SAC 2 PREP CHEM

Question 1

What are the factors that affect the rate of reaction? (5 factors). Explain how each of these affect the reaction rate?

Answer 1

• surface area: when increased, there is greater number of exposed particles, increasing the frequency of collisions –> smaller particles react much faster.
• concentration and pressure: when increased, there are more particles in a fixed volume (less space), increase frequency of collisions
• temperature: when increased, there is more energy for particles to have collisions, increasing the probability of a successful collision as there is a higher propertion of particles with energy that can overcome the EA.
• gas pressure
• presence of a catalyst

Question 2

What is the definition of rate of reaction? What is the unit?

Answer 2

the change in the concentration of a reactant or product over time, M s^-1

Question 3

What are the conditions that must be met for Collision Theory?

Answer 3

1. collide with each other
2. collide with sufficient energy to break bonds within reactants
3. collide with the correct orientation to break the bonds within the reactants and allow formation of new products

Question 4

What is the collision theory? How does the collision theory correlate to the rate of reaction?

Answer 4

• Particles must collide with enough energy and correct orientation to form new products. Minimum energy of a collision must be possessed for a reaction to occur –> activation energy.
• to increase the rate of reaction–> the proprtion of successful collsions can be increased by either increasing the frequency of collisions or increasing the proportion of collision with enough energy to proceed.

Question 5

Why is the orientation of collision important?

Answer 5

To break the bonds within the reactants and allow formation of new products.

Question 6

What is the Maxwell-Boltzmann distribution?

Answer 6

• Particles in a substance will have a range of kinetic energies at any temperature.
• Most of the particles have similar kinetic energies but there’s always some particles with a high or low energy.

Question 7

What is the effect of temperature on the rate of reaction? Explain using the collision theory what are the different types of catalyst? When are these different types of catalyst used?

Answer 7

When temperature increases, there is more energy for particles to have collisions. Higher proportion of particles with energy that is sufficient to overcome the activation energy –> Increases the probability of more successful collisions.

Question 8

What is dynamic equilibrium?

Answer 8

1. concentration of both reactants and products is kept constant as the rate of forward and backward reactions are equal.
2. at a molecular level, the bonds are constantly being broken down and new bonds are being formed as the reactants and products continue to be converted to one another.

Question 9

what is a closed system?

Answer 9

a system in which only energy is exchanged with the surroundings.

Question 10

What is open system?

Answer 10

a system that allows matter and energy to be exchanged with the surroundings.

Question 11

What are reversible and irreversible reactions? How do you differentiate these?

Answer 11

A reversible reaction is a chemical reaction that can proceed in both directions. In other words, the products of the reaction can react with one another to form the original reactants. This means that the reaction can reach a state of equilibrium, where the rates of the forward and reverse reactions are equal and no net change occurs.

An irreversible reaction, on the other hand, is a chemical reaction that proceeds in one direction only. Once the reaction has occurred, it cannot be undone, and the products cannot react to form the original reactants.

Reversible reactions: occur under conditions of equilibrium, form liquid or solution.
Irreversible: occur under non-equilibrium conditions, form gas or solids

Question 12

Explain the term ‘extent of reaction’ in contrast to ‘rate of reaction

Answer 12

The extent of reaction refers to the amount of product that is formed at equilibrium in a chemical reaction. It is a measure of how far the reaction has progressed, and is determined by the initial amounts of reactants and the equilibrium constant for the reaction.

The rate of reaction, on the other hand, refers to the speed at which a reaction proceeds, or how quickly the reactants are consumed and the products are formed. It is a measure of the change in the concentration of reactants or products over time, and is determined by factors such as temperature, pressure, and the presence of a catalyst.

Question 13

What is the reaction quotient?
What is the equilibrium constant?

Answer 13

reaction quotient is denoted by Qc and is the ratio of concentrations of products to reactants in a reversible reaction.

equilibrium constant is denoted by Kc and remains constant for all equilibrium mixtures at a fixed temperature.

both must be aq or g

Question 14

What happens when Qc=Kc

Answer 14

equilibrium

Question 15

what happens when Qc<Kc

Answer 15

the system shifts to the right towards the products to increase Qc and to achieve quilibrium and more products are formed.

