Acids with Reactive Metals (DP IB Chemistry): Revision Note

Acids with Reactive Metals

Metals and acids

• The typical reaction of a metal and an acid can be summarised as

acid + metal  →  salt + hydrogen

• For example:

2HCl (aq)  +  Zn (s)  → ZnCl₂ (aq) + H₂ (g)

hydrochloric acid + zinc  →  zinc chloride* + hydrogen

H₂SO₄ (aq) + Fe (s) → FeSO₄ (aq) + H₂ (g)

sulfuric acid + iron → iron(II) sulfate* + hydrogen

*zinc chloride and iron(II) sulfate are salts

• A salt is an ionic compound formed when the hydrogen of an acid is replaced by a metal or another positive ion

• Clearly, the extent of the reaction depends on the reactivity of the metal and the strength of the acid

• Very reactive metals would react dangerously with acids and these reactions are not usually carried out

• Metals low in reactivity do not react at all

  • For instance, copper does not react with dilute acids

• Stronger acids will react more vigorously with metals than weak acids

• What signs of reaction would be expected to be different between the two?

  • Faster reaction seen as:

    • more effervescence

    • the metal dissolves faster

Ionic equations

• The reactions of acids and metals can be written as ionic equations showing only the species that has changed in the reaction

• Zinc reacts with hydrochloric acid to give a salt and hydrogen:

2HCl (aq) + Zn (s) → ZnCl₂ (aq) + H₂ (g)

• The ionic equation, including all species, is:

2H⁺ (aq) + 2Cl^– (aq) + Zn (s) → Zn²⁺ (aq) +  2Cl^– (aq) + H₂ (g)

• The chloride ions are spectator ions and can be removed to give the full ionic equation:

2H⁺ (aq)  + Zn (s) → Zn²⁺ (aq) + H₂ (g)

• The full ionic equation is made from 2 half-equations:

2H⁺ (aq)  + 2e^- → H₂ (g)

Zn (s) → Zn²⁺ (aq) + 2e^-

• H⁺ (aq) in HCl (aq) is being reduced to H₂ (+1 to 0)

• Zn (s) is being oxidised to Zn²⁺ (0 to +2)

Relative reducing power of metals

• The ability of a metal to act as a reducing agent depends on how easily it loses electrons

• This list orders metals from strongest to weakest reducing agents based on their tendency to lose electrons and become oxidised:

  • Magnesium (Mg)

  • Aluminium (Al)

  • Zinc (Zn)

  • Iron (Fe)

  • Lead (Pb)

  • Hydrogen (H) (reference point)

  • Copper (Cu)

  • Silver (Ag)

• The metals at the top of the list are the strongest reducing agents

  • They lose electrons most easily and are readily oxidised

• The metals at the bottom are the weakest reducing agents

  • They lose electrons less easily and are not readily oxidised

• This order also helps predict displacement reactions

  • More reactive metals displace less reactive metal ions from solution