#### Oxidation Reduction

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Stamping an identification number into the steel frame of a bicycle compresses the crystal structure of the metal. If the number is filed off, there are scientific ways to reveal the number.

One method is to apply aqueous copper(II) chloride to the number area. The Cu2+ ions react with some iron atoms in the steel frame, producing copper atoms that show the pattern of the number. The ionic equation below represents this reaction.

Fe(s) + Cu2+(aq) → Fe2+(aq) + Cu(s)

Another method is to apply hydrochloric acid to the number area. The acid reacts with the iron, producing bubbles of hydrogen gas. The bubbles form faster where the metal was compressed, so the number becomes visible. The equation below represents this reaction.

2HCl(aq) + Fe(s) → FeCl2(aq) + H2(g)

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Fossil fuels produce air pollution and may eventually be depleted. Scientists are researching ways to use hydrogen as an alternate fuel.

A device called an artificial leaf was invented to produce hydrogen and oxygen using sunlight and water. The artifical leaf is an electrochemical cell. Equations 1 and 2 below represent the reactions taking place in the leaf. Equation 3 represents a reaction of hydrogen when used as fuel.

Equation 1: 2H2O + energy from sunlight → O2 + 4H+ + 4e

Equation 2: 4H+ + 4e → 2H2

Equation 3: 2H2(g) + O2(g) → 2H2O(g) + energy

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Early scientists defined oxidation as a chemical reaction in which oxygen combined with another element to produce an oxide of the element. An example of oxidation based on this definition is the combustion of methane. This reaction is represented by the balanced equation below.

Equation 1: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

The definition of oxidation has since been expanded to include many reactions that do not involve oxygen. An example of oxidation based on this expanded definition is the reaction between magnesium ribbon and powdered sulfur when heated in a crucible. This reaction is represented by the balanced equation below.

Equation 2: Mg(s) + S(s) → MgS(s)

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The diagram and ionic equation below represent an operating voltaic cell. 14 