6. Chemical Reactions

Question 1

State 3 features of a Physical change that differentiate it from a Chemical change.

Answer 1

3 differences of a Physical change from a Chemical change are:
1. No new substance is formed
2. Constituent species maintain their properties
3. The change is easily reversible

These features of a physical change can be used to identify it in a reaction.

Question 2

State 3 features of a Chemical change that differentiate it from a Physical change.

Answer 2

3 differences of a Chemical change from a Physical change are:
1. A new substance is formed
2. Properties of the new substance are different from {those of} constituent species
3. The change is not easily reversible

These features of a chemical change can be used to identify it in a reaction.

Question 3

Give 4 examples of Chemical Changes.

Answer 3

4 examples of Chemical Changes are:
1. Combustion of a fuel
2. Photosynthesis
3. Respiration
4. Neutralisation

Question 4

Give 4 examples of Physical Changes.

Answer 4

4 examples of Physical Changes are:
1. Melting
2. Condensation
3. Freezing
4. Evaporation

All the changes of state are physical changes.

Question 5

Define rate of reaction.

Answer 5

Rate of reaction is the speed at which a reaction proceeds.

Question 6

State the 5 factors influencing rate of reaction.

Answer 6

The 5 factors that affect the rate of a reaction are:
1. Concentration of solutions
2. Pressure of gases
3. Surface area of solids
4. Temperature
5. Catalyst

Question 7

Explain why the rate of reaction is high at the start of the reaction.

Answer 7

The rate of reaction is high at the start of the reaction because:
1. Concentration of reactants will be high
2. High concentration = more reactant molecules per unit volume
3. This results in a higher frequency of collisions

At the start of a reaction, the rate is high because the concentration of reactants will be high and at a high concentration, there are more reactant molecules per unit volume resulting in a higher frequency of collisions.

Question 8

Describe {1} and justify {2} how concentration of solutions affects a reaction.

Answer 8

At a higher concentration, the rate is higher.

This is because:
1. Higher concentration = more reactant molecules per unit volume
2. This results in a higher frequency of collisions.

When particles are colliding more frequently, the rate is faster.
as concentration decreases, the rate decreases.

Question 9

What is the effect of a change (increase and decrease) in pressure of reactions involving gases? [2]

Only applying to reactions involving gases.

Answer 9

In a reaction involving gases;
1. an increase in the pressure of the gases increases the rate of reaction
2. a decrease in the pressure of the gases decreases the rate of reaction.

Question 10

Explain why in a reaction involving gases, an increase in the pressure of the gases results in an increase in the rate of reaction. {2}

Answer 10

In a reaction involving gases, an increase in pressure results in an increase in rate of reaction because:
1. At a higher pressure there are more gas molecules per unit volume.
2. This results in a higher frequency of collisions, leading to a higher rate of reaction

Question 11

What has a greater surface area, a powder or a lump of the same solid?

Answer 11

A powder has a greater surface area than a large lump of a solid.

Question 12

How would an increase in the surface area of a solid impact the rate of reaction?

Answer 12

An increase in the surface area of a solid would increase the rate of reaction.

The opposite action has the opposite effect.

Question 13

Explain why an increase in surface area of a solid increases the rate of reaction.

Answer 13

Crushing a solid to a powder increases the surface area of the solid, and this increases its rate of reaction.
This is because the greater the surface area exposed, the higher the rate of reaction.

Question 14

How does a change in temperature (increase and decrease) influence the rate of a reaction?

Answer 14

The influence of temperture on the rate of a reaction is, i. the higher the temperature, the greater the rate of reaction
and
ii. the lower the temperature, the lesser the rate of reaction.

Question 15

Explain why the rate of reaction increases with an increase in temperature. {3}

Answer 15

The higher the temperature, the greater the rate.
This is because:
1. the higher the temperature, the more kinetic energy particles have
2. more kinetic energy results in a higher frequency of collisions
3. more particles have energy greater or equal to the activation energy.

Question 16

What is a catalyst?

Answer 16

A catalyst is a substance that increases the rate of reaction and is unchanged at the end of the reaction.

or without undergoing a permanent change.

Thus the addition of a catalyst to a reaction increases the rate of reaction.

Question 17

What does a catalyst do in a reaction?

Pertaining to Activation Energy, Eₐ.

Answer 17

A catalyst decreases the activation energy, Eₐ, of a reaction.

