What is the thermal effect of the reaction

Although familiarity with the term "thermal effect of a chemical reaction" in the majority occurs in chemistry lessons, nevertheless it is applied more widely. It is difficult to imagine any area of activity where this phenomenon would not be used.

Let us give an example of only some of them, where it is necessary to know what the thermal effect of the reaction is. Currently, the automotive industry is developing at a fantastic pace: the number of cars annually increases several times. At the same time, the main source of energy for them is gasoline (alternative designs so far find their embodiment only in single prototypes). To correct the flare force of the fuel, special additives are used that reduce the intensity of detonation. A vivid example is monomethylaniline. When it is received, the thermal effect of the reaction is calculated, which in this case is -11-19 kJ / mol.

Another field of application is the food industry. No doubt, any person paid attention to the indication of caloric content of a particular product. In this case, the caloric value and the thermal effect of the reaction are directly related, since during the oxidation of food, heat is released. Correcting your diet based on these data, you can achieve a significant reduction in body weight. Despite the fact that the thermal effect of the reaction is measured in joules, there is a direct relationship between them and calories: 4 J = 1 kcal. For food products, the estimated amount (mass) is usually indicated.

Let us now turn to the theory and give a definition. So, the heat effect indicates the amount of heat released or absorbed by the system when chemical processes occur in it . It should be borne in mind that in addition to heat, radiation can be generated. The thermal effect of the chemical reaction is numerically equal to the difference between the energy levels of the system: the initial and residual. If in the process of the reaction heat is absorbed from the surrounding space, then the endothermic process is said. Accordingly, the release of thermal energy is characteristic of the exothermic process. They are quite simple to distinguish: if the value of the total energy released as a result of the reaction turns out to be greater than the one spent for it (for example, the thermal energy of the burning fuel), then this is an exotherm. But for the decomposition of water and coal into hydrogen and carbon monoxide, it is necessary to spend additional energy on heating, so its absorption (endothermy) takes place.

The thermal effect of the reaction can be calculated using known formulas. In calculations, the thermal effect is denoted by the letter Q (or DH). The difference in the type of process (endo or exo), therefore Q = - DH. Thermochemical equations assume an indication of the thermal effect and reagents (correct and reverse calculation). The peculiarity of such equations is the possibility of transferring the magnitude of thermal effects and the substances themselves to different parts. It is possible to perform term-by-term subtraction or addition of the formulas themselves, but taking into account the aggregate states of the substances.

Let us give an example of combustion reactions of methane, carbon, hydrogen:

1) CH4 + 2O2 = CO2 + 2H2O + 890 kJ

2) C + O2 = CO2 + 394 kJ

3) 2H2 + O2 = 2H2O + 572 kJ

Now we subtract 2 and 3 from 1 (the right-hand sides from the right, the left-from the left).

As a result, we get:

CH4 - C - 2 H4 = 890 - 394 - 572 = - 76 kJ.

If all the parts are multiplied by - 1 (remove the negative value), then we get:

C + 2H2 = CH4 + 76 kJ / mol.

How can you interpret the result? The thermal effect that occurs when methane is formed from hydrogen and carbon is 76 J for each mole of the produced gas. It also follows from the formulas that thermal energy will be released, that is, it is an exothermic process. Such calculations make it possible to avoid the need for direct laboratory experiments, which often involve difficulties.