Chemical Kinetics

Half-Life

Concepts

The variation in concentration with time provides a highly detailed description of how fast a reaction is occurring. In many circumstances, though, it is desirable to have a simple, approximate measure of the reaction rate, and the half-life provides such a measure.

The half-life, t1/2, is the time it takes for one-half of the original amount of material to react (assuming the compound in question is a limiting reactant). If the initial concentration of a reactant A is 0.100 mole L-1, the half-life is the time at which [A] = 0.0500 mole L-1.

Intuitively, the faster the reaction, the shorter the half-life. The rate of the reaction is proportional to the rate constant; thus the larger the rate constant, the shorter the half-life.

The following exercises illustrate how the half-life depends upon the concentration of reactant for zero-, first-, and second-order reactions.



Zero-Order Reaction

The graph at the left shows how the concentration of reactant A varies with time for a zero-order reaction. To create the plot, enter an initial concentration for A in the box and click the "Create Plot" button.

Use the cursor to determine the half-life of the reaction. Do this by locating the point on the concentration-time curve that corresponds with one-half the original amount of reactant. While the cursor is over this point, press the left mouse button to display the time, which is the half-life.

Perform this measurement for several different initial concentrations. For each initial concentration, enter the half-life in the appropriate box and add this point to the graph at the right. The dependent variable in the graph at the right is the half-life. Select an option for the independent variable: [A]0, ln [A]0, or 1/[A]0.

Which plot gives a straight line?

Write the integrated rate law for a zero-order reaction, and derive an expression for the half-life as a function of the initial concentration.

Is your relationship consistent with the plot you found to yield a straight line?

After at least five points are plotted, click on the "Plot Line" button, which will plot the line-of-best-fit on the graph at the right. Use the slope and/or intercept to determine the rate constant for the reaction.


Initial Concentration:   mole/L      

Half-Life:   sec            

C     ln C     1/C      

slope:         intercept:  



First-Order Reaction

Perform the same measurements as described for the zero-order reaction.

Which plot gives a straight line?

Write the integrated rate law for a first-order reaction, and derive an expression for the half-life as a function of the initial concentration.

Is your relationship consistent with the plot you found to yield a straight line?

After at least five points are plotted, click on the "Plot Line" button, which will plot the line-of-best-fit on the graph at the right. Use the slope and/or intercept to determine the rate constant for the reaction.


Initial Concentration:   mole/L      

Half-Life:   sec            

C     ln C     1/C      

slope:         intercept:  



Second-Order Reaction

Perform the same measurements as described for the zero-order reaction.

Which plot gives a straight line?

Write the integrated rate law for a second-order reaction, and derive an expression for the half-life as a function of the initial concentration.

Is your relationship consistent with the plot you found to yield a straight line?

After at least five points are plotted, click on the "Plot Line" button, which will plot the line-of-best-fit on the graph at the right. Use the slope and/or intercept to determine the rate constant for the reaction.


Initial Concentration:   mole/L      

Half-Life:   sec            

C     ln C     1/C      

slope:         intercept:  


Integrated Rate Laws                     Method of Initial Rates

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