Water and Air - IGCSE Chemistry Notes

Water

Chemical Tests for Water

• Cobalt (II) chloride turns blue to pink on the addition of water. This test is usually done using cobalt chloride paper. The equation is: $CoCl<em>2(s) + 6H</em>2O(l) \rightarrow CoCl<em>2.6H</em>2O(s)$

• Anhydrous copper (II) sulfate turns white to blue on the addition of water. The equation is: $CuSO<em>4(s) + 5H</em>2O(l) \rightarrow CuSO<em>4.5H</em>2O(s)$

Water Treatment

• Untreated water contains soluble and insoluble impurities.

• Insoluble impurities include soil, pieces of plants, and other organic matter.

• Soluble impurities include dissolved calcium, metallic compounds, and inorganic pollutants.

• Filtration is the process used to remove large insoluble particles by passing the water through layers of sand and gravel filters that trap larger particles.

• Bacteria and other microorganisms are too small to be trapped by the filters, so chlorination is used.

• Chlorination involves the careful addition of chlorine to the water supply, which kills bacteria and other unwanted microorganisms.

• Cholera and typhoid are examples of bacterial diseases that can arise from the consumption of untreated water.

Uses of Water

Water in industry

• As a coolant to reduce the temperature of some industrial processes, e.g., in nuclear power plants.

• Watering crops.

• As a solvent in many chemical production processes.

• Hydroelectric power stations to generate electricity.

• As a first raw material for many processes, e.g., the production of ethanol from ethene and steam (water).

Water in homes

• Drinking, cooking, and washing.

• General sanitation.

• In car radiators, for gardens and plants.

An Inadequate Supply of Water

• Clean and safe water supply is very important to mankind.

• Many problems arise in the event of an inadequate water supply, including:

  • Food shortages and famine due to a lack of crops which cannot grow without a clean water supply.

  • Poor sanitation leads to the spread of bacteria and disease as drinking water becomes infected.

Air

Composition of Air

• Clean, dry air is approximately 78% nitrogen, 21% oxygen, and the remainder is a mixture of noble gases and carbon dioxide.

Common Pollutants in the Air

• Carbon monoxide, sulfur dioxide, oxides of nitrogen, and lead compounds.

Sources of Pollutants:

• Carbon monoxide: Incomplete combustion of carbon-containing substances.

• Sulfur dioxide: Combustion of fossil fuels which contain sulfur compounds (leading to 'acid rain'). $S + O<em>2 \rightarrow SO</em>2$

• Oxides of nitrogen: Car engines.

• Lead compounds: Leaded petrol.

Adverse Effects of Pollutants:

• On buildings and on health, and are of global concern.

Separation of Oxygen and Nitrogen from Liquid Air

• By fractional distillation.

Oxides of Nitrogen in Car Engines and Their Catalytic Removal

• Description and explanation.

Conditions Required for the Rusting of Iron

• Description.

Methods of Rust Prevention

• Paint and other coatings to exclude oxygen.

• Sacrificial protection in terms of the reactivity series of metals.

• Galvanizing as a method of rust prevention.

Uses of Air

• The gases available in the air have many important applications.

• Oxygen is used in steel making, welding, and in breathing apparatus.

• Nitrogen is used in food packaging, the production of ammonia, and in the production of silicon chips.

• Both of these gases are separated from air by fractional distillation.

Fractional Distillation of Air

• The air is first filtered to remove dust, and then cooled in stages until it reaches $-200°C$.

• At this temperature, the air is in the liquid state.

• Water vapor and carbon dioxide freeze at higher temperatures and are removed using absorbent filters.

• The Noble gases are still in the gaseous state at $-200°C$, leaving a mixture of liquid nitrogen and oxygen.

• The liquefied mixture is passed into the bottom of a fractionating column.

• The column is warmer at the bottom than it is at the top.

• Oxygen liquefies at $-183°C$ and nitrogen liquefies at $-196°C$.