Question 16

what happens when Qc>Kc

Answer 16

the system shifts to the left towards the reactants to decrease Qc and to achieve equilibrium and more reactants are formed

Question 17

Explain what happens to the equilibrium constants when
a) When one equation is the reverse of another
b) The coefficient of an equation is doubled
c)The coefficient of the equation is halved

Answer 17

(a)The numerator and denominator of the equilibrium constant are switched, resulting in a new equilibrium constant. equilibrium constant is the reciprocal of the original equilibrium constant.
(b) The new equilibrium constant (Kc’) is equal to the original constant (Kc) raised to the power of the change in the coefficient
(c)
1. If the coefficient of a reactant is halved, the equilibrium constant will increase. This is because the concentration of the reactant in the equilibrium expression will be smaller, and the equilibrium will shift to favor the side with the smaller concentration.
2. If the coefficient of a product is halved, the equilibrium constant will decrease. This is because the concentration of the product in the equilibrium expression will be smaller, and the equilibrium will shift to favor the side with the larger concentration.

Question 18

What is the equilibrium yield?

Answer 18

If the pressure is increased, the equilibrium position moves to the right, so the yield of ammonia increases. The rate of reaction also increases because the gas molecules are closer together, so successful collisions are more frequent.

Question 19

What is the effect of temperature on the equilibrium constant?

Answer 19

Endothermic forward reaction: Kc increase because system changes to partially decrease the temperature.
Exothermic forward reaction: Kc decreases because system changes to partially decrease the temperature.

Question 20

What is Le Chatelier’s principle?

Answer 20

• if an equilibrium system is subjected to a change, the system will adjust itself to partially oppose the effect of the change.
• the addition of a catalyst and inert gas has no change because they both affect the forward and reverse reaction.

Question 21

What is the position of equilibrium? How do you change the position of equilibrium?

Answer 21

• amounts of reactants and products at equilibrium, varying depending on the extent of the reaction.
  Changing position:
  1. adding or removing a reactant or product
  2. changing pressure
  3. changing concentration
  4. changing temperature
  5. adding a catalyst (changes only the rate of the reaction)

Question 22

Explain the effects of
a) Adding a reactant
b) Adding a product
c)Removing a product

Answer 22

(a) form more products, net forward reaction, shifts to right
(b) form more reactants, net reverse reaction, shifts left
(c) form more products, net forward reaction, shifts to right

• adding a reactant is the same as removing a product

Question 23

What is competing equilibria?

Answer 23

equilibria have a common reactant, each equilibrium can be regarded as competing for the reactant. However, equilibrium with larger equilibrium constant has greater effect on the position of eq.

Question 24

What happens to the equilibrium constant when the pressure is altered?

Answer 24

no change

Question 25

What happens when an inert gas is added to an equilibrium mixture?

Answer 25

no change (for catalyst as well) because they affect both the forward and reverse reactions.

Question 26

What happens to the equilibria when dilution occurs?

adding water (dilution of solutions)

Answer 26

shifts in the direction of the most particles

Question 27

How does a catalyst affect the position of equilibrium?

Answer 27

no change

Question 28

What is the first way to optimise the reaction to obtain a high equilibrium yield?

Answer 28

• high concentrations/pressure: pressures depend on the relative numbers of reactant and product particles

Question 29

What is the second way to optimise the reaction to obtain a high equilibrium yield?

Answer 29

• high temperatures: low temperatures for exothermic reactions, high temperatures for endothermic reactions

Question 30

What is the third way to optimise the reaction to obtain a high equilibrium yield?

Answer 30

• high surface area of solids: addition of excess reactant

Question 31

What is the fourth way to optimise the reaction to obtain a high equilibrium yield?

Answer 31

• use a catalyst: removal of product as it forms

Question 32

How do you calculate the percentage yield?

Answer 32

percentage yield= actual yield/theortical yield x 100

Question 33

In an industry setting, what are the two consideration for the best economic outcome?