Collision Theory And Catalysis

Question 1

What is a catalyst?

Answer 1

A substance that increases the rate of reaction without modifying the overall standard Gibbs energy change in the reaction

Undergoes no nit chemical change - reusable

Provides an alternative reaction pathway - most commonly lowering Ea

Question 2

What assumptions are made in the kinetic gas model?

Answer 2

Molecules are hard spheres whose motion is predicted by classical mechanics

Size of molecules is negligible and distance travelled is&raquo_space; molecule size

Only elastic collisions, molecules do not interact except during collisions

Question 3

What equation defines the root mean square speed of molecules?

Answer 3

Question 4

What is collision density (Z(AB))?

Answer 4

It is the number of collisions per unit time and volume

Question 5

What is the relationship between reaction rate and speed of molecules in collision theory?

Answer 5

They are proportional to each other

Question 6

What is Z(AB) equal to?

Answer 6

Z(AB) = ∂(AB) v(rel) Na^2 [A][B]

Question 7

What is the collision cross section of two molecules?

Answer 7

∂(AB) = 3.14r^2 = 3.14( Ra + Rb)^2

Where Ra is the radius of molecule A, a hard sphere

Question 8

What is the mean relative speed when calculating the collision density of two molecules?

Answer 8

Question 9

What is the number density of two molecules in a reaction vessel?

Answer 9

Number density of a species is the concentration multiplied by Avogadro’s constant, Na

Number density = Na[A] x Na[B]

Question 10

What does the Maxwell-Boltzmann distribution do?

Answer 10

It shows the probability of finding a particle with a given kinetic energy, depending on temperature and its molecular mass.

Question 11

What does integrating the Maxwell-Boltzmann distribution give?

Answer 11

It gives the mean relative speed, i.e. the mean speed one molecule approaches another

= Vrel

Question 12

What does collision density actually mean?

Answer 12

It is the number of collisions per unit time and volume, i.e. number of products forming per unit time and volume

Question 13

What is the collision density divided by Avogadro’s, Z(AB)/NA = ?

Answer 13

The rate constant of a reaction, k

Question 14

For products to form, what must happen?

Answer 14

Colliding reactants must have enough energy to overcome the energy barrier of the transition state.

Only some collisions lead to reactions

Question 15

What’re the key points from the Maxwell-Boltzmann distribution?

Answer 15

A fraction of molecules with a very high speed is small as exp(-v^2) becomes very smaller with large velocity.

When M is large the exponential factor goes more rapidly to zero, heavy molecules are less likely to have high speed

As T increases, expect more molecules at higher speeds

Question 16

What does f(react) = ?

Answer 16

f(react) = exp(-Ea/RT)

Question 17

What does ‘f(react) =‘ mean?

Answer 17

f(react) gives the probability that a molecule will have a specific energy (Ea)

Not every collision = reaction

Question 18

What’re the geometric and steric requirements for a reaction?

Answer 18

Molecules must collide in correct conformation, this is accounted for by P = experimental k / theoretical k

Question 19

Why does K + Br2 have a steric factor, P of 4.8 (high)?

Answer 19

Because the number of collisions have been underestimated - ignoring that the molecule don’t interact

There is a “harpoon” effect that gives rise to K+ and Br2- due to a long distance electron transfer. This coulombic attraction draws the two charged species together.

Question 20

What is the normal estimation of the steric factor P?

Answer 20

Normally P < 1, less reactions per second with correct alignment and energy than predicted value.

Question 21

Why is P sometimes &laquo_space;1?

Answer 21

This is due to a highly specific conformation required for the reaction to occur - usually for larger / more complicated reactions.

Question 22

Why would P > 1?

Answer 22

Indicates that the molecules are interacting with each other which results in more collisions.

Question 23

How does the Arrhenius equation and collision theory differ?

Answer 23

Arrhenius equation is an empirical relationship

Collision theory is derived from (mostly) fundamental parameters for gas phase I molecular reactions

Question 24

What’re the disadvantages of using collision theory? What’s better?

Answer 24

Limited to gas phase, bimolecular reactions

Experimental value of P

More accuracy given by transition state theory

Question 25

Does pre-exponential factor A of the Arrhenius equation rely on temperature? Why?

Answer 25

It depends on the square root of T. However, since a change in T has very little effect on A, we assume in the Arrhenius equation that A does not in fact change with T.