CHEMICAL EQUILIBRIUM, PART 1, CLASS 11 CHEMISTRY

IN A CHEMICAL REACTION, CHEMICAL EQUILIBRIUM IS THE STATE IN WHICH BOTH THE REACTANTS AND PRODUCTS ARE PRESENT IN CONCENTRATIONS WHICH HAVE NO FURTHER TENDENCY TO CHANGE WITH TIME, SO THAT THERE IS NO OBSERVABLE CHANGE IN THE PROPERTIES OF THE SYSTEM. THIS STATE RESULTS WHEN THE FORWARD REACTION PROCEEDS AT THE SAME RATE AS THE REVERSE REACTION. SO, THE REACTION RATES OF THE FORWARD AND AND BACKWARD REACTIONS ARE EQUAL. THUS, THERE ARE NO CHANGE IN THE CONCENTRATIONS OF THE REACTANTS AND PRODUCTS. SUCH A STATE IS KNOWN AS DYNAMIC EQUILIBRIUM.

1. CLASS 11 CHEMISTRY CHEMICAL EQUILIBRIUM [PART 1]

2. Q. What is equlibrium state ? Explain it . A. The state at which the velocity of forward reaction becomes equal to the velocity of backward reaction or reverse reaction is called equlibrium state. If we consider a reversible reaction:A+B ↔ C+D is taking place in a closed vessel. At the beginning, we have only the reactants A and B. Their concentrations are maximum. As the reaction proceeds, the reactants A and B change into the products C and D. The equlibrium concentrations of the products increase gradually. The rate of forward reaction diminishes while the reverse reaction sets in and proceeds with increasing speed. Contd. 2

3. A state is soon reached where the speed of forward and backward reaction become equal. The system is then said to have a state of equilibrium . Once equilibrium is reached there is no further change in the composition of the system. The products are formed by forward reaction just as fast as they change back into reactants by the reverse reaction. The state at which the rate of forward reaction is equal to the rate of the reverse reaction in a reversible reaction is known as the equilibrium state . 3

4. Q. What is physical equilibrium ? A. In a reaction, when there is a change in physical state only, it is known as physical process. When equilibrium is established in a physical process, it is called physical equilibrium. So, an equilibrium between two different physical states of the same substance is referred to as physical equilibrium. An example of physical equilibrium is as follows: H 2 O[s] ↔ H 2 O[l] 4

5. Q. What is chemical equilibrium ? A. In a reversible process, a state will be attained such that the rate of forward reaction is equal to the backward rate of reaction. Such a state is known as the equilibrium state. If there exists an equilibrium in between ractants and products, such reactions are said to be chemical equilibrium process. As for example: 1. N 2 + 3H 2 ↔ 2NH 3 2. 2SO 2 + O 2 ↔ 2SO 3 5

6. Q. What are the characteristics of physical equilibrium ? A. The characteristics of physical equilibrium are: 1. Equilibrium can be established only in case of a closed system. 2. The equilibrium is dynamic in nature i.e. the process does not stop after the establishment of the equilibrium but the rate of the forward reaction becomes equal to the rate of the backeard reaction. Contd. 6

7. 3. The measurable properties of the system such as melting point, boiling point, vapour pressure and solubility remain constant since the concentration of the substances remains constant. 4. When equilibrium is attained, the concentration of the different substances participating in the physical process becomes constant at constant temperature. 7

8. Q. What are the characteristic of chemical equilibrium ? A. The characteristic of chemical equilibrium are: 1. At equilibrium, the rates of forward and reverse reactions are equal. 2. All the reactants and products are present at equilibrium. 3. The concentrations of the reactants and products reach constant values and these are called equilibrium concentrations. 4. A catalyst does not affect the position of equilibrium. 8

9. Q. Explain dynamic nature of equilibrium . A. A reversible process is said to be in dynamic equilibrium, when the forward and reverse processes occur at the same rate, resulting no observable change in the system. Although the chemical reaction obtained equilibrium, there is conversion of reactants to products and products to reactants, in same amount. The reaction doesn’t stop. But there is no change in concentrations of reactant and products. So, the equilibrium is also called dynamic equilibrium. 9

10. Q. What is the ‘Law of Mass Action ’? A. The ‘Law of Mass Action’ states that: At constant temperature, the rate of chemical reaction is directly proportional to the products of the active masses of reacting species, with each active mass raised to the power equal to the stoichiometric coefficient of that species in the chemical reaction. If we consider, a simple equation: aA+bB → Products, According to law of mass action: Rate of reaction α [A] a [B] b or, Rate of reaction = k[A] a [B] b , where k is a constant of proportionality, and is known as rate constant. 10

