Physical Chemistry Part 2 Flashcards
what is a catalyst?
a catalyst is a substance that increases the rate of reaction without being used up in the process
how does a catalyst work?
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
What is a homogeneous catalyst
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
what is a heterogeneous catalyst
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
What is the economic importance of catalysts, how do they benefit us economically
- 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
describe the economic importance of catalysts in regards to the Haber process
- 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
what are the disadvantages of catalysts
- toxic substances
- hard to obtain
making Ethanoic acid the old version
- 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
How to make ethanoic acid the new way
- 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- Cativa process superseded the Monsanto process - iridium catalyst is used - cheaper than rhodium and cuts cost - releases less carbon dioxide into the atmosphere therefore has high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
DRAW A BOLTZMANN DISTRIBUTION GRAPH
DRAW IT
What is the Boltzmann distribution
this is the distribution of energies of molecules at a particular temperature this is often shown as a graph
why is it an average distribution of molecular energies - describe why the Boltzmann distribution graph is shaped the way that it is
- some molecules move fast and have high energy
- some molecules move slowly and have low energy
- majority of molecules have an average energy
Describe the important features of a Boltzmann distribution graph
- 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
Describe the effect temperature has on a rate of reaction
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
DRAW HOW A CATALYST EFFECTS THE BOLTZMANN DISTRIBUTION CURVE
DRAW THIS
Describe how a catalyst effects the rate of reaction
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
what is a reversible reaction
this is when a reaction can take place in the forward or reverse direction
What is a state of equilibrium
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
A dynamic equilibrium is when….
- 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
when does an equilibrium only apply
an equilibrium only applies when the system remains isolated
What is an isolated system
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
when is the position of the equilibrium established
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
A reversible reaction only remains in dynamic equilibrium when…
it is isolated and in a closed system
what are the factors that affect the position of the equilibrium
- concentrations of the reactants or products
- pressure in reactions involving gases
- temperature
What is Le Chatelier’s Principle
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
What effect does concentration have on equilibrium
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
What is the effect of pressure on equilibrium
- 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……
what is the effect of temperature on the equilibrium
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
What is the effect of a catalyst on equilibrium
- 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
describe where the products for the Haber process come from
- nitrogen is obtained from the air via fractional distillation
- hydrogen is prepared by reacting together methane from natural gas and water
What are the ideal conditions for the Haber process
- 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
What are the drawbacks for using the ideal conditions for the Haber process
- 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
describe the modern ammonia plant
- 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
what happens when an reaction reaches equilibrium (concentrations)
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
if the concentration of the products is much higher to that of the reactants then we say
the equilibrium lies to the right or favours the products
if the concentration of the reactants is much higher than that of the products we say
the equilibrium lies to the left or favours the reactants
write out the equilibrium constant equation
WRITE IT
what does the Kc indicate
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
When Kc is greater than 1
the reaction favours the products
the products on the right hand side are predominate at equilibrium
When Kc is smaller than 1
the reactions favours the reactants
the reactants on the left hand side are predominate at equilibrium
How to measure reaction rates
- 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
How to measure the change to the position of equilibrium for changes in concentration and temperature
- coloured compounds
- measure intensity of the colour
- using a colorimeter - this measures how much light is absorbed
