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Equilibrium is a province in which there are no discernible alterations as times goes by. When a chemical reaction has reached the equilibrium province, the concentration of reactant and merchandises remain changeless over clip, and there are no seeable alterations in the system. However, there is much activity at the molecular degree because reactant molecule to organize merchandise molecule while the merchandise molecule react to give reactant molecule. This dynamic state of affairs is the topic of term equilibrium.As few chemical reactions proceed in merely one way. Most are reversible, at least to some extent. At shortly as some merchandise molecule are formed, the contrary procedure begins to take topographic point and the react molecule reactant molecule are formed from merchandise molecule. Chemical equilibrium is achieved when the rates of forward and contrary reactions are equal and the concentrations of the reactants and the merchandise remain changeless.

Chemical EQUILIBRIUM is a dynamic procedure. As such, it can be linked to the motion of skiers at a busy sky resort, where the figure of skiers carried up the mountain on the chair lift is equal to the figure coming down the inclines. Although there is changeless transportation of skiers, the Numberss of people at the top and the figure at the underside of the incline do non alter.

Note that the chemical equilibrium involves different substances as reactant and merchandise. Equilibrium between two stage of the same substances is called physical equilibrium because the alterations that occur are physical procedure.

History: –

The construct of chemical equilibrium develops after Berthoket ( 1803 ) found that some chemical are reversible. For any reaction to be equilibrium, rate of forward reaction is equal rate of backward reaction, so at equilibrium about all the reactants are used up & A ; for to go forth it barely any merchandise formed from reactant.

In 1864 Guldberg and pay showed by experimentation that in chemical reaction an equilibrium is reached that can be approached from either way. They were seemingly the first to recognize that there is a mathematical relation between the concentration of reactants and merchandises at equilibrium. In 1877 va n’t hoff suggested that in the equilibrium expressions the concentration of each reactant should look to the first power, matching with the stochiometric Numberss in the balanced chemical equation.

The construct of P [ H ] was foremost introduced by Soren Peder Lauritz Sorensen at the carlsberg research lab in 1909 and revised to the modern pH in 1924 after it became evident that electromotive force in cells depended on activity instead than concentration of H ions.


Physical Equilibrium: –

The equilibrium affecting merely physical alterations called physical equilibrium. Examples are vaporization of H2O, runing ice, disintegration of ice disintegration of sugar in H2O etc.


It can be established merely in instance of closed system i.e. the system should neither derive affair from environing nor lose affair to environing.

Procedure is dynamic in nature, non halt but alterations take topographic point in forward or backward way with same velocity.

The mensurable belongings of system become changeless at equilibrium.

They may in signifier of: –

Solid- liquid Equilibrium.

Liquid- Gas Equilibrium.

Solid- Vapour Equilibrium.

Chemical Equilibrium: –

Equilibrium apparatus in chemical reaction called chemical reaction. Example reaction between N and H, Decomposition of Ca carbonate, reaction between sulfur and O.


It is dynamic in nature.

The discernible belongingss of the system become changeless equilibrium and remain unchanged thereafter.

The equilibrium can be approached from either way.

The equilibrium attain merely in the system is closed one.

The free energy at changeless force per unit area and temperature is zero.

They are of two types-

Irreversible reaction.

Reversible reaction.


Homogeneous equilibrium: – The term homogenous equilibrium applies to reactions in which all reacting species are in the same stage.

Heterogeneous equilibrium: – A heterogenous equilibrium consequences from a reversible reaction and merchandises that are in different stages.


It is applicable merely when concentration of reactant and merchandise have attained their equilibrium province, the value of tantamount invariable is independent of the original concentration of reactant. The value of equilibrium invariable is the same irrespective of way from which equilibrium has been attained, The presences of accelerator can non change the place of equilibrium so the value of equilibrium invariable, and it besides non alter in presence of inert stuff.

Application: –

Predicting the extent of a reaction: – The magnitude of a equilibrium changeless K indicates the extent of reaction can travel. The larger the value of K, greater will be equilibrium concentration of constituents on right manus side of reaction to those on left manus side i.e. the proceed to greater extent, The value of K is really little which means that the forward reaction has proceed to little extent merely.

Predicting the way of reaction: – The equilibrium changeless helps in foretelling the way in which reaction can continue at any phase. If Q is reactive Quotient, K is equilibrium changeless.

Q is more than K= Reaction in forward way.

Q is less than K= Reaction in back ward way.

Q is equal to K= Reaction is at equilibrium.


