Gases in the Atmosphere

Welcome to the study of Gases in the Atmosphere. In this chapter, we explore the composition of clean, dry air, the experimental methods used to find the percentage of oxygen in the air, the combustion reactions of elements in oxygen, and the formation and environmental impact of carbon dioxide.

Key Learning Objectives:

  • State the approximate percentages by volume of the four most abundant gases in dry air.
  • Describe how to determine the percentage by volume of oxygen in air using metals (copper, iron) and non-metals (phosphorus).
  • Describe the combustion of magnesium, hydrogen, and sulfur in oxygen, including the acid-base character of the oxides formed.
  • Describe the formation of carbon dioxide from the thermal decomposition of metal carbonates, such as copper(II) carbonate.
  • Explain the role of carbon dioxide as a greenhouse gas and how increasing amounts may contribute to climate change.

1. Composition of Dry Air

Clean, dry air is a mixture of different gases. You must know the approximate percentages by volume of the four most abundant gases:

  • Nitrogen ( ext{N}_{2}): Approximately 78%
  • Oxygen ( ext{O}_{2}): Approximately 21%
  • Argon ( ext{Ar}): Approximately 0.9%
  • Carbon dioxide ( ext{CO}_{2}): Approximately 0.04%

Note: Air also contains variable amounts of water vapour depending on humidity and temperature.

2. Determining the Percentage of Oxygen in Air

Oxygen can be removed from a known volume of air by reacting it with an excess of a metal or a non-metal. The decrease in volume represents the volume of oxygen originally present.

Method A: Using Copper Metal

  • A sample of air (e.g. \(100\text{ cm}^3\)) is passed repeatedly back and forth between two gas syringes across heated copper turnings in a silica tube.
  • The copper reacts with oxygen to form solid black copper(II) oxide:

    \[2\text{Cu(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{CuO(s)}\]

  • The apparatus is allowed to cool to room temperature before taking the final volume reading (because gases expand when heated).
  • The volume of gas decreases by approximately \(21\%\) (e.g. from \(100\text{ cm}^3\) to \(79\text{ cm}^3\)).

Method B: Using the Rusting of Iron

  • Wet iron wool is placed at the bottom of an inverted measuring cylinder or test tube placed in a beaker of water.
  • Over several days, the iron reacts with oxygen and water to form hydrated iron(III) oxide (rust):

    \[4\text{Fe(s)} + 3\text{O}_2\text{(g)} + x\text{H}_2\text{O(l)} \rightarrow 2\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O(s)}\]

  • As oxygen is consumed, the water level rises to take its place.

Method C: Using Phosphorus (Non-Metal)

  • A piece of phosphorus is placed on an evaporating dish floating on water under an inverted bell jar.
  • The phosphorus is ignited, and it reacts with oxygen to form solid phosphorus oxide, which dissolves in water:

    \[4\text{P(s)} + 5\text{O}_2\text{(g)} \rightarrow 2\text{P}_2\text{O}_5\text{(s)}\]

  • The water level rises up the bell jar by about \(\frac{1}{5}\) (\(21\%\)) of the initial air volume.

Calculating the Percentage of Oxygen

\[\text{Percentage of } \text{O}_2 = \frac{\text{Initial volume of air} - \text{Final volume of air}}{\text{Initial volume of air}} \times 100\]

3. Combustion of Elements in Oxygen

Combustion is an exothermic reaction where a substance burns in oxygen. Elements react with oxygen to form oxides, which can be classified as basic or acidic.

A. Combustion of Magnesium

  • Observations: Magnesium burns with a bright, blinding white flame to form a white powder (magnesium oxide, \(\text{MgO}\)).
  • Equation:

    \[2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)}\]

  • Acid-base character: Magnesium oxide is a basic oxide. It reacts slightly with water to produce an alkaline solution (magnesium hydroxide, \(\text{pH} \approx 9\text{--}10\)):

    \[\text{MgO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Mg(OH)}_2\text{(aq)}\]

B. Combustion of Hydrogen

  • Observations: Hydrogen burns with an almost invisible pale blue flame, producing water vapour. A squeaky pop test confirms the presence of hydrogen.
  • Equation:

    \[2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O(l)}\]

  • Acid-base character: Water is a neutral oxide (\(\text{pH} = 7\)).

