the reactions of acids with metals, metal oxides, metal hydroxides and metal carbonates (and hydrogen carbonates), including the writing of balanced equations and an explanation of the underlying acid-base or redox process

What this dot point is asking

VCAA wants you to recognise, write equations for, and explain the four classic reactions of an acid: with a metal, a metal oxide, a metal hydroxide, and a carbonate or hydrogen carbonate. The first is redox; the other three are acid-base neutralisations. In every case the acid loses its acidic hydrogen and the metal cation appears in a soluble salt.

The answer

Acid + metal: redox

A metal above hydrogen in the activity series reacts with a dilute acid to give a salt and hydrogen gas:

Example:

This is a redox reaction, not an acid-base neutralisation in the Brønsted sense. The metal loses electrons (it is oxidised); the hydrogen ion gains electrons to form $H_2$ gas (it is reduced).

Half-equations:

Oxidation: IMATH_14
Reduction: $2H^+(aq) + 2e^- \to H_2(g)$

Metals below hydrogen ($Cu$, $Ag$, $Au$) do not react with dilute non-oxidising acids. Sulfuric acid in concentrated or hot form is an oxidising acid and behaves differently (outside VCE Unit 2 scope).

Observations: bubbles of hydrogen gas; the metal is consumed; the solution gets warmer (exothermic); test for $H_2$ with a glowing splint (it pops).

Acid + metal oxide: neutralisation

A metal oxide is a basic oxide. Adding it to an acid gives a salt and water:

Example:

This is a Brønsted-Lowry neutralisation. The $O^{2-}$ in the oxide accepts protons to form water:

Observations: the solid metal oxide dissolves; the solution turns from acidic to neutral as the acid is consumed; no gas; mild heat released.

Acid + metal hydroxide: neutralisation

A metal hydroxide is a base. Adding it to an acid gives a salt and water:

Example with a strong acid and a strong base:

Net ionic equation:

This is the canonical strong-acid strong-base neutralisation. The hydroxide ion accepts the proton from the acid. The cation and the anion from the original acid and base end up as spectator ions in the resulting salt solution.

For a strong acid plus a weak base, the weak base is written as a molecule:

Net ionic: $NH_3(aq) + H^+(aq) \to NH_4^+(aq)$.

Observations: temperature rises noticeably ($\Delta H \approx -57\ kJ\ mol^{-1}$ for a strong-acid strong-base neutralisation per mole of water formed); no gas; pH moves toward 7 (or toward the pH dictated by the salt for weak-acid or weak-base cases).

Acid + carbonate or hydrogen carbonate: salt, water and CO2

Both metal carbonates and metal hydrogen carbonates react with acids to give a salt, water and carbon dioxide gas:

Example:

The carbonate or hydrogen carbonate accepts protons; the resulting $H_2CO_3$ is unstable and decomposes to $H_2O$ and $CO_2$:

This reaction is the chemistry of antacid tablets ($CaCO_3$, $MgCO_3$, $NaHCO_3$ for stomach acid), of acid corrosion on marble and limestone buildings, and of acid rain effects on shells and corals.

Observations: vigorous bubbling (carbon dioxide); the carbonate is consumed; test for $CO_2$ by bubbling the gas through limewater $Ca(OH)_2(aq)$ and observing a milky precipitate of $CaCO_3$.

Summary table

In every case the result is a soluble salt of the metal with the acid's anion plus water (and possibly $H_2$ or $CO_2$). This is also the basis of the four ways in which a chemist makes salts in the laboratory.

Common traps

Calling the metal-plus-acid reaction a neutralisation. It is redox. No proton transfer to a base happens in the Brønsted sense; instead the proton is reduced to $H_2$.

Forgetting to balance the H atoms. $H_2SO_4$ contributes two $H^+$ per molecule, so $2NaOH$ are needed; $H_3PO_4$ contributes three. Read the formula before balancing.

Writing $H_2CO_3$ as a stable product. Carbonic acid is unstable and is shown as $H_2O + CO_2$ in the final equation.

Forgetting the gas for a carbonate or hydrogen carbonate reaction. $CO_2$ is part of the products and is a marked observation.

Confusing the two gas tests. Hydrogen burns with a pop test (a glowing splint at the mouth of a test tube). Carbon dioxide turns limewater milky and does not pop.

Assuming any metal reacts. Copper, silver and gold do not react with dilute hydrochloric or sulfuric acid. They are below hydrogen.

In one sentence

An acid reacts with a metal (above hydrogen) by redox to give a salt and hydrogen, and with a metal oxide, metal hydroxide or carbonate by acid-base proton transfer to give a salt and water (plus $CO_2$ from a carbonate), with the chemistry in every case captured by the acid donating its proton(s) and the metal cation appearing in the resulting salt.