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Chemistry in our world
Chemical reactions in the body (stomach acid, antacids, respiration and fermentation), how iron is made in a blast furnace and how hydrogen could make "green steel", and how rising carbon dioxide is acidifying the oceans.
Australia digs up more iron ore than any other country: around 900 million tonnes a year, mostly from the Pilbara in Western Australia. Almost all of it is shipped overseas and turned into steel by reactions with carbon, which release huge amounts of carbon dioxide. Some of that carbon dioxide ends up in the ocean, where another reaction is slowly changing the water around the Great Barrier Reef. The reactions in this chapter are happening at the scale of the whole planet.
Acids and bases in your body
Your stomach lining makes hydrochloric acid, keeping the stomach contents at a pH of about 1.5 to 3.5. The acid kills most microbes swallowed with food, and activates pepsin, an enzyme that digests protein. The stomach protects itself with a thick layer of mucus and a constant supply of new lining cells. When food moves into the small intestine, the pancreas releases hydrogen carbonate ions that neutralise the acid, because the enzymes there work best at about pH 7 to 8.
If acid escapes up into the oesophagus, which has no protective mucus, it causes the burning pain of reflux ("heartburn"). Antacids are weak bases that neutralise the excess acid:
Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l)
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
The carbon dioxide from carbonate antacids is why they can make you burp. A strong base such as sodium hydroxide could never be used: it would damage the mouth and throat.
The energy for all of this comes from cellular respiration, a slow, enzyme-controlled cousin of combustion, and plants make the glucose by the endothermic reaction of photosynthesis. Yeast and some bacteria release energy without oxygen by fermentation:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (respiration, exothermic)
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (photosynthesis, endothermic, needs light)
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (fermentation by yeast)
Hard-working muscles short of oxygen make lactic acid. It is cleared within an hour or so of exercise, so it isn't the cause of the stiffness you feel a day or two later; that comes from tiny tears in the muscle fibres as they repair.
Making iron and steel
Iron ore is mostly iron(III) oxide (Fe₂O₃, haematite). To get the iron, the oxygen must be removed. This happens in a blast furnace, a steel tower up to 30 metres high that runs day and night for years. Iron ore, coke (nearly pure carbon made from coal) and limestone are tipped in at the top, and hot air is blasted in near the bottom.
- Coke burns in the hot air, heating the furnace to about 1500 °C: C + O₂ → CO₂
- Higher up, carbon dioxide reacts with more coke to make carbon monoxide: CO₂ + C → 2CO
- Carbon monoxide takes the oxygen from the ore: Fe₂O₃ + 3CO → 2Fe + 3CO₂. The molten iron trickles to the bottom.
- Limestone decomposes, CaCO₃ → CaO + CO₂, and the calcium oxide reacts with sandy impurities (silica) to form a molten slag, CaO + SiO₂ → CaSiO₃, which floats on the iron and is used for road building and cement.
Iron from the blast furnace contains about 4% carbon, which makes it brittle. To make steel, pure oxygen is blown through the molten iron to burn off most of the carbon, and small amounts of other metals are added. Steel making is responsible for about 7% of the world's carbon dioxide emissions, because carbon is used to remove the oxygen from the ore. One alternative being developed, including in Australia, is to remove the oxygen with hydrogen made using renewable electricity:
Fe₂O₃(s) + 3H₂(g) → 2Fe(s) + 3H₂O(g)
The by-product is water instead of carbon dioxide, which is why this is called green steel.
Ocean acidification
The oceans have absorbed roughly a quarter of the carbon dioxide released by human activity. That has slowed global warming, but it changes the chemistry of seawater. Carbon dioxide reacts with water to form carbonic acid, a weak acid that releases hydrogen ions:
CO₂(aq) + H₂O(l) → H₂CO₃(aq) → H⁺(aq) + HCO₃⁻(aq)
Seawater is slightly basic, so it doesn't become an acid, but its pH has fallen from about 8.2 before the Industrial Revolution to about 8.1 today. On the logarithmic scale that is roughly a 30% increase in hydrogen ions. Many of the extra H⁺ ions react with carbonate ions (H⁺ + CO₃²⁻ → HCO₃⁻), and carbonate is exactly what corals, oysters, sea urchins and tiny drifting snails called pteropods need to build their calcium carbonate shells and skeletons. With fewer carbonate ions, building shells takes more energy, and if the water becomes too acidic the shells can start to dissolve.
Cold water dissolves more carbon dioxide and naturally holds fewer carbonate ions, so the Southern Ocean around Antarctica is expected to become corrosive to some shells first. On the Great Barrier Reef, acidification adds to the stress of marine heatwaves, which cause coral bleaching. Scientists measure ocean pH at long-term monitoring stations and from ships and floats, and the only lasting fix is to cut carbon dioxide emissions.
- Start at 280 ppm (the year 1800), then slide to today's level of about 425 ppm. By how much has the ocean pH fallen, and by what percentage have hydrogen ions increased?
- Set CO₂ to the ice-age level of 190 ppm. What happens to the pH and to the shell-building index Ω?
- Switch to Southern Ocean water. At about what CO₂ level does Ω fall below 1, so the water becomes corrosive to aragonite shells?
- For tropical reef water, compare Ω at 280 ppm and at 1000 ppm. Explain why coral reefs would grow more slowly even though Ω stays above 1.
Test yourself
Which substance removes oxygen from iron ore in a blast furnace?
Fe₂O₃ + 3CO → 2Fe + 3CO₂.
Why is limestone added to a blast furnace?
CaO from limestone reacts with silica to form slag.
Ocean pH has fallen by about 0.1 since 1800. The hydrogen ion concentration has risen by about
10^0.1 ≈ 1.26, roughly a 26–30% increase.
Ocean acidification harms corals and shellfish mainly because
H⁺ ions convert carbonate into hydrogen carbonate.
Which antacid reaction produces a gas?
Carbonates release carbon dioxide.
Label the blast furnace.
Put the steps in the blast furnace in order.
The stomach makes acid. Antacids are weak . In a blast furnace, impurities are removed as . Using instead of carbon to remove oxygen from iron ore makes water as the by-product. Carbon dioxide dissolves in seawater to form acid.
An Australian company plans to make "green steel" using hydrogen instead of coke. Compare the chemistry of the two methods using balanced equations, and evaluate the environmental benefits, including the effect on the oceans. Identify one challenge for the new method.
In a blast furnace, coke burns to carbon dioxide (C + O₂ → CO₂), which reacts with more coke to form carbon monoxide (CO₂ + C → 2CO). Carbon monoxide removes oxygen from the ore: Fe₂O₃ + 3CO → 2Fe + 3CO₂. Every tonne of iron therefore releases a large amount of carbon dioxide, and steel making produces about 7% of global emissions. In the green steel method, hydrogen removes the oxygen instead: Fe₂O₃ + 3H₂ → 2Fe + 3H₂O. Both are reactions that take oxygen away from iron(III) oxide, but the by-product of the second is water. If the hydrogen is made by electrolysis using solar or wind power, emissions fall sharply. Less carbon dioxide would reduce the enhanced greenhouse effect, and it would also slow ocean acidification: less CO₂ dissolving means less carbonic acid (CO₂ + H₂O → H₂CO₃), a smaller fall in pH and more carbonate ions left for corals and shellfish such as those on the Great Barrier Reef. One challenge is that making enough hydrogen needs enormous amounts of renewable electricity, which is currently expensive; hydrogen is also hard to store and transport.