10– ammonia Flashcards
Decomposition equation to form ammonia
NH4Cl (s) —> NH3 (g) + HCl (g)
HCl (g) + NH3 (g) —> NH4Cl
Haber process equation
N2 (g) + 3H2(g) —> 2NH3 (g)
2NH3 (g) —> 3H2(g) + N2 (g)
Conditions:
200/250atm
450dgC
Finely divided iron catalyst
Where to get raw materials required for the Haber process?
Nitrogen: fractional distillation of liquified air
Hydrogen: cracking of crude oil fractions
Haber process
- Nitrogen and hydrogen are mixed in the ratio 1:3 by volume
- The gas mixture is compressed to 200/250atm
- The compressed gases flow over the iron catalyst and are heated to 450dgC. Only about 15% of the mixture leaving the chamber is ammonia
- A mixture of ammonia, unreacted nitrogen and unreacted hydrogen is obtained and cooled
- Ammonia gas condenses into liquid and is stored under pressure (boiling point of ammonia higher than nitrogen and hydrogen thus when cooled, only ammonia condenses, N2 H2 remains gases)
- Unreacted nitrogen and hydrogen are transferred to the converter to be recycled for further reaction.
Explain why the optimum pressure is not used but a compromised pressure of 200/250atm is used.
The higher the pressure, the higher the yield of ammonia. Increase in pressure also increases the rate of reaction. However, maintaining a high pressure incurs a large cost as it requires expensive equipment and a large amount of electricity thus a compromise pressure of 200/250atm is used.
Describe and explain the trend between the temperature of the reaction and the percentage yield of ammonia. [2]
Explain why the Haber process is not carried out at room temperature even though it will save energy costs and the yield lol be close to 100%. [1]
As temperature increases, percentage yield of ammonia decreases [1] due to the decomposition of ammonia back into N2 and H2. [1]
Rate of reaction is slow. [1]
Thus, a compromise tempt of 450dgC is used.
Alternate ways to obtain ammonia
Displacement of ammonia from ammonium salts by reaction with alkali
NH4Cl (aq) + NaOH (aq) -> NaCl (aq) + NH3 (g) + H2O (l)
Explain in terms of collision betw particles, how a lower pressure affects the rate of reaction in the reactor
A lower pressure will decrease the number of reacting particles per unit volume, hence decreasing the frequency of effective collisions between reacting particles. Thus, the rate of reaction will decrease. Yield of ammonia decreases, relative amount of nitrogen and hydrogen leaving the main reactor increases.
Use the graph (%yield of ammonia—pressure graph) to predict how a lower pressure affects the relative amounts of ammonia, nitrogen and hydrogen that leave the main reactor. Explain your reasoning.
Using info from the graph, at 550dgC, when pressure decreases from 200atm to 100atm, the yield of ammonia decreases from approximately 15% to 7.5%, hence a lower pressure will decrease the yield of ammonia. Therefore, the relative amount of ammonia decreases. Thus, relative amount of nitrogen and hydrogen that leaves the main reactor.
What effect does a lower pressure have on the final amount of ammonia made from a fixed amount of nitrogen and hydrogen? Explain your reasoning.
A lower pressure will have no effect on the final amount of ammonia made. The final amount of ammonia obtained is dependent on the amount of nitrogen and hydrogen and not the pressure. As the nitrogen and hydrogen are recycled into the reactor and their total amount is fixed, the final amount of ammonia obtained will be fixed.
Explain why the percentages of N2 and H2 gases are different when they are measured by volume and when measured by mass.
1 mole of N2 gas reacts with 3 moles of H2 gas to produce 2 moles of ammonia gas.
Since mole ratio = volume ratio of gases, volume ratio of N2:H2 gas = 1:3
The relative molecular mass of N2, Mr = 28 is greater than relative Mr of H2, Mr=2.
Molar volume of gases is the same at room temperature and pressure. Hence, volume of 1 mole of N2 gas is 3 times smaller than volume of 3 moles of H2 gas.
Give 2 reasons why the untreated gases N2 and H2 are fed back into reactor.
To increase the yield of NH3.
Recycling unreacted gases decreases the amount of energy required to obtain the raw materials as N2 gas and H2 gas are obtained from fractional distillation of liquid air and cracking of crude oil respectively, both which requires energy.
Explain why significantly higher temperature and pressure are not used commercially. [2]
Expensive to maintain such high pressure as it requires expensive equipment [1] and a large amount of electricity.
At high temperatures, ammonia will decompose back to nitrogen and hydrogen, reducing yield of ammonia. [1]
Suggest why the atom economies of the 2 processes are different. [2]
Process 1 only forms one product, which is the useful product ammonium sulfate. [1]
Process 2 forms 2 products, and only one of the product is useful [1]
