Acid reactions and neutralisation

A reactive metal reacts with dilute hydrochloric acid or dilute sulfuric acid to produce a salt and hydrogen:

metal + acid → salt + hydrogen

For example, magnesium reacts with hydrochloric acid:

$$\mathrm{Mg + 2HCl \rightarrow MgCl_2 + H_2}$$

The magnesium is gradually used up and bubbles of hydrogen appear. With dilute sulfuric acid, zinc forms zinc sulfate instead:

$$\mathrm{Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2}$$

Metals above hydrogen in the reactivity series can release hydrogen from these dilute acids. Metals below hydrogen, such as copper, do not. Very reactive metals such as sodium are too dangerous to use for an ordinary acid–metal investigation.

To identify the gas, collect a small sample in a test tube and hold a burning splint at its mouth. Hydrogen burns with a squeaky pop. Bubbles alone show that a gas is being produced; the test identifies it as hydrogen.

Bases and neutralisation

A base is a substance that reacts with an acid to form a salt and water only. Metal oxides and metal hydroxides are bases. A reaction between an acid and a base is called neutralisation:

acid + base → salt + water

For a metal oxide, the general reaction is:

acid + metal oxide → salt + water

For example, copper oxide reacts with sulfuric acid to form copper sulfate and water:

$$\mathrm{CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O}$$

For a metal hydroxide, the general reaction is:

acid + metal hydroxide → salt + water

For example, sodium hydroxide reacts with hydrochloric acid:

$$\mathrm{NaOH + HCl \rightarrow NaCl + H_2O}$$

Unlike the acid–metal reaction, neither of these reactions produces hydrogen gas.

What makes a base an alkali?

An alkali is a soluble base. Sodium hydroxide dissolves in water, so it is an alkali. Copper oxide is insoluble in water: it is a base, but not an alkali. All alkalis are bases, but not all bases are alkalis.

Acids provide hydrogen ions, $\mathrm{H^+}$, in solution. Alkalis provide hydroxide ions, $\mathrm{OH^-}$. During acid–alkali neutralisation, these ions combine to form water:

$$\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}$$

This ionic equation shows the particles that actually react. In the hydrochloric acid–sodium hydroxide reaction, sodium ions and chloride ions remain in solution; they are not changed by the neutralisation.

Neutralisation removes hydrogen ions from the acidic solution. Its pH therefore rises as alkali is added. The mixture is neutral when the acid and alkali have reacted in the correct proportions; simply mixing any amounts does not guarantee a final pH of 7.

Acids reacting with metal carbonates

A metal carbonate reacts with an acid to produce three products:

acid + metal carbonate → salt + water + carbon dioxide

For example, sodium carbonate reacts with nitric acid to form sodium nitrate:

$$\mathrm{Na_2CO_3 + 2HNO_3 \rightarrow 2NaNO_3 + H_2O + CO_2}$$

The carbon dioxide causes effervescence: visible bubbling or fizzing. This distinguishes a carbonate reaction from the oxide and hydroxide reactions above.

Test the gas by bubbling it through limewater, which is a solution of calcium hydroxide. Carbon dioxide turns the initially clear limewater milky or cloudy white.

Investigating pH as a powdered base is added

The core practical investigates adding powdered calcium hydroxide or calcium oxide to a fixed volume of dilute hydrochloric acid. Both substances react with the acid to form calcium chloride and water:

$$\mathrm{Ca(OH)_2(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + 2H_2O(l)}$$ $$\mathrm{CaO(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l)}$$

One suitable method uses calcium hydroxide, a 100 cm³ beaker, a 50 cm³ measuring cylinder, a balance, a spatula and a stirring rod. Universal indicator paper, a white tile and a pH colour chart allow you to estimate pH; a pH probe can be used instead. Wear eye protection: dilute hydrochloric acid is an irritant, and calcium hydroxide can irritate skin and seriously damage eyes. Wash your hands afterwards.

1. Measure 50 cm³ of dilute hydrochloric acid into the beaker and record its initial pH before adding any powder.
2. To estimate pH using indicator paper, place a fresh piece on the white tile. Dip the glass rod into the mixture and transfer a drop onto the paper. Wait 30 seconds, compare its colour with the pH chart, then rinse the rod.
3. Weigh 0.3 g of calcium hydroxide powder, add it to the acid and stir. Allow it to react before measuring and recording the pH again. Stirring brings the reactants together and makes the mixture more uniform.
4. Add seven further weighed 0.3 g portions, stirring and measuring pH after each addition. This gives a total of 2.4 g added. If using a pH probe, wait for a stable reading each time.
5. Record results in a table with columns headed Total mass of calcium hydroxide added / g and pH. Include the initial reading at 0 g, followed by readings at 0.3 g, 0.6 g and so on.

The mass added is the independent variable; pH is the dependent variable. Keep the starting volume and concentration of acid constant when repeating the investigation. Use the same powdered substance throughout each run, and repeat the investigation to check that the pattern is reproducible.

Interpreting and improving the investigation

Plot pH on the vertical axis against total mass of powder added in grams on the horizontal axis, then draw a smooth curve of best fit. To estimate the mass needed for neutralisation, trace horizontally from pH 7 to the curve, then vertically down to the mass axis. This may give an estimate between two measured masses rather than exactly at one of them.

Initially, the acid has a pH below 7. As the base reacts, hydrogen ions are removed and the pH rises. Around neutralisation, a small further addition can produce a large change in pH. Once the acid has been used up, excess base makes the mixture alkaline, and eventually further additions cause little change in pH. Calcium hydroxide is only sparingly soluble, so excess solid may remain visible.

Smaller weighed portions near pH 7 give more closely spaced measurements where the pH changes rapidly, improving the estimate of the mass needed for neutralisation. A pH probe avoids judging colours and gives more precise readings than universal indicator paper. The graph connects these measurements to the progress of the neutralisation reaction.

Hydrochloric acid → chlorides; sulfuric acid → sulfates; nitric acid → nitrates.

Neutralisation

Acid + base → salt + water. Alkalis are soluble bases; not every base is an alkali.

$$\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}$$