1. Hydrogen ions unlock, to wreak havoc

Acids burn. Acid rain corrodes. Acidifying sea kills. The only thing more dangerous than an acid is perhaps a secondary school student handling acid. While this is worrying, it is also intriguing. In this article, we will explain the power of acids by looking at how acids react with metals, carbonates, and bases.

Dissociation of hydrogen chloride molecules to form hydrogen ions and chloride ions in water
Hydrochloric acid dissociates in water to produce hydrogen ions (red spheres) and chloride ions (green spheres)

But first, we have to recap on the dissociated hydrogen ions that acids produce in water. All acids dissociate in water to produce hydrogen ions. Positively charged and provided of any electron, hydrogen ions are eager to react. We shall witness its reactivity by looking at the ferocity of acid rains.

2. Gone are the metals

Adam and Eve and the Acid Rain, painting by Eddie Hara (© 2019 National Heritage Board)

Like how the venom of a serpent burns, acid rain eats away metallic statues and structures. This is due to the strong acids dissolved in the rain. Their hydrogen ions react with metals, to form hydrogen gas and and an ionic compound we call salt. The salt is made up of the cation from the metal and the anion from the acid.

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

Let’s take the reaction between sulfuric acid found in acid rain and iron as an example. It produces hydrogen gas, alongside iron(II) sulfate salt. This salt is made up of iron(II) cation from the metal, and sulfate anion from the acid. As nitrate salt is soluble, the product remains dissolved in the reaction mixture.

Reactivity Series of Metals

In the reactivity series of metals, only lead and the metals above can react with acids.

However, metals have different reactivity. While some react violently with acids, others like copper, silver, platinum, and gold are so stable that they do not react. To help us identify what react, we can refer to the reactivity series. As we move up the series, the metals above are more reactive. They have a greater propensity to lose electrons to the hydrogen ions of acid during the metal-acid reaction.

Acids react with reactive metals to form a salt and hydrogen gas.

3. And also the carbonates

Besides corroding metallic structures, acid rain also gnaws away stone monuments made up of limestone, which is essentially calcium carbonate. The hydrogen ions of acid react with the carbonate ions of calcium carbonate. This forms water and carbon dioxide gas, turning the monument into thin air rather literally.

A salt is also produced as a by-product. Like in the acid-metal reaction, the exact chemical composition of the salt depends on the type of acid and carbonate used. The anion of the acid will become the anion of the salt, while the cation of the carbonate will be the cation of the salt.

2HNO3 (aq) + CaCO3 (s) ⟶ Ca(NO3)2 (aq) + CO2 (g) + H2O (l)

For example, nitric acid reacts with calcium carbonate to form calcium nitrate salt, carbon dioxide gas, and water. This salt is made up of calcium cation from the carbonate, and nitrate anion from the acid.

Besides calcium carbonate, all carbonates react with acids to give the same trio of products: a salt, carbon dioxide gas, and water.

4. Don’t cry over spilled acids

The average pH of rain in Singapore is around 4, which is not low enough to be considered as acid rain (Hu, Balasubramanian & Wu, 2003).

Thankfully, we have not had any serious occurrence of acid rain in Singapore. The pH of our rain is only slightly acidic, due to atmospheric carbon dioxide and some pollutants.

However, we are not completely spared from the environmental danger of strong acid. Singapore has a big chemical industry that requires acid as one of the many raw materials. To meet the demand, we import large amounts of concentrated sulfuric acid and hydrochloric acid from Malaysia via the Tuas Second Link. This poses a considerable risk of an accidental spill.

2HCl (aq) + Na2CO3 (s) ⟶ 2NaCl (aq) + CO2 (g) + H2O (l)

In 2019 the Singapore Civil Defence Force conducted a drill simulating the spillage of concentrated hydrochloric acid. They cleaned up the acid, but not by sweeping it away. Instead, they added sodium carbonates to react with the hydrochloric acid, turning it into relatively harmless salt, water, and carbon dioxide gas.

5. Turning acids into water: neutralisation

Another way of neutralising the danger of acids is to neutralise acids with bases. It is termed neutralisation as the products are salt and water only, which usually gives a pH of around 7. In fact, this reaction is so iconic that we define a base as any compound that undergoes neutralisation reaction with acids.

During neutralisation, acids react with bases to form salt and water only.

6. The union of hydrogen and hydroxide ions

A common group of bases that we can use to neutralise acids are the alkalis. They are the soluble bases that can dissolve in water to produce hydroxide ions. During neutralisation, each of these hydroxide ions combines with a hydrogen ion of acids to form a molecule of water. We can represent this with an ionic equation, which focuses on the reactive ions.

The anion of the acid and the cation of the alkali are left out in the ionic equation as they are but calefares. Chemically speaking, they are the spectator ions that do not directly take part in neutralisation. They simply chill and remain dissolved in the solution, to form a salt as a by-product.

HCl (aq) + NaOH (aq) ⟶ NaCl (aq) + H2O (l)
H2SO4 (aq) + 2KOH (aq) ⟶ K2SO4 (aq) + 2H2O (l)

Regardless of the exact acid or alkali used, the ionic equation is the same. Simply ignore the spectator ions and highlight the hydrogen and hydroxide ions as shown above.

7. Other types of bases that can neutralise acids

Types of Bases Examples
alkalis sodium hydroxide, NaOH
potassium hydroxide, KOH
ammonia, NH3
insoluble hydroxides copper(II) hydroxide, Cu(OH)2
metal oxides calcium oxide, CaO
iron(II) oxide, FeO
zinc oxide, ZnO

2HCl (aq) + Cu(OH)2 (s) ⟶ CuCl2 (aq) + 2H2O (l)

Besides the soluble hydroxides that we call alkalis, insoluble hydroxides are another group of bases that can undergo neutralisation with acids.

2HCl (aq) + CaO (s) ⟶ CaCl2 (aq) + H2O (l)

Another group of bases is the metal oxides. Metal oxides are ionic compounds made up of a metal cation and an oxide anion. Examples include calcium oxide, iron(II) oxide and zinc oxide. They too react with acids to form salt and water only.