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Chemical Equilibrium

Polycythaemia  (raised RBC count) at high altitude: Polycythaemia (raised RBC count at high altitude) Red blood cells are red only because they contain a protein chemical called hemoglobin(Hb), which is bright red in color. Hemoglobin contains the element Iron (Fe), making it an excellent vehicle for transporting oxygen and carbon dioxide.
There is an equilibrium between oxygen and hemoglobin in RBC during transportation which is given by,
Hb (aq) + 4O2 (g) Hb (O2)4 (aq).
As long as there is sufficient oxygen in the air, a healthy equilibrium is maintained; but at high altitudes, the oxygen levels are low, so a backward reaction is favored (according to Le Chatelier's principle). So the availability of oxygenated blood is low. Without adequate oxygen fed to the body's cells and tissues, a person tends to feel light‐headed. To shift the equilibrium towards right side, the level of RBC count is raised (and hence Hb concentration) leads to polycythaemia.

Learning Objectives

After completing the topic, the student will be able to:

  • Define a reversible reaction, equilibrium state, give a graphical representation of an equilibrium state and list the types of equilibrium based on phase of the system.
  • Comment on dynamic nature of equilibrium.
  • Define equilibrium constant and express the equilibrium constant in terms of partial pressures and concentrations of the reactants and products.
  • State Le Chatelier's principle and discuss the factors effecting equilibrium state.
  • Give the relation between equilibrium constant and temperature.
  • Calculate the solubility product of sparingly soluble salts and predict the formation of precipitate in solution by comparing ionic product and solubility product.
  • Distinguish exothermic and endothermic reactions.
  • Define and calculate heat (energy) changes involved in a given reaction.
Attainment of equilibrium in reversible reaction Attainment of equilibrium in reversible reaction Starting from the beginning of the reaction, represented by the Y–axis (or when time = 0), the rate of forward reaction rises sharply, while the rate of the reverse reaction decreases. This is due to the reaction consisting of pure reactants.

In order to advance the reaction, reactants are converted to products, and it is only until a large enough concentration of products are available, that the reverse reaction becomes a factor. It is at this point that we reach equilibrium, where the forward and reverse rates converge at the same point, forming the equilibrium state.
Reversible reactions

Depending on the extent of reaction, reactions are of two types: reversible and irreversible reactions. Reactants, written on the left hand side of a chemical equation, proceed in their reactions. The products are written on the right hand side of the chemical equation. This is indicated by the direction of the arrow in a chemical equation. This is a direct reaction.

For example when steam is passed over hot iron, hydrogen gas and ferroso‐ferric oxide are produced.

3Fe  + 4H2””/  Fe3O4 + 4H2 ‐‐‐‐‐‐‐‐[Direct reaction]

In some chemical reactions, as soon as the products are formed they begin to react among themselves and actually a backward or reverse reaction starts.

Fe3O4 + 4H2 ””/  3Fe + 4H2O ‐‐‐‐‐‐‐‐[Reverse reaction]

It is interesting to note that if carried out in a open vessel, the reaction cannot go in the backward direction because, one of the products of the reaction, the hydrogen gas escapes. But if the reaction is carried out in a closed vessel, the reaction becomes reversible, because when hydrogen gas is passed over hot ferroso‐ferric oxide, steam and iron are produced.

These two reactions may be represented by a single equation as

Two Reactions in one Equation

Thus a chemical reactions that proceeds in two directions is known as a reversible reaction.

Reversible and irreversible reactions Irreversible and reversible reactions Image at the top represents decomposition of calcium carbonate under open condition, which is an irreversible reaction. While image at the bottom represents decomposition and combination reactions, which is a reversible in nature. (This experiment can also be used to verify the 'Law of conservation of mass').
Examples of reversible reactions

To repeat, a chemical reaction in which substances react together to produce resultants and the resultants in turn react with one another to produce the original substances is known as reversible chemical reaction. For example, decomposition followed by combination reactions of calcium carbonate.

Examples of Reverse reaction

When we heat calcium carbonate (CaCO3) in a closed vessel the evolved carbon dioxide will not go out; it reacts with another product calcium oxide (CaO) to give the calcium carbonate (CaCO3) in reverse. The direction of reaction is affected by like reaction temperature,concentration of reactants, stability of products,etc

When the decomposition reaction is carried out in an open condition, evolved carbon dioxide gas escapes. Therefore, uniting of CO2 gas with calcium oxide is not observed resulting in an irreversible reaction.


Rate of the Reverse Reaction

Reversible reactions do not proceed to completion in any one direction. Usually the rate of the reverse reaction is slower than that of the forward reaction.

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