Question

Which of the following processes are spontaneous?

(a) Methane burns in air.

(b) A teaspoonful of sugar dissolves in a cup of hot coffee.

(c) A soft-boiled egg becomes raw.

Short answer

(a) Methane burning in air is a spontaneous process.

(b) A teaspoonful of sugar dissolving in a cup of hot coffee is a spontaneous process.

(c) A soft-boiled egg becoming raw is a non-spontaneous process.

Step-by-step solution

Concept Introduction

A spontaneous process can move in a specific direction without the assistance of an external source of energy. When an external source of energy drives a non-spontaneous process, it moves in a specific direction.

Methane burning in air

(a)

The methane gas upon small energy gain (temperature elevation) burns spontaneously in the air until it is all burned.

Therefore, this is a spontaneous process.

Sugar dissolving in hot coffee

(b)

As a spoon of sugar is put in the hot coffee, sugar molecules gain more energy from the heat and the particles go to more energy states, so the crystalline sugar structure is eventually dissolved in the coffee. With mixing, the process is only facilitated as sugar molecules are more separated and thus may have more even heat distribution (and faster dissolution).

Therefore, this is a spontaneous process.

Soft boiled egg becoming Raw

(c)

Upon boiling the raw egg, as the temperature is elevated, the quaternary structure of proteins in the egg de-naturate and the sulphur bridges, for example, are broken. Therefore, the boiled egg cannot be changed spontaneously to raw, as a lot of energy is necessary to return all the proteins to its original state.

Therefore, this is a non-spontaneous process.