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
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
H_{2} + I_{2} ——–› 2HI
rate = k [H_{2}] [I_{2}]
First order with respect [H_{2}]
First order with respect to [I_{2}]
In other words, the rate depends equally on the initial concentration of each reactant.
Overall: 1 + 1 = 2, therefore, second order overall
Example 2
2NO_{2} + F_{2} ———› 2NO_{2}F
Rate = k [NO_{2}] [F_{2}]
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?