Carbon and Its Inorganic Compounds

Lessons

Senior 3 · Chemistry

Carbon and Its Inorganic Compounds

5 lessons

Lesson 5 of 5

The Carbon Cycle and the Carbon Oxides in the Environment

25 min

Where was your carbon last year?

Follow one carbon atom. Today it is in a lump of charcoal in a cooking stove. When the charcoal burns, the atom leaves as part of a carbon dioxide molecule and drifts away in the air. Weeks later a eucalyptus leaf on a hillside takes it in and builds it into sugar, then into wood. One day that wood may be cut and turned into charcoal again, or a goat may eat the leaf and breathe the atom out. Carbon atoms are never used up or destroyed: they keep moving between the air, living things, the oceans and the rocks. So why is the amount of carbon dioxide in the air going up every year?

Definition

Carbon cycle: the continuous movement of carbon atoms between the atmosphere (as carbon dioxide), living things (as carbohydrates, proteins and fats), the oceans and lakes (as dissolved carbon dioxide and hydrogencarbonate ions) and the rocks (as carbonates and fossil fuels).

Processes that remove carbon dioxide from the air

Photosynthesis by green plants and algae: $\mathrm{6CO_2(g) + 6H_2O(l) \rightarrow C_6H_{12}O_6(aq) + 6O_2(g)}$ .
Dissolving in oceans and lakes. Sea creatures use the dissolved carbon to build shells of calcium carbonate, which settle and, over millions of years, become limestone and chalk.

Processes that add carbon dioxide to the air

Respiration by plants, animals and decomposers (bacteria and fungi that feed on dead matter): $\mathrm{C_6H_{12}O_6(aq) + 6O_2(g) \rightarrow 6CO_2(g) + 6H_2O(l)}$ .
Combustion of wood, charcoal and fossil fuels, for example natural gas (methane): $\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)}$ .
Volcanoes, and heating limestone in lime kilns and cement works: $\mathrm{CaCO_3(s) \rightarrow CaO(s) + CO_2(g)}$ .

Figure 5.1: The carbon cycle.

A cycle out of balance

For thousands of years the processes that add carbon dioxide and those that remove it were almost balanced, and the air held about 280 parts per million (ppm) of carbon dioxide. Since about 1750 people have burned ever more coal, oil and gas. These fossil fuels hold carbon that was taken out of the air hundreds of millions of years ago, and burning them returns it in a few decades, far faster than plants and oceans can take it back. Clearing forests makes it worse, because it removes plants that would absorb carbon dioxide and often releases their carbon by burning.

Since 1958, scientists have measured carbon dioxide in the clean air on the mountain of Mauna Loa, in Hawaii. Figure 5.2 shows their yearly averages.

Figure 5.2: Yearly average carbon dioxide in the air at Mauna Loa, 1960–2025 (data: NOAA Global Monitoring Laboratory).

Definition

Greenhouse gas: a gas in the atmosphere that lets sunlight through but absorbs the infrared radiation (heat) given off by the warm ground, then sends part of it back down. Carbon dioxide, methane and water vapour are greenhouse gases. The warming this causes is the greenhouse effect; without it the Earth would be frozen. Extra greenhouse gas strengthens the effect and warms the planet: global warming, one part of climate change.

The environmental issues of the carbon oxides

Carbon dioxide is not poisonous at the levels found in air, but as a greenhouse gas it drives global warming, which brings more intense rain and droughts, rising seas and shifting growing seasons. More of it dissolves in the oceans too, making seawater slightly more acidic, which harms shell-building sea life. Rwanda has promised action: in 2020 the Government announced a plan to cut greenhouse gas emissions by 38% by 2030, compared with the "business as usual" amount: a cut of up to 4.6 million tonnes of carbon dioxide equivalent in 2030.

Carbon monoxide is a local and immediate danger rather than a global one. It comes from incomplete combustion in stoves, generators and engines, and it poisons people nearby by blocking haemoglobin (Lesson 2). In a car's catalytic converter, carbon monoxide in the exhaust reacts with nitrogen monoxide to form carbon dioxide and nitrogen:

$\mathrm{2CO(g) + 2NO(g) \rightarrow 2CO_2(g) + N_2(g)}$

Table 5.1: Reducing the harm from carbon oxides

Gas	Main harm	Ways to reduce it
carbon dioxide	greenhouse gas: global warming and more acidic oceans	use less fossil fuel; hydroelectric and solar power; efficient cooking stoves that use less charcoal; plant and protect forests; public transport and electric vehicles
carbon monoxide	poisons people in closed spaces and near traffic	burn fuels with plenty of air; never use charcoal stoves or generators indoors; maintain engines; catalytic converters

Worked example

Problem. Use Figure 5.2. (a) Calculate the average rise in carbon dioxide per year from 1960 to 2025. (b) Calculate the average rise per year from 2000 (369.7 ppm) to 2025. (c) What do your answers show?

