Physical Chemistry Part 2

Question 1

what is a catalyst?

Answer 1

a catalyst is a substance that increases the rate of reaction without being used up in the process

Question 2

how does a catalyst work?

Answer 2

a catalyst increases the rate of reaction and is not used up in the process, the catalyst lowers the activation energy and provides an alternative route for the catalyst to follow, this alternative route has an lower energy

Question 3

What is a homogeneous catalyst

Answer 3

this is a catalyst which catalyses a reaction in the same phase as the reactants - e.g. a liquid catalyst mixed with liquid reactants or a gaseous catalyst mixed with gaseous reactants

Question 4

what is a heterogeneous catalyst

Answer 4

this is a catalyst for a reaction which is in a different phase from the reactants, e.g. a solid catalyst used in liquid reactants

Question 5

What is the economic importance of catalysts, how do they benefit us economically

Answer 5

• lower energy demands of the processes - they reduce costs and help the environment - less fossil fuels are needed for the required process this reduces the amount of carbon dioxide
• catalytic converters - they improve air quality by reducing toxic emissions from vehicles and prevent photo chemical smog

Question 6

describe the economic importance of catalysts in regards to the Haber process

Answer 6

• a lot of energy is needed to break the triple bond in Nitrogen, therefore this needs a high activation energy
• iron is used to catalyse the reaction by weakening the nitrogen bonds and lowering the activation energy therefore costs

Question 7

what are the disadvantages of catalysts

Answer 7

• toxic substances

- hard to obtain

Question 8

making Ethanoic acid the old version

Answer 8

• important chemical that is used in the production of goods and services
• important made easily and at low cost
• most ethanoic acid was made by oxidising butane or light naphtha via hydration of ethene
• butane was heated in air using catalyst of manganese, cobalt and chromium ions to produce ethanoic acid
• this was inefficient and produced a low yield of ethanoic acid

Question 9

How to make ethanoic acid the new way

Answer 9

• Monsanto process
• methanol reacted with carbon monoxide
• used cobalt as a catalyst and iodide ions
• orignally carried out at 300 degrees and 700 atm, there were high energy costs to generate temperature and pressure
• used rhodium as a catalyst instead - lower temperature of 150 degrees and lower pressure of 30 atm
• Cativa process superseded the Monsanto process - iridium catalyst is used - cheaper than rhodium and cuts cost - releases less carbon dioxide into the atmosphere therefore has high environmental benefits

Question 10

DRAW A BOLTZMANN DISTRIBUTION GRAPH

Answer 10

DRAW IT

Question 11

What is the Boltzmann distribution

Answer 11

this is the distribution of energies of molecules at a particular temperature this is often shown as a graph

Question 12

why is it an average distribution of molecular energies - describe why the Boltzmann distribution graph is shaped the way that it is

Answer 12

• some molecules move fast and have high energy
• some molecules move slowly and have low energy
• majority of molecules have an average energy

Question 13

Describe the important features of a Boltzmann distribution graph

Answer 13

• area under the curve is equal to the total number of molecules in the sample - area does not change with conditions
• there are no molecules in the system with zero energy the curve starts at the origin
• no maximum amount of energy for a molecule the curve gets closer to but does not cross the energy axis
• ## only molecules with a energy greater than the activation energy are able to react

Question 14

Describe the effect temperature has on a rate of reaction

Answer 14

higher the temperatures the faster the kinetic energy of all the molecules, the more frequent the collisions and the more likely the molecule is likely to overcome the activation energy therefore the rate of reaction increases due to more successful collisions over a certain period of time
- Boltzmann graph shows a second line with a lower activation energy - draw this

Question 15

DRAW HOW A CATALYST EFFECTS THE BOLTZMANN DISTRIBUTION CURVE

Answer 15

DRAW THIS

Question 16

Describe how a catalyst effects the rate of reaction

Answer 16

the catalyst lower the activation energy by following an alternative route that requires a less amount of energy
- more molecules will overcome the activation energy and react so there will be a high number of successful collisions in a certain amount of time so the rate of reaction will increase

Question 17

what is a reversible reaction

Answer 17

this is when a reaction can take place in the forward or reverse direction

Question 18

What is a state of equilibrium

Answer 18

this is when there is no observable change in the reaction and nothing appears to be happening
- however the system is dynamic and in constant motion

Question 19

A dynamic equilibrium is when….

