Although familiarity with the term "standard enthalpy of formation" the majority going to the chemistry lessons, though it is used more widely.It is hard to imagine any area of activity, where it is not used to this phenomenon.
Here is an example of just some of them, which is necessary to know what is the heat of reaction.Currently, the automotive industry is developing at a fantastic rate: the number of cars is increasing annually by several times.At the same time the main source of energy for them is gasoline (alternative development is still embodied only in a few prototypes).To adjust the flashing power fuel use special additives that reduce the intensity of the detonation.A striking example - monomethylaniline.Upon receipt calculated heat of reaction, which in this case is -11-19 kJ / mol.
Another area of application - food industry.Without a doubt, anyone who paid attention to the calorie indication of a product.In this case, caloric content and the heat of reaction directly connected, as in the oxidation of food generates heat.Correcting their diet, based on these data, it is possible to achieve a significant reduction in body weight.Despite the fact that the heat of reaction is measured in joules, and between them there is a direct relationship calories: 4 J = 1 kcal.With respect to food products is usually indicated the estimated amount (weight).
Let us now turn to the theory and give a definition.Thus, the thermal effect is indicative for the amount of heat released or absorbed by the system, it flows in chemical processes.It should be borne in mind that in addition to heat radiation can be generated.Standard enthalpy of formation is numerically equal to the difference between the energy levels of the system: initial and residual.If the reaction heat is absorbed from the surrounding space, we speak of the endothermic process.Accordingly, the allocation of thermal energy is characteristic of the exothermic process.They are fairly easy to distinguish: if the value of the total energy released by the reaction, is more than it spent for the start (for example, thermal energy burning fuel), this exotherm.But for the decomposition of water into hydrogen and carbon monoxide and the need to spend more energy to heat, so it holds the absorption (endothermy).
Calculate the heat of reaction is possible using known formulas.In the calculations of the thermal effect is designated by the letter Q (or DH).The difference in the type of process (endo or exo), so that Q = - DH.Thermochemical equations assume an indication of the effect of heat and reagents (faithful and reverse calculation).The peculiarity of these equations in the possibility of transferring value of thermal effects and substances themselves in different parts.Carrying out by subtracting or adding formulas themselves, but with the physical state of matter.
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 subtract 2 and 3 from 1 (right side of the right-wing, left-wing - from the left).
As a result, we obtain:
CH4 - C - 2 H4 = 890 - 394 - 572 = - 76 kJ.
If all parts multiplied by - 1 (remove negative value), we get:
C + 2H2 = CH4 + 76 kJ / mol.
How can you interpret the results?The thermal effects occurring during the formation of methane from carbon and hydrogen, was 76 J per mole of product gas.Also from the formulas that the thermal energy will be released, that is, we are talking about an exothermic process.Similar calculations can avoid the need for immediate laboratory experiments that are often difficult.
