4.4.2.1,4.4.2.2,4.4.2.4 - Acids, neutralisation and pH

Acids reacting with metals

Some metals react with dilute acids to make a salt and hydrogen gas.

For the required reactions, use magnesium, zinc or iron with dilute hydrochloric or sulfuric acid. The metal gradually disappears and bubbles of hydrogen form. More reactive magnesium reacts more rapidly than comparable zinc or iron under the same conditions. Copper does not displace hydrogen from these dilute acids.

The word pattern is metal + acid → salt + hydrogen. Hydrochloric acid gives a chloride, whereas sulfuric acid gives a sulfate:

Iron forms iron(II) salts in these reactions: $\ce{FeCl2}$ or $\ce{FeSO4}$. The number II states that the iron ions have charge $2+$. The salt dissolves if it is soluble, so disappearance of the metal does not mean the metal element has ceased to exist. Its atoms are now present as ions in the solution.

Hydrogen is identified by a burning splint giving a squeaky pop in a small collected sample. Eye protection, small quantities and controlled gas testing matter because acid can splash and hydrogen is flammable. Never place an ignition source beside the reaction vessel.

Electron transfer in metal–acid reactions (Higher)

A metal–acid reaction transfers electrons from metal atoms to hydrogen ions.

For zinc reacting with hydrochloric acid, the two changes are:

Zinc atoms lose electrons and are oxidised. Hydrogen ions gain electrons and are reduced. Both happen in the same redox reaction: the electrons lost by zinc are the electrons gained by hydrogen ions. Chloride ions remain dissolved and unchanged, so cancelling them gives:

The starting species reduced is the hydrogen ion, not the hydrogen gas formed. Magnesium and iron reacting with the specified dilute acids follow the same electron-transfer pattern. Their ions have charge $2+$, so one metal atom loses two electrons and two hydrogen ions accept them. Sulfate ions are spectators when sulfuric acid is used.

Bases, alkalis and carbonates

An acid can be neutralised without producing hydrogen if the other reactant is a base or a carbonate.

Many metal oxides and hydroxides are bases. Copper oxide is insoluble in water, so it is a base but not an alkali. Sodium hydroxide dissolves and is an alkali. An acid reacting with a metal oxide or hydroxide makes a salt and water:

The first example removes black copper oxide as a soluble salt forms; it does not release hydrogen because copper oxide is a compound, not copper metal.

Metal carbonates react with acid to produce a salt, water and carbon dioxide:

Effervescence shows a gas is forming, but bubbles alone cannot identify it. Passing carbon dioxide through limewater turns the limewater cloudy. This distinguishes it from hydrogen produced by a metal–acid reaction. Predicting the right gas therefore requires identifying the reactant class, rather than applying “acid makes hydrogen” to every reaction.

Choosing and naming salts

The acid supplies the salt's negative ion, while the metal-containing reactant supplies its positive ion.

Hydrochloric acid forms chlorides, nitric acid forms nitrates, and sulfuric acid forms sulfates. For example, magnesium oxide with nitric acid produces magnesium nitrate, whereas the same oxide with sulfuric acid produces magnesium sulfate. Changing the acid changes the negative ion; changing magnesium oxide to copper oxide changes the positive ion.

A salt formula must have zero overall charge. Common negative ions are chloride $\ce{Cl-}$, nitrate $\ce{NO3-}$, and sulfate $\ce{SO4^2-}$. Use the given positive-ion charge with these charges to choose the ratio.

Magnesium is $\ce{Mg^2+}$. It needs two nitrate ions to balance its charge, so magnesium nitrate is $\ce{Mg(NO3)2}$. The brackets show that there are two complete nitrate ions; changing the formula of nitrate would make a different ion. Magnesium needs only one sulfate ion, giving $\ce{MgSO4}$. Sodium is $\ce{Na+}$, so two sodium ions balance one sulfate ion: $\ce{Na2SO4}$.

Once formulae are correct, balance the reaction using coefficients. For magnesium oxide and nitric acid:

Both sides contain one magnesium atom, two nitrogen atoms, seven oxygen atoms and two hydrogen atoms. This check separates two decisions: ion charges determine the formulae, then conservation of atoms determines coefficients.