10.1: Spontaneity

Learning Objectives

• Define spontaneous and nonspontaneous processes.
• Distinguish between spontaneity and rate of reaction.

Physical and chemical processes proceed in a particular direction under a given set of conditions. Water flows downhill without assistance, but moving it uphill requires a pump. Ice melts at room temperature, but liquid water does not spontaneously freeze under those same conditions. Iron rusts when exposed to air and moisture, but rust will not convert back to metallic iron without intervention.

These examples illustrate the concept of spontaneity. A spontaneous process is one that occurs on its own under a given set of conditions, without the need for continuous input of energy from the surroundings. Importantly, a spontaneous process may require an initial input of energy to begin, but once started, it proceeds without ongoing external energy. In contrast, a nonspontaneous process cannot occur under the given conditions unless it is continuously driven by energy from an external source.

If a process is spontaneous in one direction under certain conditions, the reverse process is nonspontaneous under those same conditions. For example, at room temperature and atmospheric pressure, ice melts spontaneously, but water does not spontaneously freeze.

It is important to distinguish between spontaneity and rate of reaction. A spontaneous process may be very fast or extremely slow. Spontaneity tells us whether a process can occur under certain conditions, not how quickly it will proceed.

For example, the conversion of diamond to graphite is thermodynamically spontaneous at room temperature and pressure (Figure \(\PageIndex{1}\)):

\[C (s \text {, diamond }) \longrightarrow C (s \text {, graphite }) \nonumber \]

However, the process occurs so slowly that it is essentially unobservable on a human timescale. In contrast, combustion reactions are often both spontaneous and rapid. These examples highlight the distinction between thermodynamics, which addresses whether a process is energetically favorable, and kinetics, which describes how fast it happens.

Figure \(\PageIndex{1}\): The conversion of carbon from the diamond allotrope to the graphite allotrope is spontaneous at ambient pressure, but its rate is immeasurably slow at low to moderate temperatures. This process is known as graphitization, and its rate can be increased to easily measurable values at temperatures in the 1000–2000 K range. (credit "diamond" photo: modification of work by "Fancy Diamonds"/Flickr; credit "graphite" photo: modification of work by images-of-elements.com/carbon.php)

What determines if a process is spontaneous?

Many spontaneous chemical reactions release energy in the form of heat — they are exothermic. For instance, the combustion of fuels, the freezing of water, and the condensation of steam all occur spontaneously under appropriate conditions and give off heat. However, some exothermic processes are non-spontaneous and some endothermic processes may be spontaneous depending on the reaction conditions.

For example, ice melting at room temperature is spontaneous and endothermic. Instant cold packs become cold when activated, due to a spontaneous endothermic dissolution process (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): The molecular view of a solid dissolving in water. When a solute dissolves, the individual ions of the solid become surrounded by solvent particles and detach from the remaining solid. The ions are surrounded by solvent molecules in solution. Source: Photo © Thinkstock

These examples show that the release of energy is not the only factor that determines whether a process is spontaneous; they involve a deeper common feature related to particle behaviour.

What do the melting of ice and the dissolution process in an ice pack have in common?

In both cases, the resulting particles become more spread out and have greater freedom of motion than in the starting materials. For example, when ice melts, the molecules change from vibrating about fixed positions to moving and rotating freely in the liquid state.

In the next section, entropy, the measure of dispersion of energy in a system, will be described. We will see that both the change in enthalpy and the change in entropy of a process will affect the spontaneity.

Summary

Spontaneous processes proceed without continuous input of energy under given conditions, while nonspontaneous processes require ongoing external energy. If a process is spontaneous in one direction under certain conditions, the reverse process is nonspontaneous under those same conditions.

Spontaneity does not imply a fast rate of reaction. Although many spontaneous processes are exothermic, energy release alone does not determine spontaneity.