Topic 2: Managing Chemical Processes

Question 1

What is the law of conservation of energy?

Answer 1

Energy cannot be created or destroyed, only converted from one form to another
Therefore, the energy of a system is conserved

Question 2

What is the law of conservation of mass?

Answer 2

Mass is neither created nor destroyed in chemical reactions; it is always conserved
Mass of reactants = mass of products

Question 3

What is the collision theory?

Answer 3

All particles are moving (have kinetic energy)
For a reaction to take place, particles must collide with each other, but not all collisions result in a reaction
Successful collisions require:
Sufficient Ea (the minimum amount of energy required for a reaction to occur)
Particles to collide in the correct orientation, allowing the reactants to break bonds and form new ones

Question 4

What is the rate of reaction? How can it be increased? What factors can affect it?

Answer 4

The rate of chemical reaction measures how fast or slow reactants are forming products per unit time (i.e. how many successful collisions per unit time)
The rate can be increased by:
Increasing the frequency of collisions per unit time
Increasing the proportion of collisions reaching (or exceeding) Ea
Factors that affect the rate of a chemical reaction are a change in:
Concentration
Pressure
Surface area
Temperature
Presence of a catalyst

Question 5

How does change in concentration affect rate of reaction?

Answer 5

If the concentration of a species of reacting particle is increased, there are more reacting particles in the system
This increases the frequency of collisions and therefore increases the frequency of successful collisions per unit time resulting in an increase in the rate of reaction

Question 6

What is pressure? How does change in pressure affect rate of reaction?

Answer 6

The pressure of a system is determined by the movement of particles within a given volume
Change in pressure is effectively a change in concentration of a gaseous system
Whether the pressure is increased by introducing more particles to a fixed volume or by reducing the volume of the system, the frequency of collisions will increase, resulting in more successful collisions per unit time resulting in an increase in the rate of reaction

Question 7

How does change in surface area affect rate of reaction?

Answer 7

In chemicals that involve solids, the interactions resulting in successful collisions occur on the surface of the solid
If the surface area of a species of reacting particles is increased, more particles are exposed due to a greater SA:V
This results in an increase in the frequency of collisions as more particles are exposed, resulting in more successful collisions per unit time, resulting in an increase in the rate of reaction

Question 8

How does change in temperature affect rate of reaction?

Answer 8

If the temperature of a system is increased, this results in the average kinetic energy of the particles increasing
This increases the movement of the particles and therefore the frequency of collisions
Also, a larger proportion of collisions will reach (or exceed) the required Ea
This means that there is an increased frequency of successful collisions occurring per unit time, resulting in an increase in the rate of reaction

Question 9

What is a catalyst? How does the presence of a catalyst affect rate of reaction?

Answer 9

Catalysts are substances that are used to increase (positive catalyst) or decrease (inhibitor) a rate of reaction, such as an enzyme
If a positive catalyst is introduced to a system, this would provide an alternate reaction pathway for the reacting particles with a lower Ea, resulting in a larger portion of collisions reaching (or exceeding) the required Ea, increasing the frequency of successful collisions per unit time, resulting in an increase in the rate of reaction

Question 10

What is enthalpy?

Answer 10

Enthalpy is the energy content of a system at constant temperature and pressure, represented by the letter H
The overall change of energy in a system for the reaction pathway is represented by deltaH

Question 11

What can deltaH be represented on?

Answer 11

DeltaH can be represented on an energy profile diagram
These represent the overall conversion of reactants into products and their relative energies as the reaction proceeds

Question 12

What is an endothermic reaction?

Answer 12

An endothermic reaction is when the overall enthalpy of the products is greater than the enthalpy of the reactants and the system has gained energy (deltaH is positive)

Question 13

What is an exothermic reaction?

