Free Hydrogen Bonding MCQs with Answers

1. A hydrogen bond forms when hydrogen is covalently bonded to

• A. carbon, silicon or sulphur
• B. any halogen
• C. a metal
• D. a small, highly electronegative atom such as fluorine, oxygen or nitrogen

Explanation: The electronegative atom pulls electron density away, leaving the hydrogen nucleus unusually exposed so that it attracts a lone pair on a neighbouring molecule. Only fluorine, oxygen and nitrogen are small and electronegative enough for the effect to be strong. Carbon is not electronegative enough, which is why hydrocarbons show no hydrogen bonding and are gases or volatile liquids.

2. The boiling point of water is far higher than that of hydrogen sulphide, although sulphur lies below oxygen in the same group, because

• A. water molecules are hydrogen bonded to one another
• B. water has a larger molar mass
• C. hydrogen sulphide is an ionic compound
• D. water molecules are non polar

Explanation: Extensive hydrogen bonding between water molecules must be overcome before they can escape, so a great deal of energy is needed and water is liquid at room temperature. Hydrogen sulphide has a larger molar mass yet boils at minus 60 degrees Celsius, which proves that molar mass is not the deciding factor. Sulphur is too large and not electronegative enough to support hydrogen bonding.

3. Hydrogen bonding is responsible for all of the following EXCEPT

• A. the high boiling point of ammonia compared with phosphine
• B. the solubility of ethanol in water
• C. the low density of ice compared with water
• D. the electrical conductivity of molten sodium chloride

Explanation: Molten sodium chloride conducts because it is an ionic compound whose ions become mobile when the lattice melts, and there is no hydrogen in the compound at all. The other three are classic consequences of hydrogen bonding: raised boiling points, mutual solubility with water and the open lattice of ice. Recognising when a substance simply cannot hydrogen bond is the quickest route to the answer.