SAT Subject Test Chemistry
REVIEW OF MAJOR TOPICS
Chemical Reactions and Thermochemistry
ADDITIVITY OF REACTION HEATS AND HESS’S LAW
Chemical equations and ΔH0 values may be manipulated algebraically. Finding the ΔH for the formation of vapor from liquid water shows how this can be done.
Since we want the equation for H2O(l) → H2O(g), we can reverse the second equation. This changes the sign of ΔH.
Simplification gives the net equation:
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 ΔH’s that cannot be easily determined experimentally. An example is the determination of the ΔH of CO from the ΔH0f of CO2.
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:
This relationship can also be shown schematically as follows:
Some commonly used standard heats of formation (enthalpy), designated as ΔH0f, are listed in Table in the Tables for Reference section.
An alternative (and easier) method of calculating enthalpies is based on the concept that ΔH0reaction 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 ΔH0 reaction for the decomposition of sodium chlorate.