5. Metals

10.1 Where do metals fit into a table of the elements? Chemists have for more than 150 years arranged elements into a table so that they form Groups with similar properties. There are several groups of metals in such a periodic table. The chemical elements can be arranged in order of the relative atomic masses of their elements. This list can then be arranged in rows so that elements with similar properties are in the same columns, known as Groups. The resulting table is known as the periodic table. Although most elements are in appropriate Groups, a few are not. Argon atoms, for example, have a greater relative atomic mass than potassium atoms but argon is better placed before the potassium in the periodic table so that it is in Group 0 and potassium is in Group 1. More than three-quarters of the elements are metals. In the periodic table metals are mainly found: - in the two left hand columns (Group 1 and Group 2); - in the central block (transition elements). The elements in Group 1 of the periodic table (known as the alkali metals): - are metals with low density (the first three in the Group are less dense than, and therefore float on, water); - react with non-metals to form ionic compounds. The compounds are white solids which dissolve in water to form colourless solutions; - react with water releasing hydrogen; - form hydroxides which dissolve in water to give alkaline solutions. In the centre of the periodic table is a block of metallic elements. These elements, which include iron and copper, are known as transition metals. Like other metals, transition metals are good conductors of heat and electricity and can easily be bent or hammered into shape. Compared to alkali metals: - they have high melting points (except for mercury, which is a liquid at room temperature); - they are hard, tough and strong; - they are much less reactive and so do not react (corrode) so quickly with oxygen and/or water. These properties make transition metals very useful as structural materials (e.g. iron, usually in the form of steel) and for making things which must allow heat or electricity to pass through them easily (e.g. copper for electrical cables). Most transition metals form coloured compounds. These can be seen: - in pottery glazes of various colours; - in weathered copper (green). Many transition metals, including iron and platinum, are used as catalysts. 10.2 How are metals extracted from their ores? How a metal is extracted from its ore depends on how reactive the metal is. Because aluminium is more reactive than iron it has to be extracted from its oxide in a different way. Some metals are more reactive than others. By observing whether or not various metals react: - with air, to produce metal oxides; - with water (cold, hot or as steam) to produce metal hydroxides (or oxides) and hydrogen; - with dilute acids, to produce metal salts and hydrogen; and by observing how vigorous any of these reactions are, a reactivity series can be determined. The Earths crust contains metals and metal compounds. These are always found mixed with other substances. In ores, the metal or metal compound is concentrated enough to make it economic to extract the metal. Gold, an unreactive metal, is found in the Earth as the metal itself. Chemical separation is not needed. The reactivity series of metals lists metals in order of their reactivity, the most reactive metal being placed at the top of the list and the least reactive at the bottom. A more reactive metal can displace a less reactive metal from its compounds. The non-metal elements carbon and hydrogen will also displace less reactive metals from oxides of those metals. Candidates should be able to use the position of a metal in the reactivity series to predict how the metal could be extracted from a compound. Often an ore contains a metal oxide or a substance which can easily be changed into a metal oxide. To extract the metal, the oxygen must be removed from the metal oxide. This is called reduction. How a metal is extracted from its ore depends on how reactive the metal is. A metal such as iron, which is less reactive than carbon, can be extracted from its ore using carbon. The solid raw materials used in the blast furnace are iron ore (haematite), coke and limestone. Hot air is blown into the furnace and this causes the coke to burn forming carbon dioxide and releasing energy. At the high temperatures in the furnace the carbon dioxide reacts with coke to form carbon monoxide. The carbon monoxide reduces the iron oxide in the iron ore into molten iron which flows to the bottom of the furnace. The carbon monoxide combines with the oxygen from the iron ore to produce carbon dioxide. This is called oxidation. Limestone is added to remove acidic impurities forming a molten slag that floats on the surface of the molten iron. Reactive metals such as aluminium are extracted by electrolysis. When substances which are made of ions are dissolved in water, or melted, they can be broken down (decomposed) into simpler substances by passing an electric current through them. This process is called electrolysis. When an ionic substance is melted or dissolved in water the ions are free to move about. During electrolysis, positively charged ions for example, metal ions move to the negative electrode, and negatively charged ions move to the positive electrode. During electrolysis, gases may be given off, or metals deposited at the electrodes. The raw material for producing aluminium is aluminium oxide, purified from aluminium ore (bauxite). Because aluminium oxide has a very high melting point it is dissolved in a molten aluminium compound called cryolite at a much lower temperature. The electrodes are made of carbon. The aluminium forms at the negative electrode and oxygen forms at the positive electrodes. This makes the positive electrodes burn away quickly so that they frequently have to be replaced. Copper can be purified by electrolysis using a positive electrode made of the impure copper and a negative electrode of pure copper in a solution containing copper ions. Higher Tier At the negative electrode positively charged ions gain electrons (reduction). At the positive electrode negatively charged ions lose electrons (oxidation). In a chemical reaction if oxidation occurs reduction also occurs. These reactions are called redox reactions. 10.3 How can metals be prevented from reverting to their oxides? When we use metals as structural materials we dont want them to corrode (oxidise) back to their oxides. There are various ways of preventing this. Iron (or steel) corrodes more quickly than most other transition metals. This corrosion can be prevented by connecting iron to a more reactive metal (e.g. zinc or magnesium) (sacrificial protection) or by mixing in other metals (e.g. chromium) to make non-rusting alloys called stainless steel. Aluminium does not oxidise (corrode) as quickly as its reactivity would suggest. Once a thin oxide layer has formed on the surface, it forms a barrier to oxygen and water and so prevents further corrosion. Aluminium is a useful structural metal. It can be made harder, stronger and stiffer by mixing it with small amounts of other metals (e.g magnesium) to make alloys. 10.4 How can metal compounds be made? Metal compounds called salts can be prepared by reacting metal hydroxides or metal oxides with acids. When a substance dissolves in water it forms an aqueous solution which may be acidic, alkaline or neutral. Water itself is neutral. Indicators can be used to show whether a solution is acidic, alkaline or neutral by the way their colours change. The pH scale is used to show how acidic or alkaline a solution is. Compounds of alkali metals called salts can be made by reacting solutions of their hydroxides, which are alkaline with acids. In these neutralisation reactions: - acid + alkaline hydroxide solution a neutral salt solution + water The particular salt produced in any reaction between an acid and an alkali depends on: - the acid used; - the metal in the alkali. Neutralising hydrochloric acid produces chlorides. Neutralising nitric acid produces nitrates. Neutralising sulphuric acid produces sulphates. Ammonia also dissolves in water to produce an alkaline solution. This can be neutralised with acids to produce ammonium salts. An indicator can be used to show when acidic and alkaline solutions have completely reacted to produce a neutral salt solution. Salts of transition metals, as with salts of some other metals, can be made by reacting their oxides or hydroxides with acids. Transition metal oxides and hydroxides do not dissolve in water and are called bases. To produce a solution of a soluble transition metal salt, the metal oxide (or hydroxide) is added to an acid until no more will react. The excess metal oxide (or hydroxide) can then be filtered off. Hydrogen ions H+(aq) make solutions acidic. Hydroxide ions OH-(aq) make solutions alkaline. Higher Tier In neutralisation reactions: H+(aq) + OH (aq) H2O(l)