5 - Kinetics

Question 1

What is kinetics?

Answer 1

Study of factors affecting rates of chemical reactions

Question 2

What 2 conditions are needed for a reaction to take place

Answer 2

Particles colliding with enough energy to break bonds. (Energy greater than activation energy)
Sterics - collisions at the correct alignment and orientation.

Question 3

What effect does increasing temperature have on the rate of reaction and why?

Answer 3

Increase
Increases the kinetic energy of the particles therefore there is a greater frequency of successful collisions.

Question 4

What effect does increasing pressure have on the rate of reaction and why?

Answer 4

Increase
As there are more particles in a given volume collisions are more likely (frequency of collisions increases)

Question 5

What effect does increasing concentration have on the rate of reaction and why?

Answer 5

Increase
As there are more particles in a given volume the frequency of collisions increases. (Collision more likely)

Question 6

What effect does increasing surface area have on the rate of reaction and why?

Answer 6

Increase
As there are more particles (solid) available to collide with molecules (gas/ liquid) there are more sites for reaction therefore a greater frequency of collisions.

Question 7

What effect does a catalyst have on the rate of reaction and why?

Answer 7

Increase
Provides an alternative pathway with lower activation energy.
(Without being used up)

Question 8

What is activation energy?

Answer 8

The minimum energy for a collision to result in a reaction/ for bonds to break.

Question 9

What are exothermic reactions?

Answer 9

When the reactants have more energy than the products.
Thermal energy is released

Question 10

What are endothermic reactions?

Answer 10

When the products have more energy than the reactants.
Thermal energy is absorbed.

Question 11

What is the transition state?

Answer 11

The species at the top of the curve of an enthalpy diagram.
Where some bonds are being made and some broken so it has extra energy and is unstable.

Question 12

What does a concentration against rate of reaction graph look like?

Answer 12

As the reaction proceeds the reactants are used up so their concentration falls.
The rate of reaction drops as the reaction proceeds as there is less reactants to use.

Question 13

What is the Maxwell Boltzmann distribution?

Answer 13

The distribution of energy amongst particles of a gas at a certain temperature

Question 14

MBD -What is the most probable energy?

Answer 14

The peak of the distribution.
Most particles have this value.

Question 15

MBD - What is the mean energy?

Answer 15

To the right of the most probable energy.
The average energy of all the particles.

Question 16

MBD - What does the area under the curve represent?

Answer 16

The number of particles.

Question 17

MBD - Why does the distribution start at zero?

Answer 17

No particle can have zero energy

Question 18

MBD - What is on the x and y axis?

Answer 18

y - number of molecules
x - energy

Question 19

What effect does increasing temperature have on the MBD?

Answer 19

The peak lowers and shifts to the right.
Area under the curve stays the same.
Number of particles with very high energy increases.
More particles with Ea so greater proportion of successful collisions.

Question 20

What effect does decreasing temperature have on the MBD?

Answer 20

The peak rises and shifts to the left.
Area under the curve stays the same.
Less particles with Ea so proportion of successful collisions decreases.

Question 21

What effect does a catalyst have on the MBD?

Answer 21

Activation energy shifts left.

Question 22

What is a heterogenous catalyst?

Answer 22

A catalyst in a different phase to the reactants.

Question 23

How do heterogenous catalysts work?

Answer 23

Adsorption - species lands on active site, forming a weak bond with the catalyst.
Reaction - adsorbed gases held at right orientation on the surface, increasing the chance of a favourable collision.
Desorption - there is a re-arrangement of electrons and the product is released from the active site.

Question 24

Why is the strength of adsorption critical for heterogenous catalysts?

Answer 24

Needs to be strong enough to hold gases for long enough to react but weak enough to release the products.

Question 24

How do heterogenous catalyst increase the rate of reaction?

Answer 24

Adsorption allows bonds to break and fragments react quicker.
Two reactants adsorbed along side each other increases concentration.
Adsorption increases likelihood of a collision.
Reactants held at right orientation so more favourable collisions.

Question 25

How to maximise a catalyst?

Answer 25

Increase surface area by finely dividing or honeycomb/ mesh pattern to provide more collision sites.

Question 26

How could a heterogenous catalyst determine the products?

Answer 26

The distance between active sites determines the method of adsorption, affecting which bonds are weakened.

Question 27

What is poisening?

Answer 27

When impurities in a reaction mixture adsorb onto the surface of a catalyst therefore reducing potential sites, decreasing the efficiency.

Question 28

What is a homogenous catalyst?

Answer 28

Where the catalyst and reactants are in the same phase.

Question 29

How does a homogenous catalyst work?

Answer 29

The reaction proceeds through an intermediate species with lower activation energy, usually in multiple reaction steps.

Question 30

Why are transition metals used as homogenous catalyst?

Answer 30

Their ability to change oxidation state.

Question 31

What is a state?

Answer 31

Any matter’s form that exists at a specific temperature and pressure.
Can be a number of phases.

Question 32

What is a phase?

Answer 32

Any matter’s form that has a uniform set of chemical and physical properties.

Question 33

What is a zeolite?

Answer 33

An open pore structure that ions or molecules fit into.
Used in the petrochemical industry.

Question 34

What is a catalytic converter?

Answer 34

A honeycomb ceramic structure coated in platinum or rhodium metals which act as the catalyst.

Question 35

What are 2 examples of a catalyst?

Answer 35

Iron (Fe) - Haber Process
Aluminium oxide (Al2O3) - cracking hydrocarbons from crude oil.