C6 The rate and extent of chemical change

Question 1

How can the rate of a chemical reaction can be found.

Answer 1

It can be found by measuring the quantity of a reactant used over time (divided by time taken to change) or the quantity of product formed over time (divided by time taken to change)

Question 2

Write an equation for the rate of a chemical reaction.

Answer 2

mean rate of reaction = quantity of reactant used/

time taken

Question 3

What two units can be used for the rate of a reaction. Explain what each symbol stands for.

Answer 3

The units of rate of reaction may be given as g/s or cm³/s.

g = grams

s = time in seconds

cm³ = volume in centimeters cubed

Question 4

For the Higher Tier, students only.

How can you express rate in terms of moles.

Answer 4

The units for rate of reaction in terms of moles is mol/s.

Question 5

What Factors affect the rate of chemical reactions.

Answer 5

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 6

In what way does temperature affect the rate of a chemical reaction.

Answer 6

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

Question 7

Explain how changing the concentration of reactants the pressure of reacting gases and the surface area of solids affects the rate of a chemical reaction.

Answer 7

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 8

Explain what is meant by surface area.

Answer 8

The area of a surface which is exposed.

Question 9

Give an example where a reaction is faster when the surface area is increased.

Answer 9

A specific mass of calcium carbonate ( marble chips). reacts with hydrochloric acid at a much slower rate than the same mass of marble chips which has been crushed into a powder.

Question 10

How would you measure the rate of the reaction between Zinc and HCl.

Answer 10

Measure the volume of gas collected every 20 seconds in a gas syringe.

Question 11

How would you measure the rate of the reaction between sodium thiosulphate and HCl.

Answer 11

Place HCl into a conical flask, place a cross underneath the flask add the Sodium thiosulphate and start the clock. Record the time it takes for the cross to no longer be visable, repeat this for each variable.

Question 12

What is minimum activation energy.

Answer 12

It is the minimu amount of energy that particles must posses when they collide for a reaction to happen.

Question 13

Explain why a gate made from Iron spindles rusts faster then a gate made from soild iron.

Answer 13

The spindles have more surface area.

Question 14

What are catalysts ?

Answer 14

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

Enzymes act as catalysts in biological systems.

Question 15

What are enzymes ?

Answer 15

Enzymes act as catalysts in biological systems.

Question 16

Give an example of an enzyme ?

Answer 16

Amylase is an enzyme that breaks down starch into glucose

Question 17

How do catalysts work ?

Answer 17

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

Question 18

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

Answer 18

Question 19

What are reversible reactions.

Answer 19

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

reversible reactions.

Question 20

How can a reversible reaction be represented ?

Answer 20

The reversible sign is the last equation on the card.

Question 21

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

Answer 21

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

Question 22

Exothermic reactions……………………….heat

Answer 22

Give out

Question 23

Endo thermic reactions………………………………….. Heat

Answer 23

Take in heat.

Question 24

An exothermic reaction is represented by………?

Answer 24

an negative sign in front of the delta H

►

Question 25

An endothermic reaction is represented by………?

Answer 25

Put a positive sign in front of the delta H

Question 26

If you increase the temperature for an exothermic reaction which way will the reaction speed up ?

Answer 26

The backward reaction will speed up..

Question 27

If you increase the temperature for an endothermic reaction which way will the reaction speed up ?

Answer 27

The forward reaction will speed up.

Question 28

If you increase the temperature for an exothermic reaction how will it affect the yield of product. ?

Answer 28

The product yield decreases.

Question 29

If you increase the temperature for an endothermic reaction which way will the reaction speed up ?

Answer 29

The product will increase.

Question 31

Higher tier only

What are two disadvantages of carrying out a reaction at a high temperature, if the reaction is exothermic ?

Answer 31

There will be a low yield of product and it is expensive to generate the extra heat required.

Question 32

Higher tier only

What are two disadvantages of carrying out a reaction at a low temperature, if the reaction is endothermic ?

Answer 32

The yield will be low and the rate of the reaction slow.

Question 33

Higher tier only

What are two advantages of carrying out a reaction at a high temperature, if the reaction is endothermic ?

Answer 33

The yield will be high and the rate of the reaction will be fast.

Question 34

Higher tier only

How can scientists make a reaction go faster at a lower temperature ?

Answer 34

Use a catalyst

Question 35

Higher tier only

What are two disadvantages of doing a reaction at a high pressure?

Answer 35

It can be dangerous, could be explosions and it is expensive to generate high pressures.

Question 36

Higher tier only

Explain how an increase in pressure affects the position of a reaction in equilibrium that has gases as products or reactants?

Answer 36

An increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules of gas.

Question 37

Higher tier only

Explain how a decrease in pressure affects the position of a reaction in equilibrium that has gases as products or reactants ?

Answer 37

A decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules of gas.

Question 38

Higher tier

What is Le Chatelier’s Principle.

Answer 38

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

Question 39

Higher tier only

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

Answer 39

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 40

Higher tier

What happens if the concentration of a reactant is increased ?

Answer 40

If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.

Question 41

Higher tier

What happens if the concentration of a reactant is decreased ?

Answer 41

If the concentration of a product is decreased, more reactants will react until equilibrium is reached again.

Question 42

Higher tier

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

Answer 42

If the temperature of a system at equilibrium is increased:

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 43

Higher tier

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

Answer 43

If the temperature of a system at equilibrium is decreased:

• 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 44

Higher tier

For gaseous reactions at equilibrium: what happens if the pressure is increase ?

Answer 44

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 45

Higher tier only

For gaseous reactions at equilibrium: what happens if the pressure is decreased ?

Answer 45

A decrease in pressure causes the equilibrium position to shift towards the side with the larger number of moles of gas.