Exam practice

The ozone layer in the stratosphere absorbs most of the harmful ultraviolet rays coming from the Sun (all UV-C and most UV-B), so they don’t reach the Earth’s surface. UV rays would cause skin cancer, cataracts and weak immunity in people, and harm plants, animals and ecosystems. Because it shields life on Earth from these rays, it is called the protective layer.

Hydrogen is almost insoluble in water and oxygen is only slightly soluble, so very little of either is lost by dissolving. Each pushes the water down and out of an upside-down jar full of water. This also gives a pure gas, unmixed with air, and lets us see when the jar is full.

Because of global warming. Greenhouse gases such as carbon dioxide, methane and CFCs, released by burning fuels, cutting forests and other human activities, trap more heat in the atmosphere, so the Earth’s temperature is rising. Snow and glaciers on the mountains melt faster than new snow can replace them. Soot settling on the snow darkens it, so it absorbs more sunlight and melts faster still.

Granulated zinc and dilute hydrochloric acid:
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
(With dilute sulphuric acid: Zn + H₂SO₄ → ZnSO₄ + H₂.)

Draw Figure 16.2: a Woulfe’s bottle with granulated zinc and dilute HCl, a thistle funnel dipping into the acid, a delivery tube from just below the cork to a beehive shelf in a trough of water, and an upside-down gas jar on the shelf. Label each part.
Procedure:

1. Put some granulated zinc in the Woulfe’s bottle.
2. Fit a thistle funnel through one neck and a delivery tube through the other, with airtight corks.
3. Lead the delivery tube under water to the beehive shelf in the trough.
4. Turn a gas jar full of water upside down on the beehive shelf.
5. Pour dilute hydrochloric acid through the thistle funnel until its end dips into the acid.
6. Hydrogen is given off (Zn + 2HCl → ZnCl₂ + H₂) and collected by downward displacement of water. Let the first bubbles (air) escape before collecting.

Physical properties: (1) it is colorless, odorless and tasteless; (2) it is the lightest gas, about 14 times lighter than air; (3) it hardly dissolves in water; (4) it is neutral to litmus; (5) it liquefies at −253 °C.

Uses: (1) to make ammonia (Haber process); (2) to make vanaspati ghee from vegetable oil (hydrogenation); (3) as rocket fuel; (4) in the oxy-hydrogen flame for welding and cutting metals; (5) to get metals from their oxides.

Physical properties: (1) it is colorless, odorless and tasteless; (2) it is slightly lighter than air; (3) it dissolves only a little in water; (4) it is neutral to litmus; (5) it neither burns nor helps things burn; (6) it turns liquid at −196 °C.

Uses: (1) to make ammonia and nitric acid, from which fertilizers such as urea are made; (2) to fill food packets, so the food doesn’t go stale; (3) to fill electric bulbs, so the filament doesn’t burn away; (4) as liquid nitrogen, to freeze and keep blood, cells and semen.

Bring a burning splint to the mouth of the jar. If the gas burns with a “pop” sound and a pale blue flame (and the splint goes out when pushed inside), the gas is hydrogen.

• i.Sodium hydride: 2Na + H₂ → 2NaH
• ii.Water: 2H₂ + O₂ → 2H₂O
• iii.Calcium hydride: Ca + H₂ → CaH₂
• iv.Iron and water (reduction): Fe₂O₃ + 3H₂ → 2Fe + 3H₂O

Draw Figure 16.5. Oxygen is made by decomposing hydrogen peroxide with manganese dioxide as a catalyst: 2H₂O₂ → 2H₂O + O₂ (MnO₂ over the arrow).

1. Put some manganese dioxide in a clean conical flask.
2. Fit a thistle funnel and a delivery tube through a two-holed cork, airtight; lead the delivery tube to a beehive shelf in a trough of water, under an upside-down gas jar full of water.
3. Pour hydrogen peroxide through the thistle funnel until the funnel’s end is under the liquid.
4. Oxygen is given off at once, without heating, and is collected by downward displacement of water.

Draw Figure 16.9. Nitrogen is made by gently heating a solution of sodium nitrite and ammonium chloride: NaNO₂ + NH₄Cl → NaCl + N₂ + 2H₂O (Δ over the arrow).

1. Put about 5 g of sodium nitrite and 4 g of ammonium chloride in a clean round-bottom flask and dissolve them in about 50 mL of water.
2. Set the flask on wire gauze on a tripod, hold it with a clamp, and fit an airtight delivery tube through the cork.
3. Lead the delivery tube to the beehive shelf in a trough of water, under an upside-down gas jar full of water.
4. Heat the solution gently with a burner. Once the first bubbles (the air from the flask) have escaped, collect the nitrogen by downward displacement of water.

