Metal & Acid Reactions as Redox Reactions (OCR GCSE Chemistry A (Gateway Science Suite)): Revision Notes
4.2.2 Metal & Acid Reactions as Redox Reactions
When metals react with acids, the reaction is an example of a redox reaction. In these reactions, the metal is oxidised (loses electrons), and the hydrogen ions in the acid are reduced (gains electrons), producing hydrogen gas and a salt.
Understanding the Reaction
The general reaction between a metal and an acid can be written as:
For example, when zinc reacts with hydrochloric acid, zinc chloride and hydrogen gas are produced:
- Oxidation: Zinc (Zn) loses two electrons to form , which means zinc is oxidised.
- Reduction: The hydrogen ions from the acid gain electrons to form hydrogen gas , meaning the hydrogen is reduced.
Redox Process in Metal-Acid Reactions
In this redox reaction:
- The metal acts as the reducing agent because it donates electrons (undergoes oxidation).
- The acid acts as the oxidising agent because its hydrogen ions accept electrons (undergo reduction).
Reactions with Pure Metals
Some metals will react with acids to produce salts and hydrogen gas, depending on their reactivity.
The identity of the salt produced depends on the metal and acid that react. For example, sodium (Na) and hydrochloric acid (HCl) will react to form the salt sodium chloride (NaCl):
If, instead of hydrochloric acid, sulfuric acid is used, the salt sodium sulphate (will be produced:
These reactions show that salts generally have a consistent structure, being formed from the combination of a metal cation (positive ion) and an acid anion (negative ion).
Not all metals will react with acids. The reactivity of the metal and the strength and concentration of the acid determine whether a reaction will occur:
- Highly reactive metals: Sodium, lithium, and magnesium react readily, even with weak or dilute acids.
- Less reactive metals: Copper, mercury, and silver require strong and concentrated acids to react.
- Inert metals: Platinum is so unreactive that it won't react with acids under any conditions.
Predicting Reaction Outcomes
The Reactivity Series helps predict which metals will react with acids and how vigorously:
- Higher reactivity: Metals like magnesium and zinc react readily with acids.
- Lower reactivity: Metals like copper and silver react more slowly or not at all.