3.1.5- Kinetics (PAPER 2)

Question 1

What is the collision theory?

Answer 1

If 2 particles collide, there may not be a reaction. In order for a reaction to occur, the particles must collide in the right direction and they must have energy greater than or equal to the activation energy.

Question 2

What is the difference between an effective collision and an ineffective/ inelastic collision?

Answer 2

An effective collision is a type of collision that results in a reaction whereas an ineffective collision is when the particles do not have enough energy for a reaction or do not collide with the same orientation.

Question 3

What is activation energy?

Answer 3

The minimum amount of energy required for a collision to be successful- different for different reactions as it depends on the bonds that are being broken.

Question 4

Why do most collisions not lead to a reaction?

Answer 4

Because the combined energy of the particles is lower than the activation energy or the colliding particles are in the wrong orientation.

Question 5

What is the equation for rate of reaction?

Answer 5

The amount of reactant used/ product formed divided by
time taken

Question 6

What is the definition for rate of reaction?

Answer 6

The change of concentration/ amount of reactant or product per unit time.

Question 7

What is a different definition for rate of reaction?

Answer 7

The number of successful collisions per unit time.

Question 8

How to use a tangent to work out rate of reaction?

Answer 8

Draw a line to the point

Work out change in y/ change in x

Question 9

Why is activation energy important (Ea)?

Answer 9

Bonds in a molecule stretch as they have more kinetic energy when heated. Activation energy is the difference between the reactants and the top of reaction profile- at this point the bonds have sufficient energy for them to break and for the reaction to occur.

Question 10

Draw a Maxwell–Boltzmann distribution of molecular energies in gases.

Answer 10

Energy on the bottom (x axis)
Number of molecules on left (y axis)

Curve going up quickly then gradually down.

Peak= most probable energy of a particle in a sample.

To the right of the peak= average/mean energy of the particles.

Activation energy= on the far right of the energy on the bottom (x axis)- represents the number of particles with sufficient energy to react.

Question 11

Why does Maxwell-Boltzmann distribution start at (0,0)?

Answer 11

There are no particles with zero energy.

Question 12

What does the area under the Maxwell-Boltzmann distribution represent?

Answer 12

The total number of particles.

Question 13

What does Maxwell-Boltzmann distribution show?

Answer 13

The particles of gas in a sample have different amounts of kinetic energy.

No particles have no energy.
Most particles have intermediate energies (between probable and average energy)
Few particles have high energies.

Question 14

What does the distribution curve of molecular energies in a gas show?

Answer 14

The mean energy of the molecules is greater than the most probable energy of the molecules.

Question 15

What is most probable energy?

Answer 15

The energy possessed by the maximum number of reactant molecules at a temperature T, shown as a peak on the Maxwell-Boltzmann energy distribution.

Question 16

Which 4 factors affect rate of reaction?

Answer 16

1. Temperature
2. Concentration of a solution / Pressure of a gas reaction
3. Surface area of solid reactants
4. Adding a catalyst

Question 17

Draw a Maxwell-Boltzmann distribution of higher temperature.

Answer 17

The peak is lower and shifts to the right, the line only crosses the original line once!

Particles have more kinetic energy when they are heated so a larger proportion of the molecules will have energy greater than/ equal to the activation energy.

Area under BOTH curves is the same, but area under curve beyond Ea increases with a higher temperature.

CURVE DOES NOT TOUCH X AXIS

Question 18

How can a small increase in temperature lead to a large increase in rate of reaction?

Answer 18

Many more molecules will have energy greater than or equal to the activation energy so have enough energy to react. There will be more collisions and more collisions with greater energy.

Question 19

Draw a Maxwell-Boltzmann distribution of lower temperature.

Answer 19

The peak is higher and shifts to the left, new line only crosses original line once!

Particles have less kinetic energy so a smaller proportion of the particles will have energy greater than/ equal to the activation energy, less particles taking part in the reaction.

Area under BOTH curves is the same but area under curve beyond Ea decreases with a lower temp.

Question 20

What is concentration?

Answer 20

A measure of the number of particles per unit volume- expressed in moldm-3. The greater the concentration, the higher the number of particles per unit volume.

Question 21

Draw a Maxwell-Boltzmann distribution at higher concentration/ pressure.

Answer 21

NO CHANGE TO GRAPH.

The particles are closer together and collide more often, there are more frequent collisions so a higher chance of a reaction.

More particles with energy greater than/ equal to activation energy, there is no effect on the value for most probable or average energy.

Despite the increased rate of reaction, the energy of the particles is not increased by these changes.

Question 22

Draw a Maxwell-Boltzmann distribution at half the number of particles.

Answer 22

The new line would be under the original, it would NOT cross it.

The particles are spread further apart, so there are fewer collisions between the gas particles so fewer successful collisions.

Fewer particles with energy greater than/ equal to Ea.

Question 23

How does surface area affect rate of reaction?

Answer 23

Only the particles on the surface of a solid will collide with particles of the other reactant.

Increasing SA means more particles will be on the surface and able to collide with the particles of the other reactant. There are more collisions so more successful collisions.

Question 24

How can you increase surface area?

Answer 24

By decreasing the size of the reactant particles- eg small marble chips will produce more gas than large marble chips in the same amount of time.

Powdered is smaller than granuled.

Question 25

What is a catalyst?

Answer 25

A substance that increases the rate of a chemical reaction without being changed in chemical composition or amount by providing an alternative reaction route of lower activation energy.

Question 26

How do catalysts increase rate of reaction?

Answer 26

More molecules are able to meet the activation energy so there are more successful collisions per unit time and the reaction happens faster. Activation energy on Maxwell-Boltzmann distribution shifts to the left.

They can also be used to lower the temperature required for a reaction, saving energy and money.

Question 27

Why do catalysts not affect enthalpy change or equilibria?

Answer 27

They increase the rate of reaction of both the forward and reverse reactions.

Question 28

What are the 2 types of catalysts?

Answer 28

Heterogenous and homogenous.

Question 29

What are heterogenous catalysts?

Answer 29

Catalysts which are in a different phase to the reactants. Catalyst is usually solid and reactants are liquids and gases.

Eg- solid catalysts for gas reactions in catalytic converters.

Question 30

What are homogenous catalysts?

Answer 30

Catalysts which are in the same phase as the reactants. Catalyst and reactants are usually liquids.

Eg- hardener added to fibreglass resin.

Question 31

What is a good example of a catalyst?

Answer 31

Zeolite catalyst

Has a large surface area due to its micropores, the reaction can occur faster.

Question 32

What is the full method of how a catalyst works?

Answer 32

1) The gases form weak bonds with the metal atoms of the catalyst through adsorption- this holds the gases in the right place for them to react.
2) The product breaks away from the metal atoms through desorption- freeing up room on the catalyst surface for more gases to react.
3) The bonds holding the gases on the metal surface need to be strong enough to hold them for a long time but weak enough to release the products.

Question 33

How to measure rate of reaction by how much precipitate is formed?

Answer 33

Place a cross on paper and time how long it takes for the cross to disappear/ precipitate to form.

It may be difficult to determine when the cross disappeared- use the same observer to minimise errors.

Question 34

How to measure rate of reaction by how much mass is lost?

Answer 34

For reactions that produce a gas- place reaction on balance and measure the mass lost as gas is lost.

This is accurate but a fume cupboard should be used if the gas is harmful or toxic.

Question 35

How to measure rate of reaction by the volume of gas produced?

Answer 35

Measure the amount of gas produced using a gas syringe over a period of time at regular intervals.