Use these half equations below to explain why six hydrogen ions are reduced for every two aluminum atoms oxidized.
Aluminum metal can reduce the hydrogen ions in a solution of hydrochloric acid to hydrogen gas, according to the equation:
2Al (s) + 6H+ (aq) --> 2Al3+ (aq) + 3H2 (g)
Al (s) --> Al3+ + 3e-
2H+ (aq) + 2e- --> H2 (g)
The chemical reaction represented by the equation: 2Al (s) + 6H+ (aq) --> 2Al3+ (aq) + 3H2 (g) is a redox reaction where Al is losing electrons and H+ is gaining electrons.
As the Al3+ ions have a positive charge of 3, the neutral Al atoms have to lose 3 electrons to yield an Al3+ ion. One H+ ion only has a single positive charge. This is neutralized by a single negative charge from an electron. The total number of charges both the sides of the equation has to be equal. On the left hand side of the equation, there are 6 positive charges which are provided by the 6 H+ ions. This is equal to the 6 positive charges on the right hand side provided by the 2 Al3+ ions. The reason behind 6 hydrogen atoms being reduced for 2 Al atoms that are oxidized is that Al forms positive ions that have thrice the charge as compared to a positive ion produced by hydrogen.