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Lecturer, Department of Pharmacy
Chemical Equilibrium
Reversible reaction A reaction which can go in the forward and backward direction simultaneously is called a ‘Reversible Reaction’. Such a reaction is represented by writing a pair of arrows between the reactants and products.
A + B ⇌ C+D
The arrow pointing the right indicates the forward reaction, while that pointing left shows the reverse reaction. Examples of reversible reactions Some common examples of reversible reactions are listed below: y 2NO 2 ( g ) ⇌ N 2 O 4 ( g ) y H 2 ( g ) + I 2 ( g ) ⇌ 2HI ( g ) y PCl 5 ( s ) ⇌ PCl 3 ( s ) + Cl 2 ( g ) y CaCO 3 ( s ) ⇌ CaO ( s ) + CO 2 ( g ) y CH 3 COOH ( l ) + C 2 H 5 OH ( l ) ⇌ CH 3 COOC 2 H 5 ( l ) + H 2 O ( l ) Equilibrium Equilibrium is a state in which there are no observable changes as time goes by. Chemical equilibrium The state of a reversible reaction when the two opposing reactions occur at the same rate and the concentrations of the reactants and products do not change with time.
Lecturer, Department of Pharmacy More clearly, chemical equilibrium is achieved when:
- the rates of the forward and reverse reactions are equal and
- the concentrations of the reactants and products remain constant. Chemical Equilibrium is Dynamic Equilibrium A + B ⇌ C+D As this reaction attains equilibrium, the concentrations of A and B, as also of C and D remains constant with time. Apparently, it appears that the equilibrium is dead. But it is not so. The equilibrium is dynamic. Actually, the forward and the reverse reactions are taking place at equilibrium but the concentrations remain unchanged. Consider the following reversible reaction, H 2 ( g ) + I 2 ( g ) ⇌ 2HI ( g ) The dynamic nature of chemical equilibrium can be demonstrated by adding small amount of radioactive iodine (I 2 ) to the above reaction in the state of equilibrium. It is noticed after some time that a mixture contains radioactive hydrogen iodide (HI). It indicates that the reaction is going on even at equilibrium. H 2 ( g ) + I* 2 ( g ) ⇌ 2HI* ( g ) Characteristics of chemical equilibrium
- Constancy of concentrations. When a chemical equilibrium is established in a closed vessel at constant temperature, concentrations of the various species in the reaction mixture become constant. The reaction mixture at equilibrium is called Equilibrium mixture. The concentrations at equilibrium are called Equilibrium concentrations. The equilibrium concentrations are represented by square brackets. Thus, [A] denotes the equilibrium concentration of substance A in moles per litre (mol L-^1 ).
- Equilibrium can be initiated from either side. The state of equilibrium of a reversible reaction can be approached whether we start with reactants or products, for example, the equilibrium H 2 ( g ) + I 2 ( g ) ⇌ 2HI ( g ) is established if we start the reaction with H 2 and I 2 , or 2 HI.
- Equilibrium cannot be attained in an open vessel. The equilibrium can be established only if the reaction vessel is closed and no part of the reactants or products is allowed to escape out. In an open vessel, the gaseous reactants and/or products may escape into the atmosphere leaving behind no possibility of attaining equilibrium. However, the equilibrium can be attained when all the reactants and products are in the same phase i.e., ethanol and ethanoic acid: CH 3 COOH ( l ) + C 2 H 5 OH ( l ) ⇌ CH 3 COOC 2 H 5 ( l ) + H 2 O ( l )
Lecturer, Department of Pharmacy Equilibrium Constant: Equilibrium Law (Source: Essentials of Physical Chemistry by Bahl and Tuli, Chapter 17, page 627-628 of e-book)
Lecturer, Department of Pharmacy where Kc is the Equilibrium constant. The general definition of the equilibrium constant may thus be stated as : ‘the product of the equilibrium concentrations of the products divided by the product of the equilibrium concentrations of the reactants, with each concentration term raised to a power equal to the coefficient of the substance in the balanced equation’. How to write Equilibrium Constant Expression? (steps 1-4 from Bahl & Tuli book) Step 1. Write the balanced chemical equation for the equilibrium reaction. By convention, the substances on the left of the equation are called ‘reactants’ and those on the right are called ‘products. Step 2. Write the product of concentrations of the ‘products’ and raise the concentrations of each substance to the power of its numerical quotient in the balanced equation. Step 3. Write the product of concentrations of the ‘reactants’ and raise the concentrations of each substance to the power of its numerical quotient in the balanced equation. Example: N 2 ( g ) + 3H 2 ( g ) ⇌ 2 NH 3 ( g ) This is a balanced chemical equation. For this equation,
- The Numerical quotient of N 2 is 1, H 2 is 3 and NH 3 is 2.
