Transition metals, alloys and corrosion

The transition elements are metals with similar properties which are different from those of the elements in Group 1. They occupy the central block of the periodic table, between Group 2 and Group 3.

They are still metals, so they still form positive ions and conduct heat and electricity. What is being tested here is the contrast with the alkali metals, which behave very unlike a metal in everyday use — soft enough to cut with a knife, and violently reactive with water.

You should be able to describe the difference compared with Group 1 in melting points, densities, strength, hardness and reactivity with oxygen, water and halogens.

You should also be able to exemplify these general properties by reference to Cr, Mn, Fe, Co, Ni and Cu — chromium, manganese, iron, cobalt, nickel and copper. Naming one of those six turns a general statement into an answer that scores: not "transition metals are less reactive", but "copper does not react with water, whereas potassium reacts violently with it".

Many transition elements have ions with different charges, form coloured compounds and are useful as catalysts. Those three are the typical properties, and each one separates the transition metals from Group 1, where the ion is always 1+, the compounds are white and the metals are not used as catalysts.

You should be able to exemplify these general properties by reference to compounds of Cr, Mn, Fe, Co, Ni and Cu. As with the previous section, name the compound — copper(II) sulfate solution is blue; iron(II) sulfate solution is pale green — rather than only stating that transition metal compounds are coloured.

The Roman numeral in a name such as iron(III) chloride exists precisely because of the first property: it says which of the possible charges the ion carries. Iron(II) chloride is FeCl₂ and iron(III) chloride is FeCl₃, and the compounds are different colours.

Corrosion is the destruction of materials by chemical reactions with substances in the environment. Rusting is an example of corrosion. Both air and water are necessary for iron to rust.

Corrosion can be prevented by applying a coating that acts as a barrier, such as greasing, painting or electroplating. Aluminium has an oxide coating that protects the metal from further corrosion.

Some coatings are reactive and contain a more reactive metal to provide sacrificial protection — for example, zinc is used to galvanise iron.

You should be able to describe experiments and interpret results to show that both air and water are necessary for rusting.

The standard experiment uses three test tubes, each with an iron nail:

• Tube 1: air and water — the nail is half in water and half in air. This is the control, and the nail rusts. • Tube 2: water but no air — the nail is covered with water that has been boiled to drive out dissolved air, with a layer of oil on top to keep air out. The nail does not rust. • Tube 3: air but no water — the nail sits in dry air over a drying agent such as anhydrous calcium chloride, with the tube stoppered. The nail does not rust.

Interpreting it: the nail rusts only in the tube where both are present, so neither air nor water alone is sufficient — both are necessary.

You should also be able to explain sacrificial protection in terms of relative reactivity.

The explanation: the coating metal is more reactive than iron, so it reacts with the air and water in preference to the iron. It corrodes instead of the iron — it is sacrificed — and the iron beneath is protected. Zinc galvanising works this way, and it keeps working even if the coating is scratched, because the zinc is still the more reactive metal in contact with the iron.

That is what separates sacrificial protection from a simple barrier. Paint or grease stops rusting only while the coating is unbroken; a scratch exposes the iron and rusting starts there.

Electroplating coats an object with a thin layer of another metal, using electrolysis. It is done for two reasons, and a question usually wants both: to improve the appearance of the object, and to protect it from corrosion by keeping oxygen and water away from the metal underneath.

The set-up follows one rule. The object being plated is the negative electrode, the cathode, because metal ions are positive and are attracted to it. The positive electrode, the anode, is made of the plating metal. The electrolyte is a solution of a salt of that same metal.

Because the anode dissolves at the same rate as the cathode gains metal, the concentration of the solution stays the same throughout. The anode gets lighter by the same mass the cathode gains — a favourite calculation.

Most metals in everyday use are alloys.

Bronze is an alloy of copper and tin. Brass is an alloy of copper and zinc.

Gold used as jewellery is usually an alloy with silver, copper and zinc. The proportion of gold in the alloy is measured in carats, 24 carat being 100% — pure gold — and 18 carat being 75% gold.

Steels are alloys of iron that contain specific amounts of carbon and other metals. High carbon steel is strong but brittle. Low carbon steel is softer and more easily shaped. Steels containing chromium and nickel — stainless steels — are hard and resistant to corrosion.

Aluminium alloys are low density.

You should be able to recall a use of each of the alloys specified, and to interpret and evaluate the composition and uses of alloys other than those specified given appropriate information.

When an unfamiliar alloy appears in a question, work from the properties in the data you are given to the use, and say which property decides it. That is the same reasoning as the table above: aircraft bodies need low density, cutlery needs corrosion resistance, blades need hardness.