Chemical equilibrium represents a balance between forward and contrary reactions. In most instances, this balance is quit delicate. Change in experimental status may upset the balance and switch the equilibrium place so that variable can be controlled by experimentation are concentration, force per unit area, volume and temperature. We will analyze the consequence of a accelerator


When concentration of any component additions, the system will seek to cut down the additions in concentration. Therefore, additions in concentration of reactant favors forward reaction and lessenings in concentration of merchandise favors backward reaction.


Change in force per unit area normally do non consequence the concentration of the responding species in condensed stages ( say, in an aqueous solution ) because liquids and solids are virtually in compressible. On the other manus, concentration of gases are greatly affected by alteration in force per unit area. The greater the force per unit area, the smaller the volume, and frailty versa. Note, excessively, that the term ( n/V ) is the concentration of the gas in mol/L, and it varies straight with force per unit area.

In general an addition in force per unit area ( lessening in volume ) favours the net reaction that decreases the entire Numberss of moles of gases ( the contrary reaction, in this instance ) , and a lessening in force per unit area ( increase in volume ) favours the net reaction that increases the entire Numberss of moles of gases ( here the forward reaction ) .For reaction in which there is no alteration in the Numberss of moles of gases, a force per unit area ( or volume ) alteration has no consequence on the place of equilibrium.


A alteration in concentration, force per unit area, or volume may change the equilibrium place, that is, the comparative sums of reactants and merchandises but it dose non alter the value of equilibrium changeless.Only a alteration in temperature can change the equilibrium invariable. At equilibrium at a certain temperature, the heat consequence is zero because there is no net reaction.If we treat heat as though it were a chemical reagent, than a rise in temperature “ adds ” heat to the system and a bead in a temperature “ removes ” heat from the system.As with a alteration in any other parametric quantity, ( concentration, force per unit area, or volume ) , the system displacement to cut down the consequence of the alteration. Therefore, a temperature addition favours the endothermal way ( from left to compensate of the equilibrium equation ) and a temperature lessening favours the exothermal way.

In drumhead, a temperature addition favours an exothermal way and a temperature lessening favours the exothermal reaction.


We know that a accelerator enhance the rate of a reaction by take downing the energy of the forward reaction and contrary reactions to the same extant.We can therefore conclude that the presence of a accelerator does non change the equilibrium invariable, nor does it switch the place of an equilibrium system.Adding a accelerator to a reaction mixture that is non at equilibrium will merely do the mixture to make Equilibrium Oklahoman.


Because the concentrations of H+ and OH- in aqueous solution are often really little Numberss and hence inconvenient to work with, Soren Sorensen in 1909 proposed a more practical step called pH.

The pH of a solution is defined as the negative logarithm of H ion concentration in ( mol/L ) :

pH = -log [ H3O+ ] or pH = -log [ H+ ]

The negative logarithm gives us a positive figure for Ph, which otherwise would be negative due to little value of [ H+ ] . Furthermore the term [ H+ ] permits merely to the numerical portion of the look for H ion concentration, for we ca n’t take the logarithm of units. Therefore, like the equilibrium invariable, the pH of a solution is a dimensionless measure.

Acidic solutions: pH is less than 7.00.

Basic solutions: pH is more than 7.00.

Impersonal solutions: pH is equal to 7.00.

Notice that pH additions as [ H+ ] lessenings.

For existent solutions, activity normally differs from concentrations, sometimes appreciably. Knowing the solute concentration, there are dependable ways based on thermodynamics for gauging its activity, but the inside informations are beyond the range of text.

Keep in head, hence, that the mensural pH of a solution is normally mot same as that, because the concentration of the H+ ion in molar concentration is non numerically equal to its activity value. Although we will go on to utilize concentration in our treatment, it is of import to cognize that this attack will give us merely an estimate

Of the chemical procedure that really take topographic point in solution stage. In the research lab, pH of a solution is measured with a pH metre.

List of some pH of a figure of common fluid, the pH of a organic structure unstable varies greatly, depending on location and map. The low pH ( high sourness ) of stomachic juices facilitates digestion whereas a higher pH of blood is necessary for conveyance of O.


P.W. Atkins, Physical Chemistry, 3rd edition, Oxford University Press, 1985.

F.van Zeggeren and S.N Steery, the calculation of chemical equilibrium,1920.

W.R. Smith and R.W. Mission, chemical equilibrium Analysis.

RAYMOND CHANG, Williams college

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