金属

Physical properties

Metals 金属 and non-metals 非金属 behave very differently:

Chemical properties

Metals react in three main ways:

• with dilute acids 酸 → a salt 盐 + hydrogen 氢气
• with cold water or steam 蒸汽 → a metal hydroxide (or oxide) + hydrogen
• with oxygen 氧气 → a metal oxide 氧化物

A metal is chosen for a job because of its physical properties.

• Aluminium 铝 is used to make aircraft because of its low density 密度 (it is light).
• Aluminium is used for overhead electrical cables because of its low density and good electrical conductivity.
• Aluminium is used for food containers because it resists corrosion 腐蚀.
• Copper 铜 is used for electrical wiring because of its good electrical conductivity and its ductility (it can be drawn into wires).

An alloy 合金 is a mixture 混合物 of a metal with one or more other elements.

• Brass 黄铜 is a mixture of copper and zinc 锌.
• Stainless steel 不锈钢 is a mixture of iron 铁 with other elements such as chromium 铬, nickel 镍 and carbon 碳.

Alloys are usually harder and stronger than the pure metals, which makes them more useful. For example, stainless steel is used for cutlery because it is hard and does not rust.

Why alloys are harder. In a pure metal, the atoms 原子 are all the same size, so the layers 层 can slide over each other easily. In an alloy, the different-sized atoms stop the layers from sliding, so the alloy is harder and stronger.

The reactivity series 金属活动性顺序 lists metals in order of how reactive they are. Carbon and hydrogen are included for comparison:

potassium 钾, sodium 钠, calcium 钙, magnesium 镁, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold

(most reactive at the top, least reactive at the bottom)

The higher a metal is, the more easily it forms positive ions 离子. This explains its reactions:

• potassium, sodium and calcium react with cold water.
• magnesium reacts with steam (but only very slowly with cold water).
• magnesium, zinc and iron react with dilute hydrochloric acid; copper, silver and gold do not.

Displacement reactions

A more reactive metal will displace a less reactive metal from a solution of its ions (a displacement reaction 置换反应). For example, zinc displaces copper from copper(II) sulfate solution, because zinc is more reactive than copper:

Be ready to say what you would see, because those are separate marks:

• the blue colour of the solution fades - the blue $\text{Cu}^{2+}$ ions are being used up (zinc sulfate is colourless).
• a pink-brown solid coats the zinc - that is the displaced copper.
• the grey zinc dissolves and gets smaller.
• the mixture warms up - displacement is exothermic.

Metal-and-acid reactions have their own list: effervescence 泡腾 (the hydrogen escaping) and the solid dissolves. With copper(II) oxide or carbonate and acid, add that the solution turns blue. Never write "hydrogen is given off" as an observation - you cannot see the gas, only the bubbles, so write effervescence.

Worked example. Zinc and aluminium are both found as oxides. Which can be extracted by heating with carbon, and why? Find each metal's place relative to carbon in the reactivity series. Zinc sits below carbon, so carbon is the more reactive of the two and can take the oxygen away from it: zinc oxide is reduced by heating with carbon. Aluminium sits above carbon, so carbon is not reactive enough to displace it, and aluminium must be extracted by electrolysis instead - which is why aluminium costs far more to extract. The line in the series that decides the method is carbon's, not the top of the list.

The special case of aluminium

Aluminium seems less reactive than its position suggests. This is because it is covered by a thin, strong oxide layer 氧化层 that stops other substances reaching the metal underneath.

Rusting 生锈 is the corrosion of iron and steel. Two things are needed for rusting: oxygen (from the air) and water. The rust 铁锈 formed is hydrated iron(III) oxide.

Stopping rust

Barrier methods 隔离法 keep oxygen and water away from the iron:

• painting, greasing (covering with oil), and coating with plastic.

Galvanising 镀锌 means coating iron with a layer of zinc. This works in two ways:

• it is a barrier (the zinc keeps out air and water);
• it gives sacrificial protection 牺牲保护. Because zinc is more reactive than iron, the zinc loses electrons 电子 and corrodes instead of the iron — even if the surface is scratched.

How a metal is taken from its ore 矿石 depends on its place in the reactivity series. Metals below carbon can be extracted by heating with carbon (which removes the oxygen). Metals above carbon are too reactive for this and must be extracted by electrolysis 电解.

Iron from the blast furnace

Iron is extracted from its ore hematite 赤铁矿 (iron(III) oxide) in a blast furnace 高炉:

• Carbon (coke) burns in hot air to give carbon dioxide and lots of heat.
• This carbon dioxide 二氧化碳 reacts with more carbon to form carbon monoxide 一氧化碳.
• The carbon monoxide reduces the iron(III) oxide to iron (this is reduction 还原).
• Limestone 石灰石 (calcium carbonate) breaks down in the heat to form calcium oxide 氧化钙.
• The calcium oxide reacts with sandy impurities to form slag 炉渣, which is removed.

Aluminium by electrolysis

Aluminium is extracted from its ore bauxite 铝土矿 (purified to aluminium oxide) by electrolysis:

• The aluminium oxide is dissolved in molten cryolite 冰晶石 to lower its melting point and save energy.
• At the cathode 阴极, aluminium ions gain electrons to form aluminium metal.
• At the anode 阳极, oxygen is formed. This oxygen reacts with the hot carbon anodes and burns them away, so they must be replaced regularly.

Common mistakes.

• Saying coke (carbon) reduces the iron(III) oxide in the blast furnace. The coke burns to give the heat and the gas; carbon monoxide is what reduces the iron(III) oxide.
• Saying cryolite is the ore of aluminium. The ore is bauxite; molten cryolite is used to dissolve the aluminium oxide, so the electrolysis runs at a lower temperature.
• Saying aluminium is unreactive because it does not corrode. It is quite reactive; its thin oxide layer protects the metal underneath.
• Using "rust" for any metal. Only iron and steel rust; other metals corrode.
• Getting sacrificial protection the wrong way round. The metal used must be more reactive than iron, so it loses electrons and corrodes instead of the iron.
• Saying the carbon anodes wear away in the electrolysis of aluminium oxide because of the heat alone. The oxygen made at the anode reacts with the hot carbon to form carbon dioxide.

• Learn the reactivity series (potassium → gold). A more reactive metal displaces a less reactive one from a solution of its ions.
• Metals above carbon are extracted by electrolysis; those below carbon (like iron) can be extracted by heating with carbon.
• Rusting needs both water and oxygen. Barrier methods (paint, oil) keep them out; galvanising also gives sacrificial protection, because the more reactive zinc corrodes instead of the iron.
• Alloys are harder than pure metals because the different-sized atoms stop the layers of ions sliding over each other.