Two of the compounds shown below react together to form a new compound with a coordinate covalent bond - HSC - SSCE Chemistry - Question 12 - 2004 - Paper 1

In this question, we need to identify which pair of compounds can react to form a new compound with a coordinate covalent bond. The key to solving this is to understand what a coordinate covalent bond is: a bond formed when one atom donates both of the electrons to the bond.

Among the options:

• Methane (CH₄) does not have a lone pair to donate, and thus cannot form a coordinate bond with any of the listed compounds.
• Water (H₂O) contains oxygen with two lone pairs, but bonding with methane does not lead to a coordinate bond.
• Hydrogen fluoride (HF) has a lone pair on the fluorine that can participate in a coordinate bond. In this case, it can react with ammonia (NH₃), which has a lone pair on the nitrogen.
• Hydrogen fluoride and methane would also not form a coordinate bond since methane lacks the lone pair needed for donation.

Thus, the pair that can react this way is: (C) Hydrogen fluoride and ammonia.