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KINETICS PACKET

Collision Theory & Catalysts

Collision Theory ANIMATION Ad wes CD Rom 19.22

What information do reaction rates give you?

What needs to happen for a rxn to occur?

If all that was needed was a collision, then the rxn rate for rxns in liquids or gases would be almost instantaneous, but they

are not. Very few collisions produce products.

The rxn rate is proportional to the number of effective collisions (those that yield products).

Collision theory explains why rates vary.

According to collision theory,

If a change occurs that affects the rate of collisions, it will in turn affect the reaction rate.

How does concentration affect the collision rate?

Use collision theory to explain why surface area changes rxn rate.

Properties of collision:

ANALOGIES

What energy changes occur when a tennis ball hits a wall & bounces back?

Is the tennis ball changed?

Many collisions between molecules are perfectly elastic, which means…

What is the difference in energy & changes between a fender bender vs wrapping a car around a tree?

Same phenomenon is true for collisions between molecules, if the combined KE of the colliding molecules is high enough

the result is a change in the molecular arrangement to produce products.

What force must be overcome in order for molecules to come in contact w/ one another & collide?

Can molecules collide in any orientation to produce products?

In order for a collision to result in products two requirements must

be met

1)

2)

Eg. NaOH + HCl

Collisions & Energy

Activation Energy:

If the energy of the collision is at least that of the activation energy, then

some of the KE is changed in to PE as the bonds rearrange.

ANALOGY a ball ‘rolling” over a hill,

If ball does not have enough KE, then the ball does not make it to the

peak & falls back down. Similarly if the colliding molecules do not have

enough KE they will “fall back down” to reactants again.

If the ball has enough KE, then it will reach the peak & roll over the hill.

If the colliding reactants have enough KE & correct orientation, then

they will form an activated complex at the peak, where some of the

product bonds have formed, but not all reactant bonds have been broken.

The rxn will then progress toward the products if there is enough energy

to break the old bonds.

Activated Complex or Transition state:

Interpreting the energy graphs:

Exothermic rxn:

What happens to the PE that is ‘lost’?

Endothermic rxn:

Rate of rxn does NOT depend on the energy difference btw reactants & products, but depends on the activation energy &

# of molecules possessing that amount of KE.

For an endothermic rxn, the greater the H between the reactants & products the greater the value of Ea,

If a rxn is exothermic, then the reverse rxn is endothermic & less likely because of the Ea required for the reverse rxn.

Effect of Temperature What happens to the curve if T increases?

An increase in temperature causes the curve to shift so that more of the curve

is to the left of Ea, therefore there is a greater area under to curve that meets

the energy requirement for the reaction to occur. Thus the number of

molecules that can react (reacting fraction) is higher.

Use collision theory to explain why T changes rxn rate.

How does a refrigerator slow food spoilage?

Arrhenius came up with an equation to relate the rate constant to the temp of rxn. Equation includes activation energy & the

frequency of the collisions that have the proper orientation. Using this equation, you can solve for values of k that are very similar

to those obtained experimentally which provides support for collision theory. We will NOT be using the equation.

Catalysts Figures from Zum 12.15, 12.16

Why do catalysts cause the rxn rate to increase?

Catalysts do not change the temperature of a rxn or the energy

difference btw reactants & products.

Heterogeneous catalysts:

Homogeneous catalysts:

From Brady

Factors Affecting Rxn Rates, Rates, & Rate Laws

Rate:

Eg.

Factors that affect rxn rate Temperature:

reactants nature:

Reactant ability to come in contact w/ each other

Concentration:

Surface Area:

Eg. lighting a fire w/ kindling, grain elevator explosion

State of matter:

Eg. liquid vs vapor gasoline rxn w/ O2

Catalyst:

Why do we study rates? To determine how a rxn occurs & the key “players” in the rxn

Eg. Ozone. The reaction between freon & O3 is being blamed for partial depletion of the protective ozone layer.

The steps of the reaction occur as follows

CCl2F2 CClF2 + Cl

Cl + O3 ClO + O2

ClO + O Cl + O2

We study rxn rates for info about rxn mechanics. We can determine the interaction between components and occur what reacts

first and which has the greatest effect on the rate.

