Dynamic Equilibria - Topic 5

Question 1

Haber process:

Answer 1

• 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

Question 2

Enthalpy change:

Answer 2

• heat energy change at constant pressure
• △H

Question 3

How does temperature affect the rate of attainment of equilibrium?

Answer 3

• 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

Question 4

How does pressure/concentration affect the rate of attainment of equilibrium?

Answer 4

• higher pressure/concentration → gas molecules closer together/more concentrated → more frequent collisions → more frequent successful collisions
• equilibrium reached at faster rate

Question 5

How do catalysts affect the rate of attainment of equilibrium?

Answer 5

• 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

Question 6

Conditions required for the Haber process:

Answer 6

• iron catalyst - which purified gases are passed over
• temperature 450 degrees C
• pressure 200 atmospheres (ATM)

Question 7

How are the conditions of industrial reactions e.g. the Haber process related to the availability and cost of raw materials and energy supplies?

Answer 7

• 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

Question 8

How does the control of temperature and pressure produce an acceptable yield in an acceptable time for the Haber process?

Answer 8

• 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

Question 9

What are the conditions that give highest percentage of ammonia and why is it a problem?

Answer 9

• low temperature + high pressure
• low temperature - rate of react v. slow
• high pressure - dangerous + expensive

Question 10

How does an iron catalyst produce an acceptable yield in an acceptable time for the Haber process?

Answer 10

• does not change position of equilibrium
• increases the rate of attainment of equilibrium

Question 11

Reasons that a low yield is not a problem in the Haber process:

Answer 11

• unreactive gas is recycled
• rate is high

Question 12

What may fertilisers contain and why?

Answer 12

fertilisers may contain nitrogen, phosphorous and potassium compounds to promote plant growth

Question 13

NPK fertilisers:

Answer 13

nitrogen, phosphorous and potassium fertilisers

Question 14

Why is phosphorous used in fertilisers?

Answer 14

DNA, growth, roots

Question 15

Why is potassium used in fertilisers?

Answer 15

promotes fruiting + flowering

Question 16

Why is nitrogen used in fertilisers?

Answer 16

• 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

Question 17

Problems with nitrogen compounds:

Answer 17

• eutrophication
• fertilisers washed into rivers + lakes
• algae grows
• kills plants + marine life

Question 18

Why are ammonium nitrate compounds used as fertilisers?

Answer 18

• 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

Question 19

Why is nitrogen a gas at room temperature?

Answer 19

weak intermolecular forces → require little E to over → low m.p. and b.p.

Question 20

Why is nitrogen so unreactive?

Answer 20

has strong tripple covalent bond between atoms → large amount of E required to overcome the bonds

Question 21

Why are harmful NOx gases produced in car engines if nitrogen is unreactive?

Answer 21

car engines involve v. high temps and pressure so N2 will react with O2

Question 22

How does ammonia react with nitric acid to produce a salt that is used as a fertiliser?

Answer 22

• ammonia can be used to manufacture salts with nitric acid
• ammonia acts as a base
• ammonia + nitric acid —> ammonium nitrate
• NH3 + HNO3 → NH4NO3

Question 23

How is the reaction of ammonia and an acid different from other neutralisation reactions?

Answer 23

• normally acid + base ——> salt + water
• no water produced

Question 24

Why are the ammonium salts better fertilisers than ammonia?

Answer 24

• neutral - not alkaline
• less toxic
• don’t escape/evaporates quickly

Question 25

Method for laboratory preparation of ammonium sulphate:

Answer 25

- 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)

Question 26

Reactants for the laboratory preparation of ammonium sulphate:

Answer 26

- ammonia solution + dilute sulphuric acid - bought from chemical manufacturers

Question 27

What is the scale of the laboratory preparation of ammonium sulphate?

Answer 27

- small scale - very little is produced - ammonia and sulphuric acid solutions were below concentration (0.1M) for safety reasons - milligrams

Question 28

What is the equipment used for the laboratory preparation of ammonium sulphate?

Answer 28

burette, pipette, flasks etc. were made out of glass

Question 29

How fast is the laboratory preparation of ammonium sulphate?

Answer 29

- 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

Question 30

What are the conditions required for the laboratory preparation of ammonium sulphate?

Answer 30

- room temperature + pressure - dilute solutions

Question 31

What type of reaction was the laboratory preparation of ammonium sulphate?

Answer 31

batch

Question 32

How is ammonium sulphate produced industrially?

Answer 32

- **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)**

Question 33

Reactants for the industrial preparation of ammonium sulphate:

Answer 33

natural gas, air, water (to make ammonia) and sulphur, air, water (to make sulphuric acid)

Question 34

What is the scale of the industrial preparation of ammonium sulphate?

Answer 34

- large scale - produces a lot - tonnes

Question 35

What are the hazards of the industrial preparation of ammonium sulphate?

Answer 35

- dangerous - high pressure, explosive - corrosive

Question 36

What is the equipment used for the industrial preparation of ammonium sulphate?

Answer 36

thick stainless steel

Question 37

What are the conditions required for the industrial preparation of ammonium sulphate?

Answer 37

- 60 degrees C - high pressure

Question 38

What type of process is the industrial preparation of ammonium sulphate?

Answer 38

continuous

Question 39

Similarities between industrial and laboratory preparation of ammonium sulphate:

Answer 39

- both use sulphuric acid - both neutralisation

Question 40

Différences between industrial and laboratory preparation of ammonium sulphate:

Answer 40

- 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

Question 41

Dynamic equilibria:

Answer 41

Both forward and backward reactions taking place at same time