Product- and Reactant-Favored Reactions

Some chemical reactions seem to proceed entirely on their own: for example, dropping potassium into water immediately causes a vigorous, and sometimes explosive, reaction. Other reactions need a small trigger, such as lighting a hydrogen balloon. Still other reactions don't proceed under normal conditions, like water spontaneously boiling. Finally, some reactions never occur at all, such as water and carbon dioxide mixing to form propane. What do these reactions have in common, and how can we predict how a reaction will behave?

Complete answers to these questions will not be forthcoming until the "Thermodynamics" chapter, but we can make some generalizations:

• Some reactions proceed spontaneously at STP, or require a small "trigger" to proceed
• Some reactions are dependent on conditions
• Some reactions never proceed under any conditions

We refer to the first category of reactions as product-favored, because when the reaction proceeds, most of the reactants are converted into products. Reactions in the second category can be either product-favored or reactant-favored, depending on conditions, but are usually assigned to the appropriate category based on their behavior at STP. The third set of reactions are reactant-favored, since very few (if any) products are generated.

Another property can be assigned to reactions: spontaneity. A reaction can be said to occur spontaneously if it needs no input of energy to occur. As we will see, all of the properties above can be explained using one simple model. However, first you must be familiar with the types of reaction; hence, the practice problems below.

Example Problem 1 Identify each reaction or process as product-favored or reactant-favored at STP A. Ice melting B. Wood burning C. Generating gasoline from car exhaust D. Apples oxidizing Answers: A. At zero degrees Celsius, this process is reactant-favored; ice will not melt at zero degrees (it will melt, however, at one degree). B. This is reactant-favored at STP C. Obviously, this reaction never occurs, and so is reactant-favored. D. This reaction is product-favored, as you can see if you leave an apple out for long--the brown substance is fruit sugar that has been oxidized.

Now, we will proceed to a discussion of why reactions are product-favored or reactant-favored, why some happen spontaneously, some require a trigger, and some don't occur at all. And surprisingly, we can explain all this with a simple picture!

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