Collision Theory and Rates of Reaction

Question 1

What are the 4 methods of measuring rates?

Answer 1

1. Changes in pH
2. Changes in conductivity
3. Changes in mass or gas volume
4. Changes in colour

Question 2

What is the Collision Theory?

Answer 2

Kinetic Molecular Theory states that all particles move randomly due to Kinetic E. This means not all particles share the same Kinetic E, and because of this, we use averages related to absolute temperatures (increased temp means increased Kinetic E).

Question 3

What is a Maxwell - Boltzmann Distribution Diagram?

Answer 3

The range of Kinetic E is shown in graph. The area under the curve is the total number of particles. Increased temp will stretch the graph horizontally.

Question 4

Collision theory in reactants

Answer 4

Kinetic E causes reactants to collide breaking and forming bonds. The reaction depends on the number of successful collisions. Depends on E and the geometry of the collision.

Question 5

E of collision

Answer 5

Collison requires sufficient energy to overcome the electron repulsion and break bonds (endo). When Ea is reached, reactants enter a transition and then move to products as new bonds form. The rate depends on % of particles with enough Kinetic E (more E = higher rate)

Question 6

Geometry of collision

Answer 6

Particles must collide in the correct orientation in 3D space.

Question 7

Factors affecting the rate of the reaction

Answer 7

1. Temperature: high temp means more particles with the required Ea
2. Addition of a catalyst: will not be consumed, provides rxn a pathway using a lower Ea to increase rate
3. Increased concentration of particles: more particles = more collisions = higher rate
4. Particle size: increased surface area = more particles for rxn

Question 8

Heterogenous catalyst

Answer 8

Different state from reactants

Question 9

Homogenous catalyst

Answer 9

Same state as reactants

Question 10

How does the concentration of reactants affect the rate?

Answer 10

More particles mean a higher collision rate.

Question 11

What is the rate expression?

Answer 11

rate = k [A] [B] [C]

k - rate constant, varries
- exponents above concentrations are not coefficients and can only be determined through experiment

Question 12

What is a rate?

Answer 12

Speed of change per unit time. (how fast [] of product or reactant changes). Measured as:
r = delta[] / delta T

Question 13

What is the rate of reaction?

Answer 13

rate of rxn (+ve) = -1/A = -1/B = 1/C = 1/D
- products are always +ve
- reactants are always -ve

Question 14

How to determine the instantaneous rate?

Answer 14

Find the tangent of a [] v t graph.

Question 15

How to determine rate expression?

Answer 15

• Compare [] of a reactant b/w two experiments
• Use form A^m = r to find exponent (m)
• Repeat for all reactants
• Sub into rate expression (r= k [a]…) and simplify

Question 16

How to determine K in the rate expression?

Answer 16

Rearrange for K and sub in values from one experiment

Question 17

Define the different orders

Answer 17

Zero: to the exponent of 0, eliminates the term
First: to the exponent of 1, have a constant half-life
Second: to the exponent of 2
- Possibly third

Question 18

What is a half-life?

Answer 18

T (subscript)1/2. The time for the [] of the reactant to decrease by half. lower half-life = faster reaction

Question 19

What is a reaction mechanism?

Answer 19

A series of elementary steps involving a small number of particles. The sum of steps = overall rxn. Intermediate steps appear in elementary steps but not in overall reaction

Question 20

What is the molecularity of an elementary step?

Answer 20

Number of reactants in that step

Question 21

What is the RDS?

Answer 21

The slowest step. Overall reaction can only appear as fast as the slowest step. This will be given. If RDS is not in overall rxn, find the elementary step where it is produced and sub.

Question 22

What is the rate of expression for elementary steps?

Answer 22

r = k[A]^m …
- [] is raised to the power of their coefficient

Question 23

Mechanism w.r.t the RDS

Answer 23

1. Zero order reactant does not appear in the mechanism until after RDS
2. First or second order is in the RDS
3. called ‘possible mechanism’ as it can’t be proven only supported by data.

Question 24

Units of K

Answer 24

K = mol dm^-3 s^-1
Zero order = mol dm^-3 s^-1
1st order = s^-1
2nd order = mol^1 dm^3 s^-1
3rd order = mol^-2 dm^6 s^-1

Question 25

Activation Energy and Temperature

Answer 25

Temp affects rate. Large Ea = increased temp has a big affect. Small Ea = increased temp has a lower affect

Question 26

Arrhenius Formula

Answer 26

K = A e^-Ea/RT - exponential factor
- R = gas constant
- A = Arrhenius constant/frequency factor
- takes into account the frequency that successful collsisons occur based on geometry and E.
- Can remove e-factor through natural logs