Answer 19

• the concentrations of the reactants and products remain constant
• the rate of the forward direction is the same as the rate of the reverse direction

Question 20

when does an equilibrium only apply

Answer 20

an equilibrium only applies when the system remains isolated

Question 21

What is an isolated system

Answer 21

an isolated system is when no materials are being added or taken away and no external conditions such as temperature or pressure are being altered

Question 22

when is the position of the equilibrium established

Answer 22

when the reaction has been occurring for a period of time - this is when the forward and backward reactions are stable and the same therefore the equilibrium is established

Question 23

A reversible reaction only remains in dynamic equilibrium when…

Answer 23

it is isolated and in a closed system

Question 24

what are the factors that affect the position of the equilibrium

Answer 24

• concentrations of the reactants or products
• pressure in reactions involving gases
• temperature

Question 25

What is Le Chatelier’s Principle

Answer 25

the principle states that when a system in dynamic equilibrium is subjected to change, the position of the equilibrium will shift to minimise the change
- changing conditions will help the reaction to adopt the pathway that minimises the change - carry out the forward or reverse reaction more

Question 26

What effect does concentration have on equilibrium

Answer 26

increasing the concentration of a REACTANT causes the equilibrium to shift in the direction that decreases the reactants concentration
- the system opposes the change by decreasing the concentration of the reactant by removing it
- the equilibrium will move to the right hand side and make more products to minimise change
Increasing the concentration of a PRODUCT causes the equilibrium to shift in the direction that decreases this products concentration
- the system opposes the change by decreasing the concentration of the product by removing it
- the position of the equilibrium will move to the left hand side to form more reactants the minimise change

Question 27

What is the effect of pressure on equilibrium

Answer 27

• only change if gases are present
  increasing the total pressure of the system causes the position of the equilibrium to move to the side with the fewer gas molecules in order to decrease pressure and minimise change, It will move,…….
  decreasing the total pressure of the system cause the equilibrium to move to the side with the greater number of gas molecules in order to increase pressure and minimise change, it will move……

Question 28

what is the effect of temperature on the equilibrium

Answer 28

Increasing the temperature causes the position of the equilibrium to move in the direction that decreases the temperature
- the system opposes the change and takes in heat and the position of the equilibrium moves in the endothermic direction to minimise change
decreasing the temperature of the system causes the positon of the equilibrium to move in the direction that increases the temperature
- the system opposes the change so by releasing heat and the positon of the equilibrium moves in the exothermic direction in order to minimise change

Question 29

What is the effect of a catalyst on equilibrium

Answer 29

• catalyst speeds up the rate of the forward and backward reactions equally
• a catalyst increases the rate at which the equilibrium Is established by lowering the activation energy and providing an alternative route which requires less energy but it does not affect the positon of the equilibrium

Question 30

describe where the products for the Haber process come from

Answer 30

• nitrogen is obtained from the air via fractional distillation
• hydrogen is prepared by reacting together methane from natural gas and water

Question 31

What are the ideal conditions for the Haber process

Answer 31

• Ammonia is produced in the forward direction which is exothermic therefore optimum conditions are low temperatures
• the forward reaction produces fewer gas molecules therefore high pressures are favoured

Question 32

What are the drawbacks for using the ideal conditions for the Haber process

Answer 32

• low temperatures produce a high yield but the reaction would take place at a slow rate, and few nitrogen and hydrogen molecules have enough energy to overcome the activation energy, therefore 450 degrees is used in increase the rate of reaction and make more molecules overcome the activation energy my increasing the frequency of collisions as the particles will have a higher kinetic energy, but this produces a lower yield
• a high pressure increases the concentration of the gases and increases the reaction rate so produces a high equilibrium yield at a fast rate but 200atm is used as large amounts of energy is required to supress the pressure and this increases costs, moreover it is a danger to the workforce and environment, big heavy expensive equipment is needed, therefore compromise is made

Question 33

describe the modern ammonia plant

Answer 33

• temperature - must be high enough to allow reaction to happen at a realistic rate whilst still producing a realistic yield - 450 degrees
• pressure - a high pressure must be used but not too high so that the workforce is in danger or energy costs are high so 200atm
• a catalyst is added - this is an iron catalyst and it speeds up the rate of reaction this allows the equilibrium to be established and faster and lower temperatures to be used therefore less energy needed to generate heat so reduces costs
• recycling of unconverted nitrogen and hydrogen is used again

Question 34

what happens when an reaction reaches equilibrium (concentrations)

Answer 34

the concentrations of each species involved remains constant

• reaction is still occurring but concentrations do not change rate of forwards and backwards reactions are equal
• does not mean the concentrations are equal

Question 35

if the concentration of the products is much higher to that of the reactants then we say

Answer 35

the equilibrium lies to the right or favours the products

Question 36

if the concentration of the reactants is much higher than that of the products we say

Answer 36

the equilibrium lies to the left or favours the reactants

Question 37

write out the equilibrium constant equation

Answer 37

WRITE IT

Question 38

what does the Kc indicate

Answer 38

the magnitude of Kc indicates the extent of a chemical reaction
- the Kc value of 1 would indicate that the position of the equilibrium is halfway between the reactants and products

Question 39

When Kc is greater than 1

Answer 39

the reaction favours the products

the products on the right hand side are predominate at equilibrium

Question 40

When Kc is smaller than 1

Answer 40

the reactions favours the reactants

the reactants on the left hand side are predominate at equilibrium

Question 41

How to measure reaction rates

Answer 41

• gas syringe
• monitoring a reactions mass during a reaction
• recording time for particular change to occur
• using a colorimeter to monitor changes of coloured reactants or product

Question 42

How to measure the change to the position of equilibrium for changes in concentration and temperature

Answer 42

• coloured compounds
• measure intensity of the colour
• using a colorimeter - this measures how much light is absorbed