C6: The rate and extent of chemical change

Question 1

Mean rate of reaction =

Answer 1

_{<u>quantity of reactant used OR product formed</u>}

^{time taken}

Question 2

What are the possible units for rate of reaction?

Answer 2

• g/s
• cm³/s
• mol/s

Question 3

How would you measure of rate of reaction on a graph at a given time?

Answer 3

Calculate the gradient of a tangent to the curve at that point.

Question 4

Collision theory explains how various factors affect rates of reactions. What is it?

Answer 4

Collision theory states that chemical reactions can only occur when particles collide with each other at the right orientation and with sufficient energy (i.e. the activation energy).

Question 5

What 5 factors affect the rate of chemical reactions?

Answer 5

• concentrations of reactants if in solution
• surface area of solid reactants
• temperature
• pressure of gaseous reactants
• presence of catalysts

Question 6

Predict and explain, using collision theory, the effects of changing the concentration of a reactant on the rate of a reaction.

Answer 6

Increasing the concentration of a reactant increases the number of particles of this in a given volume.

Therefore, particles of reactant collide more frequently, so the rate of reaction increases.

Question 7

Predict and explain, using collision theory, the effects of changing the pressure of reacting gases on the rate of a reaction.

Answer 7

Increasing the pressure of reacting gases increases the number of gas particles in a given volume.

Therefore, this increases the frequency of collisions, and so increases the rate of reaction.

Question 8

Predict and explain, using collision theory, the effects of decreasing the size of pieces of a solid reactant on the rate of a reaction (still with the same mass of solid).

Answer 8

Decreasing the size of pieces of solid reactant increases the surface area to volume ratio of this solid.

Therefore, this increases the frequency of collisions and so increases the rate of reaction.

Question 9

Predict and explain, using collision theory, the effects of increasing the temperature of pieces of a solid reactant on the rate of a reaction.

Answer 9

Increasing the temperature increases the frequency and energeticness of collisions, and so increases the rate of reaction.

Question 10

investigate how changes in concentration affect the rates of reactions by a method involving measuring the volume of a gas produced and a method involving a change in colour or turbidity

Question 11

What are catalysts?

Answer 11

Chemicals which increase the rate of chemical reactions without being used up during the reaction.

Different reactions need different catalysts.

Question 12

Predict and explain, using collision theory, the effect of adding a catalyst on the rate of a reaction.

Answer 12

Catalysts provide a different pathway for a reaction with a lower activation energy.

Therefore, adding a catalyst, for a given temperature, increases the rate of reaction.

Question 13

If a reversible reaction is exothermic in one direction, it is __ in the opposite direction.

Answer 13

endothermic

Question 14

What is meant by “equilibrium” in the context of reversible reactions?

Answer 14

When a reversible reaction occurs in apparatus which prevents the escape of reactants and products, equilibrium is reached when the forward and reverse reactions occur at exactly the same rate.

Question 15

What is le Chatelier’s principle?

Answer 15

If a reversible reaction is at equilibrium, and a change is made to any of the conditions, the system responds to counteract the change.

Question 16

• Nitrogen + hydrogen ⇌ ammonia*
• The forward reaction is exothermic.*

Predict and explain the effect of increasing the temperature on this reaction at equilibrium.

Answer 16

If you change the conditions of a reversible reaction at equilibrium, the system will counteract that change.

• forward reaction is exo, so backward = endo
• increase in temp mean system wants to decrease it again
• so rate of backward (endothermic) reaction will increase
• so equilibrium will shift to the left.

Question 17

Nitrogen + hydrogen ⇌ ammonia

Predict and explain the effect of increasing the pressure on this reaction at equilibrium.

Answer 17

If you change the conditions of a reversible reaction at equilibrium, the system will counteract that change.

• mole ratio of reaction = 1 : 3 ⇌ 2 = 4 : 2
• increase in pressure means system wants to decrease it again by decreasing total no. gas molecules
• fewer gas molecules on right side
• so rate of forward reaction will increase
• so equilibrium will shift to the right.

Question 18

Hydrated copper sulfate ⇌ anhydrous copper sulfate + water

Predict and explain the effect of increasing the concentration of hydrated copper sulfate on this reaction at equilibrium.

Answer 18

If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again.

• increasing the concentration of the reactant means system wants to decrease the overall concentration
• will do this by increasing amount of product formed
• so rate of forward reaction will increase
• so equilibrium will shift to the right.