8. Kinetics

Question 0

What are the optimum conditions for a fast reaction rate?

Answer 0

We need plenty of rapidly moving particles in a small volume.

Question 1

For a reaction to take place between two particles what conditions must be fulfilled?

Answer 1

They must bond have enough energy (more than the activation energy.
They must both be orientated correctly.

Question 2

List the factors which increase the rate of reaction

Answer 2

• Increasing the temperature
• Increasing the concentration of a solution
• Increasing the pressure of a gas reaction
• Increasing the surface area of solid reactants
• Using a catalyst

Question 3

How does increasing the temperature increase the rate of reaction?

Answer 3

This increases the speed if the molecules which in turn increases both their energy and the number of collisions.

Question 4

How does increasing the concentration of a solution increase the rate of reaction?

Answer 4

If there are more particles present in a given volume, collisions are more likely. However, as a reaction proceeds the reactants are up and their concentration falls so it means the rate drops as the reaction continues.

Question 5

How does increasing the pressure of a gas reaction speed up the rate of reaction?

Answer 5

There are more molecules or atoms in a given volume so collisions are more likely

Question 6

How does increasing the surface area of solid reactants increase the rate of reaction?

Answer 6

The greater the total surface area of a solid, the more of its particles are available to collide with molecules in a gas or liquid.

Question 7

How does using a catalyst increase the rate of reaction?

Answer 7

It can change the rate if a chemical reaction without being chemically changed itself.

Question 8

What kind of proportion of collisions result in a reaction?

Answer 8

Only a very small proportion.

Question 9

What is the minimum energy called that a molecule must have to start a reaction?

Answer 9

The activation energy

Abbreviation: Ea

Question 10

Reactions which need large activation energies take place…

Answer 10

Extremely slowly at room temperature because very few collisions will have sufficient energy to bring about a reaction.

Question 11

Reactions which need small activation energies take place…

Answer 11

Rapidly at room temperature because many collisions will have enough energy to bring about a reaction.

Question 12

What is the species that exists at the top of the curve of an enthalpy diagram called?

Answer 12

A transition state or activated complex

Question 13

Where is the activation energy measure from?

Answer 13

The activation energy is measured from the reactants to the top of the curve.

Question 14

What do we end up with if we plot a graph of ‘energy’ against ‘fraction of molecules’?

Answer 14

The Maxwell-Boltzmann distribution

Question 15

What does the Maxwell-Boltzmann distribution tell us about the distribution of energy amongst particles?

Answer 15

• no particles have zero energy
• most particles have intermediate energies - the peak of the curve
• a few have very high energies (Infact there is no upper limit)

NB: the average energy is not the same as the most probable energy

Question 16

If we mark Ea on a MB distribution what does the area to the right of the line under the graph represent?

Answer 16

The number of particles with enough energy to react.

Question 17

What does the need of an activation energy to be present before a reaction can take place explain?

Answer 17

The fact that exothermic reactions do not occur spontaneously at room temperature.

Question 18

At higher temperatures how is the shape of the Maxwell-Boltzmann distribution changed?

Answer 18

The peak of the curve is lower and moves to the right. The number of particles with very high energy increases.

Question 19

The area under the curve stays…

Answer 19

The same for each temperature.

Question 20

The total number of collisions in a given time increases a little as the particles move faster. However, this is not as important to the…

Answer 20

rate of reaction as the increase in the number of effective collisions due to a higher proportion of molecules have energy above the activation energy.

Question 21

How do catalysts work?

Answer 21

Catalysts work because they provide a different pathway for the reaction, one with a lower activation energy. Thus they reduce the activation energy that is needed to start the reaction.

Question 22

Do catalysts change the shape of the Maxwell-Boltzmann distribution?

Answer 22

No, they just lower the Ea so more molecules have enough energy to start a reaction.

Question 23

What two things do catalysts not effect?

Answer 23

The enthalpy change of the reactions

The position of equilibrium in a reversible reaction (both forward and reactions are speed up equally as much)

Question 24

How were most catalysts discovered?

Answer 24

By trial and error

Question 25

What two categories are catalysts divided into?

Answer 25

Heterogeneous catalysts

Homogeneous catalysts

Question 26

What is a heterogenous catalyst?

Answer 26

A catalyst which is in a different phase to the reactants - usually a solid catalyst and liquid or gaseous reactants

Question 27

What is a homogeneous catalyst?

Answer 27

Where catalyst and reactants are in the same phase

Question 28

See catalytic materials notes

Answer 28

This is in unit 2 as well as unit 1

Question 29

What reactions take place on catalytic converters?

Answer 29

1. The gases first form weak bonds (the strength is critical) with the metal atoms of the catalyst, this process is called adsorption. This means the gases are in just the right position to react together.
2. The products break away from the metal atoms (desorption). This frees up room on the catalyst surface for more gases to take their place and react.

Question 30

What are widely used as catalysts in the petrochemical industry?

Answer 30

Zeolites - minerals which molecules fit into which changes their structure and reactivity.

Question 31

Margarines are hardened by…

Answer 31

Bubbling hydrogen through them

Using a nickel catalyst

Question 32

CFCs were used in chemical applicants. They escaped to the upper atmosphere because they are relatively unreactive due to the strength of the carbon-halogen bond. What do the CFCs eventually decompose into?

Answer 32

Chlorine atoms which act as catalysts to break down the ozone layer which protects from ultraviolet radiation

Nitrogen monoxide acts in a similar way to chlorine atoms.