Lecturer, Department of Pharmacy Here, Kp is the equilibrium constant. The subscript p is referring to partial pressure. Partial pressures are expressed in atmospheres (atm). Mathematical problems: 1) From Bahl and Tuli, Chapter 17: SOLVED PROBLEM: 1 , 2 , 3 (page no. 629- 630 ) 2) From Huque and Mollah, Chapter 10: Example 10.5 (page no. 260-261) How are Kc and Kp related? (Source: Essentials of Physical Chemistry by Bahl and Tuli, Chapter 17, page 630 of e-book)
Lecturer, Department of Pharmacy Mathematical problems: 1) From Bahl and Tuli, Chapter 17: SOLVED PROBLEM: 1 (pg. 630), 2 (pg. 632), 4 (pg. 633), 2 (pg. 635), 1, 2 (pg. 637), 1 (pg. 638) 2) From Huque and Mollah, Chapter 10: Example 10.6, 10.7 (pg. 261-262) Units of Equilibrium Constant (Reading Only) (Source: Essentials of Physical Chemistry by Bahl and Tuli, Chapter 17, page 642 of e-book) Homogenous Equilibria A homogeneous equilibrium is one in which all of the reactants and products are present in the same phase (by definition, a homogeneous mixture). For the homogenous reaction, C 2 H 2 ( aq ) + 2Br 2 ( aq ) ⇌ C 2 H 2 Br 4 ( aq ) The equilibrium constant is K = [C 2 H 2 Br 4 ] / [C 2 H 2 ] [Br 2 ]^2 The chemical equilibrium in which all the reactants and products are in the liquid phase, are referred to as the liquid equilibria. Like the gas- phase equilibria, the liquid equilibria are also called homogeneous equilibria. More example: The reaction between Acetic acid and Ethyl alcohol to form Ethyl Acetate: CH 3 COOH ( l ) + C 2 H 5 OH ( l ) ⇌ CH 3 COOC 2 H 5 ( l ) + H 2 O ( l ) Heterogeneous Equilibria A heterogeneous equilibrium is a system in which reactants and products are found in two or more phases. The phases may be any combination of solid, liquid, or gas phases, and solutions. When dealing with these equilibria, remember that solids and pure liquids do not appear in equilibrium constant expressions. The decomposition of calcium carbonate upon heating to form calcium oxide and carbon dioxide is an example of heterogeneous equilibrium. If the reaction is carried in a closed vessel, the following equilibrium is established. CaCO 3 ( s ) ⇌ CaO ( s ) + CO 2 ( g )
Lecturer, Department of Pharmacy Le Chatelier’s Principle In 1884, the French Chemist Henry Le Chatelier proposed a general principle which applies to all systems in equilibrium. This important principle called the Le Chatelier’s principle may be stated as: ‘ when a stress is applied on a system in equilibrium, the system tends to adjust itself so as to reduce the stress.’ There are three ways in which the stress can be caused on a chemical equilibrium: (1) Changing the concentration of a reactant or product. (2) Changing the pressure (or volume) of the system. (3) Changing the temperature. Thus, when applied to a chemical reaction in equilibrium, Le Chatelier’s principle can be stated as: ‘ if a change in concentration, pressure or temperature is caused to a chemical reaction in equilibrium, the equilibrium will shift to the right or the left so as to minimize the change.’ Effect of a Change in Concentration When concentration of any of the reactants or products is changed, the equilibrium shifts in a direction so as to reduce the change in concentration that was made. N 2 ( g ) + 3H 2 ( g ) ⇌ 2 NH 3 ( g ) When N 2 (or H 2 ) is added to the equilibrium already in existence (equilibrium I), the equilibrium will shift to the right so as to reduce the concentration of N 2 (Le Chatelier’s principle). The concentration of NH 3 at the equilibrium II is more than at equilibrium I. Obviously, the addition of N 2 (a reactant) increases the concentration of NH 3 , while the concentration of H 2 decreases. Thus, to have a better yield of NH3, one of the reactants should be added in excess. A change in the concentration of a reactant or product can be affected by the addition or removal of that species. Let us consider a general reaction, A + B ⇌ C When a reactant A is added at equilibrium, its concentration is increased. The forward reaction alone occurs momentarily. According to Le Chatelier’s principle, a new equilibrium will be established so as to reduce the concentration of A. Thus, the addition of A causes the equilibrium to shift to right. This increases the concentration (yield) of the product C. Following the same line of argument, a decrease in the concentration of A by its removal from the equilibrium mixture, will be undone by shift to the equilibrium position to the left. This reduces the concentration (yield) of the product C.
Lecturer, Department of Pharmacy The addition of an inert gas has no effect on the position of the equilibrium. The addition of an inert gas, without changing the volume of the vessel, increases the total pressure but the partial pressures of the reactants and products are not changed. Effect of a Change in Pressure To predict the effect of a change of pressure, Le Chatelier’s principle may be stated as: when pressure is increased on a gaseous equilibrium reaction, the equilibrium will shift in a direction which tends to decrease the pressure. The pressure of a gaseous reaction at equilibrium is determined by the total number of molecules it contains. If the forward reaction proceeds by the reduction of molecules, it will be accompanied by a decrease of pressure of the system and vice versa. Let us consider a reaction, A + B ⇌ C The combination of A and B produces a decrease of number of molecules while the decomposition of C into A and B results in the increase of molecules. Therefore, by the increase of pressure on the equilibrium it will shift to right and give more C. A decrease in pressure will cause the opposite effect. The equilibrium will shift to the left when C will decompose to form more of A and B.
Lecturer, Department of Pharmacy Application of Le Chatelier’s Principle to Ensure Optimum Conditions for Maximum Yields in Industrial Processes (Note: Read the names and examples only (No description) for 1 - mark Q/A, e.g. MCQ, Fill in the blanks, True/False, One-two word Q/A)
- Synthesis of Ammonia ( Haber Process) The manufacture of ammonia by Haber process is represented by the equation: N 2 ( g ) + 3H 2 ( g ) ⇌ 2 NH 3 ( g ) + 22.0 kcal
- Manufacture of Sulphuric acid ( Contact Process) The chief reaction used in the process is: 2SO 2 ( g ) + O 2 ( g ) ⇌ 2 SO 3 ( g ) + 42.0 kcal
- Manufacture of Nitric acid ( Birkeland-Eyde process) Nitric acid is prepared on a large scale by making use of the reaction: N 2 ( g ) + O 2 ( g ) ⇌ 2NO ( g ) – 43.2 kcal