11. Q. What is ‘ Equilibrium Constant’ ? A. ‘Equilibrium Constant’ K is the ratio of the product of the active masses of the products to that of the reactants in equilibrium with each active mass term raised to the power equal to the stoichiometric coefficient of the substance in the balanced chemical equation. The ‘Equilibrium Constant’ K is also represented as K c and K p depending upon the unit of active mass used in the expression. When active masses are expressed in terms of concentration unit, K is written as K c . In case, active mass is expressed in partial pressure unit, K is written as K p . 11

12. Q. How is the equilibrium constant found? Explain. A. The numerical value of an equilibrium constant is obtained by letting a single reaction proceed to equilibrium and then measuring the concentrations of each substance involved in that reaction. The ratio of the product concentrations to reactant concentrations is calculated. Let us consider a general reaction: A+B ↔ C+D, in which an equilibrium exists between reactants [A and B] and products [C and D]. Let, [A], [B], [C] and [D], represent the active masses of A, B, C and D respectively at equilibrium. Contd. 12

13. According to law of mass action, Rate of forward reaction α [A][B] = k 1 [A][B] Rate of backward reaction α [C][D] = k 2 [C][D] Where, k 1 and k 2 are rate constants for forward and backward reactions respectively. At equilibrium, the rate of two opposing reactions become equal. Therefore, at equilibrium, Rate of forward reaction = Rate of backward reaction, Or, k 1 [A][B] = k 2 [C][D] Contd. 13

14. Or, k 1 /k 2 = [C][D]/[A][B] At a particular temperature, k 1 and k 2 are constants. Therefore, the ratio k 1 /k 2 will be a constant. This is represented by K and is called Equilibrium Constant. Thus, K = k 1 /k 2 = [C][D]/[A][B] For a general reaction of the type: aA+bB → cC+dD The expression for Equilibrium Constant can be written as: K = [C] c [D] d /[A] a [B] b 14

15. Q. What are the characteristics of Equilibrium Constant ? A. The important characteristics of Equilibrium Constant are as follows: 1. The value of equilibrium constant for a particular reaction is constant at a particular temperature. On changing temperature, the value of equilibrium constant changes. 2. The value of equilibrium constant does not depend upon the initial concentration of the reactants. Whatever be the initial concentration of the reactants, the value of equilibrium costant is always the same for a particular reaction at a particular temperature. Contd. 15

16. 3. The value of the equilibrium constant gets reversed on reversing the mode of representation of the equilibrium. If, the mode of representation of the equilibrium is reversed, the value of equilibrium constant also gets reversed. 4. The value of the equilibrium constant does not change by the presence of a catalyst. Since, a catalyst catalyses the forward and backward reaction to the same extent, the stated of equilibrium is not affected by its presence. Consequently, the value of equilibrium constant remains unchanged. 5. The value of the equilibrium constant changes on changing the stoichiometric coefficients. 16

17. Q. What is La Chatelier’s principle ? A. A system attains equilibrium under a specific set of conditions and remains in the same state of equilibrium so long as these conditions are maintained constant. A changes in these conditions disturbs the equilibrium and causes a net reaction to occur. This reaction helps the system to attain the equilibrium again although the new state of equilibrium is different from the earlier one. In order to predict the effect of changing the conditions of the system at equilibrium, Henry Louis Le Chatelier, a French chemist proposed a general principle in 1884. This principle is commonly known as La Chatelier’s principle. 17

18. Q. What does La Chatelier’s principle state? Explain it. A. La Chatelier’s principle states that ‘When a chemical system at equilibrium is subjected to a change in concentration, pressure or temperature, the system will adjust to a new equilibrium by counteracting the effect of the change as much as possible’. For example: H 2 + I 2 ↔ 2HI If we remove H 2 from the reaction, then to maintain the equilibrium, reaction will shift backwards that is towards H 2 to maintain the concentration of H 2 . 18

19. Q. What are the factors that affect equilibrium ? A. Changes in concentration, temperature and pressure can affect the position of equilibrium of a reversible reaction. Q. Which factor does not affect equilibrium ? A. A catalyst does not affect equilibrium reaction of a reversible reaction. This is because it increases the rate of forward reaction and rate of backward reaction to the same extent. Thus, a catalyst does not affect the position of equilibrium, but simply helps to achieve the equilibrium in a shorter time i.e. quickly. 19

