IGCSE Chemistry Paper-1: Specimen Questions with Answers 35 - 36 of 99

The apparatus shown can be used to measure the rate of some chemical reactions.

For which two reactions would this apparatus be suitable?

reaction 1 AgNO₃ (aq) + HCl (aq) ⇾ AgCl (s) + HNO₃ (aq)

reaction 2 2H₂O₂ (aq) ⇾ 2H₂O (l) + O₂ (g)

reaction 3 MgO (s) + 2HCl (aq) ⇾ MgCl₂ (aq) + H₂O (I)

reaction 4 ZnCO₃ (s) + 2HCl (aq) ⇾ ZnCl₂ (aq) + CO₂ (g) + H₂O (I)

Answer

b.

Explanation

This type of apparatus is used so that the gas can be collected in the syringe and the quantity can be measured from the same. In the above reactions Oxygen can be collected in the syringe and in the second reaction carbon dioxide can be collected.

When pink cobalt (II) chloride crystals are heated they form steam and a blue solid. When water is added to the blue solid, turns pink and becomes hot.

Which terms describe the pink cobalt (II) chloride crystals and the reactions?

Answer

b.

Explanation

The hydration reaction may be represented by the following chemical reaction:

Reaction of pink cobalt (II) chloride crystals when heated

CoCl₂ . 6H₂O (heat) ⇾ CoCl₂ + 6 H₂O

When water is added to the blue solid

CoCl₂ + 2H₂O ⇾ CoCl₂. 2 H₂O

Purple

CoCl₂ . 2H₂O + 4 H₂O ⇾ CoCl₂ . 6 H₂O

Pink

As the humidity increases further, the crystal structure again changes, this time rearranging itself to let four more water molecules in to surround each cobalt atom, forming the hexahydrate. Chemists use the raised dot symbol before the H₂O to indicate the number of water molecules that have become incorporated into a compound at the atomic level. Heating the hydrated forms of cobalt chloride reverses the reactions above, returning cobalt chloride to the blue, water-free, or anhydrous, state. Water is “liberated” in these reactions, known as dehydration reactions.