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- Describe a chemical test for water.
One test to identify water is to pour the substance being tested to anhydrous copper(II)sulfate. Anhydrous means there is no water in the compound. When you add water to anhydrous copper(II)sulfate, it will turn blue .
CuSO4(s) + 5H2O(l) ———–> CuSO4.5H2O(s)
anhydrous copper(II) sulfate + water ———> hydrated copper(II)sulfate
(white) (blue)
Alternatively, you can also use anhydrous cobalt chloride which changes from blue to pink when water is added.
CoCl2(s) + 9H2O(l) ———–> CoCl2.6H2O(s)
anhydrous cobalt(II) chloride + water—————> hydrated cobalt(II) chloride
(blue) (pink)
- Describe and explain, in outline, the purification of the water supply by filtration and chlorination.
Water from lakes and rivers are not pure. It may contain bacteria, dead animals and plants, dirt etc. Filtration is used to separate insoluble substances from the water. Afterwards, the water is chlorinated (treated with chlorine) to get rid of bacteria that couldn’t be removed through filtration.
- State some of the uses of water in industry and in the home.
Water is used in factories as a solvent for many chemicals and as a coolant to stop industrial processes from getting too hot. Water is also used to generate electrical power (through hydroelectric power stations or by using steam to drive turbines).
In agriculture, water is used on farms for watering crops and feeding animals.
At home, water is used for drinking, cooking, washing and cleaning purposes.
- Describe the separation of oxygen and nitrogen from liquid air by fractional distillation.
Gases in the air as used for various purposes, but they need to be separated from one another first. Let’s look at how oxygen and nitrogen are seperated using fractional distillation (to recall what fractional distillation is, click here).
The air is cooled and compressed several times until it becomes liquid air. This liquid air is fed into the distillation column. The nitrogen evaporates first because it has a lower boiling point and rises to the top and collected. The liquid oxygen is collected from the bottom.
- Describe the composition of clean air as being a mixture of 78% nitrogen, 21% oxygen and small quantities of noble gases, water vapour and carbon dioxide.
That’s pretty self-explanatory!
- State the common air pollutants as carbon monoxide, sulfur dioxide and oxides of nitrogen, and describe their sources.
Carbon monoxide, sulfur dioxide and oxides of nitrogen pollute the air.
Carbon monoxide is released when there is incomplete combustion- where there is not enough oxygen to burn a fuel completely, water and carbon monoxide are formed.
Sulfur dioxide and nitrogen oxide both come from the burning of fossil fuels. Sulfur dioxide is also released by volcanoes
- Explain the presence of oxides of nitrogen in car exhausts and their catalytic removal.
Nitrogen oxides formed in car exhausts due to heat and pressure can combine with hydrocarbons (compound that contain hydrogen and carbon) and other compounds in the atmosphere to form smog- a smoky fog. This has lots of medical consequences. So in order to stop nitrogen oxides from being released, a catalytic converter is placed in exhausts of vehicles.
The catalytic converter first reduces the nitrogen oxides to nitrogen.
2NO(g) ———–> N2(g) + O2(g) & 2NO2(g) ————> N2(g) + 2O2(g)
nitric oxide——–> nitrogen + oxygen nitric dioxide———> nitrogen + oxygen
Carbon monoxide, which is also produced in the exhaust is oxidised to carbon dioxide
2CO(g) + O2(g) ———-> 2CO2(g)
- Explain why the proportion of carbon dioxide in the atmosphere is increasing, and why this is important.
Carbon dioxide is naturally present in the atmosphere. But it is also released from burning of fossil fuels in power stations, factories and vehicles.
A greenhouse gas is a gas that absorbs heat energy and stop the heat from escaping into space. These greenhouse gases, thus keep the earth hot. If there were no greenhouse gases, the earth would be very cold. However, increasing levels of greenhouse gases such as carbon dioxide can increase the heat on earth and cause what is known as global warming. You will have heard of this. Due to global warming over the years, polar ice caps are melting, leading to a rise in sea levels causing flooding on land. Global warming also reduces rainfall leading to less food production. The climate will completely change.
- State the adverse effect of common air pollutants on buildings and on health.
Carbon monoxide is a poisonous gas. When you inhale carbon monoxide , it replaces the oxygen that is normally carried by the hemoglobin in your red blood cells. As a result, your brain and other tissues get less oxygen.
