TOPIC 9 - Kinetics I

Question 1

According to collision theory, what two things must happen for a reaction to occur?

Answer 1

Particles must collide

• in the right direction.
• with a minimum amount of kinetic energy.

Question 2

What is he rate of reaction?

Answer 2

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

Question 3

What is activation energy?

Answer 3

The minimum amount of energy required for a reaction to occur.

Question 4

What does the Maxwell-Boltzmann distribution show?

Answer 4

The distribution of energy in gas particles.

Question 5

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

Answer 5

No particles have 0 kinetic energy.

Question 6

What is A?

Answer 6

The most likely energy of a particle.

Question 7

What is B?

Answer 7

The mean energy the particles have.

Question 8

What is C?

Answer 8

Activation energy

Question 9

In a Maxwell-Boltzmann graph, what does the area under the curve represent?

Answer 9

Total number of molecules

Question 10

How does the Maxwell-Boltzmann graph change when you increase temperature?

Answer 10

• Curve shifts to the right.
• Peak is lower.
• Area under the curve increases.
• Area beyond the activation energy increases.
  *

Question 11

How does the Maxwell-Boltzmann curve change when temperature is lowered?

Answer 11

• Curve shifts to the left.
• Peak is higher.
• Area under te curve is the same.
• Area beyond activation energy decreases.

Question 12

Why does increasing the temperature increase the rate of reaction?

Answer 12

Particle have higher kinetic energy at higher temperatures.

Particles move faster so there will be more collisions per second and they will be more energetic.

The combination of more collisions and more energetic collisions increases the rate of successful collisions.

Question 13

How does increasing pressure increase the rate of reaction?

Answer 13

Particles are closer together and therefore collide more often.

If collisions are more frequent, there’s an increase chance of successful collisions happening.

Question 14

How does increasing concentration increase rate of reaction?

Answer 14

Particles are closer together so collide more often.

Therefore there’s a higher chance of a successful collision happening.

Question 15

What is a catalyst?

Answer 15

A substance that increases rate of reaction by providing an alternative pathway that has a lower activation energy.It is chemically unchnaged at the end of the reaction.

Question 16

What is an heterogeneous catalyst?

Answer 16

A catalyst that is in a different phase (state) from the reactants.Increasing its surface area will increase the rate of reaction.

Question 17

What is an homogeneous catalyst?

Answer 17

A catalyst that is in the same phase as the reactants. They from an intermediate with the reactants to form the products. The catalyst is then reformed.

Question 18

How do heterogeneous catalysts work?

Answer 18

1. Reactants bond to the surface of the catalyst (Adsorption)
2. Bonds in the reactants weaken and break to form radicals (unpaired electron), which then react with each other to make the product.
3. The products are then released from teh catalyst (desorption).

Question 19

How do both heterogeneous and homogeneous catalysts affect the Maxwell-Boltzmann curve?

Answer 19

Lowers the activation energy so area beyond the activation energy increases.

Question 20

How do heterogeneus catalyst affect the energy profile diagram?

Answer 20

Lower the activation energy.

Question 21

How do homogeneous catalysts affect the energy profile diagram?

Answer 21

There’s a lower activation energy to form the intermediates, then there’s a lower activation energy to form the final products.

Question 22

What are the advantages of using catalysts?

Answer 22

• Lower temperature and pressure needed: less money spent on energy + CO2 emissions.
• Speeds up rate of reaction: more products are made in a given amount of time.
• Can change properties of product i.e. Ziegler-Natta catalyst turns poly(ethane) to a more dense and rigid plastic.
• Can improve atom economy (so more efficient).

Question 23

How do catalytic converters in cars work?

Answer 23

Are made from a rhodium, platinum and palladium alloy (heterogeneous). they catalyse the following reaction:

2CO + 2NO > 2CO2 + N2

Question 24

How can rate of reaction be measured?

Answer 24

• Time taken for precipitate to form: Place a cross below the beaker and measure time taken for the cross to dissappear.
• Amount of mass lost: For reactions that produce a gas. Put cotton wool to avoid splashes. Use fume cupboard if gas is toxic.
• Measure volume of gas produced with a gas syringe.

Question 25

How can you calculate the initial rate of reaction from a graph?

Answer 25

Draw a tangent at time 0.

Calculate the gradient of the tangent.

Question 26

When measuring the amount of hydrogen gas given off, which mehod is preferred, measuring the mass or using a gas syringe? Why?

Answer 26

Using a gas syringe because the Mr of hydrogen gas is very small so a little decrease in mass would be detected.