New 21st Century Chemistry
Suggested answers to in-text activities and unit-end exercises
Topic 14 Unit 47
In-text activities
Discussion (page 108)
1 a) Ammonia is too alkaline and this makes the soil unsuitable for plants to grow.
Ammonia is a toxic gas.
Ammonia is volatile and difficult to store.
b) Ammonium nitrate is very soluble in water and thus easily taken in by plants.
Ammonium nitrate has a high percentage by mass of nitrogen. The higher the percentage by mass of nitrogen in a fertilizer the better because less material needs to be stored and transported.
2 a) When the temperature is increased, the system will respond by reducing the temperature.
As the backward reaction is endothermic, the system will undergo a net backward reaction.
Thus, the yield of N2O4(g) will decrease.
b) The cost of building and running the plant is lower.
Discussion (page 120)
When production is on an industrial scale, there are many issues to consider and if the present system is working well, why change it? Issues such as cost effectiveness, purity of the product, environmental impact and the disposal of the mercury if a different type of cell is to be employed, all need to be considered. It may cause more environmental damage to change a system than to keep the present system well maintained.
However, when setting up a new processing plant, the environmental concerns are much higher up the agenda today than they were at the start of the 20th century. The membrane cell will be the favourite in the majority of cases.
Discussion (page 124)
a) i) When the temperature is increased, the system will undergo a net reaction so as to lower the temperature.
As the forward reaction is exothermic, the system will undergo a net backward reaction.
Hence the yield of methanol will decrease.
ii) An increase in pressure will bring about a net reaction that decreases the number of moles of gas. This helps to reduce the pressure.
A net forward reaction will occur.
Thus, the yield of methanol will increase.
b) A low temperature increases the yield of methanol.
However, the reaction proceeds too slowly when the temperature is too low.
A temperature of 300 °C is a compromise between rate and yield.
A high pressure increases the yield of methanol.
However, using a high pressure increases the cost of building and running the plant.
A pressure of 100 atmospheres is a compromise.
Decision Making (page 126)
Students may choose location A due to the following reasons:
• It is near to the industrial area and the demand for the acid should be great.
• It is close to the power station and thus there is a reliable supply of electricity for the production process.
• It is near to the residential area and labour is readily available.
• Water required for the production can be obtained from the lake.
However, the prevailing wind would blow air pollutants produced towards the residential area nearby. Thus, measures should be taken to reduce air pollutants.
Students may choose location B due to the following reasons:
• It is near to the sulphur deposits and thus easy to obtain the raw material.
• It has a good transportation network as it is located along the railway and near to the pier. This is essential for the transportation of raw materials, products and labour.
• It is very far away from the residential areas. A plant located there should have the least environmental impact on the residents. However, labour is less readily available.
Location C is probably not a good choice as it is very near to the farm and residential area. The environment of the farm and residential area is likely to be affected by a plant in location C. It is far away from the industrial area where the demand for the acid should be great. It is also very far away from the power station where there is a reliable supply of electricity for production.
STSE Connections (page 128)
1 a) i) Between 6 a.m. and 9 a.m.
ii) These times correspond to the morning rush hours, when traffic is at a peak.
b) i) NO
ii) This might suggest that NO is involved in ozone formation.
2 • Avoid using aerosol consumer products such as hairsprays, air fresheners, deodorants and insecticides that often use VOCs as their propellants. Non-aerosol consumer products are usually in pump, solid, liquid, gel or roll-on forms.
• Replace solvent-based paints with water-based paints.
• Avoid buying paints with labels indicating that their VOC contents will exceed the future limits prescribed by the law.
• Avoid using VOC-containing products such as organic cleansing solvents and ask suppliers for non-VOC or low-VOC alternatives, or products with a green label (e.g. awarded by Green Council).
• Store VOC-containing products in air-tight containers.
• Buy products with less packaging as the printing of packaging materials generates VOCs.
• Avoid refuelling during hotter period of the day to minimize escape of petrol vapour due to petrol ‘cooking’ in the fuel tank.