Careful: heat gently, because the reaction gives out heat itself; and don’t heat dry ammonium chloride, which sublimes.

Draw Figure 16.4. Oxygen is made by heating potassium chlorate with manganese dioxide as a catalyst: 2KClO₃ → 2KCl + 3O₂ (MnO₂ and heat over the arrow).

1. Mix four parts of potassium chlorate with one part of manganese dioxide in a clean, dry hard-glass test tube.
2. Close it with a cork carrying a delivery tube, and clamp it with its mouth slightly lower than its closed end.
3. Lead the delivery tube to a beehive shelf in a trough of water, under an upside-down gas jar full of water.
4. Heat the mixture gently with a burner. Oxygen is given off and collected by downward displacement of water.
5. At the end, remove the delivery tube from the water before stopping the heating, so that water isn’t sucked back.

Physical properties: (1) colorless, odorless and tasteless; (2) slightly heavier than air; (3) slightly soluble in water; (4) neutral to litmus; (5) liquefies at −183 °C to a pale blue liquid.

Uses: (1) respiration in living things; (2) welding and cutting metals in the oxy-acetylene flame; (3) for patients with difficulty breathing, in hospitals; (4) in cylinders for mountaineers, divers, miners and astronauts; (5) in steel making.

Put a glowing splint into the jar. If it bursts into flame, the gas is oxygen. (A burning splint or candle burns more brightly in it than in air.)

• i.Magnesium oxide: 2Mg + O₂ → 2MgO
• ii.Carbon dioxide: C + O₂ → CO₂
• iii.Carbon dioxide and water: CH₄ + 2O₂ → CO₂ + 2H₂O + energy
• iv.Carbon dioxide and water: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

The ozone layer is a layer of ozone gas (O₃) in the stratosphere, roughly 15 to 35 km above the Earth, which absorbs most of the Sun’s ultraviolet rays.
Formation: UV rays split oxygen molecules into oxygen atoms, and each atom joins an oxygen molecule to form ozone:
O₂ → O + O (UV rays)
O₂ + O → O₃
Depletion: CFCs and similar chemicals rise into the stratosphere, where UV rays break them and release chlorine atoms. Each chlorine atom breaks ozone into oxygen and is set free again, so one atom destroys thousands of ozone molecules:
CFCl₃ → CFCl₂ + Cl (UV rays)
Cl + O₃ → ClO + O₂
ClO + O → Cl + O₂

The ozone layer absorbs most of the Sun’s harmful ultraviolet rays. In doing so it protects people from sunburn, skin cancer, cataracts and weakened immunity; protects plants, whose growth, photosynthesis and crops UV would reduce; protects plankton and young animals, and so the food chains and ecosystems on land and in water; and it helps keep the climate of the atmosphere in balance. Without it, life on land would hardly be possible.

The extra UV radiation reaching the ground comes from the thinning of the ozone layer, and that thinning was caused by chemicals people made and used. For example, CFCs from old refrigerators, air conditioners, aerosol sprays and foam rose into the stratosphere, where their chlorine destroyed ozone; this opened the ozone hole over Antarctica, first reported in 1985. Halons from fire extinguishers, methyl bromide pesticides and nitrous oxide from over-used fertilizers did the same. Nature doesn’t make these chemicals in large amounts, so the increase is mainly people’s doing. (That is also why, when people stopped making CFCs under the Montreal Protocol, the ozone layer began to heal.)

Causes: the ozone layer above that place has become thin. Ozone-depleting chemicals such as CFCs, halons, carbon tetrachloride, methyl bromide and oxides of nitrogen reach the stratosphere, where UV rays release chlorine and bromine atoms that destroy ozone. With less ozone, more ultraviolet rays get through. (Height matters too: high places have less air above them, so they always get more UV.)

Effects: more sunburn, eye irritation, cataracts and skin cancer, and weaker immunity in people; slower growth, less photosynthesis and smaller harvests in plants; harm to plankton and young animals, which upsets food chains and ecosystems; and plastics, paints and rubber that crack and fade faster.

2H₂ + O₂ → 2H₂O

2Na + H₂ → 2NaH (sodium hydride)

2KClO₃ → 2KCl + 3O₂↑ (heated, with MnO₂ as catalyst)

CH₄ + 2O₂ → CO₂ + 2H₂O + energy (methane)

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

2Mg + O₂ → 2MgO

4P + 5O₂ → 2P₂O₅ (phosphorus pentoxide)

Zn + H₂SO₄ → ZnSO₄ + H₂ (dilute sulphuric acid)