20. Q. What is the effect of pressure on equilibrium ? Explain it. A. The effect of change in pressure is significant only for those systems which involve gaseous substances and the equilibrium reaction involves a net increase or decrease in the number of moles of the gaseous substances. For this specific case, La chatelier’s principle can be restated as follows: When pressure is increased (or decreased) on a system containing gaseous substances in equilibrium, then the equilibrium shifts in the direction which tends to decrease [or increase] the pressure and hence to decrease or increase the number of moles. Contd. 20

21. On increasing the pressure, the number of moles per unit volume increases and thus according to La Chatelier’s principle the equilibrium shifts towards the side where the number of moles per unit volume decreases in order to nullify the effect of an increase in pressure. For example, N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g), there is a total of four moles of gaseous molecules in the left direction and two gaseous molecules in the right direction of the reaction. When the pressure increases, the equilibrium position shifts to the right in order to decrease the number of gaseous molecules. 21

22. Q. What is the effect of temperature change on equilibrium? Explain it. A. A temperature change occurs when temperature is increased or decreased by the flow of heat. This shifts chemical equilibrium towards the products or reactants, which can be determined by studying the reaction and deciding whether it is endothermic or exothermic. At equilibrium both forward and backward reaction occurs simultaneously and in opposite directions. If one of the reaction is exothermic, the other must be endothermic i.e. if the forward reaction proceeds with an evolution of heat, the backward reaction must involve an absorption of heat. Contd. 22

23. Therefore, according to La Chatelier’s principle, ‘When the temperature of a system at equilibrium is increased, the equilibrium shifts in the direction in which heat is absorbed’ . For an exothermic reaction , A + B ↔ C + D + Heat – the forward reaction is exothermic and involves evolution of heat whereas the backward reaction is endothermic and involves an absorption of heat. If such a reaction is subjected to an increase in temperature, the equilibrium must shift in the backward direction to absorb the heat given in the process of increasing the temperature. Hence, in an exothermic reaction, the increase in temperature causes the backward reaction to occur resulting in an increase in the concentration of the reactants. Contd. 23

24. For an endothermic reaction , the reaction can be represented as A + B ↔ C + D -Heat. This reaction is endothermic in the forward reaction and involves an absorption of heat, whereas it is exothermic in the backward direction and is accompanied by the evolution of heat. When such a reaction is subjected to an increase in temperature, heat is supplied to it from the outside. According to La Chatelier’s principle, the equilibrium must shift in the direction of absorption of heat . Obviously, the equilibrium will shift in the forward direction. Thus, an increase in temperature in an endothermic reaction wil help more reactants to combine together and to change in the products resulting in a higher yield of products. 24

25. Q. How does the change in concentration affect equilibrium? A. According to La Chatelier’s principle , if a stress[such as change in concentration, temperature or pressure] is applied to a system in equilibrium, the equilibrium shifts in a way to undo or nullify the effect of imposed stress. Therefore, when the concentration of a substance is increased, the equilibrium shifts in a direction in which the effect of the change of concentration could be reduced. For example, if the concentration of a particular reactant is increased, the system would like to reduce the amount of the added substance. Contd. 25

26. This is possible only when the reactants combine to form the products. Thus forward reaction occurs and the equilibrium shifts towards right. In case, the concentration of a product is increased, the equilibrium shifts towards left in order to decrease the amount of the added substance. Similarly, a decrease in concentration of any of the reactants will cause the equilibrium to shift towards left [backward reaction will occur], while a decrease in the concentration of any of the products will cause the equilibrium to shift towards right [i.e. forward reaction will occur]. 26

27. Q. What is ionic equilibrium ? A. The equilibrium established between the unionized molecules and the ions in the solution of weak electrolytes is called ionic equilibrium . For example, acetate ions and hydrogen ions, broken by the acetic acid: CH 3 COOH ↔ CH 3 COO - + H - . Q. What do you mean by ionization of acid and bases ? A. Ionization of a compound refers to a process in which a neutral molecule splits into charged ions when exposed in a solution. The degree of ionisation of acids and bases depends on the degree of dissociation of compounds into their constituent ions. Strong acids and bases have high degree of ionization in comparison to weak acids and bases. 27