Question 18

How does the number of particles per unit volume of a reaction involving gases or solutions influence the rate of reaction? {2}

the number of particles per unit volume = concentration in solutions and pressure in gases

Answer 18

1. The greater the number of particles/unit volume, the greater the rate of reaction.
2. This is because, more particles/unit volume = higher frequency of collisions

Question 19

How does the frequency of collisions between particles affect the rate of reaction.

Answer 19

In a reaction the rate increases if the frequency of collisions between particles increases and decreases if the frequency decreases.

Question 20

How does the kinetic energy of particles in a reaction affect the rate? {2}

Answer 20

The greater the amount of kinetic energy of particles, the higher the rate of reaction.
This is because, when the kinetic energy is more, there is a higher frequency of collisions of particles in the reaction.

More Kinetic Energy In Particles = More Frequent Collisions Between Particles

Question 21

State two methods of investigating the rate of reaction.

Answer 21

1. Monitoring the change in mass of a reactant or product with time
2. Monitoring volume of gas produced

Question 22

Describe in 2 steps the investigation of the rate of reaction by monitoring the change in mass of reactant or product with time.

Answer 22

1. Measure initial mass of a reactant
2. Find mass of the reactat at different time intervals

Question 23

What are the 2 characteristics of a reversible reaction in a closed system at equilibrium?

Answer 23

The 2 characteristics of a reversible reaction at equilibrium in a closed system:

1. the rate of the forward reaction is equal to the rate of the reverse reaction
2. the concentrations of reactions and products are no longer charging

• rate forward = rate backward
• concentrations of reactants and products are constant

Question 24

What is a closed system?

Answer 24

A closed system is a system in which no substances are lost or gained.

Question 25

What effect does heat have on the hydrated compound, Copper (ii) Sulfate, CuSO₄*6H₂O?

Answer 25

Heating CuSO₄*6H₂O removes the water of crystallisation through evaporation.

This reaction is reversible

6H₂O is water of crystallisation

• A colour change occurs from blue to white.
• This reaction can be used to test for water.
• This is also applicable to hydrated Cobalt (ii) Chloride.

Question 26

How does adding water to an anhydrous compound such as anhydrous Cobalt (ii) chloride affect its direction as a reversible reaction?

Answer 26

Adding water to Anhydrous CoCl₂
shifts the reaction backward.

Anhydrous means without water.

• This is also applicable to anhydrous Copper (ii) Sulfate.
• In CoCl₂, this results in a colour change from blue to pink.

Question 27

What colour change occurs when water is added to anhydrous Copper (ii) Sulfate, CuSO₄
to form hydrated Copper (ii) Sulfate, CuSO₄*6H₂O?

Adding water shifts the reaction of an anhydrous compound backward.

Answer 27

The colour changes from White to Blue

Anhydrous means without water.

i.e anhydrous Copper (ii) Sulfate, CuSO₄ is White in colour
and hydrated Copper (ii) Sulfate, CuSO₄*6H₂O is blue in colour

Heating this in its hydrated state gives the opposite colour change.

Question 28

What colour change occurs when water is added to anhydrous Cobalt (II) chloride, CoCl₂ to form hydrated Cobalt (II) chloride, CoCl₂*6H₂O?

Adding water shifts the reaction of an anhydrous compound backward.

Answer 28

The colour changes from blue to pink.

Anhydrous means without water.

i.e anhydrous Cobalt (II) chloride, CoCl₂, is blue in colour
and hydrated Cobalt (II) chloride, CoCl₂*6H₂O is pink in colour

Heating this in its hydrated state gives the opposite colour change.

Question 29

What is the effect on a reaction of
i. adding a catalyst
ii. removing a catalyst?

Answer 29

The effect on a reaction of:
i. adding a catalyst is that the rate of reaction increases
ii. removing a catalyst is that the rate of reaction decreases

Question 30

Explain the effect of a catalyst in increasing the rate of reaction.

Answer 30

A catalyst increases the rate of reaction by decreasing the activation energy, Eₐ, of a reaction.

Question 31

How does activation energy i. low and ii. high, affect a reaction?

Answer 31

i. (1)Low activation energy, achieved with a catalyst, results in a faster reaction. (2) This is because particles will reach the minimum amount of energy needed to react in a collision with more ease.
ii. (1) High activation energy, in a case without a catalyst, leads to a slower reaction. (2) The minimum amount of energy required for reaction upon collision will be harder to reach, thus it takes longer.

Might delete later.

Question 32

What is shown by the symbol ⇌ ?