Sulfur dioxide can mix with water vapour in the air precipitate down on earth as acid rain. Acid rain can damage vegetation (leaves are damaged and photosynthesis can no longer take place), the soil (it becomes too acidic to grow crops), buildings and bridges (the carbonate rocks and metals used to make them will be eroded away by acid rain and it crumbles).
- Describe the formation of carbon dioxide:
• as a product of complete combustion of carbon-containing substances
Most fuels contain carbon and so when they burn they produce carbon dioxide:
CH4(g) + 2O2(g) ———–> CO2(g) + 2H2O(l)
methane + oxygen ——> carbon dioxide + water
• as a product of respiration
Respiration is the process by which living beings gain energy from food. Glucose in food combines with the oxygen we breathe in to produce carbon dioxide and water:
C6H12O6 + 6O2 ————> 6CO2 + 6H20
glucose + oxygen ———> carbon dioxide + water
• as a product of the reaction between an acid and a carbonate
when an acid reacts with a carbonate, carbon dioxide is produced:
MgCO3(s) + H2SO4(l) ——–> MgSO4(aq) + CO2(g) +H2O(l)
magnesium carbonate + sulfuric acid ——> magnesium sulfate + carbon dioxide + water
• as a product of thermal decomposition.
Thermal decomposition is the breaking down of compounds into two or more products using heat. When compounds containing carbon are thermally decomposed, they produce carbon dioxide.
CaCO3(s) ——-heat——> CaO(s) + CO2(g)
calcium carbonate ———> calcium oxide + carbon dioxide
- Describe the essential conditions for the manufacture of ammonia by the Haber process, including the sources of the hydrogen and nitrogen, i.e. hydrocarbons or steam and air.
Ammonia (NH3) is manufactured from hydrogen and nitrogen in a process known as the Haber process.
The hydrogen required for the process is made from reacting natural gas (methane) and steam:
CH4(g) + H2O(g) ———–> CO(g) + H2(g)
methane + steam ———-> carbon monoxide + hydrogen
Alternatively, it can also be produced by ‘cracking’ hydrocarbons such as ethane:
C2H6(g) ————–> C2H4(g) + H2(g)
ethane —————> ethene + hydrogen
Nitrogen is extracted from the oxygen through fractional distillation, as described previously.
The hydrogen and nitrogen obtained is then reacted to form ammonia at 450°C of temperature and 200 atmospheres of pressure. This is a reversible reaction:
N2(g) + 3H2(g) <————-> 2NH3(g)
Under these conditions about 15% of nitrogen and hydrogen are converted to ammonia. To produce a higher yield, the following conditions are needed:
• low temperature
• high pressure
• catalyst of iron to speed up the reaction
This is an important process because ammonia is used in the manufacture of fertilisers used in agriculture. Fertilisers speed up the growth of plants and gives them additional nutrients, so that they grow quickly and healthily.
- Describe the rusting of iron in terms of a reaction involving air and water, and simple methods of rust prevention, including paint and other coatings to exclude oxygen.
Iron rusts when it comes into contact with both air and water. Water reacts with the iron over time and the oxygen in the air oxidises the iron. Combined, this forms a layer of rust.
4Fe(s) + 3O2(g) + 2H2O(l) ———-> 2Fe2O2.H2O(s)
There are several ways to prevent rust from forming:
- Paint the surface
- Covering the surface in plastic
- Metal plating (through electroplating)
- Greasing and oiling the surface from time to time
- Zinc galvanising: coating the iron/steel with zinc; the zinc will react with the oxygen rather than iron because it is more reactive. Using a more reactive metals to protect a less reactive metal from corrosion is called sacrificial protection.
- Describe the need for nitrogen- phosphorus- and potassium-containing fertilisers.
The nitrogen-phosphorus-potassium fertiliser is called the NPK fertiliser (based on it’s symbols!).
Nitrogen helps with the production of proteins required for the plants’ growth.
Phosphorus helps balance the pH of the soil so that is is just right for the plants’ healthy growth
Potassium helps with the production of chloroplasts that produce chlorophyll, which is required for photosynthesis.
- Describe the displacement of ammonia from its salts by warming with an alkali.
Many fertilisers are slightly acidic and many crops may not grow well in acidic conditions. So farmers sometimes add lime (calcium hydroxide) to the soil to neutralise it. When the lime reacts with the ammonium salts in the soil, ammonia gas is released.
ammonium chloride + calcium hydroxide ——–> ammonia + calcium chloride + water
Thus, we can deduce that when an ammonium salt is warmed with an alkali, the ammonia in the salt is displaced by ammonia gas.
Notes submitted by Lintha
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