Metals: Reactivity Series and Metal Extraction
AdvancedThe reactivity series ranks metals from most to least reactive. Potassium to aluminium must be extracted by electrolysis of molten ores; zinc, iron, lead and copper yield to heating with carbon; gold, silver and mercury are so unreactive that some occur free in nature.
- Reactivity series: K Na Ca Mg Al, (C), Zn Fe Pb, (H), Cu Hg Ag Au — higher means more reactive, more easily oxidised
- Metals above hydrogen release hydrogen with dilute acids; those below do not
- Displacement: a reactive metal displaces a less reactive one from its salt, e.g. Fe + CuSO₄ → FeSO₄ + Cu
文體小知識：科學知識說明文——從金屬與氧、水和稀酸的反應劇烈程度建立活動序，再依金屬在序中的位置講解碳還原與電解兩種提取方法。
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1A magnesium ribbon burns with dazzling sparks, while a copper wire in a flame merely blackens slowly — metals differ enormously in reactivity. Chemists compare how vigorously metals react with oxygen (or air), water and dilute acids, then rank them from most to least reactive: potassium, sodium, calcium, magnesium, aluminium, (carbon), zinc, iron, lead, (hydrogen), copper, mercury, silver, gold. Carbon and hydrogen, though not metals, are included as reference lines.
2The higher a metal sits, the more easily it loses electrons to become a cation, and the harder it is to reduce from its compounds. Potassium, sodium, calcium, magnesium and aluminium react vigorously with water or dilute acids and occur almost entirely as compounds in nature. Copper, mercury, silver and gold, below hydrogen, do not react with dilute acids; gold and silver even occur free in nature, which is why people of antiquity could pick up nuggets of native gold.
3Extraction means reducing the metal out of its compound, and the method follows reactivity. The five metals above aluminium bind oxygen so strongly that even carbon cannot reduce them; they require electrolysis of molten compounds, as in making aluminium from molten aluminium oxide. Zinc, iron, lead and copper, below carbon, can be extracted by heating their ores with carbon, or by reduction with carbon monoxide at high temperature — the blast furnace for iron is the classic case.
4The series also predicts displacement reactions: a more reactive metal displaces a less reactive one from its salt solution. For example, an iron nail dipped into copper(II) sulfate solution dissolves while its surface becomes plated with reddish-brown copper. Master the series and you hold the route map of metal chemistry: whether a metal reacts with acid, who displaces whom, and how to extract it — the practical value of this topic.
1Summary 重點摘要
The reactivity series ranks metals from most to least reactive. Potassium to aluminium must be extracted by electrolysis of molten ores; zinc, iron, lead and copper yield to heating with carbon; gold, silver and mercury are so unreactive that some occur free in nature.
2Vocabulary cards Vocabulary
Metals ranked from most to least reactive, also called the metal activity series; carbon and hydrogen are included as references.
A reaction in which a more reactive metal displaces a less reactive metal from its salt solution.
Rock bearing metal compounds from which the metal is extracted, e.g. haematite, bauxite.
Reducing a metal from its ore; the method depends on the metal's reactivity.
Gaining electrons (or losing oxygen); extracting a metal means reducing its ions to the metal.
An iron ore of mainly iron(III) oxide (Fe₂O₃), the feedstock of the blast furnace.
A large furnace in which iron ore is reduced by carbon monoxide at high temperature.
Decomposition driven by an electric current; reactive metals are extracted by electrolysis of molten compounds.
Metals occurring as compounds in nature; reactive metals exist almost entirely in this state.
Metals occurring as the uncombined element in nature, e.g. native gold and silver.
3Step by step Steps
- Step 1: Ranking the metals from experimentsCompare reactivity through three kinds of reaction: with oxygen (or air), with water and with dilute acids. For example, potassium and sodium fizz and dart about on water; magnesium barely reacts with cold water yet bubbles fiercely in dilute acid; copper ignores dilute acid altogether. Collating such evidence, metals rank from most to least reactive: K, Na, Ca, Mg, Al, Zn, Fe, Pb, Cu, Hg, Ag, Au, with carbon after aluminium and hydrogen after lead as reference lines.
- Step 2: What position in the series meansThe higher the position, the more easily the metal loses electrons to form cations, the more firmly it binds oxygen, and the harder its compounds are to reduce. Hydrogen is the key line: metals above hydrogen react with dilute hydrochloric or sulfuric acid, releasing hydrogen; copper, mercury, silver and gold, below hydrogen, do not. For example, zinc granules bubble briskly in dilute acid while a copper piece sits inertly — this single contrast shows zinc lies above hydrogen and copper below.
- Step 3: Displacement — the series in actionA more reactive metal pushes a less reactive one out of its salt solution — the most direct use of the series. Dip an iron nail into copper(II) sulfate solution: iron lies above copper, so iron atoms lose electrons and enter solution as iron(II) ions while copper ions gain electrons and plate onto the nail. The equation is Fe + CuSO₄ → FeSO₄ + Cu; the ionic equation is Fe + Cu²⁺ → Fe²⁺ + Cu. Both observations — reddish-brown coating and the blue fading — must be stated.
