Chemical reactions only take place when there are collisions between reactants

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Depends on:

Number of collisions per unit time

Fraction of collisions which are successful

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There is a relationship/pattern between reaction rate and the factors that affect it

This pattern can only be determined empirically (through experimentation)

Rate Law

Empirical evidence supports that the reaction rate will always be proportional to the product of the initial concentration(s) of the reactant(s). These concentrations are raised to some exponential values that are determined empirically.

For example:

xA  +  yB  ——–›  AB

Note:  x and y are the coefficients used in balancing the chemical reaction and A and B represent formulas for reactant molecules.

Rate Law:        rate =  k [A]m [B]n

where;     – [ ] is in mol/L

m and n are the exponents that describe the mathematical dependence on initial [reactants]

k is the rate constant (specific for a reaction and is temperature dependent)

Note:  m and n can have any real number value (including zero and fractions)

Overall Reaction Order

The sum of the exponents in the rate law (m and n) gives the overall reaction order.

Example 1

H2 +  I2 ——–›  2HI

rate =  k [H2]  [I2]

First order with respect [H2]

First order with respect to [I2]

In other words, the rate depends equally on the initial concentration of each reactant.

Overall:  1 + 1 = 2, therefore, second order overall

Example 2

2NO2 + F2 ———›  2NO2F

Rate = k [NO2] [F2]

Think-Pair-Share

Compare examples 1 and 2.  What do you notice about the values of m and n as it relates to the coefficients?

Answer:  m and n do not have to equal the coefficients (x and y) in the balanced chemical reaction

Consider the following theoretical reaction:

2 X  +  2 Y  +  3Z   ———-›   2 XYZ

rate = k [X] [Y]2 [Z]0

Think-Pair-Share

Consider the following questions as it relates to this rate law:

1)  What will happen to the rate if the concentration of X is

  • Doubled?
  • Tripled?

2)  Consider the same question for the concentration of Y and then Z.

3)  Can you summarize the patterns in the following chart:

Order of Reaction

Concentration Change 0 1 2 3
X 1
X 2
X 3

4)  What is the overall reaction rate?

5)  Does the rate depend on the initial concentration of Z?  Rewrite the rate law to reflect this understanding?

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