Answer 32

The symbol, ⇌, shows a chemical reaction that is reversible.

Question 33

In an exothermic reversible reaction, how is the position of equilibrium influenced by:
i. an increase in temperature
ii. a decrease in temperature

If the forward reaction is exothermic, then the backward reaction is endothcermic.

Answer 33

In an exothermic reversible reaction:
i. an increase in temperature promotes the backward reaction
ii. a decrease in temperature promotes the forward reaction

EXOTHERMIC REACTIONS ARE FAVOURED BY LOW TEMPERATURES, WHILE ENDOTHERMIC REACTIONS ARE FAVOURED BY HIGH TEMPERATURES
I.E. REDUCING TEMPERATURE PROMOTES THE EXOTHERMIC REACTION - in this case, that is the forward reaction, while the backward reaction is the opposite, endothermic.

Question 34

In an endothermic reversible reaction, how is the position of equilibrium influenced by:
i. an increase in temperature
ii. a decrease in temperature

If the forward reaction is exothermic, then the backward reaction is endothcermic.

Answer 34

In an endothermic reversible reaction:
i. an increase in temperature promotes the forward reaction
ii. a decrease in temperature promotes the backward reaction

EXOTHERMIC REACTIONS ARE FAVOURED BY LOW TEMPERATURES, WHILE ENDOTHERMIC REACTIONS ARE FAVOURED BY HIGH TEMPERATURES
I.E. REDUCING TEMPERATURE PROMOTES THE EXOTHERMIC REACTION - in this case, that is the forward reaction, while the backward reaction is the opposite, endothermic.

• promoting forward reaction = shifting position of equilibrium to the right
• promoting backward reaction = shifting position of equilibrium to the left

Question 35

Why does an an increase in temperature shift the equilibrium of an exothermic reversible reaction to the left, while a decrease shifts it to the right?

Answer 35

A decrease in temperature shifts an exothermic reversible reaction to the right while an increase shifts it to the left because EXOTHERMIC REACTIONS ARE FAVOURED BY LOW TEMPERATURES.

Question 36

Why does an an increase in temperature shift the equilibrium of an endoothermic reversible reaction to the right, while a decrease shifts it to the left?

Answer 36

A decrease in temperature shifts an endoothermic reversible reaction to the left while an increase shifts it to the right because ENDOTHERMIC REACTIONS ARE FAVOURED BY HIGH TEMPERATURES.

Question 37

What is the effect of i. increasing and ii. decreasing pressure on a reversible reaction in which the product has a higher pressure than the reactants?

Higher pressure = more gas molecules

Answer 37

The effect of
i. increasing the pressure of a reversibe reaction in which the product has a higher pressure than the reactants - the reaction shifts to the left, promoting the production of the reactants
ii. decreasing the pressure of a reversibe reaction in which the product has a higher pressure than the reactants - the reaction shifts to the right, promoting the production of the product

• promoting forward reaction = shifting position of equilibrium to the right
• promoting backward reaction = shifting position of equilibrium to the left

Question 38

What is the effect of i. increasing and ii. decreasing pressure on a reversible reaction in which the product has a lower pressure than the reactants?

Higher pressure = more gas molecules
Lower pressure = less gas molecules

Answer 38

The effect of
i. increasing the pressure of a reversibe reaction in which the product has a lower pressure than the reactants - the reaction shifts to the right, promoting the production of the product
ii. decreasing the pressure of a reversibe reaction in which the product has a lower pressure than the reactants - the reaction shifts to the left, promoting the production of the reactants

The effect of changing pressure is predicted using the stoichiometric equation of a reaction - it shows which side of a reaction has a higher pressure and which has a lower pressure.

• promoting forward reaction = shifting position of equilibrium to the right
• promoting backward reaction = shifting position of equilibrium to the left

Question 39

Why does an increase in pressure in a reversible reaction in which the product is low pressure promote the forward reaction, while a decrease in pressure in the same case promotes the backward reaction?

The opposite is true for a reaction in which the reactants are of higher pressure.

Answer 39

This is because increasing pressure shifts the equilibrium to where there are less gas molecules, while decreasing pressure shifts the equilibrium to where there are more gas molecules.

Question 40

What is the effect of increasing concentration of a substance on the position of equilibrium on a reversible reaction?

Answer 40

The effect of increasing the concentration of a substance in a reversible reaction is that, it favours the direction which reduces the concentration of the substance.

• Thus by increasing reactants, the forming of products is promoted, and by increasing products, the forming of reactants is promoted.