C. Combustion of Sulfur

  • Observations: Sulfur burns with a blue flame to form a colourless, pungent, and choking gas (sulfur dioxide, \(\text{SO}_2\)).
  • Equation:

    \[\text{S(s)} + \text{O}_2\text{(g)} \rightarrow \text{SO}_2\text{(g)}\]

  • Acid-base character: Sulfur dioxide is an acidic oxide. It dissolves in water to form sulfurous acid (\(\text{H}_2\text{SO}_3\)), turning blue litmus red (\(\text{pH} \approx 2\text{--}3\)):

    \[\text{SO}_2\text{(g)} + \text{H}_2\text{O(l)} \rightarrow \text{H}_2\text{SO}_3\text{(aq)}\]

4. Carbon Dioxide from Thermal Decomposition

Thermal decomposition is the breakdown of a compound into simpler substances using heat.

Decomposition of Metal Carbonates

Many metal carbonates decompose upon heating to produce a metal oxide and carbon dioxide gas:

\[\text{Metal carbonate} \xrightarrow{\text{Heat}} \text{Metal oxide} + \text{Carbon dioxide}\]

Example: Copper(II) Carbonate

  • Observation: Green copper(II) carbonate powder decomposes upon heating to form a black powder (copper(II) oxide) and a colourless gas (\(\text{CO}_2\)).
  • Equation:

    \[\text{CuCO}_3\text{(s)} \xrightarrow{\Delta} \text{CuO(s)} + \text{CO}_2\text{(g)}\]

  • Testing for Carbon Dioxide: Bubble the gas through limewater (aqueous calcium hydroxide, \(\text{Ca(OH)}_2\)). The limewater turns milky/cloudy due to the formation of a white precipitate of calcium carbonate:

    \[\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}\]

5. Carbon Dioxide and Climate Change

The Greenhouse Effect

  1. The Sun emits short-wavelength radiation (such as visible light and ultraviolet) which penetrates the atmosphere and warms the Earth's surface.
  2. The Earth's surface cools by emitting longer-wavelength infrared radiation.
  3. Greenhouse gases in the atmosphere, including carbon dioxide (\text{CO}_{2}), absorb and re-emit some of this infrared radiation back towards the Earth, keeping the planet warmer than it would otherwise be.

Human Activity and Climate Change

  • Human activities, notably the combustion of fossil fuels (coal, oil, gas) and deforestation, have significantly increased the concentration of \(\text{CO}_2\) in the atmosphere.
  • This enhanced greenhouse effect traps additional thermal energy, which may lead to global warming and climate change (resulting in rising sea levels, extreme weather events, and habitat loss).

Summary of Key Facts

  • Dry air composition: \(78\%\ \text{N}_2\), \(21\%\ \text{O}_2\), \(0.9\%\ \text{Ar}\), \(0.04\%\ \text{CO}_2\).
  • Determining \(\%\ \text{O}_2\): Measured by reacting air with copper, iron, or phosphorus and observing volume reduction.
  • Combustion in \(\text{O}_2\): Magnesium forms basic \(\text{MgO}\) (white flame/powder); hydrogen forms neutral \(\text{H}_2\text{O}\); sulfur forms acidic \(\text{SO}_2\) (blue flame).
  • Thermal decomposition of \(\text{CuCO}_3\): Green \(\text{CuCO}_3\) turns into black \(\text{CuO}\) and \(\text{CO}_2\) gas.
  • \(\text{CO}_2\) & Climate: Increasing \(\text{CO}_2\) from fossil fuels enhances the greenhouse effect and contributes to climate change.