Physical - Kinetics

Question 1

What is reaction rate?

Answer 1

• the rate of a chemical reaction is a measure of how fast a reaction takes place
• it is defined as the change in concentration of a reactant or product per unit time

Question 2

What is collision theory?

Answer 2

• for a reaction to take place between two particles, they must collide with enough energy to break bonds.
• the collision must also take place between the parts of the molecule that are going to react together, so orientation also has a part to play.
• to get a lot of collisions, you need a lot of particles in a small volume, and for the particles to have enough energy to break bonds, they need to be moving fast.
• for a fast reaction rate you need plenty of rapidly moving particles in a small volume.

Most collisions between molecules or other particles do not lead to reaction. They either do not have enough energy, or they are in the wrong orientation.

Question 3

What are the factors that affect the rate of a chemical reaction?

Answer 3

• temperature
• concentration of a solution
• pressure of a gas reaction
• surface area of solid reactants
• use of catalyst

Question 4

How does increasing the temperature increase the rate of reaction?

Answer 4

The higher the temperature, the faster the reaction. When a substance is heated, thermal energy is transferred to it. This energy is converted to kinetic energy and the molecules of the substance move faster and further. Particles have more energy and so a greater proportion of the collisions are successful. The particles also move faster and so collisions are more frequent. Therefore there are more frequent successful collisions.

Increasing temperature gives a more significant increase in the rate of reaction than concentration.

Question 5

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

Answer 5

The higher the concentration, the faster the reaction (unless the reagent is zero order). If there are more particles present in a given volume, then collisions are more likely and the reaction rate would be faster. However, as a reaction proceeds, the reactants are used up and their concentration falls. So, in most reactions, the rate of reaction drops as the reaction goes on.

Changing the concentration does not affect the position of the most probable energy or the activation energy, it just means more or less particles will have that energy.

Question 6

How does increasing the pressure of a gas reaction increase the rate of reaction?

Answer 6

This has the same effect as increasing the concentration of a solution - there are more molecules or atoms in a given volume so collisions are more likely.

Question 7

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

Answer 7

The greater the total surface area of a solid, the more of its particles are available to collide with molecules in a gas or a liquid. This means that breaking a solid lump into smaller pieces increases the rate of its reaction because there are more sites for reaction, and so there are more frequent successful collisions.

Question 8

How does using a catalyst increase the rate of reaction?

Answer 8

A catalyst is a substance that can change the rate of a chemical reaction without being chemically changed itself. It provides an alternative route/mechanism with a lower activation energy and therefore a greater proportion of the collisions are successful.

Question 9

What is the activation energy?

Answer 9

Only a very small proportion of collisions actually result in a reaction.

For a collision to result in a reaction, the molecules must have a certain minimum energy, enough to start breaking bonds. The minimum energy needed to start a reaction is called the activation energy (Ea).

Question 10

What is a transition state?

Answer 10

The species that exists at the top of the curve of an enthalpy diagram is called a transition state or activated complex. Some bonds are in the process of being made and some bonds are in the process of being broken. If there is not enough energy, the activated complex is not formed.

Question 11

What is the Maxwell-Boltzmann distribution?

Answer 11

The particles in any gas (or solution) are all moving at different speeds - a few are moving slowly, a few very fast but most are somewhere in the middle. The energy of a particle depends on its speed so the particles also have a range of energies.

If you plot a graph of energy against the fraction of particles that have energy, you get a curve with a particular shape called the Maxwell-Boltzmann distribution.

Question 12

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

Answer 12

• the particles in substance do not all gave the same amount of energy
• no particles have zero energy
• most particles have intermediate energies (around the peak of the curve)
• a few have very high energies (right hand side of the curve)
• there is no upper limit
• the peak of the curve gives the energy that more particles have than any other energy
• the average energy is not the same as the most probable energy (ae is on the right side of the mpe)
• the area under the curve represents the total number of particles, and this area never changes unless concentration changes

Question 13

What is the activation energy on the Maxwell-Boltzmann curve?

