C6

Question 1

How can the mean rate of a chemical reaction be found?

Answer 1

mean rate of reaction = quantity of reactant used / time taken
mean rate of reaction = quantity of product formed / time taken

Question 2

How can the quanity of reactant or product be measured in?

Answer 2

The quantity of reactant or product can be measured by the mass in grams or by a volume in cm^3

Question 3

What is the unit for rate of reaction?

Answer 3

The units of rate of reaction may be given as g/s or cm^3/s

Question 4

Note:

Answer 4

For the Higher Tier, students are also required to use quantity of reactants in terms of moles and units for rate of reaction in mol/s.

Question 5

Note:

Answer 5

AQA says that students should be able to:

•calculate the mean rate of a reaction from given information about the quantity of a reactant used or the quantity of a product formed and the time taken
• draw, and interpret, graphs showing the quantity of product formed or quantity of reactant used up against time
• draw tangents to the curves on these graphs and use the slope of the tangent as a measure of the rate of reaction
• (HT only) calculate the gradient of a tangent to the curve on these graphs as a measure of rate of reaction at a specific time.

(Check page 142)

Question 6

What factors affect the rate of chemical reactions?

Answer 6

Factors which affect the rates of chemical reactions include: the concentrations of reactants in solution, the pressure of reacting gases, the surface area of solid reactants, the temperature and the presence of catalysts.

Question 7

How does changing the concentrations of reactants in solution affect the rate of chemical reactions?

Answer 7

The concentration of reactants affects the rate of a reaction because increasing the concentration of reactants will increase the number of reactant particles that are available to collide and react with each other. This results in more successful collisions and a faster rate of reaction.

Question 8

How does changing the pressure of reacting gases affect the rate of chemical reactions?

Answer 8

If the pressure of gaseous reactants is increased, there are more reactant particles for a given volume. There will be more collisions and so the reaction rate is increased. The higher the pressure of reactants, the faster the rate of a reaction will be.

Question 9

How does changing the solid area of solid reactants affect the rate of chemical reactions?

Answer 9

Surface area affects the rate of a reaction because increasing the surface area of the reactants will increase the number of available sites for collision. This results in more successful collisions between reactant particles and a faster rate of reaction.

Question 10

How does changing the temperature and the presence of catalysts affect the rate of chemical reactions?

Answer 10

When temperature, pressure and surface area of reactants are increased, then the rate of reaction will also be increased. Similarly, when the concentration of reactants is increased or a catalyst is present, then the rate of reaction will also be increased. Rate of reaction can be decreased.

Question 11

Practical 11:

Answer 11

AAA

Question 12

What is the collision theory?

Answer 12

Collision theory explains how various factors affect rates of reactions. According to this theory, chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.

Question 13

What is the activation energy?

Answer 13

The minimum amount of energy that particles must have to react is called the activation energy.

Question 14

What factors increase the rate of reaction?

Answer 14

Increasing the concentration of reactants in solution, the pressure of reacting gases, and the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction.

Question 15

What are more things that increase the rate of reaction?

Answer 15

Increasing the temperature increases the frequency of collisions and makes the collisions more energetic, and so increases the rate of reaction.

Question 16

Note:

Answer 16

AQA says that students should be able to predict and explain using collision theory the effects of changing conditions of concentration, pressure and temperature on the rate of a reaction

Question 17

Predict and explain the effects of changes in the size of pieces of a reacting solid in terms of surface area to volume ratio

Answer 17

If the surface area to volume ratio of a reacting solid is increased: more reactant particles are exposed at the surface. the frequency of successful collisions between reactant particles increases. the rate of reaction increases.

Question 18

Use simple ideas about proportionality when using collision theory to explain the effect of a factor on the rate of a reaction.

Answer 18

Collision theory states that the rate of a chemical reaction is proportional to the number of collisions between reactant molecules. The more often reactant molecules collide, the more often they react with one another, and the faster the reaction rate.

Question 19

What do catalysts change?

Answer 19

Catalysts change the rate of chemical reactions but are not used up during the reaction. Different reactions need different catalysts.

Question 20

What do enzymes act as in biological systems?

Answer 20

Catalysts

Question 21

What do catalysts increase and how?

Answer 21

Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy.

Question 22

A reaction profile for a catalysed reaction can be drawn in the following form:

Answer 22

Check CGP Page 143

Question 23

Note:

Answer 23

Students should be able to identify catalysts in reactions from their effect on the rate of reaction and because they are not included in the chemical equation for the reaction.

Question 24

Explain catalytic action in terms of activation energy

Answer 24

A catalyst provides an alternative reaction pathway that has a lower activation energy than the uncatalysed reaction. This does not change the frequency of collisions. However, it does increase the frequency of successful collisions because a greater proportion of collisions now exceeds this lower activation energy.

Question 25

What are reversible reactions?

Answer 25

In some chemical reactions, the products of the reaction can react to produce the original reactants. Such reactions are called reversible reactions

Question 26

How are reversible reactions represented?

Answer 26

A + B ⇌ C + D

Question 27

How can the direction of reversible actions be changed and state an example

Answer 27

By changing conditions

                                Heat ammonium chloride ⇌ ammonia + hydrogen chloride
                                Cool

Question 28

What does it mean if a reversible reaction is exothermic in one direction and state an example?

Answer 28

If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. The same amount of energy is transferred in each case.

For examples:

                                            Endothermic hydrated copper sulfate (blue) ⇌ anhydrous copper sulfate (white) + water
                                             Exothermic

Question 29

What happens when a reversible reaction occurs in an apparatus?

Answer 29

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 30

What do the relative amounts of all the reactants and products at equilibrium depend on?

Answer 30

The conditions of the reaction

Question 31

What happens when a system is at equilibrium and a change is made to any of the conditions?

Answer 31

Then the system responds to counteract the change.

Question 32

What is Le Chatelier’s Principle

Answer 32

It predicts the effects of changing conditions on a system at equilibrium

Question 33

Note:

Answer 33

AQA says that students should be able to make qualitative predictions about the effect of changes on systems at equilibrium when given appropriate information.

(Check CGP Page 146)

Question 34

What happens if the concentration of one of the reactants or products is changed?

Answer 34

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.

Question 35

What happens if the concentration of a reactant is increased?

Answer 35

More products will be formed until equilibrium is reached again

Question 36

What happens if the concentration of a reactant is decreased?

Answer 36

More reactants will react until equilibrium is reached again.

Question 37

Note:

Answer 37

AQA says that students should be able to interpret appropriate given data to predict the effect of a change in concentration of a reactant or product on given reactions at equilibrium.

(Check page 142)

Question 38

What happens if the temperature of a system at equilibrium is increased?

Answer 38

• the relative amount of products at equilibrium increases for an endothermic reaction
• the relative amount of products at equilibrium decreases for an exothermic reaction.

Question 39

What happens if the temperature of a system at equilibrium is decreased?

Answer 39

• the relative amount of products at equilibrium decreases for an endothermic reaction
• the relative amount of products at equilibrium increases for an exothermic reaction.

Question 40

Note:

Answer 40

AQA says that students should be able to interpret appropriate given data to predict the effect of a change in temperature on given reactions at equilibrium.

Question 41

What happens if there is an increase in pressure for gaseous reactions at equilibrium?

Answer 41

An increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction

Question 42

What happens if there is an decrease in pressure for gaseous reactions at equilibrium?

Answer 42

A decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction.

Question 43

Note:

Answer 43

AQA says that students should be able to interpret appropriate given data to predict the effect of pressure changes on given reactions at equilibrium.