5.08 – Bond Enthalpies

As we wrap up the bonding chapter, we need to extend our understanding of covalent bonds to include thermodynamic data.  In this lesson we will connect the energy to make or break a bond, with the length of the bond and number of bonds between the atoms.

Lesson Objectives

  • Predict the relationship between bond type (single, double, and triple), bond strength (or enthalpy), and bond length. (8.8)
  • Use bond enthalpies to estimate enthalpy changes for reactions involving gas-phase reactants and products. (8.8)

To become familiar with the topics presented in this mission, view the slides below and take note of the key ideas.  These are from section 8.8 of your text.

Now work through the practice problems, and post your work to OneNote.

Work out these mastery problems and check your answers in OneNote.  Post your work when finished, including any corrections.  These questions are from the text and a released AP Exam.

8.66    Using the table above, estimate ΔH for the following gas-phase reactions:

8.68    Use bond enthalpies from Table 8.4 to estimate the enthalpy change for each of the following reactions:

a) C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)

b) C2H5OH(g) + 3 O2(g) → 2 CO2(g) + 3 H2O(g)

c) 8 H2S(g) → 8 H2(g) + S8(s)


1996 FRQ #3

C2H2(g) + 2 H2(g) → C2H6(g)

Substance             S° (J/mol K)            ΔH°f (kJ/mol)         Bond      Bond Energy (kJ/mol)

C2H2(g)                  200.9                      226.7                     C-C                         347

H2(g)                      130.7                      0                             C=C                        611

C2H6(g)                  --------               -84.7                     C-H                        414

H-H                        436

a. If the value of the standard entropy change, ΔS°, for the reaction is -232.7 joules per mole Kelvin, calculate the standard molar entropy, S°, of C2H6

b. Calculate the value of the standard free-energy change, ΔG°, for the reaction. What does the sign of ΔG° indicate about the reaction above?

c. Calculate the value of the equilibrium constant, K, for the reaction at 298 K.

d. Calculate the value of the C≡C bond energy in C2H2in kilojoules per mole.


Now that we have tied bonds to energy, let's start looking into more depth at molecule shapes and polarity.