• Drive less, share rides and use public transportation.
Reference:
http://www.epd.gov.hk/epd/english/environmentinhk/air/prob_solutions/vocs_smog.html#point_7
Unit-end exercises (pages 133 – 142)
Answers for the HKALE questions are not provided.
1 a) Nitrogen, phosphorus and potassium
b) Direct combination of nitrogen and oxygen in the air during lightning flashes.
The action of bacteria.
The manufacture of ammonia by the Haber process.
2 Main types of cells used for the electrolysis of brine in industry:
• mercury electrolytic cell;
• diaphragm cell;
• membrane cell.
3 Main stages in the production of methanol in industry:
• syngas (synthesis gas) generation;
• methanol synthesis;
• crude methanol purification.
4 biomass — organic matter such as urban wood waste, agricultural residues and animal waste that can be made into fuel.
activation energy — the minimum kinetic energy required by colliding particles for a reaction to occur.
enzyme — protein-based catalyst.
batch process — a process in which feedstocks are put into a reaction and allowed to react. When the reaction is completed, the product is separated from the reaction mixture.
fermentation — the slow breakdown of large organic molecules into small molecules by microorganisms in the absence of oxygen.
ammonia — a chemical manufactured via the Haber process.
heterogeneous catalysis — a process in which the reactants and the catalyst are in different phases.
gasification —– a process used to convert biomass to a product gas.
feedstocks — reactants which go into a chemical production process.
methanol — the first member of the alcohol series.
yeast — a living organism containing enzymes which can break down glucose to ethanol and carbon dioxide.
activated complex — the highly energetic and unstable species that exist briefly at the point of maximum potential energy during a reaction.
chlorine — a gas used as a disinfectant in water treatment.
homogeneous catalysis — a process in which the reactants and the catalyst are in the same phase.
petrochemicals — chemicals derived from petroleum or natural gas.
5 a) CH4(g) + H2O(g) 3H2(g) + CO(g)
b) 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)
c) Neutralization
d) % by mass of nitrogen in NH4NO3 = x 100%
= 35.0%
% by mass of nitrogen in CO(NH2)2 = x 100%
= 46.7%
∴ urea (CO(NH2)2) is the more effective fertilizer.
6 a) By fractional distillation of liquid air
b) N2(g) + 3H2(g) 2NH3(g)
c) Iron
d) i) To condense the ammonia gas into liquid.
ii) Pump the unreacted nitrogen and hydrogen back to the catalytic converter to react again.
e) For the manufacturing of fertilizers and explosives.
7 a) The mercury electrode is the cathode because sodium is produced at the mercury electrode.
b) Sodium is produced at the mercury cathode. The sodium dissolves in mercury to form an amalgam. The amalgam then flows into a second cell and reacts with water to form hydrogen gas and sodium hydroxide solution. Mercury is recovered.
2Na/Hg(l) + 2H2O(l) 2NaOH(aq) + H2(g) + 2Hg(l)
sodium amalgam
c) The process produces waste which contains poisonous mercury compounds. These waste products will cause serious pollution problems if they are discharged into rivers and seas. The marine animals may die of mercury poisoning.
8 a) No
Air contains about 20% of oxygen.
A mixture of oxygen and hydrogen is explosive.
b) Allow natural gas to react with steam.
CH4(g) + H2O(g) 3H2(g) + CO(g)
c) When the temperature is increased, the percentage of ammonia in the equilibrium mixture decreases.
It can be deduced that when the temperature is increased, the system will undergo a net backward reaction so as to lower the temperature.
Hence the production of ammonia from nitrogen and hydrogen should be an exothermic reaction.
d) i) 400 – 450 °C
ii) A lower temperature increases the yield of ammonia.
However, the reaction proceeds too slowly when the temperature is too low.
A temperature between 400 and 450 °C is a compromise between rate and yield.
e) i) 200 atmospheres
ii) A high pressure increases the yield of ammonia.
However, using a high pressure increases the cost of building and running the plant.