Answer 13

If you mark Ea on the Maxwell-Boltzmann distribution graph, then the area under the graph to the right of the activation energy line represents the number of particles with enough energy to react. Often only a small proportion of molecules have energy greater than or equal to the activation energy.

Question 14

Why don’t all reactions that are exothermic occur spontaneously at room temperature?

Answer 14

because of the need for the activation energy to be present before a reaction takes place

Question 15

What happens when particles collide with each other?

Answer 15

When particles collide with each other they gain or lose energy, and so the energy of each individual particle is constantly changing. This means that a particle that does not have enough energy to react may gain enough in a collision so that it can react in a further collision.

Question 16

How does temperature affect the shape of the Maxwell-Boltzmann graph?

Answer 16

• as the temperature increases, the average energy of the particles increases and so the distribution curve changes
• the total area under the curve stays the same as there are the same number of particles
• the peak of the curve is lower and moves to the right, as the number of particles with higher energy increases but less particles will have that energy
• a small increase in temperature has a significant increase in the number of molecules
• both curves should be asymptotic on close inspection

Question 17

How does the number of collisions change with an increase in temperature?

Answer 17

• total number of collisions increases because particles move faster
• but more importantly, the number of effective collisions increases because more particles will have the activation energy required, increasing the rate of reaction

Question 18

What are catalysts?

Answer 18

Catalysts are substances that affect the rate of chemical reactions without being chemically changed themselves at the end of the reaction. Catalysts are usually used to speed up reactions so they are important in industry. It is cheaper to speed up a reaction by using a catalyst than by using high temperatures and pressures. This is true, even if the catalyst is expensive, because it is not used up.

Question 19

How do catalysts work?

Answer 19

Catalysts work because they provide a different pathway for the reaction, one with a lower activation energy. Therefore they reduce the activation energy of the reaction (the minimum amount of energy that is needed to start the reaction).

Question 20

What are catalytic converters?

Answer 20

All new petrol-engine cars are now equipped with catalytic converters in their exhaust systems. These reduce the levels of a number of polluting gases.

The catalytic converter is a honeycomb, made of a ceramic material coated with platinum and rhodium metals - the catalysts. The honeycomb shape provides an enormous surface area, on which the reactions take place, so a little of these expensive metals goes a long way.

As they pass over the catalyst, the polluting gases react with each other to form less harmful products.

Question 21

How do the reactions take place on the surface of the catalyst?

Answer 21

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

The strength of the weak bonds holding the gases onto the metal surface is critical. They must be strong enough to hold the gases for long enough to react, but weak enough to release the products easily.

Question 22

What are zeolites?

Answer 22

Zeolites are minerals that have a very open pore structure that ions or molecules can fit into. Zeolites confine molecules in small spaces, which causes changes in their structure and reactivity. Synthetic zeolites are widely used as catalysts in the petrochemical industry.

Question 23

How do catalysts bring about the destruction of ozone?

Answer 23

Until recently, a group of apparently unreactive compounds called chlorofluorocarbons were used for a number of applications. They escaped high into the atmosphere where they remain because they are so relatively unreactive. This is partly due to the strength of the carbon-halogen bonds.

CFCs do eventually decompose to produce separate chlorine atoms. These act as catalysts in reactions that bring about the destruction of ozone. Ozone is important because it forms a layer in the atmosphere of the Earth that acts as a shield. The layer prevents too much ultraviolet radiation from reaching the Earth’s surface.

O3(g) + O(g) —–(chlorine atom catalyst)–> 2O2(g)

Nitrogen monoxide acts as a catalyst in a similar way to chlorine atoms.

Question 24

In what two ways does a catalyst increase the rate of a reaction?

Answer 24

A catalyst increases the rate of a reaction in 2 ways:

• it can change the orientation of a particle to make a successful collision more likely
• it can react with one of the reactants to form an intermediate which then reacts to make the final product - this chemical intermediate is sometimes called an activated complex

Question 25

Do catalysts lower the activation energy of a reaction?

Answer 25

No, they provide an alternate pathway for the reaction with a lower activation energy.

Question 26

Why do transition metals make good catalysts?

Answer 26

because they can provide an alternate route by changing the oxidation state very easily