Oxygen and Oxides
Composition of Dry Air
The approximate percentages by volume of the four most abundant gases in dry air:
| Gas | Approximate % by volume |
|---|---|
| Nitrogen (N2) | 78% |
| Oxygen (O2) | 21% |
| Argon (Ar) | 0.9% |
| Carbon dioxide (CO2) | 0.037% (approx. 0.04%) |
Water vapour is also present in variable amounts but is excluded from the values for “dry air”.
Determining the Percentage of Oxygen in Air
The percentage of oxygen in air can be measured by reacting a metal or non-metal with the oxygen in a fixed volume of air in a closed system.
Using Iron (Rust Method)
Iron reacts slowly with oxygen and water to form iron oxide (rust). Iron wool or iron filings placed in an inverted burette of air over water will gradually rust, consuming the oxygen. The water level rises as oxygen is removed.
Calculation:
- Initial volume of air = recorded
- Final volume of air = recorded (after iron has rusted)
- Volume of oxygen = initial volume - final volume
- % oxygen = (volume of oxygen / initial volume of air) x 100
Expected result: approximately 20% (about one-fifth of the air).
Using Phosphorus (Bell Jar Method)
Phosphorus burns readily and uses up all available oxygen. It is burned inside a sealed bell jar sitting in a trough of water. As oxygen is consumed, the water level inside the bell jar rises.
Advantage: phosphorus reacts quickly with oxygen until it is all used up.
Disadvantage: phosphorus is toxic and must be handled with great care.
Using Copper (Syringe Method)
100 cm3 of air is passed back and forth over heated copper. The copper reacts with oxygen to form black copper(II) oxide: 2Cu(s) + O2(g) → 2CuO(s)
The volume of air decreases as oxygen is consumed. The experiment ends when the volume stops decreasing.
Expected result: approximately 79 cm3 remains from 100 cm3, showing about 21 cm3 (21%) was oxygen.
Combustion Reactions
Combustion is the reaction of a substance with oxygen to produce oxides. It is always an exothermic reaction and an example of oxidation (gain of oxygen).
| Element | Observation | Equation |
|---|---|---|
| Magnesium | Burns with an intense white flame; white powder (magnesium oxide) produced | 2Mg(s) + O2(g) → 2MgO(s) |
| Hydrogen | Burns quietly; water is produced (check by seeing condensation or using anhydrous CuSO4) | 2H2(g) + O2(g) → 2H2O(l) |
| Sulfur | Burns with a blue flame; colourless, toxic gas (sulfur dioxide) is produced | S(s) + O2(g) → SO2(g) |
| Carbon | Burns with a glowing red/orange; carbon dioxide (and possibly carbon monoxide) produced | C(s) + O2(g) → CO2(g) |
Thermal Decomposition of Metal Carbonates
Thermal decomposition is a reaction in which a substance breaks down when heated.
Metal carbonates decompose on heating to produce the metal oxide and carbon dioxide:
General equation: metal carbonate → metal oxide + carbon dioxide
Example: Copper(II) carbonate: CuCO3(s) → CuO(s) + CO2(g)
- The green copper(II) carbonate powder turns black as copper(II) oxide is formed.
- The carbon dioxide produced can be identified by bubbling the gas through limewater, which turns milky white.
Other metal carbonates that undergo thermal decomposition include zinc carbonate (ZnCO3), calcium carbonate (CaCO3) and sodium carbonate (Na2CO3, though it requires a very high temperature).
Carbon Dioxide and the Greenhouse Effect
Carbon dioxide (CO2) is a greenhouse gas. Other greenhouse gases include methane (CH4) and water vapour (H2O).
How the Greenhouse Effect Works
- Short-wavelength radiation (visible light and UV) from the Sun passes through the Earth’s atmosphere and is absorbed by the Earth’s surface.
- The Earth’s surface warms up and re-emits energy as infrared (IR) radiation (longer wavelength).
- Greenhouse gases in the atmosphere absorb some of this IR radiation and re-radiate it in all directions, including back towards the Earth.
- This keeps the Earth warm enough to support life: the natural greenhouse effect.
The Enhanced Greenhouse Effect
Human activities (burning fossil fuels, deforestation, agriculture) are increasing the concentrations of CO2 and CH4 in the atmosphere. This traps more heat energy, causing:
- An increase in average global temperatures (global warming)
- Climate change: changes in weather patterns, rising sea levels, more extreme weather events
Exam Tip
Distinguish between the natural greenhouse effect (essential for life; maintains average temperature of ~15 °C) and the enhanced greenhouse effect (caused by increased greenhouse gas concentrations from human activity; leads to global warming). They are different things.
Exam Questions
Q: State the approximate percentage of oxygen in dry air.
Approximately 21% by volume.
Q: Describe how iron can be used to determine the percentage of oxygen in air.
Iron wool is placed in an inverted burette filled with air and submerged in water. The iron rusts over several days, consuming the oxygen in the air. The water level rises into the burette. The increase in water level equals the volume of oxygen consumed. % oxygen = (volume of oxygen / initial volume of air) x 100.
Q: Describe what you would observe when magnesium is burned in oxygen.
Magnesium burns with an intense white flame and produces a white powder (magnesium oxide).
Q: Describe the test for carbon dioxide and state the result.
Bubble the gas through limewater (calcium hydroxide solution). If carbon dioxide is present, the limewater turns milky white (a white precipitate of calcium carbonate forms).
Q: Explain how increasing atmospheric carbon dioxide contributes to climate change.
Carbon dioxide is a greenhouse gas that absorbs infrared radiation emitted by the Earth’s surface and re-radiates it back towards the Earth. Increasing CO2 concentrations trap more heat energy, raising average global temperatures, which is the enhanced greenhouse effect that drives climate change.
Exam Tip
When describing combustion reactions, always include balanced equations with state symbols. Note that sulfur dioxide (SO2) produced from burning sulfur is a toxic gas that contributes to acid rain. This is why sulfur emissions from power stations are an environmental concern.