CHEMICAL EQUILIBRIUM [PART 2], CLASS 11 CHEMISTRY

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 CHANGE 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 SYSTEM OF EQUILIBRIUM IS DIFFERENT FROM THE EARLIER ONE.

1. CLASS 11 CHEMISTRY CHEMICAL EQUILIBRIUM [PART 2]

2. Q. What is meant by degree of ionization ? Explain it. A. The ratio of the number of ions or molecules produced to the total number of molecules dissolved in water, is known as the degree of ionization or dissociation. It is denoted by the Greek symbol ɑ. Some substances when dissolved in water form positive and negative ions. The breaking of the substances into ions, when dissolved in a solvent, is called ionization. The extent to which an electrolyte undergoes ionization is expressed in terms of degree of ionization. 2

3. Q. What are the factors on which does the degree of ionization depend on? A. The degree of ionization depends on : 1. The nature of the solute. 2. The nature of the solvent. 3. The temperature. Q. On which factor, the degree of ionization does not depend on ? A. The degree of ionization does not depend on the volume of electrolyte . 3

4. Q. What is the formula of degree of ionization ? A. Degree of ionization[ ɑ] = number of molecules ionized/ number of molecules taken. Q. What is the difference between degree of ionization and degree of dissociation? A. Ionization and dissociation basically express the same theory i.e. separation of constituents. The main difference between ionization and differentiation is that ionization always forms electrically charged particles whereas dissociation may or may not form charged particles. 4

5. Q. How does temperature affect the degree of ionization? A. The extent of ionization varies with temperature. It increases with increase in temperature. Q. How does the degree of ionization depend on the nature of electrolyte ? A. Weak electrolytes dissolve less than strong electrolytes, whereas strong electrolytes dissociates more. 5

6. Q. How does the degree of ionization depend on the nature of solvent ? A. The dielectric constant of a solvent determines its ionizing power. The higher the dielectric constant, the stronger the solvent’s ionising power and the degree of ionization. For example, water has a high dielectric constant and is frequently employed as a solvent. Q. How does the degree of ionization depend on dilution ? A. The degree of ionization increases as the dilution increases. 6

7. Q. What are polybasic acids ? A. Polybasic acids are those acids which are capable of yielding more than one hydronium ion per molecule. These may be called dibasic, tribasic etc. According to the number of hydrogen they can replace. Some example of polybasic acids are sulphuric acid and phosphorus acid as they contain more than one ionizable ion per molecule. 7

8. Q. Explain ionization of polybasic acid . A. The ionization process of a typical polybasic acid is as follows: H 2 X[aq] ↔H + [aq]+HX - [aq] HX - [aq]↔H + [aq]+X 2- [aq] A diabasic acid is dissociated into its constituent ions in this example. For the aforementioned reaction, the equilibrium constant may be written as: Contd. 8

9. H 2 X[aq] ↔H + [aq]+HX - [aq] K a1 = [H + ][HX - ]/H 2 X HX - [aq]↔H + [aq]+X 2- [aq] K a2 = [H + ][X 2- ]/[HX - ] Here, K a1 and K a2 are the first and second ionization constants of the acid H 2 X . So, the dissociation constant of polybasic acid is the multiplication of the constituent ions. Contd. 9

10. K a = K a1 × K a2 = {[H + ][HX - ] / H 2 X} × {[H + ][X 2- ] / [HX - ]} = [H + ] 2 [X 2- ] / [H 2 X] Example: Ionization of H 2 SO 4 is as follows: H 2 SO 4 [aq] ↔ 2H + [aq] + SO 4 2- [aq] K a = [H + ] 2 [SO 4 2- ] / [H 2 SO 4 ] 10

11. Q. What is polyacidic base ? A. A base is a substance which gives [OH - ] ions when dissolved in water. The strength of the base depends on [OH - ] ions concentration. The number of ionisable hydroxide ions [OH - ] present in one molecule of base is called the acidity of bases. On the basis of acidity, bases can be classified as monoacidic base, diacidic base, triacidic base etc. As for example, bases like calcium hydroxide, aluminium hydroxide contain more than one ionisable ion per molecule of base. Such bases are termed as polyacidic base. 11

12. Q. Explain ionisation of polyacidic base . A. Ionization reaction of typical polyacidic base is: M(OH) 2 (aq) ↔ M 2+ (aq) + 2OH - (aq) K b = {[M 2+ (aq)] [OH - ] 2 } / [M(OH) 2 ] Examples: Ca[OH] 2 , Zn[OH] 2 etc. Ca(OH) 2 (aq) ↔ Ca 2+ (aq) + 2OH - (aq) K b = {Ca 2+ (aq)] [OH - ] 2 } / [Ca(OH) 2 ] 12