Calculating rates: Rates for rctnts & prod are not the same but are related through stoichiometry. If you have data that allows you to calculate the rate of one ‘player’ then using the balanced chemical equation you can determine

the rate of the rest of the species.

Eg. 1 Continue w/ [HI] data. Determine the rate for H2, I2

Eg. 2 N2 + 3H2 2NH3 If the initial rate of NH3 formation is 1.15 M/h, what is the rate of H2 disappearance?

Eg. 3 2C4H10 + 13O2 8CO2 + 10H2O The initial rate of disappearance of butane formation is 0.20M/s, calculate the initial rate

of disappearance of O2.

Collecting Data to determine rxn rates Rxn rate: progress of rxn

DEMO Eg. Mg + HCl How can you find the rate?

DEMO Eg. [ ] aq color Al + CuSO4

Using a data table, how do you find the rate of the rxn?

RATE IS ALWAYS EXPRESSED AS A POSITIVE

VALUE

The sign of a rate can be an indication of formation or

disappearance of a species, but we don’t typically include the

sign.

[ ] / t provides the average rate over a given

interval. We usually want instantaneous rate.

How do you find the instantaneous rate?

FIGURE 14.1 HEATH pg 572

What does the slope of the graph indicate about the rate?

Think about how rxn occurs.

As the rxn continues [rctnt] increases/decreases so the chances of hitting each other increases/decreases so the rxn rate

increases/decreases.

H2 is a product of the rxn above. Predict what a graph of Mg disappearance would look like. Draw it below.

On a graph when is average rate equal to instantaneous?

What is the graph depicting?

Determine the instantaneous rate at 100 s.

Compare this value to the average rate calculated from data table.

How do you calculate the initial rxn rate for the rxn?

Rate Laws & Order of Rxn What affects the [ ] of a species in a rxn?

When we are investigating rates we need to get rid of this complication/confusion. Chemists set up conditions so that the rxn

occurs in one direction. Rxn between products needs to be negligible. The rate then is only affected by the reactants.

The rate of a reaction USUALLY depends on the concentration of the reactants. Therefore, as a reaction proceeds, the

concentration of the reactants decline, and thus the rate declines. For MOST reactions, rate varies with time and approaches zero

as equilibrium is approached. The rate is largest at the start of the reaction, and it is called the initial reaction rate. The value of

the initial reaction rate is obtained as the slope of the line tangent to the concentration vs. time curve at t=0.

ANALOGY: cube & variables related to length of a side (l)

The only way to determine the relationship btw rate & [rctnt] is by experimentation!!

Eg. 4 SO2Cl2(g) SO2(g) + Cl2(g)

Initial [SO2Cl2] initial rate formation [SO2]

1 0.100 2.2 X 10-6

2 0.200 4.4 X 10-6

3 0.300 6.6 X 10-6

The Rate Law is a written expression of the relationship between [reactants] & rate of rxn, initial [ ] is usually used

Rate = k [ ]n

k & n can only be determined through experimentation

k:

Rate law =

n: rxn order for that rct, notice relation btw [ ] & rate is NOT obtained from the balanced chemical equation

Calculate the value & units for k.

Eg 5

Process for determining reaction order for reactants

Determining the order for each reactant using experimental data. Evaluate the data by looking at the ratio between the two [ ] & the

rates.

Eg. 6 Determine the rate law & the value for the rate constant for the experiment below.

Initial [A] initial [B] initial rate products

1 0.10 0.10 0.20

2 0.20 0.10 0.40

3 0.30 0.10 0.60

4 0.30 0.20 2.40

5 0.30 0.30 5.40

Rate laws only include those [rctnt] that influence the reaction, not all rctnts do

Another way of thinking….

Rate 1 = k[A]n[B]m

Rate 2 = k[A]n[B]m

Sum of exponents is order for entire rxn

Eg. 7 If the rate law for a reaction is Rate = k[A] [B]2 then the overall rxn order is

Exponents can be fractions & negatives (belongs in the denominator).

What does a negative exponent mean for the rate?

What does an exponent of 0 mean with regard to the rate?

Eg. 8

Eg. 9

Eg. 10 2A + 2B C Determine the rate law & rate constant.

Initial [A] Initial [B] rate (M/s)

0.10 0.20 300

0.30 0.40 3600

0.30 0.80 14460

Quick way to determine the units on rate constant M[1-(m+n+..)]

/ t