Answer 13

An exothermic reaction is when the overall enthalpy of the products is less than the enthalpy of the reactants and the system has lost energy (deltaH is negative)

Question 14

Compare an open and closed chemical system

Answer 14

An open chemical system is one where both chemical matter and energy can be transferred into and out of the system
A closed chemical system has no net loss or gain of matter and only energy can be transferred in and out of the system e.g., heat and light energy

Question 15

Compare non-reversible and reversible chemical reactions

Answer 15

A non-reversible reaction is one where the chemical reaction proceeds to completion and the products cannot revert to the reactants
A reversible reaction is one where the reactants can combine to form the products, and the products can combine to form the reactants

Question 16

When is a state of equilibrium achieved?

Answer 16

A state of chemical equilibrium is reached when the rate of the forward reaction is equivalent to the rate of the backward reaction in a reversible reaction in a closed system at a fixed temperature, and so the net concentrations of reactants and products do not change over time.
On a large scale, there is no observable change, but the reaction has not stopped as there are constant forward and backward reactions occurring at the same rate, indicated on a graph with a slope of 0

Question 17

What do different Kc values indicate?

Answer 17

A Kc value much greater than 1 indicates that the concentration of the products is greater than reactants, so the forward reaction is being favoured and the position of equilibrium lies to the right
A Kc value much less than 1 indicates that the concentration of the reactants is greater than the products so the backward reaction is being favoured and the position of equilibrium lies to the left

Question 18

What does Le Chatelier’s principle describe?

Answer 18

According to Le Chatelier’s principle, the system will shift the position of equilibrium in order to counteract a change placed upon it (concentration, pressure, temperature), allowing the system to re-establish equilibrium

Question 19

How does a change in concentration affect the position of equilibrium in a system?

Answer 19

If the concentration of a reactant is increased, according to Le Chatalier’s principle, the system will try to counteract this and reduce its concentration by favouring the forward reaction, converting more reactants into products, shifting the equilibrium to the right until equilibrium is re-established
If the concentration of a reactant is decreased, according to Le Chatalier’s principle, the system will try to counteract this and increase its concentration by favouring the backward reaction, converting more products into reactants, shifting the equilibrium to the left until equilibrium is re-established
The same goes for if the concentration of the products is increased/decreased

Question 20

How does a change in pressure affect the position of equilibrium in a system?

Answer 20

If the pressure of a system is increased by reducing the volume of the reaction vessel, according to Le Chatelier’s principle, the system will attempt to counter the change and reduce the pressure of the system by shifting to the side of the equilibrium which results in the formation of the least moles of gaseous particles until equilibrium is re-established
If the pressure of a system is decreased by increasing the volume of the reaction vessel, according to Le Chatelier’s principle, the system will attempt to counter the change and increase the pressure of the system by shifting to the side of the equilibrium which results in the formation of the most moles of gaseous particles until equilibrium is re-established
If the number of moles of gaseous particles is equivalent on both sides of the reaction, there is no change to the position of the equilibrium

Question 21

How does a change in temperature affect the position of equilibrium in a system?

Answer 21

If the temperature of a system is increased, according to Le Chatelier’s principle, it will attempt to counter the change by absorbing the additional thermal energy, shifting the position of equilibrium in the endothermic (deltaH is positive) direction
If the temperature of a system is decreased, according to Le Chatelier’s principle, it will attempt to counter the change by releasing thermal energy, shifting the position of equilibrium in the exothermic (deltaH is positive) direction

Question 22

What are the main components of a flow chart?

Answer 22

Raw materials - the materials added to the reaction pathway
By-products - substances produced that are either reused in the reaction pathway or collected and used for another purpose
Waste products - substances disposed of or released into the environment

Question 23

What is yield?

Answer 23

The yield of a chemical reaction refers to the efficiency of the reaction
Theoretical yield refers to the quantity of the product that is expected to be obtained (calculated using stoichiometry)
Actual yield refers to the quantity of the product that was obtained through the chemical process
Percentage yield is the percent ratio of actual yield to theoretical yield

Question 24

Explain ‘compromises’ in chemical processes

Answer 24

By shifting the position of the equilibrium of a chemical process the actual yield can be increased, however, the maximum reaction rate (high temp.) and maximum yield is not always possible, so the industry makes compromises (balances) between factors to maximise profit