Chemical Changes

Metals react with oxygen to form metal oxides. These reactions are oxidation reactions because the metal gains oxygen. Reduction can be described as the loss of oxygen from a substance. Oxidation and reduction often occur together in chemical reactions.

The reactivity series places metals in order of how readily they form positive ions. A typical order includes potassium, sodium, lithium, calcium, magnesium, carbon, zinc, iron, hydrogen and copper. Metals higher in the series are generally more reactive than those below them.

The position of a metal in the reactivity series can be investigated using its reactions with water and dilute acids. Highly reactive metals react more vigorously. Metals above hydrogen can usually react with dilute acids to produce hydrogen gas, while metals below hydrogen generally do not.

A displacement reaction occurs when a more reactive metal displaces a less reactive metal from its compound. For example, zinc can displace copper from copper sulfate because zinc is more reactive than copper. The reactivity series can therefore be used to predict whether a displacement reaction will occur.

Very unreactive metals such as gold may occur naturally as the metal itself, while more reactive metals are usually found as compounds. Metals less reactive than carbon can often be extracted from their oxides by reduction with carbon. Reduction in this context involves removing oxygen from the metal oxide.

At Higher Tier, oxidation and reduction can also be explained in terms of electron transfer. Oxidation is loss of electrons and reduction is gain of electrons. Ionic equations and half equations can be used to show which species loses electrons and which species gains them.

Acids react with some metals to form a salt and hydrogen gas. For example, magnesium reacting with hydrochloric acid produces magnesium chloride and hydrogen. The name of the salt depends on the acid used and the metal involved.

Acids are neutralised by bases and alkalis. Acid reacting with a metal oxide or metal hydroxide produces a salt and water, while acid reacting with a metal carbonate produces a salt, water and carbon dioxide. Hydrochloric acid forms chlorides, nitric acid forms nitrates and sulfuric acid forms sulfates.

Soluble salts can be prepared by reacting an acid with an excess of an insoluble solid such as a metal oxide, hydroxide or carbonate. Excess solid is added until no more reacts, the mixture is filtered to remove unreacted solid, and the solution is then evaporated and crystallised to produce a pure dry salt.

Acids produce hydrogen ions, H+, in aqueous solution, while alkalis produce hydroxide ions, OH−. The pH scale ranges from 0 to 14. Values below 7 are acidic, pH 7 is neutral and values above 7 are alkaline. Neutralisation involves H+ and OH− ions reacting to form water.

Titration is used to determine the reacting volumes of an acid and an alkali accurately. A measured volume of one solution is reacted with the other using a suitable indicator to identify the end point. The accurate reacting volumes can then be used in concentration calculations at Higher Tier.

Strong acids are completely ionised in aqueous solution, while weak acids are only partially ionised. For solutions of the same concentration, a stronger acid has a lower pH because it produces a greater concentration of hydrogen ions. A decrease of one pH unit represents a tenfold increase in hydrogen ion concentration.

Electrolysis is the decomposition of an ionic substance using electricity. When an ionic compound is molten or dissolved in water, its ions are free to move and can conduct electricity. Positive ions move to the negative electrode, the cathode, while negative ions move to the positive electrode, the anode.

During electrolysis of a molten ionic compound, the metal is produced at the cathode and the non-metal is produced at the anode. Electrolysis is also used to extract metals that are more reactive than carbon. Aluminium is extracted by electrolysing molten aluminium oxide mixed with cryolite.

In aqueous electrolysis, the products depend on the ions present and their relative reactivity. At the cathode, hydrogen is produced if the metal is more reactive than hydrogen; otherwise the metal is deposited. At the anode, oxygen is usually produced unless halide ions are present, in which case the corresponding halogen is formed. At Higher Tier, electrode reactions can be represented using balanced half equations.