28. Q. Explain ionization of acids . A. The degree of ionization is a measure of an acid or base’s acidity or baseness. In water, a strong acid completely ionises, whereas a weak acid just partially ionizes. Due to the fact that acids have variable degree of ionisation, they also have variable degree of weakness, which may be quantified. Due to the equilibrium nature of the ionisation of a weak acid, the chemical equation and expression for the equilibrium constant are as follows: HA(aq) + H 2 O → H 3 O(aq) + A - K a = [ H 3 O][A - ]/[HA] Contd. 28

29. The Equilibrium Constant for ionization of an acid is used to define the Acid Ionsiation Constant (K a ). The greater the ionisation constant, however stronger the acid. As a result, a strong acid is a better proton donor than a weak acid. Due to the product’s concentration in the numerator of the K a constant, the larger the acid ionisation constant, the stronger the acid. 29

30. Q. Explain ionization of bases . A. Certain bases, such as lithium hydroxide or sodium hydroxide, completely dissociate into their ions in aqueous solution and are referred to as strong bases. As a result, ionisation of these bases result in the formation results of hydroxide ions, represented by the symbol [OH-]. So, expression for the equivalent constant are as follows: B + H 2 O → OH - + HB + K b = [OH - ] [HB + ]/[B] Contd. 30

31. The equilibrium constant for base ionisation is denoted by the abbreviation K b . As a result, a strong base means that that it is an excellent proton acceptor , whereas a strong acid implies that it is an excellent proton donor . In water, weak acids and weak bases dissociate as follows: CH 3 COOH + H 2 O ↔ CH 3 COO - + H 3 O + NH 3 + H 2 O ↔ NH 4 + (aq) + OH - (aq) 31

32. Q. What is the Arrhenius theory of acids and bases? A. In 1884, Arrhenius proposed the concept of acid and base, based on the theory of ionization. According to Arrhenius, the acids are the hydrogen-containing compounds which give H+ ions or protons on dissociation in water and bases are the hydroxide compounds which give OH- ions on dissociation in water. 32

33. Q. What is the Arrhenius concept of acid and base with its limitatIon ? A. Arrhenius theory is only applicable when a substance is dissolved in water. It considers only those substances as acids, which release hydrogen ions in aqueous solutions and only those substances as bases, which release hydroxide ions in aqueous solutions. So this theory fails to explain the properties of acids and bases in any solvents [like benzene or acetone or gases], other than water. 33

34. Q. What are Arrhenius acids ? Give example. A. According to Arrhenius theory, an acid is a substance that dissociates to give hydrogen ions when dissolved in water. For example, when hydrogen chloride gas dissolves in water, it gives hydrogen ions. HCl(aq) → H + (aq) + Cl - (aq) The release of hydrogen ions increases the concentration of hydrogen ions in solution and makes the solution acidic. 34

35. Q. What are Arrhenius bases ? Give example. A. According to Arrhenius, base is a substance that dissolves to give hydroxide ions when dissolved in water. For example, when sodium hydroxide dissolves in water, it releases hydroxide ion. NaOH(aq) → Na + (aq)+OH - (aq) The release of hydroxide ions increases the concentration of hydroxide ions in solution and makes the solution basic. 35

36. Q. What is an electrolyte ? A. An electrolyte is a material that dissociates into charged particles known as ions when it comes into contact with water. Cations are ions that have positive charge. Anions are ions that have a negative charge. Therefore, an electrolyte is a material that when melted or dissolved in water, conduct an electric current due to flow of ions. The electrolyte in solution separates into cations and anions, which spread evenly throughout the solvent. Such a solution is electrically neutral. When an electric potential is introduced to such a solution, the cations are pulled towards the cathode, while the anions are pulled towards the positive electrode, anode. Example: NaCl[sodium chloride], HCl[hydrogen chloride], NaOH[sodium hydroxide] etc. 36

37. Q. What are strong electrolytes ? A. Strong electrolytes are those electrolytes that dissociate or are ionized completely in their aqueous solution. These electrolytes have high electrical conductivity with a higher extension of ionization. For example: NaCl, HCl, NaOH etc. Q. What are weak electrolytes ? A. Weak electrolytes are those electrolytes that do not dissociate or ionized completely in their aqueous solution. These electrolytes have low electrical conductivity with a lesser extent of ionization. For example: CH 3 COOH, NH 4 OH etc. 37

38. Q. What are the differences between strong electrolytes and weak electrolytes? A.1. Strong electrolytes are electrolytes that are entirely ionized . Weak electrolytes are electrolytes that are partly ionized. 2. Strong electrolytes possess high conductivity of electricity. Weak electrolytes show poor conductivity of electricity . 38

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