Metals and reactivity

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Learning outcomes Describe the general physical and chemical properties of metals. Explain why metals are often used in the form of alloys Identify representations of alloys from diagrams of their structure. Deduce an order of reactivity from information given.

Alloys A metal such as iron is rarely used on its own as it rusts easily. Pure copper is not very strong so can not be used for parts of machines that are constantly in motion. We can change the properties of these metals; making them harder and more resistant to corrosion. A mixture of two or more metals, or one or more metals with a non-metal, is called an alloy.

An alloy is not just a mixture of metal crystals. The atoms of the second metal form part of the crystal lattice.

How does it work?

Uses of alloys Copper, gold and aluminium are too soft for many uses. They are mixed with other metals to make them harder for everyday use. For example: Brass Brass, used in electrical fittings, is 70 per cent copper and 30 per cent zinc. Bronze Bronze, used in bells, statues and machines, is could typically be 80% copper and 20% tin and/or other metals 18 carat gold 18 carat gold, used in jewellery, is 75 per cent gold and 25 per cent copper and other metals. Duralumin Duralumin, used in aircraft manufacture, is 96 per cent aluminium and 4 per cent copper and other metals.

Smart alloys an return to their original shape after being bent. They are useful for spectacle frames and dental braces. Nitinol is an alloy of nickel and titanium, and is known as a shape memory alloy. If nitinol is bent out of shape, it returns to its original shape when it is either heated or an electric current is passed through it. (pg165)

The reactivity series Chemically some metals are more reactive than others. This can be deduced using the reactivity series of metals, which is an arrangement of metals (and carbon and hydrogen) from the most reactive to the least reactive. The extent of reactivity of a metal is considered by its ability to form a positive ion. The reactivity series of metals was determined by experiments in laboratories involving the observation of different metals reacting with water and steam, dilute acid and oxygen.

1. Reactions with Air (Oxygen) Most metals will react with air to form a metal oxide (these are either basic or amphoteric). The most reactive metals like potassium, sodium and magnesium will burn with a very bright flame and will tarnish quickly in open air. Moderately reactive metals such as magnesium, aluminium, zinc and iron burn in strong flame but don't tarnish rapidly in open air. Iron as we know rusts to give a red-brownish compound (Fe 2 O 3 ). The least reactive metals such as lead and copper show oxidisation in strong heat but show little tarnishing in open air. Metals below lead and copper in the reactive series are considered to not react with oxygen, e.g. gold and silver.

2. Reactions with Water We can divide reactions between metals and water into two categories - hot and cold water. Some metals such as potassium and sodium are so reactive they will react vigorously with cold water to form metal hydroxides and hydrogen gas. For the moderately reactive metals they will only react with steam to produce the appropriate oxide and hydrogen gas. The less reactive metals do not react with water or steam. 2K + 2H 2 O = 2KOH + H 2 Mg + H 2 O = MgO + H 2

3. Reactions with Dilute Acids Most reactive and moderately reactive metals react with dilute acid with decreasing. An exception is that of aluminium which takes longer to ‘eat through’ the protective oxide layer. Very unreactive metals show no or very slight activity. Lead reacts very slowly with acid and stops eventually while copper does not react at all. It is this property of copper and metals below it in the reactivity series that enable people to clean coins and jewellery with dilute acids. Note that aluminium appears to be very unreactive. This is because the surface of a piece of aluminium oxidises very quickly and becomes a protective cover preventing further reaction. Other metals oxidise like this, such as iron(III) oxide (rust) but don't protect the metal underneath as the oxide layer is usually porous.

Finish questions in book Unit 13.1 and 13.2 summary questions. In your books please.