A Model for Reaction Rates

Expressing Reaction Rates Average Rate = quantity time The amount of increase or decrease depends on mole ratios Units =Molarity/s or mol/Ls

Expressing Reaction Rates Cont. As a reaction proceeds, there is a decrease in concentration of reactants and an increase in the concentration of the products N2 + 3H2  2NH3 N2 and H2 decrease in concentration over time, while NH3 increases in concentration over time.

Expressing Reaction Rates Average reaction rate Concentration M2 - Concentration M1 t2 - t1 [Concentration] time Reaction rates can be negative if concentration is decreasing.

Calculating Average Reaction Rates Given H2 + Cl2 2HCl Calculate the average reaction rate expressed in moles H2 consumed per liter per second 2. Calculate the average reaction rate expressed in moles Cl2 consumed per liter per second

The Collision Theory Summary Reacting substances (atoms, ions, or molecules) must collide. Reacting substances must collide with the correct orientation Reacting substances must collide with sufficient energy to form the activated complex.

Activation Energy The minimum amount of energy that reacting particles must have to form the activated complex and lead to a reaction Symbol: Ea Direct influence on the rate of a reaction

Exothermic F

Endothermic E

Rate Law (NOT Reaction Rates) An expression that relates rate of a reaction and reactant concentration Rate of reaction depends on reactant concentrations never includes products Given: 2A + 3B 2C In general, rate = k [A]x [B]y

Rate Constant Symbol: k (lowercase) A constant specific and unique for every reaction If k is large, products form quickly If k is small, products form slowly

Order of reaction Exponents in rate law (x and y) Determine how much the rate depends on the respective concentration(s) Can ONLY be determined by experiment Overall order of a reaction is the sum of all orders (x + y)

Example 1 2A + 3B 2C Skeleton Rate = k [A]x [B]y Overall order of reaction (x + y) If x=1, 1st order in A If y=2, 2nd order in B Overall order 1 + 2 = 3rd order

Example 2 Write the skeleton rate law Rate =K[H2O(l)]x 2H2O(l) 2H2(g) + O2(g) Rate =K[H2O(l)]x Remember you only use the reactants and the only way to get x is by getting data in an experiment

Relationship of order and rate Example 1: rate = k [A] 1st order If concentration of A doubles, rate doubles If concentration of A triples, rate triples If concentration of A is halved, rate halves Example 2: rate = k [A]2 2nd order If concentration of A doubles, rate quadruples If concentration of A triples, rate is 9 times as fast

Example 3: rate = k [A]0 0th order Recall: anything raised to the 0th power = 1 If the concentration of A changes in any way NOTHING happens to the rate Rate is not dependent on concentration Rate = k Begin your homework!