Predict and write balanced molecular, ionic and net ionic equations for reactions of acids with active metals, metal carbonates and hydrogencarbonates, and bases (including metal oxides and hydroxides)

What this dot point is asking

NESA wants you to predict the products of acid reactions with active metals, metal carbonates and hydrogencarbonates, and bases (metal oxides and hydroxides), write balanced molecular, full ionic and net ionic equations, identify spectator ions, and describe the observable evidence (bubbling, dissolution, temperature change, gas tests). The chemistry builds on the Arrhenius properties of acids and feeds directly into enthalpy of neutralisation and titration analysis.

The answer

Reaction type 1: acid with an active metal

General form: acid + metal -> salt + hydrogen.

Net ionic:

This is a redox reaction. The metal is oxidised; $H^+$ is reduced.

Activity series

Only metals more reactive than hydrogen displace it from dilute acids. K, Na, Ca, Mg, Al, Zn, Fe, Pb (slowly) react; Cu, Ag, Au do not. Lead reacts slowly because of an insoluble lead salt coating.

Observations

Bubbling (hydrogen gas), the metal disappears, the solution may warm.

Gas test

Hydrogen gives a "pop" with a lit splint.

Reaction type 2: acid with a metal carbonate

General form: acid + carbonate -> salt + water + carbon dioxide.

Net ionic (for the soluble carbonate above):

For an insoluble carbonate ($CaCO_3$, $MgCO_3$), keep the carbonate as a solid in the ionic equation:

Observations. Bubbling (carbon dioxide), the carbonate dissolves.

Gas test. Carbon dioxide turns limewater (Ca(OH)_2_{(aq)}) milky/cloudy. The chemistry: $CO_2 + Ca(OH)_2 \rightarrow CaCO_3 \downarrow + H_2O$.

Reaction type 3: acid with a metal hydrogencarbonate

General form: acid + hydrogencarbonate -> salt + water + carbon dioxide.

Net ionic:

This is the chemistry of common antacids (sodium bicarbonate neutralising stomach acid).

Reaction type 4: acid with a base (neutralisation)

General form: acid + base -> salt + water.

With a soluble hydroxide:

Net ionic:

This single net ionic equation describes every strong acid + strong base reaction.

With a metal oxide (a basic oxide):

Net ionic:

With ammonia:

Net ionic:

Observations. Heat is released. With a coloured oxide ($CuO$ black), the solid dissolves and the solution takes on the colour of the metal cation ($Cu^{2+}$ blue).

Writing ionic and net ionic equations

1. Write a balanced molecular equation with state symbols.
2. Split every aqueous strong electrolyte into its ions. Strong acids ($HCl$, $HNO_3$, $H_2SO_4$, $HClO_4$), strong bases ($NaOH$, $KOH$, $Ca(OH)_2$, $Ba(OH)_2$), and soluble salts split. Solids, liquids, gases, and weak electrolytes do not split.
3. Cancel ions that appear unchanged (same species, same coefficient) on both sides. These are the spectators.
4. Check that the net ionic equation balances for atoms and for charge.

Examples in context

Example 1. Limestone neutralisation of acid mine drainage at Captains Flat. Captains Flat in southern NSW houses a legacy lead and zinc mine site discharging acidic water rich in sulfate and dissolved metals. NSW EPA contractors built passive treatment beds packed with crushed limestone ($CaCO_3$). The dominant chemistry is $CaCO_{3(s)} + 2H^+_{(aq)} \rightarrow Ca^{2+}_{(aq)} + H_2O + CO_{2(g)}$, neutralising the proton load and releasing carbon dioxide visible as bubbling at the bed surface. The net ionic equation is the same one HSC students write for limestone plus hydrochloric acid. Officers monitor downstream pH; once it rises above 6 the heavy metals precipitate as hydroxides and are filtered out of the discharge.

Example 2. BHP Newcastle pickle line acid attack on mild steel. Steel from the Newcastle steelworks is descaled in a pickle line by dipping in 15 percent sulfuric acid. The chemistry is Fe_{(s)} + H_2SO_4_{(aq)} \rightarrow FeSO_4_{(aq)} + H_{2(g)}, the textbook acid-plus-active-metal reaction. The net ionic equation reduces to $Fe + 2H^+ \rightarrow Fe^{2+} + H_2$. Plant operators must vent the hydrogen quickly because the line runs at 80 degrees C, well above the lower flammability limit of $H_2$ in air. Spent acid is recovered as ferrous sulfate, a feedstock for water treatment chemicals. The HSC reaction-type framework explains why iron pickling proceeds at all while copper (below H in the activity series) does not.

Try this

Q1. Write balanced molecular and net ionic equations for the reaction between sulfuric acid and sodium hydrogen carbonate. [3 marks]

• Cue. Molecular: $H_2SO_4 + 2NaHCO_3 \rightarrow Na_2SO_4 + 2H_2O + 2CO_2$. Net ionic: $H^+ + HCO_3^- \rightarrow H_2O + CO_2$.

Q2. A 2.50 g sample of pure calcium carbonate is added to excess 1.00 mol L$^{-1}$ HCl. Calculate the volume of $CO_2$ produced at 25 degrees C and 100 kPa (molar volume 24.79 L mol$^{-1}$). [3 marks]

• Cue. $n(CaCO_3) = 2.50 / 100.09 = 0.0250$ mol; $n(CO_2) = 0.0250$ mol; $V = 0.0250 \times 24.79 = 0.620$ L.

Q3. Predict the products and write the net ionic equation for each of: (a) magnesium ribbon plus dilute nitric acid; (b) zinc oxide plus hydrochloric acid; (c) aqueous ammonia plus hydrochloric acid. [2+2+2 marks]

• Cue. (a) $Mg + 2H^+ \rightarrow Mg^{2+} + H_2$. (b) $ZnO + 2H^+ \rightarrow Zn^{2+} + H_2O$. (c) $NH_3 + H^+ \rightarrow NH_4^+$.