Rates of Reaction

Question 1

Describe experiments to investigate the effects of changes in surface area of a solid, concentration of solutions, temperature and the use of a catalyst on the rate of a reaction

Answer 1

Experiment for surface area (of a solid):

1. Put magnesium (of a known mass) in hydrochloric acid
2. Time the reaction
3. Change the form of magnesium to powder, strips or wires, whilst keeping the same mass

Experiment for concentration (of solutions):

1. Put marble chips (of a known mass) into dilute hydrochloric acid
2. Time the reaction
3. Increase the concentration of hydrochloric acid

Experiment for temperature:

1. Put magnesium powder (of a known mass) into a known mass of hydrochloric acid
2. Time the reaction
3. Repeat at different temperatures

Experiment for catalyst:

1. Put manganese dioxide into hydrogen peroxide
2. This will decompose into water and oxygen, without the manganese dioxide being used up
3. Add more of the catalyst, to see if the reaction quickens

Question 2

Describe the effects of changes in surface area of a solid, concentration of solutions, pressure of gases, temperature and the use of a catalyst on the rate of a reaction

Answer 2

Reactions between solids and liquids are faster if the solids are presents as a lot of small bits rather than a few big ones. The more finely divid the solid, the faster the reaction, because the surface area in contact with the gas or liquid is much greater.
If you increase the concentration of the reactants, the reaction becomes faster. Increasing the concentration increases the number of times the particles hit each other per second.
Increasing the temperature means that the particles are moving faster, and so hit each other more often. That will make the reaction got faster but it only accounts for a small part of the increase rate.
Changing the pressure on a reaction where the reactants are only solids or liquids makes virtually no difference to the rate of reaction. But increasing the pressure on a reaction where the reactants are gases does speed it up.

Question 3

What is meant by the term “activation energy”?

Answer 3

The minimum amount of energy particles need to collide for a reaction to take place

Question 4

Explain the effects of changes in surface area of a solid, concentration of solutions, pressure of gases and temperature on the rate of a reaction in terms of particle collision theory

Answer 4

Surface area: In a big lump, liquid or gas particles can’t get at the particles hidden in the middle of the solid so the reaction is slower. If the same lump is split into smaller pieces, far fewer particles are now hidden away so there will be more collisions therefore a faster reaction.
Concentration: Solutions of higher concentration or gases at higher pressure have particles that are closer together so it’s more likely that there will be successful collisions that lead to a reaction, meaning a faster reaction.
Temperature: Particles are moving faster, so they collide more frequently. Because of this, there is going to be more successful collisions, which increases the rate of reaction
Pressure: There are the same number of particles squeezed into a smaller volume so particles will collide more frequently.

Question 5

What does a catalyst do?

Answer 5

It provides an alternative pathway which has a lower activation energy therefore speeding up the rate of reaction without being used up.