• Nitrogen has a lower boiling point than oxygen, so it vaporizes first and is collected as it rises in the gaseous state to the top of the column.

• The liquid $O_2$ is then removed from the bottom of the column.

Air Pollution

• Sources:

  • Carbon monoxide - incomplete combustion of fossil fuels: $CH<em>4(l) + 6O</em>2(g) \rightarrow 3CO(g) + 5C(s) + 9H_2O(l)$

  • Sulfur dioxide - Sulfur impurity in fuels: $S + O<em>2 \rightarrow SO</em>2$

    • $2ZnS + 3O<em>2 \rightarrow 2ZnO + 2SO</em>2$

  • Oxides of nitrogen - nitrogen + oxygen from air react together.

  • Compounds of lead - old water pipes, old paints, petrol in some kinds of racing cars, lead compounds from leaded petrol.

• Adverse effects:

  • Carbon monoxide - poisonous, combining with hemoglobin in blood and prevents it from carrying oxygen.

  • Particulates - respiratory problems, global dimming, smog (reflects sunlight back into space).

  • Sulfur dioxide - acid rain (corrosion to metal structures, buildings and statues made of carbonate rocks), damage to aquatic organisms, pollutes crops and water supplies, irritates lungs, throats and eyes.

  • Oxides of nitrogen - acid rain (similar effects as $SO_2$), photochemical smog, breathing difficulties (particularly for people suffering from asthma).

  • Compounds of lead - damage to the central nervous system, young infants are particularly susceptible to lead poisoning, toxic or brain damage or reduce IQ or nausea or kidney failure or anemia.

Catalytic Converters

• Contain transition metal catalysts including platinum and rhodium.

• Metal catalysts are in a honeycomb within the converter to increase the surface area available for reaction.

• A series of redox reactions occurs which neutralizes the pollutant gases.

  • Carbon monoxide is oxidized to carbon dioxide: $2CO + O<em>2 \rightarrow 2CO</em>2$

  • Nitrogen oxides are reduced to $N<em>2$ gas: $2NO \rightarrow N</em>2 + O<em>2$, $2NO</em>2 \rightarrow N<em>2 + 2O</em>2$

  • Unburned hydrocarbons are oxidized to carbon dioxide and water: $C<em>8H</em>{18} + 12\frac{1}{2}O<em>2 \rightarrow 8CO</em>2 + 9H_2O$

The Rusting of Iron

• Corrosion is the general term used to describe the degradation of metal surfaces whereas rusting is the specific type of corrosion that happens to iron.

• Rusting is a chemical reaction between iron, water, and oxygen that forms the compound iron (III) oxide.

• Oxygen and water must be present for rust to occur.

• Rusting is a redox process and it occurs faster in salty water since the presence of sodium chloride increases the electrical conductivity of the water.

• $4Fe(s) + 3O<em>2(g) + xH</em>2O(l) \rightarrow 2Fe<em>2O</em>3.xH_2O(s)$

Methods of Rust Prevention

Barrier Methods

• Preventing oxygen and water from touching the iron.

  • Paint

  • Oil or Grease

  • Electroplating

  • Plastic

Sacrificial Methods

• Adding a more reactive metal to the iron, e.g., Aluminium, Zinc.

• Galvanizing: coating the object in a layer of zinc, forming a protective coating.

  • Zinc reacts with oxygen and carbon dioxide in the air to form $ZnCO_3$, protecting the iron by the barrier method.

  • Zinc is more reactive than iron, so it loses its electrons more readily: $Zn \rightarrow Zn^{2+} + 2e^-$

  • The iron stays protected as it accepts the electrons released by zinc, remaining in the reduced state and thus it does not undergo oxidation.

  • The electrons donated by the zinc react with hydrogen ions in the water, producing hydrogen gas: $2H^+ + 2e^- \rightarrow H_2$

  • Zinc therefore reacts with oxygen and water and corrodes instead of the iron.