Question

Give an example of spontaneous combustion.

Answer 1

Hint: Identify the process which occurs on its own without an external input of the energy. The process so identified should also be a combustion process, in which material burns and produces heat energy.

Complete Step by step answer: A spontaneous process occurs on its own. You do not need to supply an external source of energy. However, a nonspontaneous process does not occur on its own. You need to supply an external source of energy. A spontaneous process is associated with a negative value of free energy change. In other words, during spontaneous processes, there is a decrease in the value of the Gibbs free energy. On the other hand, a non-spontaneous process is associated with a positive value of free energy change. In other words, during spontaneous processes, there is an increase in the value of the Gibbs free energy.
A combustion process is an exothermic process. During this process, heat energy is released in the process. Combustion takes place in presence of air or oxygen. When organic matter is heated in the fire, it burns and produces carbon dioxide gas and water vapour. This is an example of the combustion process.
A spontaneous combustion is a process in which combustion occurs spontaneously, without input of external energy.
The following is an example of spontaneous combustion. In coal mines, the combustion of coal dust results in fire break outs. This happens several times.
Another example of spontaneous combustion includes instantaneous burning of phosphorus or sulphur at room temperature.
Thus, the spontaneous combustion of phosphorus in dry air produces phosphorus pentoxide.
$${\text{P}}_4+5{\text{O}}_2\to {\text{P}}_4{\text{O}}_{10}$$

Note: It is not necessary that all the combustion processes are spontaneous processes. This is because these processes have a large activation energy. For example, combustion of organic compounds such as methane is exothermic but it is not a spontaneous process. It needs a flame or spark to initiate the process.