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22.3: Spontaneous Processes

Created by: CK-12

Lesson Objectives

The student will:

  • define a spontaneous and non-spontaneous reaction.
  • identify processes as either spontaneous or non-spontaneous.
  • describe how endothermic and exothermic reactions can be spontaneous or non-spontaneous.
  • explain the lack of correlation between spontaneity and speed of reaction.

Vocabulary

  • non-spontaneous event
  • spontaneous event

Introduction

Some events or reactions occur without any outside forces. For example, if you drop a spoonful of sugar into a cup of water, it automatically dissolves. The sugar is said to spontaneously dissolve in water. Rusting of iron can be spontaneous under the right conditions, but to undo this process would not be spontaneous. In this lesson, we will consider spontaneous and non-spontaneous reactions in light of what we just learned with respect to enthalpy.

Change That Occurs Without Outside Assistance

A spontaneous event (or reaction) is a change that occurs under a specific set of conditions. A ball rolling down a hill, the water falling down a waterfall, and the dispersion of the smell of a perfume sprayed in a room (expansion of a gas) are all examples of spontaneous events. In comparison, a non-spontaneous event (or reaction) would be a change that will not occur under a specific set of conditions. Can you picture a cold cup of hot chocolate on your desk becoming warmer as you sit and listen to your chemistry teacher? Probably not, because an external source of heat would be required to warm up the hot chocolate. For non-spontaneous events, something else outside of the reaction must be done in order to get the event (or reaction) to occur. This might be applying a force to make a ball roll up a hill or to ski up the mountain. Other factors that could drive a reaction could be the addition of heat, addition of a catalyst, or an increase in pressure.

Example:

Which of the following would be considered spontaneous? Which would be considered non-spontaneous? Explain.

  1. cooling a cup of hot coffee at room temperature
  2. ice melting at room temperature
  3. compression of gas to fill a tire
  4. water flowing downhill

Solution:

  1. Cooling a cup of hot coffee is spontaneous because heat flows spontaneously from a hotter substance to a cooler one.
  2. Ice melting is spontaneous above 0^\circ C because above this temperature, water is normally at the liquid state.
  3. Compression of gas to fill a tire is non-spontaneous because a pressure has to be applied to a gas in order to compress it.
  4. Water flowing downhill is spontaneous because water will always flow down.

Exothermic or Endothermic Can Be Spontaneous

Spontaneous and non-spontaneous reactions can be either endothermic or exothermic. Consider the the endothermic and the exothermic potential energy diagrams shown below.

Endothermic Reaction

Exothermic Reaction

Often but not always, a spontaneous process will be one that will result in a decrease in the energy of a system. Therefore, water will spontaneously flow down a waterfall, a ball will roll down a hill, and gas will expand to fill a container. All of these changes will occur spontaneously, leaving the products (or product state) with less energy than the reactants (or reactant state). Furthermore, if a reaction is spontaneous in one direction, it is non-spontaneous in the opposite direction. For example, a ball rolling down a hill would be a spontaneous event, but a ball rolling up a hill would be non-spontaneous.

Being spontaneous doesn’t necessarily mean, however, that the reaction is exothermic. Highly exothermic reactions tend to be spontaneous, but weakly exothermic or endothermic reactions can be spontaneous under the right conditions. In other words, endothermic reactions can be spontaneous just like exothermic reactions can be non-spontaneous. Consider the equation below. This equation represents the phase change of solid water (ice) to liquid water at 25^\circ\mathrm{C}.

\mathrm{H}_2\mathrm{O}_{(s)} \rightarrow \mathrm{H}_2\mathrm{O}_{(l)} \ \ \ \ \ \triangle H = 6.01 \ \text{kJ/mol}

We know that ice will spontaneously melt above 0^\circ\mathrm{C}, and the equation above also indicates the phase change is spontaneous. Now consider the equation for combustion below. Combustion is an example of a sponataneous, exothermic reaction.

\mathrm{C}_3\mathrm{H}_\mathrm{8(g)} + 5 \ \mathrm{O}_{2(g)} \rightarrow 3 \ \mathrm{CO}_{2(g)} + 4 \ \mathrm{H}_2\mathrm{O}_{(l)} \ \ \ \ \ \triangle H = -2219.9 \ \text{kJ/mol}

Therefore, spontaneity does not dictate whether a reaction is endothermic or exothermic. A spontaneous reaction is more likely to be exothermic but can be endothermic. Non-spontaneous reactions are more likely to be endothermic but can be exothermic. The deciding factor for these systems is the temperature.

Thermodynamics and Kinetics

A spontaneous process is all about the initial and final states. Reactions are considered spontaneous if, given the necessary activation energy, reactants form the products without any external forces. Therefore, an ice cube will melt, an iron nail will rust in the present of oxygen dissolved water, and a sparkler will burn. Some of these reactions are fast, and some are slow. The oxidation of iron, for example, is slow. In comparison, after the sparkler is lit, the reaction from start to finish is quite fast. Regardless of the speed of the reaction, they are both still spontaneous reactions. The rate of these reactions is the study of chemical kinetics; the spontaneity of a reaction is the study of thermodynamics. Whether the reaction occurs quickly or slowly has little to do with the reaction being spontaneous. A spontaneous reaction only means that it occurs without any continuous outside support.

Lesson Summary

  • A spontaneous event (or reaction) is a change that occurs under a specific set of conditions and without any continuous external support.
  • A non-spontaneous event (or reaction) would then be a change that will not occur under a specific set of conditions.
  • Being spontaneous does not indicate how fast a reaction occurs or if the reaction is exothermic or endothermic.

Further Reading / Supplemental Links

The learner.org website allows users to view streaming videos of the Annenberg series of chemistry videos. You are required to register before you can watch the videos but there is no charge. The website has one video that relates to this lesson called “The Driving Forces.”

Review Questions

  1. Distinguish between spontaneous and non-spontaneous reactions.
  2. Why are spontaneous reactions usually exothermic (but still can be endothermic)?
  3. Which of the following processes would be spontaneous?
    1. dissolving table salt
    2. climbing Mt. Everest
    3. separating helium from nitrogen in a mixture of gases
    4. none of these are spontaneous
  4. Which of the following processes would be non-spontaneous?
    1. iron rusting in air
    2. ice melting at 10^\circ\mathrm{C}
    3. a wild fire
    4. the reaction of \mathrm{CO}_2 and \mathrm{H}_2\mathrm{O}
  5. Which of the following reactions are spontaneous?
    1. I and II
    2. I and III
    3. II and IV
    4. Not enough information is given
  6. If a reaction is spontaneous and fast, draw a likely potential energy diagram.

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