differential rate law	Integrated rate law	Linear plot	Half life
first order	- d[A]/dt = k [A]	[A] = [A]o exp (-k t)	ln [A] vs t slope = - k	hl=ln(2)/k
second order	- d[A]/dt = k [A]2	1     1 --- - ---- = k t [A]   [A]o	1 --- vs t [A] slope = k	hl=1/k[A]o

Further applications of rate laws will be discussed in Integrated Rate Laws. Students must analyze the problems carefully, and appropriately apply the differential or integrated rate laws to derive the desirable results.