Chemistry Answer1-12

1.(a) They were formed from remains of plants and/or animals that lived millions of years ago.
(b) CH4(g) + 2O2(g) à CO2(g) + 2H2O(l)
(c) (i) It is because coal contains sulphur. The combustion of coal thus produces sulphur dioxide.(ii) Sulphur dioxide reacts with rainwater to form acids that lower the pH value of rainwater. This gives rise to acid rain.(iii) Sulphur dioxide irritates the respiratory system.(iv) To install scrubbers in power stations.

2.(a) (i) Carbon monoxide is produced when hydrocarbon fuels burn incompletely.(ii) Carbon monoxide combines more readily with haemoglobin that will reduce the oxygen-carrying capacity of blood. When the concentration of carbon monoxide in blood is very high, haemoglobin is nearly used up in combination with arbon monoxide. The blood can no longer carry oxygen to various parts of the body, thus the person may die.
(b) (i) High temperatures reached during the combustion of petrol inside the engine cause the oxidation of nitrogen. Nitrogen oxides are formed.(ii) Nitrogen oxides dissolve in rainwater to form acid rain.(iii) Nitrogen oxides irritate and attack the respiratory system.(iv) To install catalytic converters in car exhaust systems.
(c) Carbon dioxide
(d) (i) The limewater turns milky.(ii) The citrated blood turns cherry red.(iii) The universal indicator turns red.
(e) Lead compounds in car exhaust gases are cumulative poisons. They have harmful effects on red blood cells and brain cells. The use of unleaded petrol can reduce the amount of lead compounds in air.

3.(a) Naphtha
(b) Carbon dioxideCO2(g) + Ca(OH)2(aq) à CaCO3(s) + H2O(l)
(c) (i) Hydrogen(ii) C5H12(g) + 5H2O(g) à 5CO(g) + 11H2(g) or C5H12(g) + 10H2O(g) à 5CO2(g) + 16H2(g)
(d) (i) Carbon monoxide(ii) It turns citrated blood cherry red in colour.
(e) The substance is added so that any gas leakage can be detected easily.
(f) Open all the doors and windows.
(g) Pressing the electric bell may trigger off a spark which may ignite the town gas, leading to an explosion.

4.(a) A hydrocarbon is a compound that contains only carbon and hydrogen atoms. A saturated hydrocarbon is a hydrocarbon with carbon atoms which form only carbon-carbon single bond.
(b) (i) Alkane(ii) CnH2n+2
(c) Butane has a higher boiling point. The attractive forces among alkane molecules increase as the molecules become larger and heavier. As butane molecules are larger and heavier than propane molecules, more energy is needed to separate the butane molecules during boiling. Therefore, the boiling point of butane is higher.
(d) C3H8(g) + 5O2(g) à 3CO2(g) + 4H2O(l)2C4H10(g) + 13O2(g) à 8CO2(g) + 10H2O(l)
(f) (i) Complete combustion of propane in oxygen gives carbon dioxide gas which turns limewater milky. This shows the presence of carbon in propane.(ii) Complete combustion of propane in oxygen gives water which turns blue cobalt(II) chloride pink. This shows the presence of hydrogen in propane.
(g) Carbon monoxide

5.(a) During the cracking process, the high-boiling fractions are heated in the absence of air to a high temperature. The large molecules collide with one another with greater energy as the temperature increases. Eventually, the molecules have so much energy that they begin to break up into smaller molecules.
(b) Producing extra petrolProducing alkenes
(c) (i) C8H18(ii) CnH2n+2
(d) (i) 2C8H18(l) + 25O2(g) à 16CO2(g) + 18H2O(l)(ii) Carbon monoxide produced would accumulate when running a car engine in an enclosed area. It is dangerous as carbon monoxide is a very poisonous gas.(iii) Carbon dioxide in the atmosphere acts like the glass of a greenhouse. It allows high energy radiation from the sun to penetrate the atmosphere and warms the earth’s surface. Most of the absorbed energy is re-radiated as infra-red radiation. The infra-red radiation is trapped by the carbon dioxide molecules because of its lower energy nature. The heat that is trapped keeps the earth warm. This is the greenhouse effect. Burning petrol puts extra carbon dioxide into the atmosphere. Overheating of the earth occurs because of the greenhouse effect. The problem is known as global warming.
(e) (i) A hydrocarbon is a compound that contains only carbon and hydrogen atoms.An unsaturated hydrocarbon is a hydrocarbon that contains one or more carbon-carbon multiple bonds.(ii) (iii) Alkene(iv) CnH2n

