## SAT Subject Test Chemistry

__PART 2__

__REVIEW OF MAJOR TOPICS__

__CHAPTER 8__

__Chemical Reactions and Thermochemistry__

__ADDITIVITY OF REACTION HEATS AND HESS”S LAW__

Chemical equations and Δ*H*^{0} values may be manipulated algebraically. Finding the Δ*H* for the formation of vapor from liquid water shows how this can be done.

Since we want the equation for H_{2}O(l) → H_{2}O(g), we can reverse the second equation. This changes the sign of Δ*H*.

Simplification gives the net equation:

**TIP **

Know how to use Hess”s Law.

The principle underlying the preceding calculations is known as **Hess”s Law of Heat Summation**. This principle states that, when a reaction can be expressed as the algebraic sum of two or more other reactions, the heat of the reaction is the algebraic sum of the heats of these reactions. This is based upon the **First Law of Thermodynamics**, which, simply stated, says that the total energy of the universe is constant and cannot be created or destroyed.

These laws allow calculations of Δ*H*”s that cannot be easily determined experimentally. An example is the determination of the Δ*H* of CO from the Δ*H*^{0}* _{f}* of CO

_{2}.

The calculation for the above example is shown below.

The equation wanted is

To get this, we reverse the second equation and add it to the first:

Addition yields

This relationship can also be shown schematically as follows:

Some commonly used standard heats of formation (enthalpy), designated as Δ*H*^{0}* _{f}*, are listed in Table in the Tables for Reference section.

An alternative (and easier) method of calculating enthalpies is based on the concept that Δ*H*^{0}_{reaction} is equal to the difference between the total enthalpy of the reactants and that of the products. This can be expressed as follows:

**PROBLEM:** Calculate Δ*H*^{0} _{reaction} for the decomposition of sodium chlorate.