Reaction Rates

Question 1

4 factors that affect the rate of a reaction?

Answer 1

Increasing Temperature
Increasing Concentration
Increasing Surface Area ( Decreasing Particle Size )
Use of a Catalyst

Question 2

What is Collision Theory?

Answer 2

Collision theory states that before a reaction can take place, the particles must collide with each other. Collision theory can be used to describe the effect of the 4 factors effecting the rate of reaction.

Question 3

How does increasing surface area increase the rate of reaction?

Answer 3

Collisions happen at the surface. Decreasing the particle size of any reactant results in an increase in surface area. The greater the surface area, the greater number of successful collisions, and the faster the rate of reaction.

Question 4

Average Rate Formula

Answer 4

change in quantity/change in time = __ (cm^3/g-1) s-1

Question 5

Why does increasing the concentration increase the rate of reaction?

Answer 5

As the concentration increases the number of reactant particles increases. The particles will therefore be closer together, and more likely to collide. More collisions mean more successful collisions, therefore the reaction rate increases.

Question 6

Why does the rate of reaction decrease as time goes by?

Answer 6

As the reaction proceeds, reactants are changed to products. At the beginning of a reaction, there are more reactants, meaning a fast rate. As these are used up, the rate of reaction decreases.

Question 7

What is the Relative Rate?

Answer 7

The relative rate of a reaction is the rate at any particular point in time.

Question 8

What is the relative rate formula?

Answer 8

relative rate = 1/time
units = s-1

Question 9

What is enthalpy?

Answer 9

Enthalpy (H) is the measure of the energy stored in a chemical

Question 10

What is an exothermic reaction?

Answer 10

Where chemicals’ potential energy is changed into heat energy, e.g. combustion, neutralisation.

Question 11

What is an endothermic reaction

Answer 11

Where heat energy is changed into chemical potential energy. The energy stored in the chemical increases, e.g. photosynthesis

Question 12

Enthalpy change (ΔH) formula

Answer 12

ΔH = H products - H reactants

Question 13

ΔH in exothermic reactions

Answer 13

ΔH is negative for an exothermic reaction. The unit is KJmol-1.
In an exothermic reaction, the products have less energy than the reactants, exothermic reactions give out energy to the surroundings. In an exothermic reaction the temperature inside the reactant vessel rises.

Question 14

ΔH in endothermic reactions

Answer 14

ΔH is positive for an endothermic reaction. The unit is KJmol-1.
In an endothermic reaction, the products have more energy than the reactants, endothermic reactions take in energy from the surroundings. In an endothermic reaction, the temperature inside the reaction vessel drops:
- lowering the temperature can have the effect of slowing down the rate of reaction

Question 15

What is the activation energy?

Answer 15

The activation energy (Ea) is the minimum kinetic energy required by colliding molecules for a reaction to take place, only particles with energy above this level have the potential to collide. The higher the activation energy (Ea), the higher the ‘energy barrier’ and the slower the reaction.
The activation energy (Ea) is also the energy needed by colliding particles to form the activated complex.

Question 16

In a potential energy diagram, what can the ΔH be observed as?

Answer 16

The difference between products and the reactants.

Question 17

When is an activation complex formed?

Answer 17

As a reaction proceeds from reactants to products, an intermediate state is reached at the top of the activation barrier at which a highly energetic species called an activated complex is formed.
This unstable activation complex only exists for a short period of time. From the peak of the energy barrier, it can lose energy by either forming stable products or forming reactants again.

Question 18

what conditions are necessary for particles to collide?

Answer 18

sufficient energy and also the correct geometry.

Question 19

What is temperature?

Answer 19

Temperature is a measure of the average kinetic energy of the particles in a chemical.

Question 20

In a reaction, as the temperature increases, what happens to the number of particles with enough energy greater than the activation energy?

Answer 20

The number of particles with energy greater than the activation energy increases, even a small change in temperature causes a large change in rate.

Question 21

What is the effect on the number of particles with enough energy to collide when increasing the concentration?

Answer 21

As you increase the concentration the number of particles with sufficient energy to collide increases.

Question 22

What is the definition of a catalyst?

Answer 22

A substance that increases the rate of a chemical reaction without itself undergoing a permanent change.

Question 23

What does a catalyst do?

Answer 23

A catalyst offers an alternative pathway to products and lowers the activation energy for a reaction which makes it easier for an activated complex with lower energy to form. A catalyst also helps in the formation of the activation complex by helping the particles collide with the correct geometry. A catalyst increases:
- the particles with energy more than or equal to the Ea
- the rate of successful collisions
- the reaction rate

Question 24

Why does a small increase in temperature cause a large increase in rate?

Answer 24

Because more particles have sufficient energy to react