Reactions of Acids

Strong Acids and Weak Acids

Some acids are strong acids, while some are weak acids. Strong acids are those that ionise completely to give H⁺ ions in an aqueous solution. Weak acids do not ionise completely in an aqueous solution. A solution with some amount of weak acid will be less acidic than a solution with the same amount of a strong acid.

The acidity of a solution is measured on a pH scale. The pH scale varies from 0 to 14. Solutions with pH < 7 are acidic, solutions with pH > 7 are basic, and pH = 7 indicates neutral solution. Water has a pH of 7. Very strong acids have a pH of 0 (hydrochloric acid with a concentration of 1 mol L^-1). Similarly, very strong bases have a pH of 14 (sodium hydroxide with a concentration of 1 mol L^-1).

The pH of a solution is lower (more acidic) if the concentration of H⁺ ions is more. A weak acid does not ionise completely in an aqueous solution, therefore concentration of H⁺ ions is less, therefore, pH is less. That is why it less acidic than a solution with the same amount of strong acid.

Some examples of acids are:

• Hydrochloric Acid HCl (strong).
• Hydrobromic acid HBr (strong).
• Nitric acid HNO₃ (strong).
• Sulphuric acid H₂SO₄ (strong).
• Sulphurous acid H₂SO₃ (weak).
• Carbonic acid H₂CO₃ (weak).
• Acetic acid CH₃COOH (weak).

All acids react in a similar way with particular substances. We will be studying these general reactions of acids, which include -

• Reaction of acids with metal hydroxides.
• Reaction of acids with metals.
• Reaction of acids with metal carbonates and bicarbonates.
• Reaction of acids with weak bases.

Reaction of Acids with Metal Hydroxides

Metal Hydroxides are chemicals which neutralize acids. All metal hydroxides are bases, or alkalis.

When an acid reacts with a base, the reaction produces salt and water. sodium chloride (key component of table salt) is produced by the reaction of hydrochloric acid and sodium hydroxide, a base.

$$acid\ +\ metal\ hydroxide\ \rightarrow salt\ +\ water$$

The hydrogen ion H⁺ from the acid, and the hydroxide ion OH^- from the base, combine to form water. While the anion from the acid, and the cation from the base combine to form salt. Take a look at another acid-base reaction -

$$H_{2}SO_{4\ (aq)}+Cu(OH)_{2}\rightarrow CuSO_{4}+2H_{2}O$$

Sulphuric acid reacts with copper hydroxide to produce copper sulphate and Water. The anion in sulphuric acid is sulphate (SO₄^-), and the cation in copper hydroxide is copper (Cu²⁺). Now, let's look at another type of acid reacting with a base -

$$HNO_{3}+NaOH\rightarrow NaNO_{3}+H_{2}O$$

HNO₃ is nitric acid. When it reacts with the base sodium hydroxide (NaNO₃), sodium nitrate (NaNO₃) salt is produced.

Points to be remembered about reaction of acids and bases:

• Reaction of hydrochloric acid with a base will produce chloride salts, like sodium chloride (NaCl), copper chloride (CuCl₂) etc.
• Reaction of sulphuric acid with a base will produce sulphate salts, like sodium sulphate (NaSO₄), copper sulphate (CuSO₄) etc.
• Reaction of nitric acid with a base will produce nitrate salts, like sodium nitrate (NaNO₃), copper nitrate (CuNO₃) etc.

Reactions of Acids with Metals

Acids react with metals to give a salt and Hydrogen gas.

$$Acid\ +\ Metal\rightarrow Salt+H_{2\ (g)}$$

The metal forms a compound with the anion released by the acid, and the H⁺ cation released combine to form hydrogen.

Reaction of Acids with Metal Carbonates and Metal Hydrogen Carbonates

Metal carbonates are compounds which have a metal as cation and a carbonate ion (CO₃^2-) for anion. Acids react with metal carbonates to produce salt, water, and carbon dioxide.

$$Acid\ +\ Metal\ carbonate\ \rightarrow Salt\ +\ Water\ + CO_{2\ (g)}$$

The products are are similar to as if hydrochloric acid was reacting with copper hydroxide (Cu(OH)₂), with an extra product of carbon dioxide which is evolved due to the presence of the carbonate group. Let us look at another example:

$$HBr_{(aq)}+MgCO_{3(s)}\rightarrow MgBr_{2(aq)}+H_{2}O_{(l)}+CO_{2(g)}$$

The acid in above reaction is hydrogen bromide (HBr), and it is reacting with magnesium carbonate (MgCO₃). The salt produced is magnesium bromide (MgBr₂)

Metal hydrogen carbonates are chemicals which have a metal for the cation, and HCO₃^- for the anion. Metal hydrogen carbonates are also called metal bicarbonates.

$$Acid\ +\ Metal\ bicarbonate\rightarrow Salt\ +\ Water\ +\ CO_{2 (g)}$$

In the reaction above, hydrochloric acid reacts with sodium bicarbonate (NaHCO₃) to produce sodium chloride salt, along with water and carbon dioxide gas. Look at another example - reaction of potassium bicarbonate with nitric acid.

$$HNO_{3(aq)}+KHCO_{3(s)}\rightarrow KNO_{3(aq)}+H^{2}O_{(l)}+CO_{2(g)}$$

The salt produced in this reaction is potassium nitrate (KNO₃), and as usual, carbon dioxide gas is released.

Reaction of Acids with Weak Bases

Acids neutralise weak bases, just like they do strong bases. But some weak bases do not contain hydroxide (OH^-), and so do not produce water when reacting with acids.

$$Acid\ +\ Weak base\ \rightarrow Salt\ +\ Water$$

Unlike many other bases, it does not contain a hydroxide ion (OH^-). Ammonia has a high affinity for a proton i.e. it readily accepts an H⁺ ion. Ammonia gains an H⁺ ion to become ammonium ion (NH4⁺). Acids react with with ammonia to produce ammonium salts.

Can you guess what will be the product if ammonia reacts with sulphuric acid?

$$H_{2}SO_{4(aq)}+2NH_{3(g)}\rightarrow (NH_{4})_{2}SO_{4(aq)}$$

The above reaction is of sulphuric acid and ammonia. Ammonium sulphate salt is produced in the reaction.

Another example of weak base is aluminium hydroxide. Reaction of Al(OH)₃ with an acid produces salt and water.

$$HCl+Al(OH)_{3}\rightarrow AlCl_{3}+H_{2}O$$