A pressure of 200 atmospheres is a compromise.
f) i) Iron
ii) A heterogeneous catalyst
Iron is a solid while the reactants are gases, i.e. the catalyst and the reactants are in different phases.
iii)
g) Pump the unreacted nitrogen and hydrogen back to the reaction chamber to react again.
9 a) Plants require nitrogen for the synthesis of proteins and chlorophyll.
b) % by mass of nitrogen in (NH4)2SO4 = x 100%
= 21.2%
c) NH4NO3
It contains the highest percentage by mass of nitrogen. It is the most soluble in water and thus easily absorbed by plants.
d) 2NH3(aq) + H2SO4(aq) (NH4)2SO4(aq)
e) Advantages
• Easily absorbed by plants
• Can be sprayed onto crops
Disadvantages
Any two of the following:
• Easily washed out of soil
• Causes eutrophication
• Causes blue baby syndrome
10 a) Any three of the following:
• The cost of the storage tanks.
• The value of the land required for storage.
• The value of the materials stored and the working capital tied up with them.
• Some materials may require special storage conditions.
• Additional safety concerns where materials are stored in bulk.
• Economies of scale when buying in bulk.
• Buy raw materials just when required rather than keeping stock.
• The cost to the company should stocks run out and production be brought to a halt.
b) In continuous manufacturing processes, there must be a steady flow of reagents into the reactor and finished products out of the reactor.
It is easier for materials in liquid or gaseous state to move in and out the reactor.
c) By producing fertilizers which are protected, either by external coatings or as a result of their internal compositions.
d) Fertilizers get into the sea / fertilizers are soluble.
The fertilizers encourage algae growth. Algae grow in great green-cloudy masses so that sunlight is cut off from the other water plants.
The plants die and algae are short-lived too. Bacteria cause these plants to decompose and so use up oxygen in the water.
Fishes die due to lack of oxygen.
11 a) Titanium electrode
b) Na+(aq) ions
c) Water
d) At the titanium electrode:
A hydroxide ion is a stronger reducing agent than a chloride ion. Thus, hydroxide ions should be discharged preferentially.
However, at higher concentrations of chloride ions, the discharge of chloride ions becomes more favourable.
Chloride ions are preferentially discharged (oxidized) to form chlorine gas.
At the steel electrode:
A hydrogen ion is a stronger oxidizing agent than a sodium ion. Therefore hydrogen ions are preferentially discharged (reduced) to form hydrogen gas.
Hydrogen ions and chloride ions are consumed in the electrolysis. Sodium ions and hydroxide ions remain in the solution. Thus, sodium hydroxide solution is obtained.
e) Mercury electrolytic cells produce waste which contains poisonous mercury compounds. These waste products cause serious pollution problems if they are discharged into rivers and seas. The marine animals may die of mercury poisoning.
f) i) B
ii) As chlorine is toxic, it can kill the bacteria.
g) Hydrochloric acid
12 a) i) CH4(g) + H2O(g) CO(g) + 3H2(g)
ii) 730 °C
30 atmospheres
nickel oxide catalyst
b) Conditions A
A decrease in pressure will bring about a net reaction that increases the number of moles of gas. This helps to increase the pressure.
A net backward reaction occurs.
Thus, the yield of methanol will decrease.
Conditions B
When the temperature is decreased, the system will respond by raising the temperature.
As the forward reaction is exothermic, the system will undergo a net forward reaction.
Thus, the yield of methanol will increase.
Conditions C
A catalyst increases the rates of both the forward reaction and the backward reaction to the same extent.
Thus, the yield of methanol remains the same.
c) i) Biomass is organic matter such as urban wood waste, agricultural residues and animal waste that can be made into fuel.
ii) The biomass is converted to a product gas in a process called gasification.
The product gas contains hydrogen, carbon monoxide, carbon dioxide, etc.
Part of the carbon dioxide is removed from the gas.
The addition of hydrogen gives a syngas that is suitable for methanol synthesis.
iii) Methanol can be obtained from renewable resources.