Metals

Reactivity of Metals

Across the periodic table, the reactivity of metals of increases from the top of a group to the bottom and from the right of a period to the left. Cesium (Cs) is them most reactive naturally occurring metal, though the laboratory produced element francium (Fr) is the most reactive overall. The least reactive of the metal elements is platinum (Pt). This overall trend can be seen in some of the individual groups of the periodic table. It is most prominent in groups 1 and 2. Here there is a strong increase in the reactivity of the elements as we go down the group from lithium and beryllium to francium and radium. This reactivity can be considered in terms of the ability of the metal to form positive ions in reactions. The more reactive a metal, the easier it is for it to lose electrons to become positive.

Reactivity of Metals with Acids and Water

The reactivity of metals can be investigated by studying their reactions with acids and water.

When reacted with an acid, a metal will produce a (usually) solid salt and hydrogen gas:

\text{Metal} + \text{Acid} \rarr \text{Salt} + \text{Hydrogen}

When reacted with water, metals will produce a metal hydroxide and hydrogen gas:

\text{Metal} + \text{Water} \rarr \text{Metal Hydroxide} + \text{Hydrogen}

The reactivity of a metal will determine how quickly a metal will react in water or acid, and under what conditions. More reactive metals, such as sodium or magnesium, will react readily (even violently) with dilute acids; less reactive metals will react only very slowly. 

Similarly, more reactive metals will react readily with cold water where as less reactive ones will not react.

The Reactivity Series

The Extraction of Metals from their Ores

It is fairly uncommon for metals to occur naturally in their pure elemental form. Instead, metals tend to appear in the Earth’s crust as compounds known as ores. These compounds are often (but far from exclusively) metal oxides. Metal oxide ores are formed by the reaction of elemental metals with oxygen in a process called oxidation as the metals are gaining oxygen atoms.

\text{Metal}+\text{Oxygen}\rarr\text{Metal Oxide}

To extract pure metals from their oxide ores, they must be reduced (i.e. they must lose their oxygen atoms). This is typically done in one of two ways:

1. Reduction by electrolysis
2. Reduction by reaction with carbon

Which method is used will depend on the position of the metal in the reactivity series relative to that of carbon. Metals that come higher in the reactivity series and so are more reactive than carbon will be reduced using electrolysis. Those lower in the reactivity series and so are less reactive will be reduced by reaction with carbon. This is because metal oxides will not lose their oxygen when the metal is more reactive than carbon. 

Reduction by carbon is a preferable method of extraction, as this is much less expensive than extraction using electrolysis. One common example of this method is the the reduction of iron (III) oxide to produce pure iron. In this process (known as smelting) iron (III) oxide is heated to a very high temperature (above 1200\degree \text{C}) in blast furnace. Carbon is added and a reaction takes place to form pure iron:

2\text{Fe}_2\text{O}_3 + 3\text{C} \rarr 4\text{Fe} + 3\text{CO}_2 

In some cases, such as those of gold and platinum, a metal may be so unreactive that it does not naturally form ores. In these cases, the metal can be found in its pure elemental form. 

Oxidation and Reduction

The extraction of metals from their ores are just one example of a family of chemical reactions known as redox reactions. Redox reactions are ones that involve the transfer of electrons between the reactants. The name is a combination of the two processes that take place in a redox reaction:

• Reduction: The gain of electrons by an atom or ion and thus a reduction in its charge.
• Oxidation: The loss of electrons by an atom or ion and thus an increase in its charge.

The term oxidation comes from the early understanding of these reactions involving the gain of oxygen atoms by a reactant to form an oxide. Later, as scientists began to understand more about the chemicals processes involved, the definition was changed to a more accurate one.

Redox reactions can be identified by considering the oxidation states of the elements involved. If there is a change in these across the reaction, then it is a redox process.

Take the example of the reaction between solid sodium and chlorine:

2\text{Na} + \text{Cl}_2 \rarr 2\text{NaCl}

Much like balancing a reaction, we can tally the oxidation states of the reactants and products to see if there has been a change in oxidation state:

Both reactants start out with an oxidation state of 0 (i.e. they have no charge). After the reaction, sodium has lost 1 electron to gain a +1 charge. Chlorine has gained 1 electron to gain a -1 charge. As both elements have undergone a change in their oxidation state, we can identify the reaction as a redox reaction.