N o r m a n Friedman Harford Community College Eel Air. Maryland 21014
I
Hydrogen Bonding and Heat of Solution
A typical experiment demonstrating heat of solution1 involves mixing e q u a l volumes of water and e t h y l alcohol and noting t h e increase i n t e m p e r a t u r e of the solution. A modification of t h i s experiment i s presented that clearly illustrates the role of hydrogen bond formation and its effect upon the heat of solution. T h e heat of solution produced b y mixing two hvdroeen bonded liquids, i.e. C2H50H-H20, can b e considered originate from t h e difference between an energy consuming process a n d a n energy releasing process, illustrated in eqn.
to
H-bond breaking energy consuming
H-bond formation energy releasing
(1) The exothermic heat uf ~olutionfor miring an alcohol and water results from the energy releasing pvbeing peater than the energy consuming process. Dimethyl sulfoxide (DMSO) and dimethyl farmamide (DMF)2 are water-soluble aprotic substances, neither being hydrogen bond donors but are hydrogen bond acceptors. If the hydrogen bonded alcohol in eqn. (1)is replaced by either DMSO or DMF, i t is not unreasonable to expect that the heat of solution should be more exothermic since the energy consuming process does not require H-bond breaking between molecules of the aprotic solvent whereas these solvents do participate in H-bond formation with water acting as the donor molecule. This is strikingly confirmed by observing the following temperature changes upon mixing equal volumes (25 ml each) of: CZHsOH-H20,AT = +I0C; DMF-H1O, AT = +16OC; DMSO-H?O,
calculated with the inclusion of specific heat data and the heat capacity of the calorimeter. Experlrnental The calorimeter used is a standard student-type device consisting of an aluminum cup and stirrer, a wooden or plastic cover, and a 050°C thermometer with scale divisions of 0.2'C. A Stvrofoarn CUD mav also he used except w ~ r hDMF as it rapidly dissolv& the cup. kqudl volumes of each solvent pair r25 or hO ml are convenient quantit~esl should he used. Have all equrpment and materials at the same temperature before mixing. After mixing, record the temperature a t 30 s intervals for 5 min with continual stirring of the solution. The maximum temperature increase is obtained by plotting a time-temperature curve and extrapolating to the time of mixing. ~~
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Calculations The observed increase in temperature of the solution depends on 1)the weight of eaeh liquid, 2) their specific heats, and 3) the beat absorbed by the calorimeter. In this experiment, the heat capacity of the calorimeter used was 7.0 ealloC. An illustrative calculation of A H for the DMSO-H20 system follows, where AT = + 2 3 T
h e a t absorbed b v water
h e a t absorbed b y DMSO
A T = +?R0C~ -. . -. . .
An additional system of interest is formamide-water which exhibits a temperature decrease of -2%. Formamide can act as bath a bydro& bond donor and acceptor and the endothermic nature of this &tion orocess arises from the energy .. consuming process being greater than the energy releasing process. Apparently the strength of the intermolecular hydrogen bonding in pure iormamide ispeater than the intermolecular hydrogen bonding in formamide.water. ~
The dramatic nature of these heat effects can be demonstrated by using the DMSO-HPOsystem as the heat source in place of concentrated HzSO-Hz0 in an ether tube a ~ p a r a t u s . ~ The above solvent systems are also suitable for a freshman level laboratory experiment and, if desired, AH (rather than AT) iseasily
248 1 Journal 01Chemical Education
AH-
~
heat absorbed by calorimeter
1.2 Kcal 3.3 Kcal mole 2 7 g Or
Discussion In this experiment entropy effects are assumed t o be of similar magnitudeand small (compared to AH) so that noappreciable error is introduced when comparing values of AH or AT. It is recommended that the calorimetric studies he performed using equal volumes of substances as described and not equal moles. I t is observed in the latter case that the temperature increase is some 3-5OC less than reported, even though larger volumes of substances are used. Correspondingly, the calculated values of AH are less than expected. These "inconsistencies" have been attributed to a n incomplete reaction which is avoided when water is in excess and DMSO or DMF is the limiting reagent. Alyea, H. N., "TOPS inGeneralChemistry," 3rd Ed., J. CHEM. EDUC., Easton, PA, 1967, p. 113. CAUTION: Avoid contact of DMSO and DMF with skin, or the breathing of rts vapors! 'Alyea. H. N , and Dutton, F H., "Tested i)emonstrations in Chemisrry,".l.C'HEM. EDIIC., Easton, PA, 1965.p. 17 k7.2.
