Why do some reactions happen and others dont

Why do some reactions happen and others don’t? Are the products more stable than the reactants? Thermodynamics Does the reaction go at a reasonable rate? Kinetics

What would affect how fast a reaction happens?

Control of Reactivity

Chapter 13: Chemical Kinetics Rates of Reactions How does a reaction take place? Consider: NO + O 3 NO 2 + O 2 Molecules collide Bonds are formed and break product molecules separate

So, what controls the rate of a reaction? • Number of collisions • How often they collide in a shape that allows new bonds to form • The energy of the colliding reactant molecules

Collision Theory For a reaction to take place: • Molecules must collide • They must do so in the correct orientation • They must collide with an energy greater than the “activation” energy

Concentration Dependence • It makes sense that as concentration increases, the number of collisions per second will increase • Therefore, in general, as concentration increases, rate increases • But, it depends on which collisions control the rate • So, you can’t predict concentration dependence- it must be measured experimentally

But, what do we mean by “rate? ” • In real life, rate = distance/time • This is change in position over time • In chemistry, generally change in concentration over time

Types of measured rates: • Rate over time: • Instantaneous rate: • Initial rate:

Example of rate measurement:

Rate Laws (also called Rate Equations) For the reaction: 2 N 2 O 5 4 NO + O 2 Rate = k[N 2 O 5] first order reaction For the reaction: NO 2 NO + ½ O 2 Rate = k[NO 2]2 second order reaction For the reaction: CO + NO 2 CO 2 + NO Rate = k[CO][NO 2] first order in CO and in NO 2; second order overall

Determining a Rate Law • Remember– it must be done by experiment; the reaction equation does not tell you the rate law • Two methods: Initial Rates Graphical Method

Determining a Rate Law: Initial Rate Method • Measure the rate of the reaction right at the start. • Vary the starting concentrations • Compare initial rates to initial concentrations

Determining a Rate Law: Initial Rate Method • Useful rules: Vary only one concentration at a time • If concentration doubles and: – Rate does not change, then zero order – Rate doubles, then first order – Rate quadruples, then second order • General Rule:

Initial Rate Method: Example 1 When concentration is doubled, rate increases by: Therefore reaction is second order: Rate = k[NH 4 NCO]2 Now, use one of the experiments to find the rate constant, k:

Initial Rate Method: Example 2

Initial Rate Method: Example 2 NOTICE: When [O 2] is doubled without changing [NO], the rate doubles. Therefore the reaction is first order in [O 2].

Initial Rate Method: Example 2 NOTICE: When [NO] is doubled without changing [O 2], the rate quadruples. Therefore the reaction is second order in [NO].

Initial Rate Method: Example 2 The reaction is first order in [O 2] and second order in [NO]. Rate = k[O 2][NO]2 Now we find the value of k.

Concentration-Time Relationships

Concentration-Time Relationships

Example 2. A B The reaction is first order. If [A] is initially 0. 100 M and after 18 minutes, [A] has dropped to 0. 064 M, what is k?

Graphical Method for Determining Rate Laws

Graphical Method for Determining Rate Laws A plot of concentration vs. Time will be linear. A plot of ln[R] vs. Time will be linear. A plot of 1/[R] vs. Time will be linear.

Graphical Method for Determining Rate Laws How it works: 1. Collect [R] over an interval of times. 2. Make plots of [R] vs. time ln[R] vs. time 1/R vs. time Only one will be linear. That tells you the reaction order. The slope of the linear plot is the rate constant.

Graphical Method for Determining Rate Laws Example: 2 H 2 O 2 2 H 2 O + O 2 Time(min) 0 200 400 600 800 1000 [H 2 O 2](mol/L) 0. 0200 0. 0160 0. 0131 0. 0106 0. 0086 0. 0069

Graphical Method for Determining Rate Laws Example: 2 H 2 O 2 2 H 2 O + O 2 Time(min) 0 200 400 600 800 1000 [H 2 O 2](mol/L) 0. 0200 0. 0160 0. 0131 0. 0106 0. 0086 0. 0069

Graphical Method for Determining Rate Laws time concentration

Graphical Method for Determining Rate Laws ln(conc)

Graphical Method for Determining Rate Laws Rate = k[H 2 O 2] k = 0. 0011 min-1 ln(conc)

Graphical Method for Determining Rate Laws 1/concentration Check the second order plot to be sure it doesn’t also look linear.

Half-Life • Half-Life = the time it takes for half the reactant concentration to drop to half of its original value

First Order Reaction: 2 H 2 O 2 2 H 2 O + O 2 Rate = k[H 2 O 2]; k = 1. 05 x 10 -3/min

First Order Reaction: 2 H 2 O 2 2 H 2 O + O 2 Rate = k[H 2 O 2]; k = 1. 05 x 10 -3/min

First Order Reaction: 2 H 2 O 2 2 H 2 O + O 2 Rate = k[H 2 O 2]; k = 1. 05 x 10 -3/min

First Order Reaction: 2 H 2 O 2 2 H 2 O + O 2 Rate = k[H 2 O 2]; k = 1. 05 x 10 -3/min

First Order Reaction: 2 H 2 O 2 2 H 2 O + O 2 Rate = k[H 2 O 2]; k = 1. 05 x 10 -3/min

Calculations involving Half-Life For a first order reaction: At the half-life, one half is gone, so [R]t = ½ [R]o and

Radioactive Decay Radioisotopes decay via first order reactions. Instead of concentrations, amounts are used. Measured as radioactive activity, in counts per minute (cpm) using a detector.

Radioactive Decay: Example 1 Radioactive gold-198 is used in the diagnosis of liver problems. The half-life of this isotope is 2. 7 days. If you begin with a 5. 6 -mg sample of the isotope, how much of this sample remains after 1. 0 day?

Radioactive Decay: Carbon Dating Sunlight + Nitrogen C-14 In living thing Atmospheric C-14 Sunlight + Nitrogen C-14 Dead thing Atmospheric C-14

Radioactive Decay: Example 2 The Carbon-14 activity of an artifact in a burial site is found to be 8. 6 counts per minute per gram. Living material has an activity of 12. 3 counts per minute per gram. How long ago did the artifact die? t 1/2 = 5730 years