Carbon Dioxide (CO2)

Properties of Carbon Dioxide

Carbon dioxide (CO₂) is a colourless gas with no smell or taste. It is slightly acidic when dissolved in water, forming a weak acid called carbonic acid (H₂CO₃), which can affect the pH of natural waters.

CO₂ is non-flammable, meaning it does not burn or support combustion. This property makes it useful in fire extinguishers.

Its density is greater than that of air, so CO₂ tends to sink in the atmosphere rather than rise.

Sources of Carbon Dioxide

CO₂ is naturally present in the atmosphere and is produced by several processes:

• Combustion of fossil fuels: Burning coal, oil, and natural gas releases CO₂ as carbon in the fuel reacts with oxygen.
• Respiration: All living organisms produce CO₂ when they respire, breaking down glucose to release energy.
• Decomposition: When plants and animals die, decomposers break down their bodies, releasing CO₂ into the atmosphere.
• Volcanic activity: Volcanoes emit CO₂ from deep within the Earth during eruptions.

For example, when petrol burns in a car engine, hydrocarbons (represented as C_xH_y) react with oxygen to form carbon dioxide and water:

$$\text{C}_x\text{H}_y + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$$

This combustion is a major source of atmospheric CO₂.

Role in Greenhouse Effect

Carbon dioxide is a key greenhouse gas. It absorbs infrared radiation (heat) emitted from the Earth's surface and traps it in the atmosphere. This process helps keep the planet warm enough to support life.

Without greenhouse gases like CO₂, the Earth would be too cold. However, increased CO₂ levels enhance this effect, leading to more heat being trapped and contributing to global warming.

CO₂ molecules absorb energy from infrared radiation because their bonds can vibrate in ways that interact with this radiation. This energy is then re-radiated in all directions, including back towards the Earth's surface.

For instance, sunlight passes through the atmosphere and warms the Earth's surface. The Earth then emits infrared radiation, which CO₂ absorbs and re-emits, trapping heat.

This natural greenhouse effect is essential, but human activities increasing CO₂ levels intensify it, causing climate change.

Environmental Impact

The concentration of CO₂ in the atmosphere has risen significantly since the Industrial Revolution due to increased fossil fuel burning and deforestation.

This rise contributes to climate change, causing effects such as:

• Higher global temperatures
• More extreme weather events (storms, droughts, floods)
• Melting polar ice and rising sea levels

Another important impact is ocean acidification. CO₂ dissolves in seawater and forms carbonic acid, lowering the pH of oceans. This harms marine life, especially organisms with calcium carbonate shells or skeletons, like corals and shellfish.

The chemical reaction in seawater is:

$$\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3$$

This acid dissociates, releasing hydrogen ions that reduce pH and affect marine ecosystems:

$$\text{H}_2\text{CO}_3 \rightarrow \text{H}^+ + \text{HCO}_3^-$$

Learning Example: Calculating the mass of CO₂ produced

When 12 g of carbon burns completely, how much CO₂ is produced?

Carbon reacts with oxygen to form CO₂:

$$\text{C} + \text{O}_2 \rightarrow \text{CO}_2$$

Molar mass of C = 12 g/mol, CO₂ = 12 + (16  2) = 44 g/mol.

From the equation, 1 mole of C produces 1 mole of CO₂.

So, 12 g of C (1 mole) produces 44 g of CO₂.

Answer: 44 g of CO₂ is produced.