Topic 6 - The rate and extent of chemical change

Question 1

How do you define rate of reaction? And, how do you calculate the mean rate of reaction?

Answer 1

Rate of reaction is how fast reactants are changed into products.

Mean rate of reaction = quantity of reactant used or product formed / time

Question 2

What factors affect rate of reaction? Explain each in detail.

Answer 2

Temperature:
— If temperature is increased, particles gain more kinetic energy so they move faster. This means there are more frequent collisions so therefore there are more successful collisions. This shows that increasing temperature, increases rate of reaction.

Concentration / pressure of gas:

• – If the concentration of a solution is increased then there are more particles occupying the same volume of the solution. This means particles collide more frequently so there are more successful collisions.
• – If the pressure of a gas is increased it means the same number of particles occupy a smaller space therefore there are more frequent collisions which means more successful collisions.
• – Increasing concentration of a solution or pressure of a gas, increases rate of reaction.

Surface area:

• – Breaking up a solid like a powder into smaller pieces increases its surface area to volume ratio therefore for the same volume of the solid, the particles have more area to work on which leads to more frequent collisions.
• – Increasing surface area, increases rate of reaction

Presence of a catalyst:

• – Using a catalyst decreases the activation energy by providing an alternative reaction pathway.
• – Using a catalyst, increases rate of reaction

Question 3

What does it mean when a reaction is at equilibrium?

Answer 3

When a reaction is at equilibrium, the rate of the forward reaction is the same as the rate of the backward reaction but this only applies in a closed sytem. The reaction is still happening but there’s no overall effect which is known as dynamic equilibrium.

Question 4

What is Le Chatelier’s principle?

Answer 4

Le Chatelier’s principle:

— If the conditions of a reaction at equilibrium are changed then the sytem will try to counteract the change.

Question 5

What is the effect of changing concentration on the equilibrium of a reaction?

Answer 5

• – Increasing the concentration of the reactants causes the system to form more products till equilibrium is reached again.
• – Decreasing the concentration of the products causes more reactants to react so the amount of reactants is reduced.

Question 6

What is the effect of changing temeperature on the equilibrium of a reaction?

Answer 6

• – Increasing the temperature means the equilibrium will shift in the endothermic direction therefore more products are formed for the endothermic reaction and the amount of product decreases for the exothermic reaction
• – Decreasing the temperature means the equilibrium will shift in the exothermic direction and therefore more products are formed for the exothermic reaction and the amount of product decreases for the endothermic reaction.

Question 7

What is the effect of changing pressure on the equilibrium of a reaction?

Answer 7

NOTE: this only applies to gases after the equation has been balanced

• – Increasing the pressure causes the equilibrium to shift in the direction where there are fewer molecules
• – Decreasing the pressure causes equilibrium to shift in the direction where there are more molecules