The result obtained in Example 1.15 A is the theoretically predicted maximum amount of water. In reality, the amount of water actually collected may, for at least two reasons, be somewhat less. First, some reactions may not go to completion but instead may reach an equilibrium condition. Second, other reactions may have more than one pathway and may produce secondary or tertiary products that compete with the main route. In a sense, the percent yield of a reaction is a measure of efficiency.
In either case, a percent yield may be computed as follows:
Suppose that, in Example 1.11, the actual mass of water collected or measured is 48.0 g. Compute the percent yield of the reaction.
CONSECUTIVE AND SIMULTANEOUS REACTIONS
Reactions that are carried out one after another in sequence to yield a final product are called consecutive reactions. In simultaneous reactions, two or more reactants react independently of each other in separate reactions at the same time. Said differently, a reaction may have more than one pathway to produce a spectrum of products.
An example of a consecutive reaction involves the purification of titanium dioxide, TiO2, the most widely used white pigment for paints. To free TiO2 of unwanted colored impurities, it must first be converted into TiCh and then reconverted into TiO2.
Note that the TiCl4 product in the first reaction becomes a reactant in the second. This is the connecting link. For example, one could ask how many grams of carbon are required to produce 1.0 kilogram of pure TiO2. First, it must be realized that 1 mole of TiO2 (pure) requires 1 mole of TiCh. Then 2 moles of TiCLi require 3 moles of carbon. In this way, using the factor-label method, a calculation with conversion factors can be set up, connecting TiO2 (s) with C (s).
ENERGY CHANGE FOR EXOTHERMIC VS. ENDOTHERMIC REACTIONS
Exothermic reactions are reactions that produce or generate heat to the environment or surroundings. By convention, their AH values are always negative. Endothermic reactions, on the other hand, absorb heat from the environment or surroundings (Figure 1.1). Their AH values are always positive. Units of heat energy are usually in either kilojoules per mole (kJ/mole) or kilocalories per mole (kcal/mol). The AH value of a reaction is the difference in energy between the energy of the products and the energy of the reactants as shown in the energy reaction diagram. See Section 2.7 in Chapter 2 for further clarification.
Figure 1.1. Change in potential energy for (A) exothermic reaction and (B) endothermic reaction