- Step 4: The principle of extraction — reducing the ionsIn ores the metal usually sits as cations; extraction reduces those ions back to atoms, and the method follows reactivity. The rule is clear: metals above carbon defeat chemical reduction and need electrolysis; metals below carbon yield to heating with carbon or to carbon monoxide. For example, aluminium demands electrolysis of molten aluminium oxide, iron needs only a blast furnace — a vast cost gap that explains why aluminium, though more abundant than iron, has long cost more.
- Step 5: Electrolysis — the five most reactivePotassium, sodium, calcium, magnesium and aluminium sit above carbon, binding oxygen so strongly that no chemical reducing agent manages the job; an electric current must force the ions to be reduced: melt the ore and pass electricity, and metal ions gain electrons at the cathode. For example, industry electrolyses molten aluminium oxide (Al₂O₃) on a huge scale — drink cans and aircraft bodies both start there — while sodium comes from molten sodium chloride. Electrolysis devours electricity, so aluminium smelters sit where power is cheap, and recycling cans saves far more energy than smelting new metal.
- Step 6: Carbon reduction — the blast furnaceZinc, iron, lead and copper lie below carbon and yield to carbon or carbon monoxide. The blast furnace charges haematite (mainly Fe₂O₃): coke burns to give high temperature and carbon monoxide, and the carbon monoxide strips oxygen from the iron oxide — Fe₂O₃ + 3CO → 2Fe + 3CO₂. Check the balancing stepwise: two iron atoms, three oxygens and three carbons on each side. Heating directly with carbon gives 2Fe₂O₃ + 3C → 4Fe + 3CO₂; either equation must balance to earn the marks.
- Step 7: The least reactive metals and conservationAt the foot of the series — copper, mercury, silver, gold — chemical inertness rules: copper and mercury oxides decompose to the metal on gentle heating, while gold and silver often lie free in veins, needing only mining and refining. For example, prospectors once panned native gold from rivers, yet not one gram of native sodium exists in nature — the contrast in a nutshell. Metal reserves are finite and mining is energy-hungry and environmentally costly, so recycling (aluminium cans, scrap steel) conserves both resources and energy — the positive attitude this course stresses.
4Diagrams Diagrams
5Worked example Example
Industrial context: a steelworks smelts haematite (mainly iron(III) oxide, Fe₂O₃); another plant extracts aluminium from its ore, while a student wrongly suggests reducing aluminium with carbon. Answer: (1) write the balanced equation for reducing iron(III) oxide with carbon monoxide at high temperature; (2) explain why carbon cannot extract aluminium and state the correct method; (3) list which metals in the series must be extracted by electrolysis.
- Write the equation: carbon monoxide takes oxygen from Fe₂O₃ — start with Fe₂O₃ + CO → Fe + CO₂, then balance to Fe₂O₃ + 3CO → 2Fe + 3CO₂
- Check the balance: 2 Fe, 3 O and 3 C on each side — every atom count matches
- Judge aluminium's method: aluminium sits above carbon and binds oxygen extremely strongly, beyond the reducing power of carbon
- State the method: electrolysis of molten aluminium oxide (Al₂O₃); Al ions gain electrons at the cathode
- List the electrolysis metals: K, Na, Ca, Mg and Al — all five above carbon in the series
- Answer: At high temperature iron(III) oxide is reduced by carbon monoxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂. Aluminium lies above carbon in the reactivity series and binds oxygen so strongly that carbon cannot reduce it from its oxide; molten aluminium oxide must be electrolysed, with aluminium ions gaining electrons at the cathode. The five metals above carbon — potassium, sodium, calcium, magnesium and aluminium — all require electrolysis, while zinc, iron, lead and copper, below carbon, are extracted by reduction with carbon or carbon monoxide.
6Common mistakes Mistakes
7Quick check Quick check
Q1Write the order of metals in the reactivity series, including the carbon and hydrogen reference lines.
Q2Why must metals such as potassium and sodium be extracted by electrolysis?
Q3Write the equation for the reduction of iron(III) oxide by carbon monoxide at high temperature.
Q4What is observed when an iron nail is placed in copper(II) sulfate solution? Give the equation.
Q5Why does native gold occur in nature but not native sodium?
8Recap Recap
- Reactivity series: K Na Ca Mg Al, (C), Zn Fe Pb, (H), Cu Hg Ag Au — higher means more reactive, more easily oxidised
- Metals above hydrogen release hydrogen with dilute acids; those below do not
- Displacement: a reactive metal displaces a less reactive one from its salt, e.g. Fe + CuSO₄ → FeSO₄ + Cu
- Extraction follows position: K to Al by electrolysis of molten ores; Zn, Fe, Pb, Cu by carbon or CO (Fe₂O₃ + 3CO → 2Fe + 3CO₂)
- Hg, Ag, Au are least reactive: oxides decompose on heating, gold and silver occur free; recycling conserves resources