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Redox of metals for OCR GCSE Chemistry

Redox of metals

This page covers the following topics:

1. Reactivity series
2. Extraction of metals
3. Biological extraction of metals
4. Oxidation of metals
5. Corrosion

Reactivity series is a list of metals ordered according to their reactivity. Hydrogen is commonly included in a reactivity series as a reference middle point. A more reactive metal displaces a less reactive metal in its salt solution unless the more reactive metal reacts with water in a solution. For example, iron chloride solution reacts with zinc to produce iron and a zinc chloride solution. Copper nitrate solution reacts with zinc to produce copper and zinc nitrate.

FeClโ‚‚ + Zn โ†’ Fe + ZnClโ‚‚
Zn + Cu(NOโ‚ƒ)โ‚‚ โ†’ Zn(NOโ‚ƒ)โ‚‚ + Cu

Reactivity series

Unreactive metals are commonly found as pure metals. Other more reactive metals are found naturally in their ores and need to be processed to get pure elements. The most unreactive metals can be simply heated to extract them from their ores, for example, 2Agโ‚‚O โ†’ 4Ag + Oโ‚‚. Heating an ore with carbon can be used to extract other metals that are less reactive than carbon, for example, C + 2FeO โ†’ COโ‚‚ + 2Fe. Alternatively, electrolysis can be used to extract metals from metal compounds. During the reduction of metal oxides, metal oxides lose oxygen and are reduced to pure metals.

Extraction of metals

Bacterial extraction, also called biomining, uses bacteria to reduce metal oxides from ores with a low metal concentration. Bacterial extraction has an advantage of using little energy as the extraction of the metals is a natural process within some bacteria, yet it can release toxic compounds that can have a negative impact to the environment.

Phytoextraction is a process during which plants are grown on a soil with a low metal oxide concentration to absorb them throughout their lifespan. The plants are then harvested and burnt providing ash that has an increased concentration of desired metals. This process can be considered relatively carbon-neutral due to both capturing COโ‚‚ during the growth of the plants and the release of the COโ‚‚ while burning them.

Biological extraction of metals

Reactivity series can help determine if metals react with cold water, diluted acids or just oxygen. Reactive metals tend to form positive ions in many of these reactions. Unreactive metals are less likely to react with the aforementioned compounds to form positive ions.

The most reactive metals react with water to produce metal hydroxides and hydrogen. For example:
2K + 2Hโ‚‚O โ†’ 2KOH + Hโ‚‚

Some metals react with acids to displace hydrogen forming metal salts and hydrogen. For example:
Mg + 2HCl โ†’ MgClโ‚‚ + Hโ‚‚

During reactions with oxygen metals are oxidised to produce metal oxides. For example:
4Na + Oโ‚‚ โ†’ 2Naโ‚‚O

Oxidation of metals

Corrosion is a process during which substances are damaged by reacting with chemicals in the environment. Rusting is the corrosion of iron and it occurs when iron has access to both water and oxygen.

Corrosion can be stopped or slowed down by applying a coating to a surface, such as paint, another metal (electroplating) or metal oxide layer. For example, aluminium items like cutlery are covered with an aluminium oxide layer that prevents them from reacting with water.

Sacrificial protection can be used to slow down corrosion by attaching a more reactive metal to the surface of another metal. The more reactive the metal is, the more likely it is to be oxidised first, in this way minimising the potential damage to the main metal. For example, zinc is commonly attached to large ships to prevent their corrosion.

Corrosion

1

Provide a balanced chemical equation for the reaction between lithium and water.

The most reactive metals react with water to produce metal hydroxides and hydrogen.

2Li + 2Hโ‚‚O โ†’ 2LiOH + Hโ‚‚

Provide a balanced chemical equation for the reaction between lithium and water.

2

Explain why sodium metal does not displace iron in an iron chloride solution.

A more reactive metal displaces a less reactive metal in its salt solution unless the more reactive metal reacts with water in a solution. Since sodium reacts vigorously with water, sodium does not displace iron in an iron chloride solution.

Sodium reacts with water in the solution.

Explain why sodium metal does not displace iron in an iron chloride solution.

3

Describe how the tendency to form positive ions change with a decreasing reactivity of metals.

Reactive metals tend to form positive ions in many of these reactions. Unreactive metals are less likely to react with the aforementioned compounds to form positive ions.

The tendency to form positive ions decrease with a decreasing reactivity of metals.

Describe how the tendency to form positive ions change with a decreasing reactivity of metals.

4

Explain why phytoextraction can be considered a carbon-neutral process.

Phytoextraction can be considered relatively carbon-neutral due to both capturing COโ‚‚ during the growth of the plants and the release of the COโ‚‚ while burning them.

COโ‚‚ is captured during the growth of plants and released when burning them.

Explain why phytoextraction can be considered a carbon-neutral process.

5

Provide a balanced chemical equation for the reaction that is happening in the beaker presented in the image.

A more reactive metal displaces a less reactive metal in its salt solution unless the more reactive metal reacts with water in a solution.

2Al + 3FeClโ‚‚ โ†’ 2AlClโ‚ƒ + 3Fe

Provide a balanced chemical equation for the reaction that is happening in the beaker presented in the image.

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