Kinetics

Question 1

What are the 5 factors that affect the rate of chemical reactions?

Answer 1

1. Temperature - Increase of temperature increases the speed of the molecules which in turn increases both their energy and the number of collisions
2. Concentration of a solution - If there are more particles present in a given volume, more collisions are likely and rate would be faster. However, as a reaction proceeds, the reactants are used up and their concentration falls, so rate of reaction will drop as reaction goes on
3. Pressure of a gas reaction - Same as concentration - there are more molecules/atoms in a given volume so collisions are more likely
4. Surface area of solid reactants - 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. This means that breaking a solid lump into smaller pieces increases the rate of reaction as more sites for reaction
5. Catalyst - A catalyst will change rate without being chemically changed itself. Increases reaction

Question 2

Definition of ‘activation energy’

Answer 2

The minimum amount of energy for particles to collude with for a successful reaction to take place

Question 3

What has to happen to ensure a collision is successful?

Answer 3

• Collisions must have energy greater than or equal to the activation energy of the reaction
• The particle orientation must be correct

Question 4

How does a catalyst work?

Answer 4

Provide an alternative reaction pathway with a lower activation energy

Therefore, reduce activation energy

Question 5

What is the definition of ‘rate of reaction’?

What are the units?

Answer 5

The measure of the amount of product formed or reactant used over time.

Units: g/s, cm³, mol/s

Question 6

What is the Maxwell-Bolzmann distribution?

Answer 6

Shows the distribution of the energies amongst the particles at a constant temperature.

Question 7

What does the area under the curve (Maxwell-Boltzmann) indicate?

Answer 7

The total number of particles present

Question 8

What to we know about how to understand Maxwell-Boltzmann distribution curve?

Answer 8

• No particles have zero energy
• Most particles have intermediate energies (about the peak of the curve)
• A few have very high energies (right hand side)
• No upper limit of energy
• Average energy is not the same as the most probable energy

Question 9

Are average energy and probable energy the same or different? (Maxwell-Boltzmann)

Answer 9

Different

Question 10

How do you show activation energy on a Maxwell-Boltzmann distribution curve?

What does the area to the right represent?

Answer 10

Area under the graph to the right of the activation energy represents the number of particles with enough energy to react

Question 11

With higher temperatures, what happens with the Maxwell-Boltzmann distribution curve?

Answer 11

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

Question 12

What happens to the area underneath the Maxwell-Boltzmann distribution curve when the temperature changes?

Answer 12

Nothing, area stays the same as represents the total number of particles

Question 13

What are the labelled axis’ on the Maxwell-Boltzmann distribution curve?

Answer 13

• Fraction of particles with energy
• Energy

Question 14

What is an another reason for using a catalyst?

(other than speeding up a reaction)

Answer 14

Cheaper than higher pressures and temperatures

(Yes catalysts may be expensive, but they are not used up)

Question 15

What would a Maxwell-Boltzmann distribution curve look like with the use of a catalyst?

Answer 15

Question 16

What do catalytic converters do?

Answer 16

Reduce the levels of a number of polluting gases

Question 17

How to catalytic converters work/how do the reactions take place on the surface?

Answer 17

• As they pass over the catalyst, the polluting gases react with each other to form less harmful products
• The gases first form weak bonds with metal atoms of the catalyst (adsorption)
• This holds the gases in just the right position for them to react together and the gases then react on the surface
• The products then break away from the metal atoms (desorption). This frees up room on the catalyst for more gases to take their place and react

Question 18

What are catalytic converts made of?

Answer 18

The catalytic converter is a honeycomb, made or a ceramic material coated with platinum and rhodium metals

Question 19

What are the reactions which take place in catalytic converters?

Answer 19

carbon monoxide + nitrogen oxides → nitrogen + carbon dioxide

hydrocarbons + nitrogen oxides → nitrogen + carbon dioxide + water

Question 20

How do CFCs destroy ozone?

Answer 20

• CFCs decompose to produce separate chlorine atoms
• Chlorine atoms act as catalyst in reactions that bring about the destruction of ozone (O₃)

O₃ + O → 2O₂

(chlorine atom catalyst on arrow)