Chemical reactions only take place when there are collisions between reactants

**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?*