Catalyst
 Given the following equations and Ho values, determine the heat of




reaction (kJ) at 298 K for the reaction:
B2H6(g) + 6 Cl2(g)  2 BCl3(g) + 6 HCl(g)
BCl3(g) + 3 H2O(l) H3BO3(g) + 3 HCl(g)
B2H6(g) + 6 H2O(l)  2 H3BO3(s) + 6 H2(g
1/2 H2(g) + 1/2 Cl2(g)  HCl(g)
ΔH = -112.5 kJ
ΔH = -493.4 kJ
Δ H = -92.3 kJ
Lecture 5.3 – Enthalpies of Formation and
Bond Enthalpies
Today’s Learning Targets
 LT 5.6 – For a given chemical bond, I can characterize and calculate its
bond enthalpy. Furthermore, I can apply this value to the type and
length of bond that the molecule contains.
 LT 5.8 – For a phase change, I can characterize the enthalpy of
formation and what this value indicates about a given phase change.
 LT 5.9 – I can apply Hess’s Law to a chemical reaction and calculate
the enthalpy for a combination of chemical reactions.
Enthalpies of Formation (ΔHf)
 The enthalpy change for creating a compound from its constituent
elements is known ΔH of formation (ΔHf)
2 C (s)+ H2 (g)  C2H2 ΔHf =226.7 kJ/mol
 The value of ΔHf depends on temperature, pressure, and state of
products/reactants
Standard Enthalpies
 In order to compare ΔHf of different reactions, we define
standard state of 1 atm and 298 K
 Don’t confuse with STP! (1 atm and 273 K)
 Standard Enthalpies (ΔHo) are ΔH values calculated under
these standard conditions
 ΔHfo is the change in enthalpy to produce one mole of the
compound from the constituent elements
 All elements that are stable at room temperature have a ΔHfo of
0 kJ/mol
Class Example
 Write the equation corresponding to the standard enthalpy of
formation of liquid carbon tetrachloride (CCl4).
Table Talk
 Write the equation corresponding to the standard enthalpy of
formation of gaseous propane (C3H8).
Using ΔHf to calculate ΔHrxn
 We can use known ΔHfo and Hess’s Law to calculate the ΔHrxno
for a given reaction of interest.
 For the combustion of propane gas, C3H8 (g), to CO2 (g) and
H2O (l) under standard conditions:
C3H8 (g) + 5 O2 (g)  3 CO2 (g) + 4H2O (l) ΔHrxno = ?
You know that:
3 C (s) + 4H2 (g)  C3H8 (g) ΔHfo = -103.85 kJ/mol
C (s) + O2 (g)  CO2 (g) ΔHfo = -393.5 kJ/mol
H2 (g) + ½ O2 (g)  H2O (l) ΔHfo = -285.8 kJ/mol
Shortcut for Using ΔHf to calculate ΔHrxn
 We can quickly calculate the ΔHrxn by using the equation:
H  nH (products) mH (reactants)
o
rxn
o
f
o
f
 Note that n and m represent the coefficient from the balanced
chemical reaction.
Class Example
 Calculate the standard enthalpy change for the combustion of 1




mole of benzene (C6H6) to CO2 (g) and H2O (l).
Note:
ΔHfo (CO2) = -393.5 kJ/mol
ΔHfo (C6H6) = 49.0 kJ/mol
ΔHfo (H2O) = -285.8 kJ/mol
Table Talk
 Calculate the standard enthalpy change for the combustion of 1




mole of ethanol (C2H5OH) to CO2 (g) and H2O (l).
Note:
ΔHfo (CO2) = -393.5 kJ/mol
ΔHfo (C2H5OH) = -277.7 kJ/mol
ΔHfo (H2O) = -285.8 kJ/mol
Bond Enthalpies and ΔHrxn
 We can use bond enthalpies to estimate the ΔHrxn where certain
bonds are broken and new bonds are formed.
 Allows for a quick determination of exo- or endothermic
 NOTE – This is an estimate
 This is an application of Hess’s Law
 Assume all bonds are broken and then all bonds reform:
Hrxn (enthalpies of bonds broken) -(enthalpies of bonds formed)
Class Example
 You run the reaction:






2 C2H6 (g) + 7 O2 (g)  4 CO2 (g) + 6 H2O
Using the following ΔH values, calculate ΔHrxn for the reaction
and determine if it is endothermic or exothermic:
ΔHC – H = 413 kJ/mol
ΔHC – C = 348 kJ/mol
ΔHO=O = 495 kJ/mol
ΔHC =O = 799 kJ/mol
ΔHH– O = 463 kJ/mol
Table Talk
 You run the reaction:





2 N2H4 (g)  N2 (g) + 2 H2
Using the following ΔH values, calculate ΔHrxn for the reaction
and determine if it is endothermic or exothermic:
ΔHN – H = 391 kJ/mol
ΔHN – N = 163 kJ/mol
ΔHN2 = 941 kJ/mol
ΔHH-H = 436 kJ/mol
Problem Set 5.3
 This is a sampling of free response type questions that have been
observed on past AP exams
 This will be due next Monday/Tuesday!
Rate Yourself
 We have finished almost all of chapter 5!
 Rate yourself 1 – 4 on LTs 5.1 to 5.9.
 It is your responsibility to study any areas that are a 2 or lower
over the long weekend
Happy Thanksgiving!
Closing Time
 Read 5.7, 8.8
 Do book problems: