chemistry topic 6b and 10.9 (haber process)

Haber procces 10.9, reversible reactions and le chateliers principle 6b

what is a reversible reaction

where the products of the reaction can react with themselves to produce original reactions

(represented with reversible arrow)

what is a closed system

where none of the reactants or products can escape and nothing else can get in

equilibrium always reached if reversible reaction is taken place in closed system

what does it mean if a reaction has reached equilibrium

if a reaction is at equilibrium it doesn’t mean the amount of reactant and product is the same

• if it lies to the right the concentration of products is greater than reactants

• if it lies to the left the concentration of reactants is greater than the products 

what is an endothermic reaction

chemical process that absorbs energy from its surroundings

what is an exothermic reaction

chemical process that releases energy to its surroundings

reversible reactions endo and exothermic

if the reaction is endothermic in one direction it is exothermic in the over. the energy absorbed by the endothermic reaction is the same to the energy released by the exothermic reaction

what is le chateliers principle

the idea that if you change the conditions of reversible reaction at equilibrium, the system will try to counteract the change. so the effect of any changes to a system can be predicted.

altering temperature, pressure, concentration of the reactants can alter the yield of the reaction - making sure you end up with more of the product (and less reactant)

what happens to the yield if you change the temperature 

raise the temp - yield of endothermic reaction increases and yield of exothermic reaction decreases

reduce temp - yield of exothermic reaction increases and yield of endothermic reaction decreases 

what happens if the pressure is changed

affects reactions where reactants and products are gases

lots reactions have greater volume on one side. greater volume = more gas molecules 

raising pressure favours reaction which produces less volume

lowering pressure favours reaction which produces more volume

what happens if the concentration is changed

if you change concentration of reactant or products the system will no longer be at equilibrium so the system will respond to bring itself back to equilibrium

if you increase concentration of reactant the system tries to decrease it by making more products

if you decrease concentration of product the system tries to increase it by reducing amount reactants

what is used to make ammonia in haber process

nitrogen and hydrogen gas

where does the nitrogen and hydrogen come from

nitrogen is obtained easily from air which is 78% nitrogen

hydrogen comes from reacting natural gas (methane) with steam. it can also come from crude oil

symbol equation for ammonia reaction

𝑁2+3𝐻2⇌2𝑁𝐻3

what is the haber process carried out in

reaction vessel

200atm means 200 atmospheres  

how the haber process works

hydrogen and nitrogen pumped into the compressor through the pipes mixed in 3:1 ratio

the gases are pressurised to 200atm in the compressor

the gases are pumped in a tank with trays of iron catalysts at 450c. in this condition some of the hydrogen and nitrogen will react to form ammonia

the unreacted/unused hydrogen and nitrogen are recyled by being fed back through the pipes to be put back into the tank with catalyst

in the condenser the ammonia turns from gas to liquid and can be removed from reaction vessel

what conditions are used in the haber process

450c 

200 atm - pressure 

iron catalyst

what is a factor in choosing the conditions

cost, energy and availability

as the reaction is reversible , the temperature and pressure conditions used are a compromise between yield and rate

temperature in the haber process

the forward reaction is exothermic so increasing the temp will move equilibrium the wrong way (away from ammonia toward h2 and n2) so the yield of ammonia would be greater in low temperature

but lower temp means lower rate of reaction. so temp is increased anyway - for reasonable rate of reaction

450c is a compromise between maximum yield and speed of reaction

pressure in haber process

higher pressure favours forward reaction (4 molecules on left , 2 on right) this moves equilibrium towards ammonia away from n2 and h2. so higher pressure increases yield of ammonia. high pressure also increases rate of reaction

the pressure is set to as high as possible to give best yield, without making plant to expensive - so 200 atm

iron catalyst in haber process

makes reaction go faster so it reaches equilibrium faster. but catalyst doesn’t change the amount of prroducts and reactants 

without the catalyst the temp would have to be raised more to get a quick enough reaction - this would reduce the percentage yield and mean higher energy costs.