Put these four molecules in INCREASING order (from lowest to highest) of predicted melting and boiling point temperatures:

Question

Methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀) are all substances that are used as combustible fuels.

Put these four molecules in INCREASING order (from lowest to highest) of predicted melting and boiling point temperatures:

Explain why you put these molecules in the order that you did, above.

Answer 1

Hello, Salo,

These are all "saturated" hydrocarbons, which means they consist of only carbon and hydrogen atoms, and only single bonds(no double or triple bonds). This makes them highly non-polar, so that their properties are primarily dictated by their size, or molecular weights. Both boiling and melting points will increase with increasing molecular weights: methane, ethane, propane, butane.

Bob

Answer 2

The melting point (m.p.) and boiling point (b.p.) are pretty much based on intermolecular forces (IMF) holding the molecules together. The greater the IMF, the greater the b.p. and m.p.

As you increase the number of C atoms (going from CH4 to C4H10), the dispersion force increases and that's the only IMF present in these 4 hydrocarbons. So, the order from low to high would be

CH₄ < C₂H₆ < C₃H₈ < C₄H₁₀

(note: meting points are not so easy to predict because IMFs AND crystal structure are both factors. But in general chemistry, you can probably use this order)