Rates Of Reaction

Question 1

What is the collision theory?

Answer 1

In order to react, particles must:

• Collide
• With a certain amount of energy (force)

Any change that increases the amount of collisions will increase the rate of reaction. Any change that increases the energy of the collisions (eg. Temperature) will have a huge effect on the rate of reaction.

Question 2

What is the effect of concentration on the rate of a reaction?

Answer 2

The rate of reaction in a more concentrated solution in much faster than that of a more dilute solution. This is because there are more solvent molecules per cm3 so there are more successful collisions per second.

Question 3

What is the effect of surface area of a solid on the rate of a reaction?

Answer 3

Only the particles on the surface of a solid can be collided and reacted with. If we cut the solid up, there will be more surface exposed (it will have a greater surface area), and there will be more successful collisions per second.

Question 4

What is the effect of temperature on the rate of a reaction?

Answer 4

A small increase in temperature can have a huge effect on the rate. When the temperature is increased, the particles have more kinetic energy ie. they move faster. This has the duel effect of:
- Increasing the frequency of the collisions.
- Increasing the energy of the collisions (increasing their success rate).
Therefore, there are many more successful collisions per second.

Question 5

What is the effect of catalysts on the rate of a reaction?

Answer 5

Collisions only result in a reaction if the particles collide with enough energy to get the reaction started. This minimum energy required is called the activation energy for the reaction. Catalysts provide an alternative way for the reaction to happen which has a lower activation energy, increasing the number of successful collisions per second.

Question 6

Describe an experiment to investigate the effect of concentration on the rate of a reaction.

Answer 6

1. Fill two test tubes with different concentrations of hydrochloric acid (one more concentrated, the other more dilute). The volume of acid, however, should be exactly the same (5cm3).
2. Put a magnesium ribbon in each test tube (same size).
3. Observe which test tube shows the faster reaction by observing the number of bubbles of gas formed. Both tubes should produce the same volume of gas, as the volumes of acid and magnesium are equal, but this should happen at different rates.
4. The rate of reaction in the more concentrated test tube should be much faster than that of the more dilute test tube.

Question 7

Describe an experiment to investigate the effect of surface area of a solid on the rate of a reaction.

Answer 7

1. Set up the equipment.
2. Fill up a measuring cylinder with water all the way, put your hand over it and invert it, placing it in the water tub and ensuring that there are no air bubbles. Insert the tube.
3. Put in the marble chips with the hydrochloric acid.
4. Start the timer. Measure the volume of gas produced every ten seconds.
5. Stop when no more gas is produced. Repeat with different-sized chips.

Question 8

Describe an experiment to investigate the effect of temperature on the rate of a reaction.

Answer 8

‘Disappearing cross’ experiment:

1. Set up the equipment.
2. Pour 25ml of hydrochloric acid into the conical flask.
3. Pour 25ml of sodium thiosulphate into the same conical flask, and start the stopwatch immediately.
4. Observe until cross is no longer visible, and stop the stopwatch.
5. Repeat with conical flask in a water bath with a different temperature.

Question 9

Describe an experiment to investigate the effect of a catalyst on the rate of a reaction.

Answer 9

The catalytic decomposition of hydrogen peroxide:
Hydrogen peroxide —> water + oxygen

1. Set up the equipment.
2. When you are ready to start the reaction, shake the flask so that the weighing bottle falls over and the manganese (IV) oxide comes into contact with the hydrogen peroxide. Start the stopwatch immediately.
3. Measure the volume of gas formed by observing the gas syringe. When it stops moving, all the hydrogen peroxide has been decomposed. Stop the timer.
4. Repeat the experiment with twice as much catalyst. The reaction should occur twice as fast, with the same volume of products being formed.

Question 10

What is a catalyst?

Answer 10

A substance that increases the rate of a reaction, but is chemically unchanged at the end of the reaction, however, they may change during the reaction. There is the same volume of it before and after the reaction.