Exercises: Greenhouse effect, carbon cycle and ocean acidification

The natural greenhouse effect keeps the average surface temperature of the Earth at about $15\,^{\circ}\text{C}$ rather than about $-18\,^{\circ}\text{C}$.

1. a) Name two naturally occurring greenhouse gases other than carbon dioxide.
2. b) Explain, in terms of radiation, how greenhouse gases warm the lower atmosphere.
3. c) State the difference between the natural greenhouse effect and the enhanced greenhouse effect.

The table gives the 100-year global warming potential (GWP) of three gases, relative to $\mathrm{CO_2}$.

Gas	GWP (100 years)
$\mathrm{CO_2}$	1
$\mathrm{CH_4}$	28
$\mathrm{N_2O}$	265

1. a) Explain what is meant by the term global warming potential.
2. b) Which of the three gases has the greatest warming effect per kilogram emitted over 100 years?
3. c) Calculate the $\mathrm{CO_2}$-equivalent, in kg, of an emission of $10.0\,\text{kg}$ of methane.

Carbon dioxide dissolves in sea water and reacts with water to form carbonic acid.

1. a) Write the equation for the reaction of carbon dioxide with water to form carbonic acid.
2. b) Write an equation showing the dissociation of carbonic acid into hydrogencarbonate ions and hydrogen ions.
3. c) State what happens to the pH of the oceans as more carbon dioxide dissolves.

A farm in County Cork has a herd of 120 dairy cattle. Each animal emits on average $110\,\text{kg}$ of methane per year. The 100-year GWP of methane is 28.

1. a) Calculate the total mass of methane emitted by the herd per year, in kg.
2. b) Calculate the $\mathrm{CO_2}$-equivalent of this emission, in tonnes.
3. c) A car emits $120\,\text{g}$ of $\mathrm{CO_2}$ per km. Calculate how many kilometres a car would have to travel to emit the same mass of $\mathrm{CO_2}$ as your answer to b).

The carbon cycle involves photosynthesis, respiration, combustion, decomposition and dissolution in the oceans.

1. a) Write the balanced equation for photosynthesis using glucose, $\mathrm{C_6H_{12}O_6}$, as the product.
2. b) Write the balanced equation for the complete combustion of glucose.
3. c) Explain why burning peat, coal and natural gas increases the atmospheric carbon dioxide concentration, whereas burning sustainably grown wood is described as approximately carbon neutral.
4. d) Explain why deforestation contributes to the enhanced greenhouse effect.

The mass of $\mathrm{CO_2}$ released when a fuel burns can be calculated from its equation. A household in Galway burns $50.0\,\text{kg}$ of peat briquettes per week. Assume the briquettes are $60.0\%$ carbon by mass and that all the carbon is converted to $\mathrm{CO_2}$ ($A_r$: C = 12, O = 16).

1. a) Calculate the mass of carbon burned per week.
2. b) Calculate the mass of $\mathrm{CO_2}$ produced per week, in kg.
3. c) Calculate the mass of $\mathrm{CO_2}$ produced over a 26-week heating season, in tonnes.

The first dissociation of carbonic acid in sea water is:

$\mathrm{CO_2(aq)} + \mathrm{H_2O(l)} \rightleftharpoons \mathrm{H^+(aq)} + \mathrm{HCO_3^-(aq)}$

and hydrogencarbonate ions are in equilibrium with carbonate ions:

$\mathrm{HCO_3^-(aq)} \rightleftharpoons \mathrm{H^+(aq)} + \mathrm{CO_3^{2-}(aq)}$

Marine organisms build shells of calcium carbonate: $\mathrm{Ca^{2+}(aq)} + \mathrm{CO_3^{2-}(aq)} \rightleftharpoons \mathrm{CaCO_3(s)}$.

1. a) Using Le Châtelier's principle, explain how increased atmospheric $\mathrm{CO_2}$ lowers the concentration of carbonate ions in sea water.
2. b) Explain the consequence for shell-forming organisms such as oysters and corals.
3. c) The average pH of surface sea water has fallen from 8.2 to 8.1. Calculate the factor by which $[\mathrm{H^+}]$ has increased.
4. d) Explain why a fall of only 0.1 pH units is chemically significant.

Carbon dioxide absorbs infrared radiation, but nitrogen and oxygen do not.

1. a) Explain, in terms of molecular vibration and change in dipole moment, why $\mathrm{CO_2}$ absorbs infrared radiation while $\mathrm{N_2}$ does not.
2. b) Explain why the atmospheric lifetime of a gas as well as its infrared absorption affects its GWP.
3. c) Over 20 years the GWP of methane is 84, and over 100 years it is 28. Suggest why the value is lower over the longer period.
4. d) A policy-maker states that methane reduction is more urgent for short-term warming than for long-term warming. Evaluate this statement using the data.

Atmospheric $\mathrm{CO_2}$ concentration rose from $280\,\text{ppm}$ before industrialisation to $420\,\text{ppm}$ today. Assume the atmosphere contains $1.77\times10^{20}\,\text{mol}$ of gas in total, and $M_r(\mathrm{CO_2}) = 44$.

1. a) Calculate the percentage increase in $\mathrm{CO_2}$ concentration.
2. b) Calculate the increase in the amount of $\mathrm{CO_2}$ in the atmosphere, in mol.
3. c) Calculate the corresponding mass, in gigatonnes ($1\,\text{Gt} = 10^{15}\,\text{g}$).
4. d) Humans have emitted about 1,600 Gt of $\mathrm{CO_2}$ over this period. Calculate the fraction that remains in the atmosphere and suggest where the rest has gone.