Chapter 14

 Physical state of reactants:  Reactants must come in contact with one another in order for a reaction to occur.  Concentration of reactants:  Reactions tent to proceed faster if the reactants are present in higher concentrations.  The temperature at which the reaction occurs.  In general reactions happen faster at higher temperatures.

 The speed of any even is defined as a change that occurs over a given period of time.  The rate of a reaction is stated as the change in concentration of a reactant or product per unit of time.  Example:  A  B  If we start with 1.0 moles of A and 0.0 moles of B in one liter of solution what is the concentration of each?  If after 40 seconds the concentration of A is 0.3 M and the concentration of B is 0.7 M we can calculate the average rate of the reaction.

 In a reaction between butyl chloride (C 4 H 9 Cl) and water, the concentration of C 4 H 9 Cl is 0.220M at the beginning of the reaction. At 4.00s, the concentration of butyl chloride is 0.100M. Calculate the average reaction rate as moles of C 4 H 9 Cl consumed per second.

 Take the derivative of the rate.

 In the reaction between C 4 H 9 Cl and water the rate of disappearance of C 4 H 9 Cl is the same as the rate of formation of C 4 H 9 OH because the stoichiometry is a one to one ratio.  2HI  H 2 + I 2

 In the following reaction if the rate at which O 2 appears is 6.0 x M/s what would be the rate of disappearance of O 3 ?  2O 3  3O 2

Experiment Number Initial [NH 4 + ]Initial [NO 2 - ]Initial Rate (M/s) x x x x x x 10 -7

 The rate law for most reactions have the general from:  Rate = k[reactant 1] m [reactant 2] n  m and n are called the reaction orders  The sum of m and n is called the overall reaction order.  The exponents in a rate law are sometimes the same as the coefficients in the balanced equation but must be determined experimentally.

 units of rate = (units of rate constant)(units of concentration) 2  2N 2 O 5  4NO 2 + O 2  Rate = k[N 2 O 5 ]  CHCl 3 + Cl 2  CCl 4 + HCl  Rate = k[CHCl 3 ][Cl 2 ] 1/2  H 2 + I 2  2HI  Rate = k[H 2 ][I 2 ]

Experiment number [A][B]Initial Rate( M/s) M 4.0 x M0.200 M4.0 x M0.100 M16.0 x 10 -5

 First order reactions:  A first order reaction is on whose rate depends on the concentration of a single reactant raised to the first power.  A  Products  Rate = - Δ [A]/ Δ t = k[A]  Integrated rate law  ln[A] t – ln[A] 0 = -kt  ln[A] t = -kt + ln[A] 0

 The half life of a reaction is the time required for the concentration of a reactant to drop to one half of its initial value (t 1/2 )  [A] t1/2 = ½[A] 0

 Most reactions proceed faster at higher temperatures.  One explanation of this is the collision model  Orientation factor  Activation energy:  The minimum amount of energy required to initiate a chemical reaction.

 Reacting substances must collide  Reacting substances must collide in the correct orientation.  Reacting substances must collide with enough energy to form an activated complex.

 k = Ae -Ea/RT  k = rate constant  E a = activation energy  R = J/mol-k  T = Temperature in Kelvin  A = frequency factor

Temperature ( 0 C)k (s -1 ) x x x x 10 -3

 The process by which a reaction occurs is called the reaction mechanism.  Elementary reactions  Happen in a single step.  The number of molecules that participate as reactants in an elementary reaction is called its molecularity.  Unimolecular  Bimolecular

 Multistep reaction mechanisms consist of a sequence of elementary reactions.  NO 2 + CO  NO + CO 2  The chemical equations for the elementary reactions in a multistep mechanism must always add to give the chemical equation of the overall process.

 If a reaction is an elementary reaction, then it’s rate law is based directly on its molecularity.

 If a reaction occurs as a series of elementary steps one of the steps must proceed slower than the other (or others).  This step limits the rate of the overall reaction and is called the rate limiting step.  The slowest step in a multistep reaction is the rate limiting step.