Solution.

(a) $\frac{427.4 - 316.9}{2025 - 1960} = \frac{110.5}{65} = 1.70\,\mathrm{ppm\ per\ year}$

(b) $\frac{427.4 - 369.7}{2025 - 2000} = \frac{57.7}{25} = 2.31\,\mathrm{ppm\ per\ year}$

(c) Carbon dioxide has risen every year, and the rise is getting faster: the recent rate is higher than the average over the whole period, which is why the line in Figure 5.2 curves upwards.

Worked example

Problem. A family burns 1.0 kg of charcoal a day. Taking charcoal to be pure carbon (a simplification), what mass of carbon dioxide does this release each day? Why is the mass of gas greater than the mass of charcoal? (C = 12, O = 16)

Solution. $\mathrm{C(s) + O_2(g) \rightarrow CO_2(g)}$ : 1 mol C (12 g) gives 1 mol $\mathrm{CO_2}$ (44 g).

$n(\mathrm{C}) = \frac{1000\,\mathrm{g}}{12\,\mathrm{g\,mol^{-1}}} = 83.3\,\mathrm{mol} \qquad m(\mathrm{CO_2}) = 83.3 \times 44 = 3670\,\mathrm{g} \approx 3.7\,\mathrm{kg}$

The carbon dioxide is heavier because each carbon atom has joined with two oxygen atoms taken from the air: 1 kg of carbon combines with about 2.7 kg of oxygen.

Warning

Common mistakes. Do not mix up the greenhouse effect with the "hole in the ozone layer": they are different problems caused by different gases (you will meet ozone-destroying chlorine compounds in Unit 4). Do not write that carbon dioxide is poisonous; carbon monoxide is. And the greenhouse effect itself is natural and necessary; the problem is the extra warming from the extra gas.

Tip

Two ways in, many ways out. Carbon leaves the air by photosynthesis and dissolving. It returns by respiration, decay, combustion and volcanoes and heated limestone. Check any carbon-cycle diagram against this list.

FuseSchool: What Is The Carbon Cycle, Part 2 (3 min). Watch for why burning wood returns "recent" carbon to the cycle while burning fossil fuels adds carbon that was locked away for millions of years.

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Practice

Name two processes that remove carbon dioxide from the air and three that add it.
Write balanced equations for (a) photosynthesis and (b) the complete combustion of methane.
Explain the role of decomposers in the carbon cycle.
What is a greenhouse gas? Name two.
Why does burning fossil fuels increase the carbon dioxide in the air, while the carbon dioxide breathed out by animals does not, overall?
Use Figure 5.2 to estimate the carbon dioxide concentration in 1990, then check it against the NOAA value of 354.5 ppm.
State two ways of reducing carbon monoxide pollution in a town, and explain how each works.
A friend says: "Planting trees in Rwanda cannot make any difference, because the whole world's air is mixed." Evaluate this statement.

Show answersHide answers

Remove: photosynthesis; dissolving in oceans and lakes. Add (any three): respiration; decay by decomposers; combustion of fuels; volcanoes; heating limestone.
(a) $\mathrm{6CO_2(g) + 6H_2O(l) \rightarrow C_6H_{12}O_6(aq) + 6O_2(g)}$ ; (b) $\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)}$ .
Decomposers (bacteria and fungi) feed on dead plants, animals and waste and release their carbon as carbon dioxide by respiration, so it can be used again by plants.
A gas that absorbs heat (infrared radiation) given off by the Earth and sends part of it back, warming the planet. Carbon dioxide and methane (also water vapour).
The carbon breathed out by animals came from food that plants made from carbon dioxide recently, so it is recycled. The carbon in fossil fuels was removed from the air millions of years ago, so burning it adds extra carbon dioxide to the present cycle.
About 355 ppm, read from the line at 1990; the NOAA value is 354.5 ppm.
Any two, e.g.: catalytic converters (turn CO into $\mathrm{CO_2}$ by reaction with NO); keep engines well tuned (so fuel burns completely); burn fuels with plenty of air; electric vehicles or public transport (less fuel burned).
Partly true: carbon dioxide mixes through the whole atmosphere, so one country's trees cannot change the world's level much on their own. But every tree absorbs carbon dioxide as it grows, forests also protect soil and water, and many countries acting together, as in Rwanda's 2030 target, do make a measurable difference. The statement is too pessimistic.

1Where was your carbon last year?

2Processes that remove carbon dioxide from the air

3Processes that add carbon dioxide to the air

4A cycle out of balance

5The environmental issues of the carbon oxides

Where was your carbon last year?

Processes that remove carbon dioxide from the air

Processes that add carbon dioxide to the air

A cycle out of balance

The environmental issues of the carbon oxides