Spontaneous Reaction: Definition & Examples

Ordinary, everyday things, such as salt dissolving in water, ice melting, and fruit ripening, are examples of spontaneous reactions that we normally encounter. What are spontaneous reactions? We will cover the topic in this lesson.
What Is a Spontaneous Reaction?
Imagine you’re walking past a natural waterfall. The water that tumbles downhill does so of its own accord and doesn’t need any help from levees or dams. Now imagine something even more commonplace — taking a nice, cold soda out of your fridge and putting it down next to your computer. After a while, the soda becomes room temperature; again, without any intervention on your part.

From this, we can define a spontaneous reaction as a reaction that occurs in a given set of conditions without intervention. A spontaneous reaction proceeds to completion without any outside help.

Just because a reaction is spontaneous, it does not automatically imply that these reactions occur instantaneously. An example of this is when iron nails rust due to constant exposure to moisture — this process does not occur overnight, but can take days or months. Still, the process doesn’t need any outside intervention to occur.

Spontaneous Reactions and Entropy
Let us think about it this way: Where do you spend more effort, cleaning your house or making a mess? In general, we spend more effort and energy cleaning our houses, and if we miss a week or two of cleaning, then it easily becomes messy. We can say that making a mess is definitely a more spontaneous process.

According to the Second Law of Thermodynamics, a reaction is spontaneous if the overall entropy, or disorder, increases. If we go back to our cleaning the house analogy, when our home becomes messy, then the entropy increases. The mess that accumulates is a spontaneous reaction, and as a result, the entropy increases.

If the overall entropy change, represented as dS, increases, then the reaction is spontaneous. The overall entropy change comes from the total of the change of entropy in a system and the change of entropy in the surroundings, as shown in the following equation:

Equation for Total Change in Entropy
In a spontaneous reaction, just because the overall change in entropy is positive does not mean that the entropy of the systems and surroundings should always both be positive. As long as the positive entropy of either the system or surroundings compensates for the negative entropy change of the other, then the reaction will be spontaneous.

Is a Reaction Spontaneous?
Now that we know the overall entropy change should increase for spontaneous reactions, we can use this information to tell if a reaction is spontaneous or not. To be able to tell if a reaction is spontaneous or not qualitatively, meaning without having to calculate anything, the first thing we look at is the entropy. There are ways to tell if the entropy of your system increases.

One of these ways is a phase change. The arrangement of molecules from solid phase to liquid phase to gas phase becomes more disorderly, and the movement of the molecules also increases, resulting in a higher entropy, as shown in the figure.

Solid, Liquid and Gas Molecules
So, for instance, when a substance changes from solid to liquid, the molecules move more freely and the arrangement becomes less packed. The same goes with changing from solid to gas, or liquid to gas. Evaporation is one example of this.

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Another way to tell if there’s an entropy increase is if there’s an increase in temperature. On cold days, you may just want to stay inside and read by the fire. On warmer days, with the sun out, you may want to go outside to enjoy the weather. We can think of molecules the same way — there is more motion when the temperature increases, and therefore, an increase in entropy.

In terms of a chemical reaction, if you mix two substances and heat is released to the surroundings, then the resulting entropy is increased.

There is a saying, ‘the more the merrier’. If you compare the movement and activity of two people on a date to a group of five friends having dinner together, the bigger group is usually louder, and there are multiple conversations taking place at the same time — this leads to higher entropy.

We can apply this to when a chemical reaction results in an increase in a number of moles, which also leads to higher entropy. An example of this reaction would look like this:

Number of Moles Increase, Higher Entropy

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