Thermochemistry in USE Problems
Thermochemistry is a branch of general chemistry that studies the thermal effects of chemical reactions. In the chemistry USE, it is primarily featured in problem 27, which contains three typical problem prototypes. All of them boil down to correlating the thermal effect (or enthalpy, as this quantity is called in university courses) with the amount of substance of reactants and reaction products.
However, real thermochemistry is not limited to such proportional calculations. Its main task is to determine the heat of a chemical reaction using various methods, including indirect ones. This is of immense practical importance: the energy released during the combustion of fuel (e.g., methane in a reaction with oxygen to form carbon dioxide and water) is actively used in energy generation and transportation. Methane fuel is considered environmentally cleaner than petroleum products precisely because of the known thermochemical characteristics of its combustion reaction.
Enthalpy and the Personality of G. I. Hess
Enthalpy (denoted as H) is a thermodynamic state function characterizing the heat content of a system. The enthalpy change ΔH during a reaction is numerically equal to the thermal effect at constant pressure, but has the opposite sign: for exothermic processes ΔH < 0, and for endothermic processes ΔH > 0.
Herman Ivanovich Hess (Germain Henri Hess) was a Russian scientist of Swiss origin and a subject of the Russian Empire. He led geological exploration expeditions in Irkutsk, taught at the St. Petersburg Technological and Mining Institutes, and practiced medicine. In addition to discovering the law of additivity of reaction heats, Hess compiled the first Russian chemical nomenclature and invented the alcoholometer; the concept of "Hess degrees" existed as a measure of the concentration of alcoholic solutions.
Hess's Law
Hess's law is stated as follows: the thermal effect of a chemical reaction depends only on the initial and final states of the system and does not depend on the path by which the reaction proceeds, provided that the pressure (or volume) remains constant.
Metaphorically, this can be visualized as climbing a mountain: you can go straight up or take a winding path—the subjective expenditure of effort differs, but the elevation difference between the base and the summit is identical. For a chemical reaction, the only thing that matters is which reactants were taken and which products were obtained; intermediate stages do not affect the value of ΔH.
Important consequences:
- The ΔH of the forward and reverse reactions are equal in magnitude and opposite in sign.
- A reaction can be represented as the sum of several stages, and the total ΔH is the algebraic sum of the enthalpies of these stages.
- The law makes it possible to calculate the thermal effects of reactions that are impossible or difficult to measure experimentally.
Application of Hess's Law: The Phosgene Combustion Problem
It is required to determine the thermal effect of the combustion reaction of phosgene (COCl₂) to carbon dioxide and chlorine. Direct measurement is difficult, but two reactions are known: the decomposition of phosgene and the decomposition of carbon dioxide, along with their respective ΔH values.
Solution algorithm:
- Write down the target reaction and the two auxiliary ones.
- Reverse the second reaction (the decomposition of CO₂) because in the target reaction CO₂ is a product, not a reactant. Upon reversal, the sign of ΔH changes to the opposite: it was +282.8 kJ, and becomes −282.8 kJ.
- Add the equations and their corresponding enthalpy changes. The resulting sum is the ΔH of the target reaction.
Thus, by taking a detour through intermediate products (carbon monoxide CO), it is possible to calculate the thermal effect, which is inaccessible to direct measurement.
Thermochemistry in the Organism and Dietetics
The combustion of glucose in a bonfire and its oxidation in a living organism are chemically identical processes: identical reactants (C₆H₁₂O₆ and O₂), identical products (CO₂ and H₂O), and identical stoichiometric coefficients. The difference lies in kinetics: in a bonfire, energy is released instantaneously as heat, while in a cell the process proceeds stepwise, allowing energy to be stored in the form of ATP molecules and reducing equivalents (NADH, FADH₂).
According to Hess's law, the total thermal effect of glucose oxidation is identical in both cases. This is precisely why the caloric value of food is determined by burning the product in a calorimeter (bomb calorimeter) and measuring the released heat. Based on such measurements, reference values have been established: carbohydrates yield about 17.2 kJ per gram, and fats yield about 38 kJ per gram. Dietetic meal planning relies directly on thermochemical data and Hess's law, demonstrating the fundamental connection between general chemistry, medicine, and physiology.
Key points
- Enthalpy H is a state function; ΔH characterizes the thermal effect of a reaction at constant pressure.
- Hess's law: The ΔH of a reaction depends only on the initial and final states and does not depend on the path of its reaction.
- Upon reversing a reaction equation, the sign of ΔH changes to the opposite.
- Hess's law makes it possible to calculate thermal effects indirectly—through a combination of known reactions.
- G. I. Hess was a 19th-century domestic scientist, author of the law of constant heat sums and the first Russian chemical nomenclature.
- The oxidation of glucose in a calorimeter and in a cell yields the identical total ΔH, differing only in mechanism and rate.
- The caloric content of food is determined calorimetrically, which is a direct application of Hess's law in dietetics.
- Thermochemistry is the basis for understanding bioenergetics and metabolism for a medical student.