Predicting Reactions

1. Combination

The best source of information to predict a chemical combination is the heat of formation table. A heat of formation table gives the number of calories evolved or absorbed when a mole of the compound in question is formed by the direct union of its elements. A positive number indicates that heat is absorbed and a negative number that heat is evolved or give off. It makes some difference whether the compounds formed are in the solid, liquid, or gaseous state. The values given are in kilocalories (1 kcal = 1000 calories). One calorie is the amount of heat needed to raise the temperature of 1 gram of water 1 degree Celsius. The symbol DH is used to indicate the heat of formation

If the heat of formation is a large number preceded by a minus sign, the combination is likely to occur spontaneously and the reaction is exothermic. If, on the other hand, the number is small and negative or is positive, heat will be needed to get the reaction to go at any noticeable rate. An example is:

Zn + S ---> ZnS + 48.5 kcal (DH= -48.5 kcal)

This means that 1 mole of zinc (65 g) reacts with 1 mole of sulfur (32 g) to form 1 mole of zinc sulfide (97 g) and releases 48.5 Kcal of heat

2. Decomposition

The prediction of decomposition reactions uses the same source of information, the heat of formation table. If the heat of formation is a high exothermic (DH is negative) value, the compound will be difficult to decompose since this same quantity of energy must be returned to the compound. A low heat of formation indicates decomposition would not be difficult, such as the decomposition of mercuric oxide with DH = -21.68 kcal/mol

3. Single Replacement

A simple way of predicting single replacement reactions is to check the relative positions of the two elements in a electromotive chart. An electromotive chart is a listing of elements in order of reactivity. If the element that is to replace the other in the compound is higher on the chart, the reaction will occur. If it is below, there will be no reaction.

4. Double Replacement

For double replacement reaction to go to completion, that is, proceed until the supply of one of the reactants is exhausted, one of the following conditions must be present: an insoluble precipitate is formed, a nonionizing substance is formed, or a gaseous product is given off.

5. Entropy

In many of the preceding predictions of reactions, we used the concept that reactions will occur when they result in the lowest possible energy state.

There is, however, another driving force to reactions that relates to their state of disorder or of randomness. This state of disorder is called entropy. A reaction is also driven, then, by a need for a greater degree disorder. An example is the intermixing of gases in two connected flasks when a valve is opened to allow the two previously isolated gases to travel between the two flasks. Since temperature remains constant throughout the process, the total heat content cannot have changed to a lower energy level, and yet the gases will become evenly distributed in the two flasks. The system has thus reached a higher degree of disorder or entropy.