Dynamic Equilibria - Topic 5 Flashcards
Haber process:
- a reversible reaction between nitrogen and hydrogen to form ammonia
- the reaction is reversible so ammonia breaks down again into nitrogen and hydrogen
- nitrogen + hydrogen ⇌ ammonia
Enthalpy change:
- heat energy change at constant pressure
- △H
How does temperature affect the rate of attainment of equilibrium?
- a higher temperature
- particles have more kinetic energy → move faster → more frequent collisions → more frequent and successful collisions in given time → more reactions in given time
- equilibrium reached at a faster rate
How does pressure/concentration affect the rate of attainment of equilibrium?
- higher pressure/concentration → gas molecules closer together/more concentrated → more frequent collisions → more frequent successful collisions
- equilibrium reached at faster rate
How do catalysts affect the rate of attainment of equilibrium?
- catalysts speed up chemical reactions without being used up
- they work by providing an alternative E pathway with a lower activation E
- higher proportion of collisions successful → more successful collision in given time → more reactions in given time → therefore, catalyst speeds up rate at which eqm is reached
- they do not affect the position of eqm as they speed up both the forward and reverse reactions by the same amount so does not affect yield
Conditions required for the Haber process:
- iron catalyst - which purified gases are passed over
- temperature 450 degrees C
- pressure 200 atmospheres (ATM)
How are the conditions of industrial reactions e.g. the Haber process related to the availability and cost of raw materials and energy supplies?
- catalyst is used because of the effect of having an increased rate of reaction - but also expensive
- high temperatures and pressures can be expensive and dangerous as well (particularly pressure - explosions) and the equipment required for them can be very expensive
How does the control of temperature and pressure produce an acceptable yield in an acceptable time for the Haber process?
- high temperatures and pressures are desired for industrial reactions to increase the rate of reaction and the rate at which the equilibrium is attained
- but a higher temperature shifts equilibrium towards the reactants (as the forwards reaction is exothermic) - lower yield
- therefore a compromise is required to ensure a fast rate of reaction and high yield of products - high temp → low yield → maximised product/time
- high pressure - > increased rate → reasonable cost + risk
What are the conditions that give highest percentage of ammonia and why is it a problem?
- low temperature + high pressure
- low temperature - rate of react v. slow
- high pressure - dangerous + expensive
How does an iron catalyst produce an acceptable yield in an acceptable time for the Haber process?
- does not change position of equilibrium
- increases the rate of attainment of equilibrium
Reasons that a low yield is not a problem in the Haber process:
- unreactive gas is recycled
- rate is high
What may fertilisers contain and why?
fertilisers may contain nitrogen, phosphorous and potassium compounds to promote plant growth
NPK fertilisers:
nitrogen, phosphorous and potassium fertilisers
Why is phosphorous used in fertilisers?
DNA, growth, roots
Why is potassium used in fertilisers?
promotes fruiting + flowering
Why is nitrogen used in fertilisers?
- used in biological molecules e.g. proteins and DNA in living creatures
- very few living creatures able to make use of huge amount of nitrogen in atmosphere
Problems with nitrogen compounds:
- eutrophication
- fertilisers washed into rivers + lakes
- algae grows
- kills plants + marine life
Why are ammonium nitrate compounds used as fertilisers?
- most plants require soluble nitrogen compounds to supply their nitrogen intake
- natural sources of nitrogen compounds include manure and compost - however, these natural sources of fertiliser are not sufficient to grow enough food for the world’s growing population
- ammonia is a soluble nitrogen compound but is not suitable to be used as a nitrogen fertiliser as it is alkaline and escapes easily from the soil as it is a gas
- ammonia can be turned into a suitable fertiliser by reacting it with an acid
Why is nitrogen a gas at room temperature?
weak intermolecular forces → require little E to over → low m.p. and b.p.
Why is nitrogen so unreactive?
has strong tripple covalent bond between atoms → large amount of E required to overcome the bonds
Why are harmful NOx gases produced in car engines if nitrogen is unreactive?
car engines involve v. high temps and pressure so N2 will react with O2
How does ammonia react with nitric acid to produce a salt that is used as a fertiliser?
- ammonia can be used to manufacture salts with nitric acid
- ammonia acts as a base
- ammonia + nitric acid —> ammonium nitrate
- NH3 + HNO3 → NH4NO3
How is the reaction of ammonia and an acid different from other neutralisation reactions?
- normally acid + base ——> salt + water
- no water produced
Why are the ammonium salts better fertilisers than ammonia?
- neutral - not alkaline
- less toxic
- don’t escape/evaporates quickly
Method for laboratory preparation of ammonium sulphate:
- Use glass pipette to put 25cm3 of ammonia solution into conical flask
- Add methyl orange - turns yellow
- Fill burette with H2SO4 and add to conical flask
- Near endpoint add drop-wise and swirl flask
- Indicator turns orange
- Repeat using same volumes but without indicator
- Heat/Leave water to evaporate leaving solid NH4NO3
- (only few stages - titration and then crystallisation)
Reactants for the laboratory preparation of ammonium sulphate:
- ammonia solution + dilute sulphuric acid
- bought from chemical manufacturers
What is the scale of the laboratory preparation of ammonium sulphate?
- small scale - very little is produced
- ammonia and sulphuric acid solutions were below concentration (0.1M) for safety reasons
- milligrams
What is the equipment used for the laboratory preparation of ammonium sulphate?
burette, pipette, flasks etc. were made out of glass
How fast is the laboratory preparation of ammonium sulphate?
- titration was carried out slowly and carefully until indicator just changed colour from yellow to orange
- ammonium sulphate solution was left for a several days to crystallised, producing a batch
What are the conditions required for the laboratory preparation of ammonium sulphate?
- room temperature + pressure
- dilute solutions
What type of reaction was the laboratory preparation of ammonium sulphate?
batch
How is ammonium sulphate produced industrially?
- the ammonia and sulphuric acid are imported on a very large scale (tonnes)
- The reactants are continually piped into larger reaction towers, where concentrated sulphuric acid is sprayed onto anhydrous ammonia gas at 60 degrees C
- The pipes and reaction vessels are made of thick stainless steel to withstand high pressures and highly corrosive nature of reactants
- The factory is designed to minimised risk of explosion from highly exothermic process
- A slurry of ammonium sulphate is blown through hot air to produce small pellets of the fertiliser
- Care is taken to adjust the size of the pellets so they do not jam or break farming machinery
- (many stages required - need to make and ammonia and sulphuric acids, react accurate volumes then evaporate)
Reactants for the industrial preparation of ammonium sulphate:
natural gas, air, water (to make ammonia) and sulphur, air, water (to make sulphuric acid)
What is the scale of the industrial preparation of ammonium sulphate?
- large scale - produces a lot
- tonnes
What are the hazards of the industrial preparation of ammonium sulphate?
- dangerous - high pressure, explosive
- corrosive
What is the equipment used for the industrial preparation of ammonium sulphate?
thick stainless steel
What are the conditions required for the industrial preparation of ammonium sulphate?
- 60 degrees C
- high pressure
What type of process is the industrial preparation of ammonium sulphate?
continuous
Similarities between industrial and laboratory preparation of ammonium sulphate:
- both use sulphuric acid
- both neutralisation
Différences between industrial and laboratory preparation of ammonium sulphate:
- industrial process on much larger scale than laboratory process
- industrial process involves more stages than laboratory process
- ammonia is a gas in the industrial process but a solution in the laboratory process
- laboratory preparation uses titration and crystallisation unlike industrial process
Dynamic equilibria:
Both forward and backward reactions taking place at same time