6.(a) X: FuelY: Oxidant
(b) (i) The foam keeps out the air / This excludes the oxidant.(ii) This cuts off the fuel.(iii) This cools the fire and removes the heat.
(c) (i) We can activate it by turning the boiling tube upside down, so that the dilute hydrochloric acid is mixed with the dilute sodium carbonate solution. Carbon dioxide gas is produced. The pressure of the gas pushes a stream of watery mixture out of the tube. The watery mixture cools the fire and the carbon dioxide keeps out the air. (ii) This type of fire extinguishers cannot be used on petrol fires. This is because petrol will float on the water and the fire will spread even further.

7.(a) Petroleum was formed from plants and small marine animals that lived millions of years ago. When they died, their bodies sank to the bottom of the sea and were covered by thick layers of sand and mud. The combined effect of heat, pressure (from overlying layers) and bacterial action gradually changed the dead remains of plants and animals into petroleum and natural gas.
(b) (i) Fractional distillation(ii) It is because different components in petroleum have different boiling points.(iii) (iv) •Wear safety glasses.
(c) (i) As gaseous fuel(ii) As fuel for aircraft
(d) 5-10
(f) (i) Petroleum clogs the feathers of sea birds and prevents them from flying or swimming. Petroleum interferes with the insulation provided by the feathers, so the birds may die of cold. Birds trying to clean their feathers with their beaks may be poisoned.(ii) Petroleum can spoil beaches.(iii) By adding detergent to emulsify the petroleum.The petroleum can then be carriedaway by water current.

8.(a) Cracking
(b) As a catalyst
(c) If the wool is heated directly, the medicinal paraffin may just vaporize and escape before cracking takes place.
(d) Remove the delivery tube from water before the heating is stopped.
(e) The oily liquid might be lower members of alkanes or alkenes.
(f) (i) The acidified potassium permanganate solution would be decolorized.(ii) Ethene
(g) Cracking is important because it can provide extra petrol and a source of alkenes for making a great variety of products.

9.(a) (i) Fraction 1(ii) Refinery gases(iii)
(b) Fraction 4
(c) (i) Fraction 2(ii) Petrol
(d) A high temperature and a catalyst are required.
(e) (i) •Burning of petroleum produces a lot of air pollutants. •The resource of petroleum is limited. •Supply of petroleum may become insecure in times of political unstability in petroleum-producing countries. (ii) Advantages: •Fairly large supply of uranium ore •Low air pollution Disadvantages: •Risks on safety of citizens in case of a nuclear accident •Radioactive waste disposal problem

10.(a) (i) Fuel, oxidant and heat(ii) BCF extinguishers put out a fire by pouring a non-flammable liquid on the fire. The liquid forms a dense blanket of vapour over the fire. This keeps the air out and cools the fire.(iii) BCF does not conduct electricity.(iv) A BCF extinguisher leaves a small amount of toxic fumes after putting out a fire.

11.(b) Structural formula of any alkane
(c) (i) The brown colour of bromine vapour disappears.(ii) Substitution reaction(iii) CH3CH2Br / CH3CHBr2 / CH3CBr3 / CH2BrCBr3 / CHBr2CBr3 / CBr3CBr3 / HBr (any one)
(d) Combustion of alkanes in air becomes less complete as the number of carbon atoms in the molecules increases. Candles are made from alkanes with molecules containing 25 or more carbon atoms. Unburnt carbon particles are present in the candle flame due to incomplete combustion. The carbon particles make the flame yellow.

12.Air pollutants such as oxides of sulphur and nitrogen react with rainwater to form acids that lower the pH value of rainwater. This gives rise to acid rain. Acid rain is harmful to plants and water lives. It causes damage to metalwork and building materials also. Most plants grow best when the pH value of the soil is close to 7. Acid rain makes the soil more acidic and affects the growth of plants. Acid rain makes rivers and lakes more acidic. This is harmful to water lives and even kills them. Acid rain can speed up the corrosion of metals. Many stone buildings are made of limestone and marble, which consist mainly of calcium carbonate. Acid rain reacts with calcium carbonate and causes damage to these buildings.