13. Q. What do you you mean by acid strength ? A. Acid strength is the measure of the ability of the acid to lose its H + ion. Q. What are strong acids and weak acids ? A. A strong acid is the one that completely ionizes in a solution. Strong acids are corrosive in nature and cause severe burns when they come in contact with skin. Example: Sulphuric acid, Hydrochloric acid. A weak acid is the one that only partially ionizes in a solution. Weak acids are only mildly corrosive and are even present in our food. Citric acid[present in citric fruits], acetic acid[present in vinegar] are a few examples of weak acid. 13

14. Q. What are the factors that affect acid strength ? A. Acid strength or the degree of the dissociation of an acid depends on the following factors: 1. It depends on the strength of the H-A bond . The weaker the bond, the lesser the energy required to break it. Hence the acid is strong. 2. The polarity of the H-A bond affects its acid strength. If the bond is highly polar, the proton tends to leave the molecule easily , making it a strong acid. 3. The a tomic size of A also affects the acid strength. As the atom becomes larger, the bond gets weaker. Consequently acid strength increases. 14

15. Q. Explain acid strength order . A. The strength of H-A bond is a more important factor in deciding the acidity than its polarity. As the size of A increases on descending a group, H-A bond strength decreases and therefore the acid strength increases. For example, the acid strengths of hydrides of group-17 elements increase in the order: HF<HCl<HBr<HI. When we compare the acid strengths of elements in the same row, priority is given to the polarity of the H-A bond. Contd. 15

16. Bond polarity is the ability of molecules or to separate electric charges along with their bonds, which causes a dipole moment or electric dipole to form. The electronegativity difference between atoms in a bond determines the polarity of that bond. A table of electronegativity may be used to determine which atom will attract more electrons from two atoms, when they share electrons. High polarity allows the proton to escape the molecular configuration easily, which results in strong acid . On the other hand, low polarity keeps the proton in place and results in a weak acid . 16

17. Q. What is the full form of pH ? A. The full form of pH is ‘Potential of Hydrogen’. pH is known as the negative logarithm of hydrogen ion concentration. Q. What is the use of the concept of pH ? A. The concept of pH is used to specify the acidity or basicity of an aqueous solution. Acidic solutions have a higher pH value than basic or alkaline solutions. The pH scale inversely indicates the concentrations of the hydrogen ions in a solution. Solutions at 25 °C having pH less than 7 are acidic, while solutions with pH greater than 7 are classified as basic. 17

18. Q. What is the simple definition of pH ? A. pH is a figure expressing the acidity or alkalinity of a solution on a logarithmic scale on which 7 is neutral, lower values are more acid and higher values more alkaline. The pH is equal to -log 10 c, where ‘c’ is the hydrogen ion concentration in moles per litre. The pH range goes from 0 to 14, with 7 being neutral. pH of less than 7 indicates acidity, whereas pH of greater than 7 indicates alkalinity. Therefore, pH is really a measure of the relative amount of free hydrogen and hydroxyl ions in the water. 18

19. Q. What do you mean by the hydrolysis of salt ? A. Hydrolysis of the salt is the process in which a salt and water react to form an acidic or basic solution. In the process of hydrolysis, only those salt hydrolyse which are formed by the reaction of : 1. Strong base and weak acid. 2. Strong acid and weak base. This happens because a salt formed due to a strong base and a weak acid on dissolving in water will form a basic solution. A basic solution turns red litmus blue. Contd. 19

20. Na 2 CO 3 + 2H 2 O → 2NaOH + H 2 CO 3 However, the salt formed due to a strong acid and a weak base on dissolving in water will make an acidic solution. Acidic solutions turns blue litmus red. FeCl 3 + 3H 2 O → Fe[OH] 3 + 3HCl Q. Distinguish between hydrolysis and hydration . A. Hydrolysis means breaking of a compound using water: NaCl 2 + H 2 O → NaOH + HCl Hydration means addition of water molecule to a compound. C 2 H 4 [ethene] + H 2 O → CH 3 CH 2 OH 20

21. Q. What is buffer solution ? A. A solution whose pH is not altered to any great extent by the addition of small quantities of either an acid or base is called buffer solution. Buffer is also defined as the solution of reserve acidity or alkalinity which resists change of pH upon the addition of a small amount of acid or alkali. Many chemical reactions are carried out at a constant pH. In nature, there are many systems that use buffering for pH regulation. For example, the bicarbonate buffering system is used to regulate the pH of biood, and bicarbonate also acts as a buffer in the ocean. 21

22. Q. What are the characteristics of buffer solution ? A.1. It has a definite pH. 2. Its pH does not change on standing for long periods of time. 3. Its pH does not change on dilution. 4. Its pH is slightly changed by the addition of small quantity of an acid or base. 22

