Definition:
In any chemical reaction, energy is absorbed or released (usually in the form of heat or light). This is because, by breaking and forming bonds, it is absorbed and energy is released respectively.
Basic concepts:
Heat of Reaction:
Heat of Formation:
Heat of Combustion:
As an arbitrary criterion, a negative sign is understood as an exothermic reaction, which indicates that the system loses or releases energy, and a positive sign when the reaction is endothermic, which indicates that the system absorbs or gains energy.
Reaction Energy Classification
According to the energy criteria:
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Exothermic:
The energy released in the new bonds that form is greater than that used in the bonds that break. (they release energy, they are associated with a negative sign).
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Endothermic:
The energy absorbed in the bonds that are broken is greater than the energy released in the bonds that are formed. (they absorb energy, they are associated with a positive sign).
According to its origin:
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Chemistry:
Exothermic chemical energy releases heat, which can be used as an ignition source.
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Electric:
The passage of an electric current or a spark produces heat.
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Nuclear:
Nuclear fusion and fission produce heat.
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Mechanical:
By compression or friction, the mechanical resistance of two bodies can produce heat.
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Hess’s law:
Or the law of the additive heat of the reaction that says that if a system evolves from one initial state to another, the final energy balance does not depend on the chemical path followed, but only on the initial and final stages.
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Reaction speed:
A chemical reaction can be fast or slow depending on different factors and this is called chemical kinetics. Thus, the reaction rate that indicates the speed with which a chemical reaction occurs is defined as the number of moles of a transformed substance per unit of time.
Factors Affecting the Rate of Reaction Energy
Nature of reagents:
It is clear that there are substances that tend to react more easily when they come into contact with each other and, therefore, do so at a higher rate than those that have a lower reactivity with each other. For example, if we put chlorine ions (Cl-) in contact with sodium ions (Na +), they will react quickly to form sodium chloride (NaCl). However, if two noble gases are brought into contact, there will be no reaction between them (under normal conditions, of course).
Reagent concentration:
For a reaction to take place, there must be contact between the reactants, they must collide and it is clear that the greater the number of particles that are in a given space, the greater the number of collisions that will occur. Experimentally, it was shown that the reaction rate is directly proportional to the concentration of the reagents d in such a way that if the concentration of these doubles, the rate is localized.
Temperature:
Temperature is a measure of the internal agitation of bodies, that is, the movement of particles within the body. The higher the temperature, the greater the agitation or movement and therefore the greater the possibility of collisions, which leads to an increase in the reaction rate.
Presence or absence of catalysts:
Catalysts are substances that help the reaction, increasing the speed of the reaction. After completion of the reaction, the catalysts return to their initial form.
The minimum energy principle:
As in all nature, also in chemical reactions the principle of minimum energy works, according to which material systems tend to evolve in the direction in which their potential energy decreases. A ball descends through an inclined plane until it finds the lowest position, which is the one with the lowest energy; a compressed spring expands to reach a condition of minimum deformation and therefore minimum accumulated energy, and a chemical reaction evolves to states of lower energy.
It sometimes happens that, since the energy content of the products is lower than that of the reactants, the system in question does not evolve spontaneously, as might be expected, according to the principle of minimum energy. In some cases, that is, because a certain amount of energy, usually small, is necessary to initiate the reaction, in the same way that a block of wood must be given an initial impulse to descend on an inclined plane. This initial dose of energy is called activation energy and is used to break the first few connections, which will provide enough energy to sustain the reaction on its own.
The principle of maximum disorder
According to the principle of minimum energy, considered in isolation, no endothermic reaction can be spontaneous, because in this type of reaction the energy of the system increases. However, there are reactions and processes in nature that, being endothermic, occur spontaneously. This indicates that, along with energy, another factor must condition the spontaneous nature of a chemical reaction. This additional factor is the degree of disorder, also called entropy (S).
Entropy depends on factors such as the number of particles in play or the physical state of the substances. Thus, the gaseous state is more disordered than the liquid or solid and therefore corresponds to a higher entropy.
Along with the tendency to reach the state of minimum energy, chemical systems naturally tend to reach the state of maximum disorder and both factors together control the spontaneous character of chemical reactions.
A balance between energy and disorder:
The fact that the spontaneity of chemical reactions depends not only on energy, but also on disorder, can be explained by the following equation between physical quantities:
ΔG = ΔH – TΔS; where H is the energy content or enthalpy, T is the absolute temperature, S is the entropy and G is the Gibbs free energy. This magnitude G, to which both enthalpy and entropy contribute, is what determines the spontaneous character of a chemical reaction. In all spontaneous processes, the free energy of the system decreases, that is, the final value of G is less than the initial value and therefore ΔG is negative. According to the above equation, this decrease (ΔG <0) may be due to a decrease in the H energy content (ΔH <0), an increase in the disorder (ΔS> 0), or both.
The end result of this balance between energy and disorder is responsible for the spontaneity of the reaction. If T.SS is greater than ΔH, even if the process is endothermic (ΔH> 0), it will be spontaneous (ΔG <0). Such is the case of the reaction:
N2O4 (g) → 2.NO2 (g)
which is not spontaneous at 258 K and is spontaneous at 358 K, because at this temperature, the disorder term T δ S predominates over the energetic term Δ H, with which ΔG is negative. This example shows the importance of the temperature factor in determining whether a chemical reaction is spontaneous or not.
Any exothermic reaction (ΔH <0) in which an increase in entropy occurs (ΔS> 0) is spontaneous (ΔG <0). An example is the decomposition reaction of hydrogen peroxide:
2 H2O2 (g, 1 atmosphere) → 298 K → 2.H2O (g, 1 atmosphere) + O2 (g, 1 atmosphere) + 211 kJ
In this process, the number of particles increases (with the same gaseous state of reactants and products), which increases the disturbance; but also, it releases heat (ΔH <0). Both circumstances contribute to the decrease in free energy and therefore the process occurs spontaneously.
