Chemistry, Third edition

Fig. 6.3Always add acid to
water, never water to acid.

6 · REACTIONS OF IONS IN SOLUTION

Reaction of pure sulfuric acid with water

Sulfuric acid (H 2 SO 4 ) is usually sold as concentrated sulfuric acid, which contains

98% sulfuric acid and 2% water by mass: this is sufficiently pure to justify the name

pure sulfuric acid.

Pure sulfuric acid (like hydrogen chloride) is covalent. Unlike HCl(g), however, it

is a liquid at room temperature. Pure sulfuric acid is a colourless oily liquid that looks

and pours like glycerine, but there the similarity ends. If pure sulfuric acid is added to

water, a violent reaction occurs. Much heat is produced. The resulting colourless

solution is called dilute sulfuric acid. The overall equation for the reaction is:

H 2 SO 4 (l)

H 2 O

• 2H(aq)SO 42 (aq)

The reaction of pure sulfuric acid with water is so violent that dilute sulfuric acid

should always be prepared by slowly adding the pure acid to water. Under no cir-

cumstances should water be added to the pure acid. So much heat would be

generated where the two liquids mix that some of the mixture might spit out into

your face (Fig. 6.3).

Pure sulfuric acid is so greedy for water that it is commonly used as a drying

agent. As most gases are only slightly soluble in the pure acid, they may be dried by

88

Bags of fertilizer. Fertilizers have greatly increased the
productivity of crop farmers, and may have prevented large-
scale starvation. Many fertilizers are made from ammonia
(NH 3 ), sulfuric acid (H 2 SO 4 ) and phosphoric acid (H 3 PO 4 ).

Reactions of silver nitrate

(i)iWhich of the following would give a white precipitate when added to a mixture of silver

nitrate and nitric acid?

(a)potassium sulfate

(b)magnesium nitrate

(c)calcium chloride.

(ii)If a solution containing bromide ions (Br) is added to silver nitrate solution, a cream

precipitate is produced. If a solution containing iodide ions (I) is added to silver nitrate

solution, a pale yellow precipitate is produced. What do you think these precipitates are? Write

an ionic equation for each precipitation reaction.

Exercise 6F