Rates of reaction

Question 1

What are the five factors which affect the rate of reaction?

Answer 1

• Surface area of solid reactants
• Concentration of reactants in a solution
• Gas pressure
• Temperature
• The presence of catalysts

Question 2

How can the yield of a reaction be calculated?

Answer 2

Yield is the actual yield compared with the theoretical yield

Question 3

What is the actual yield?

Answer 3

Proportion of product in the reaction mixture

Question 4

What is the theoretical yield?

Answer 4

The maximum amount of product possible (calculated from balanced chemical equation and mole ratio)

Question 5

Why is the actual yield less that the theoretical yield?

Answer 5

• Reaction rates

- The position of equilibrium

Question 6

Define activation energy

Answer 6

The minimum energy that must be absorbed in order to break the bonds of the reactants

Question 7

Name two factors that the activation energy depends on

Answer 7

1. The strength of the bonds

2. The number of bonds to be broken (single, double or triple covalent bond)

Question 8

What is the transition state?

Answer 8

When activation energy is absorbed, a new arrangement of atoms known as the transition state occurs. It is the maximum potential energy of the energy profile diagram.

Question 9

What is collision theory?

Answer 9

For a reaction to occur the reactant particles (atoms ions or molecules) must

• Collide with each other
• Collide with sufficient energy to break the bonds within the reaction (successful collisions) ek greater than or equal to Ea
• Collide with the correct orientation to break the bonds within the reactants.

Question 10

How is activation energy related to the rate of reaction?

Answer 10

• The size of the activation energy determines how easy it is for a reaction to occur and what proportion of collisions results in a successful reaction.
• The reaction rate is therefore dependent upon activation energy. The higher the activation energy, the lower the reaction rate.

Question 11

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

Answer 11

For a reaction to occur between substances, particles must collide with each other. BY crushing a solid that is involved in a reaction into smaller parts, more particles are present at the surface (or increases the surface area) and able to react at any moment. Thus the frequency of collision increases and so more successful collisions occur at a given time. But kinetic energy of collisions do not change, hence proportion of successful collisions remains the same

Question 12

How does increasing concentration of reactants affect rate of reaction?

Answer 12

Increasing the pressure of a gas or concentration of a solution increases the nu,bet of particles per unit volume, therefore, the frequency of collisions increases and so more successful collisions occur at a given time. However k.e of collisions does not change, hence the proportion of successful collisions remains the same,

Question 13

How does increasing the reaction temperature affect the rate of reaction?

Answer 13

As the temperature of the reaction mixture increases the average speed and the kinetic energy of the particle in the mixture increases. therefore the frequency of collisions increases and so more successful collisions occur at a given time. The proportion of successful collisions per unit time increases.

Question 14

How do catalysts affect the rate of reaction?

Answer 14

A catalyst is not consumed during the reaction. They peed up reactions by providing a new pathway with lower activation energy for the reaction.. The the lower activation energy more of the collisions that occur between particles will have sufficient energy to form products and the reaction will be more rapid. Therefore, the proportion of successful collisions per unit time increases.

Question 15

Explain catalytic activity

Answer 15

• Catalyst is a surface reactant molecules can collide with.
• When reactant molecules collide with the catalyst Ek is greater than the bond energy of the reactants, thus transition state can be achieved easily.
• Now the reactant molecules have free particles; can collide with each other to form the products.

Question 16

Which factors increase frequency of colliosns

Answer 16

• surface area

- concentration

Question 17

Which factors increase proportion of successful collisions

Answer 17

Catalyst added

Question 18

Which factors do both

Answer 18

Increased temperature

Question 19

Why does the gradient decrease over time?

Answer 19

• Initially there are more reactant particles per unit volume available in the reaction mixture, this frequency of successful collisions are greater between reactants, therefore rate (gradient) is higher
• As the reactants are consumed to form the products, probability of collisions between reactants decreases, Thus gradient becomes less steeper.
• Towards the end of the reaction there are minimum number of reactant particles per unit volume in the reaction mixture, Thus collisions between them with the correct orientation is impossible therefore reaction will be stopped but not complete

Question 20

How can you measure the rate of reaction in a colour changing experiment?

Answer 20

Change in colour can be measure with a uv visible spectrometer. concentration is directly proportional to absorbance