ATOC/CHEM 5151 – Fall 2016 (updated web link Aug 31, 2016)
Problem 7
Oxidation States of Carbon and Reaction Enthalpy
Answers: to be posted Thursday, September 1, 2016
Environmental chemists often use oxidation states of important nutrients (e.g., carbon,
nitrogen, and sulfur) to track energy flow in the earth system. In Earth’s atmosphere, which is
rich in oxidants (O3, O2, OH, HO2, and NO3, for example), oxidation reactions tend to
dominate over other reactions when it comes to extracting energy from compounds emitted
to the atmosphere. As a result, many compounds are oxidized to forms that produce acids
(which are highly oxidized species). Examples include CO2 (which forms carbonic acid, of
H2CO3, in water), HNO3, HONO, HSO3, and H2SO4.
(1) Using the simple rule that each H counts as +1 and each O counts as -2, determine the
oxidation state of the carbon atom and the nitrogen atom in each of the following
compounds.
______ Methane (CH4)
______ Methanol (CH3OH)
______Formaldehyde (CH2O)
______Formic acid (HCOOH)
______ Glyoxal (C2H2O2 or CHO-CHO, where each C is singly bonded to H and doubly
bonded to O)
______Carbon dioxide (CO2)
______ Carbonic acid (H2CO3)
______ Bicarbonate ion (HCO3-)
______ Carbonate ion (CO32-)
(2) Determine the energy released by the composite reaction to form formaldehyde from OH
and CH4. Use the following source for “enthalpy of formation” for each of the reactants
and products (http://jpldataeval.jpl.nasa.gov/pdf/JPL_Publication_15-10.pdf). Note – see
Table 6-2 starting on page 6-3, and use the value for H(298).
OH + CH4  CH3 + H2O
CH3 + O2  CH3O2
CH3O2 + NO  CH3O + NO2
CH3O + O2  HO2 + CH2O
HO2 + NO  OH + NO2