Which molecule has the highest boiling point: methane, ammonia, water or hydrogen fluoride? Explain why.

Answer: water, followed by hydrogen fluroide, ammonia and lowest is methane.

Explanation: First it helps to write the molecular formulae of these molecules and know their shape:

Methane: CH₄, tetrahedral

Ammonia: NH₃, pyramidal

Water: H₂O, non-linear/bent

Hydrogen fluroide: HF, linear

The boiling point of simple molecules such as these is determined by the strength of the intermolecular forces between them, i.e. the strength of the van der Waals/London dispersion forces, permanent dipole-permanent dipole forces and hydrogen bonding. Now you need your periodic table.

Methane is the easiest to discout as it is the only molecule without hydrogen bonding (only seen between N/O/F and H), and since hydrogen bonding is by far the strongest intermolecular force, methane boils the coldest. It also has no permanent-dipole permanent dipole forces due to being tetrahedral, so all dipoles cancel out, and so boils at a very low -162 ^oC.

The other three molecules all have hydrogen bonding and permanent dipole-permanent dipole forces so are more difficult to order.

Hydrogen fluroide has the greatest difference in electronegativity between its atoms (look at your periodic table) and thus has the strongest hydrogen bonds and permanent dipole-permanent dipole forces, with ammonia having the weakest of the three due to the smallest difference in electronegativity between its constituent atoms. You would naturally expect hydrogen fluoride to have the highest boiling point and ammonia the lowest of the three. This is true for ammonia (-33 ^oC) but not for hydrogen fluoride.

Water in fact has the highest boiling point because although its individual hydrogen bonds are not as strong as hydrogen fluoride's, the fact there are twice as many (two H instead of one) means the total strength of intermolecular forces between water molecules is greater than that of hydrogen fluoride, and so has the highest boiling point (100 ^oC compared to 20 ^oC). You may be wondering why ammonia having three H compared to two is not higher than both water and hydrogen fluoride, and this is because the individual hydrogen bonds are much weaker for ammonia, and this outweighs the greater number of hydrogen bonds.

Final answer: water (100 ^oC), hydrogen fluoride (20 ^oC), ammonia (-33 ^oC), methane (-162 ^oC).