Cracking the SAT Chemistry Subject Test

As you might expect, a triple bond such as the one that’s present in hydrogen
cyanide (HCN) is represented by a triple line. Take a look:

H   –   C   ≡   N;

Using Bond Energies to Predict ∆H

Naturally, different bonds have different strengths. For instance, the bond
between a carbon atom and an oxygen atom is stronger than the bond between
two chlorine atoms. When a bond is relatively strong, we say its bond energy is
high. Bond energy refers to the amount of energy it takes to break a bond. Since
a C–O bond is harder to break than a Cl–Cl bond, the C–O bond has higher bond
energy than the Cl–Cl bond.

ΔH

Remember    that    ΔH  is  the

change  in  enthalpy    that

occurs  in  the course  of  a

reaction.   A   positive    ΔH

indicates   a   net absorbance

of  energy.

Chemical reactions involve the breaking of bonds in the reactants (which
requires energy) and the formation of new bonds to make products (which
releases energy). If we know which bonds are to be made and which are to be
broken, and we know their respective bond energies, we can estimate ∆H for the
reaction.

Suppose we need to estimate ∆H for the reaction H 2 + Br 2 → 2HBr, given the

following bond energies:

H–H bond:   436 kJ/mol

Br–Br   bond:   193 kJ/mol

H–Br    bond:   366 kJ/mol

In converting H 2 and Br 2 to products, we must break 1 mole of H–H bonds and 1

mole of Br–Br bonds. This will require (1 mole) (436 kJ/mol) + (1 mole) (193
kJ/mol) = 629 kJ of energy. Since we form 2 moles of H–Br bonds, this releases
(2 moles) (366 kJ/mol) = 732 kJ of energy. The enthalpy change for the reaction,
∆H, is equal to the net energy change, which is 629 kJ − 732 kJ = −103 kJ. If 1
mole of H 2 and 1 mole of Br 2 react to form 2 moles of HBr, the reaction should