Equilibrium is usually understood as a special state of a system or body, when all the effects exerted on it cancel each other out. Or absent altogether. In chemistry, the concept of equilibrium is applied to reactions occurring between various substances, and more precisely, to the conditions of their occurrence.
The concept of equilibrium
Chemical reactions have many classifications according to various criteria, but speaking of chemical equilibrium, what are reversible and irreversible reactions should be remembered.
If as a result of the reaction products are formed that do not interact with each other, they speak of irreversible reactions, that is, they go only in the forward direction. Usually in them one of the products is a gaseous, slightly dissociating or insoluble compound. For instance:
Pb (NO 3 ) 2 + 2ฮCl <โ> PbCl 2 โ + 2HNO 3
Na 2 CO 3 + 2ฮCl <โ> 2NaCl + CO 2 โ + ฮ 2 O
NaOฮ + ฮCl <โ> NaCl + ฮ 2 O
The products of reversible reactions are able to interact with each other, thus forming the starting materials, that is, two oppositely directed reactions occur simultaneously. If, at some point in time, under certain conditions, the rate of the direct reaction will be equal to the rate of the inverse, then the chemical equilibrium is established.
It should be mentioned that such an equilibrium is characterized as dynamic. In other words, both reactions continue, but the concentrations of all its participants remain unchanged and are called equilibrium.
Mathematically, this state is expressed using the equilibrium constant (Cp). Let the interaction of substances occur, described by the equation aฮ + bB <> cC + dD. For opposite reactions, we can write the formulas for calculating their velocities through the law of mass action. Since in a state of equilibrium of velocity these will be equal, it is possible to express the ratio of the rate constants of two opposite reactions. So it will be numerically equal to the equilibrium constant.
The value of K p helps to determine the completeness of the ongoing reactions. If K p <1, then the reaction in the forward direction almost does not proceed. If K p > 1, then the equilibrium is shifted to the products.
Types of equilibrium
Chemical equilibrium is true, apparent, and false. For true balance, there are signs:
- If there is no external influence, then it is invariable in time.
- If external influences change (this applies to temperature, pressure, etc.), then the state of the system also changes. But one has only to return the initial values โโof the conditions, the equilibrium is immediately restored.
- The state of true equilibrium can be achieved both from the side of the products of the chemical reaction, and from the starting materials.
If at least one of these conditions is not satisfied, then they say that such an equilibrium is apparent (metastable). If the state of the system begins to change irreversibly with changing external conditions, then this equilibrium is called false (or inhibited). An example of the latter is the reaction of iron with oxygen.
The concept of equilibrium is somewhat different in terms of thermodynamics and kinetics. Thermodynamic equilibrium refers to the minimum value of the Gibbs energy for a particular system. True equilibrium is characterized by ฮG = 0. And about a state for which the rates of direct and reverse reactions are equal, that is, v 1 = v 2 , they say that such an equilibrium is kinetic.
Le Chatelier principle
Henri Le Chatelier studied the laws of equilibrium displacement in the 19th century, however, he summarized all these works and formulated the principle of moving equilibrium later by Carl Brown:
if you act on the equilibrium system from the outside, then the equilibrium will shift in the direction of decreasing the effect
In other words, if any effect is exerted on the equilibrium system, it tends to change in such a way that this effect is minimal.
Equilibrium shift
We will consider the consequences of the Le Chatelier principle using the reaction equation as an example:
N 2 + 3ฮ 2 <โ> 2Nฮ 3 + Q.
If you increase the temperature, the balance will shift towards the endothermic reaction. In this example, heat is released, which means the direct reaction is exothermic, and the equilibrium will shift to the starting materials.
If you increase the pressure, this will lead to a shift in equilibrium to smaller volumes of gaseous substances. In the above example, there are 4 moles of gaseous starting materials and 2 moles of gaseous products, which means that the equilibrium will shift to the reaction products.
If you increase the concentration of the starting material, then the equilibrium will shift in the direction of the direct reaction and vice versa. Thus, if you increase the concentration of N 2 or ฮ 2 , then the equilibrium will shift in the forward direction, and if ammonia - in the opposite direction.