Although the majority of people get acquainted with the term “thermal effect of a chemical reaction” in chemistry lessons, it is nevertheless used 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 a few of them, where knowledge of what the thermal effect of the reaction is is necessary. Currently, the automotive industry is developing at a fantastic pace: the number of cars annually increases several times. At the same time, gas is the main source of energy for them (alternative designs so far find their embodiment only in single prototypes). To adjust the fuel flare strength, special additives are used that reduce the detonation intensity. A striking example is monomethylaniline. Upon its receipt, the thermal effect of the reaction is calculated, which in this case is –11–19 kJ / mol.
Another area of ​​application is the food industry. No doubt, any person paid attention to the indication of the calorie content of a product. In this case, the calorific value and the thermal effect of the reaction are directly related, since heat is generated during the oxidation of food. By adjusting 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 quantity (mass) is usually indicated.
Let us now turn to theory and define it. So, the thermal effect indicates the amount of heat released or absorbed by the system during chemical processes in it . It is worth considering that in addition to heat, radiation can be generated. The thermal effect of a chemical reaction is numerically equal to the difference between the energy levels of the system: the initial and residual. If in the course of the reaction heat is absorbed from the surrounding space, then they speak of an endothermic process. Accordingly, the release of thermal energy is characteristic of an exothermic process. It is quite simple to distinguish between them: if the value of the total energy released as a result of the reaction is greater than the energy expended for its start (for example, the thermal energy of a burning fuel), then this is exothermy. But for the decomposition of water and coal into hydrogen and carbon monoxide, it is necessary to expend additional energy for heating, therefore, its absorption (endothermy) takes place.
The thermal effect of the reaction can be calculated using known formulas. In calculations, the thermal effect is indicated by the letter Q (or DH). The difference in the type of process (endo or exo), so Q = - DH. Thermochemical equations imply an indication of the thermal effect and reagents (the reverse calculation is also true). The peculiarity of such equations is the possibility of transferring the magnitude of thermal effects and the substances themselves in different parts. It is possible to carry out term by term subtraction or addition of the formulas themselves, but taking into account the state of aggregation of substances.
We give an example of the 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 subtract 2 and 3 from 1 (the right parts from the right, the left from the left).
As a result, we get:
CH4 - C - 2 H4 = 890 - 394 - 572 = - 76 kJ.
If all 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 occurring during the formation of methane from hydrogen and carbon will be 76 J for every mole of gas produced. It also follows from the formulas that thermal energy will be released, that is, we are talking about an exothermic process. Such calculations avoid the need for direct laboratory experiments, which are often difficult.