Kinetics

Chapter 15

Introduction

• Rate of Reaction – the rate at which ________ are ___________ and __________ are ______________.

• Chemical Kinetics – the study of the ____________ of ______________.

Ways to Detect Concentration

• Titrations – (Quenching)• Spectrophotometer -

Factors that affect Reaction Rates

• Nature of the Reactants –• Concentration – • Temperature – • Presence of a Catalyst –

Rate = k[A]x[B]y

• Zero order – the concentration of the reactant _________ affect the rate of production of product. The concentration of reactant is doubled, the rate of production of product __________________.

• First order – the concentration of the reactant _________ affect the rate of production of product. The concentration of reactant is doubled, the rate of production of product __________________.

• Second order – the concentration of the reactant _________ affect the rate of production of product. The concentration of reactant is doubled, the rate of production of product __________________.

• Overall order =

Example 1

• Expt [A] [B] Rate (M/sec)1 1.0 x10-2 1.0 x 10-2 1.5 x 10-6

2 1.0 x 10-2 2.0 x 10-2 3.0 x 10-6

3 2.0 x 10-2 1.0 x 10-2 6.0 x 10-6

Example 2

• Expt [A] [B] [C] Rate(M/min)1 .20 .20 .20 2.4 x 10-6

2 .40 .30 .20 9.6 x 10-6

3 .20 .30 .20 2.4 x 10-6

4 .20 .40 .60 7.2 x 10-6

Example 3

• Expt [A] [B] Rate (M/min)1 .01 .01 6.00 x 10-3

2 .02 .03 1.44 x 10-1

3 .01 .02 1.20 x 10-2

CLR Graphs

• Concentration vs Time

• ln of Concentration vs Time

• Reciprocal of Concentration vs Time

First Order Reactions

• ln([A]0/[A]) = akt

• A0 = Initial Concentration• A = Concentration at some point• a = Coefficient of A• k = Rate Constant• t = Time

Half Life of a First Order Rxn

• t1/2 = ln2/ak

• Compound A decomposes to form B and C in a reaction that is first order with respect to A and first order overall. At 25C, the specific rate constant for the reaction is 0.0450 sec-1. What is the half life of A?

A B + C

• The reaction 2N2O5 2N2O4 + O2 obeys the rate law: rate = k[N2O5], in which the specific rate constant is 0.00840 sec-1 at a certain temperature. If 2.50 moles of N2O5 were placed in a 5.00 Liter container at that temperature, how many moles of N2O5 would remain after 1.00 minutes?

• How long would it take for 90% of the original N2O5 to react?

Second Order Rxn

• 1/[A] - 1/[A]0 = akt

• t1/2 = 1/(ak[A]0)• Compounds A and B react to form C and D in a

reaction that was found to be second order in A and second order overall. The rate constant is 0.622 liters per mole per minute. What is the half life of A when 4.10 x 10-2 M A is mixed with excess B.

• The gas phase decomposition of NOBr is second order in [NOBr] with k = 0.810 M-1sec-1 at 10C. We start with 4.00 x 10-3M NOBr in a flask at 10C. How many seconds does it take to use up 1.50 x 10-3 M of this NOBr?

2NOBr 2 NO + Br2

• Consider the reaction in the previous problem. If we start with 2.40 x 10-3M NOB, what concentration of NOBr will remain after 5.00 minutes of reaction?

Zero Order Rxn

• Rate = -1/a(D[A]/Dt) = k• t1/2 = [A]0/2ak

http://genchem1.chem.okstate.edu/CCLI/Startup.html

Collision Theory of Reaction Rates

• In order for a reaction to occur the molecules, atoms, or ions must ______________.

• In order for the collision to be successful the reactants must – A.

– B.

• How does the presence of a Catalyst help with this?

The Transition StateActivation Energy

Rate Determining Step

• A reaction can never proceed faster than its ____________ step.

• Most reactions occur not in one step, but instead in several smaller steps that include fast moving steps as well as slow moving steps.

Example 1

• NO2 + NO2 N2O4 (Slow)

• N2O4 + CO NO + CO2 + NO2 (Fast)• What are the intermediates?

• What is the rate law for this reaction?

Example 2

• NO + Br2 NOBr2 (fast)

• NOBr2 + NO 2NOBr (slow)• What are the intermediates?

• What is the rate law for this reaction?

Example 3

• I2 2I (fast)

• I + H2 H2I (fast)

• H2I + I 2HI (slow)• What are the intermediates?

• What is the rate law for this reaction?

Temperature and Reactions

• As the temperature goes up __________ molecules have the energy required.

• Arrheius Equation– k = Ae-Ea/RT

– ln k = lnA – Ea/RT• A is a constant that is equal to the # of collisions• R = 8.314

• As Ea goes up what happens to reaction speed?

• As T goes up, what happens to reaction speed?

• ln (k2/k1) = Ea/R x (1/T1 – 1/T2)• The specific rate constant, k, for the following

first order reaction is 9.16 x 10-3 sec-1 at 0C. The activation energy of this reaction is 88.0 kJ/mol. Determine the value of k at 2.0C.

• N2O5 NO2 + NO3

• The gas phase decomposition of ethyl iodide to give ethylene and hydrogen iodide is a first order reaction. At 600K, the value of k is 1.60 x 10-5 sec-1. When the temperature is raised to 700K, the value of k increases to 6.36 x 10-3 sec-1. What is the activation energy for this reaction?

Catalyst

• Homogeneous Catalyst

• Heterogeneous Catalyst

• Enzymes– Substrates