Cracking the SAT Chemistry Subject Test

reaction.

Exothermic  reaction    →   energy  is  released    →   ΔH  <   0   →   enthalpy    decreases

Endothermic reaction    →   energy  is  absorbed    →   ΔH  >   0   →   enthalpy

increases

Heat of Formation

Another term you should be familiar with for the test is heat of formation—that
is, the amount of heat that’s released or absorbed when 1 mole of a compound is
formed from its elements. When we talk about heat of formation, we use the
same symbol we use for enthalpy change but we put a subscript “f” on it: ∆Hf .

Let’s consider the heat of formation of gaseous carbon dioxide, CO 2.

C(s)    +   O 2 (g) →   CO 2 (g);   ∆Hf =   −393    kJ/mol

The fact that the heat of formation is negative means that heat is released during
this reaction; this is an exothermic reaction. When 1 mole of CO 2 (g) is formed

from its elements (C(s) and O 2 (g)), 393 kJ of energy are released.

C, Ni, Cl 2 , O 2 , H 2 , and N 2 —or all other elemental atoms or molecules—have a

heat of formation of zero. Remember this for the exam.

For all elements,   the heat    of  formation   is  zero.

For this test, you’ll also need to keep in mind that for any reaction, the heats of
formation of all the products minus the heats of formation of all the reactants is
equal to ∆Hf for the whole reaction. The test writers might show you a reaction

and give you heats of formation for all of the reactants and products. Then