23. Q. What are the types of buffer solution ? A. Two common types of buffer solution are : 1. A weak acid together with a salt of the same acid with a strong base. These are called acid buffers . Example: CH 3 COOH + CH 3 COONa 2. A weak base together with a salt of the same base with a strong acid. These are called basic buffers . Example: NH 4 OH + NH 4 Cl 23

24. Q. What is buffer action ? A. The property of a buffer solution to resist any change in its pH value even when small amount of the acid or the base are added to it is called Buffer action. Q. Explain the ‘Buffer’ action of acidic buffer . A. If we consider the buffer action of acidic buffer containing acetic acid(CH 3 COOH) and sodium acetate(CH 3 COONa), acetic acid dissociates to a small extent whereas sodium acetate is almost completely dissociated in aqueous solution as: Contd. 24

25. CH 3 COOH + H 2 O ↔ CH 3 COO - + H 3 O + CH 3 COONa ↔ CH 3 COO - + Na - By common ion effect, the ionization of CH 3 COOH is further suppressed. Thus in the solution, there are excess of acetate ions and small amount of H 3 O + ions. When a few drops of an acid are added to the above mixture solution, the H 3 O + ions given by the acid combine with CH 3 COO - ions to form weakly ionized molecules of CH 3 COOH. CH 3 COO - + H 3 O + → CH 3 COOH + H 2 O Therefore, H 3 O + ions concentration and hence the pH of the solution remains almost constant. 25

26. When a few drops of a base are added , the OH - ions given by the base combine with the H 3 O + ions already present to form weakly ionized molecules of H 2 O. H 3 O + + OH - → 2H 2 O As H 3 O + ions are consumed, the equilibrium shifts towards right. Thus, more of CH 3 COOH dissociates to make up the loss of H 3 O + ion. Therefore, the H 3 O + ion concentration or pH of the solution does not change . 26

27. Q. Explain the ‘Buffer’ action of basic buffer . A. The ‘Buffer’ action of basic buffer i.e NH 4 OH + NH 4 Cl may be explained as : NH 4 OH dissociates to a small extent whereas NH 4 Cl dissociates completely in aqueous solution as: NH 4 OH ↔ NH 4 + + OH - --- [1] NH 4 Cl → NH 4 + + Cl - By common ion effect, the ionization of NH 4 OH is further suppressed. Thus, in the solution, there are excess of NH 4 + ions and a small amount of OH - ions. Contd. 27

28. When a few drop of a base are added, the OH - ions given by it immediately combine with NH 4 + ions to weakly ionized NH 4 OH. NH 4 + + OH - → NH 4 OH Therefore, H 3 O + ion concentration or pH of the solution remain unaffected. When a small amount of acid is added, the H 3 O + ions given by it combine with OH - ions already produced by NH 4 OH in equilibrium 1. Contd. 28

29. As the OH - ions are consumed, the equilibrium 1 shifts in forward direction. More of NH 4 OH dissociates to produce more of OH - ions which make up the loss of OH - ions concentration. Therefore, the H 3 + O ion concentration or pH of the solution remains fairly constant. Q. What is buffer capacity ? A. Buffer capacity is defined as the number of moles of an acid or a base required to be added to one litre of the buffer solution so as to change ite pH by one unit. 29

30. Q. What is the importance of buffer solution ? A.1. In biological processes , the pH of our blood is maintained constant in spite of various acid and base producing reactions going on in our body. The buffer action is due to the presence of carbonic acid, bicarbonate ion and carbon dioxide in the blood. 2. In industrial processes , the use of buffers is an important part of many industrial processes, e.g., in electroplating, in the manufacture of leather, dyes, photographic materials. 3. In bacteriological research , culture media are generally buffered to maintain pH required for the growth of the bacteria being studied. 30

31. Q. What is Henderson equation ? A. An equation that could calculate the pH value of a given buffer solution was first derived by the American chemist Lawrence Joseph Henderson. This equation was then re-expressed in logarihmic terms by the Danish chemist Karl Albert Hasselbalch. The Henderson-Hasselbach equation can be written as: pH = pK a + log 10 [(A - )/(HA)] where pK a is acid ionization constant, (A - ) denotes the molar concentration of the conjugate base (of the acid) and (HA) denotes the molar concentration of the weak acid. 31

32. Q. What are the limitations of Henderson’s equation ? A.1. The equation does not account for water’s self-dissociation; it cannot provide the correct pH values for highly diluted buffer solutions. 2. The assumption stated in the Henderson’s equation might not be applicable when associating with strong bases or acids. 3. The relevance of water hydrolysis and its influence on the pH of the entire solution is often overlooked. Likewise, the hydrolysis of an acid and base dissociation are overlooked. 32

33. Q. What is the difference between K a and pK a ? A. The acid dissociation constant is K a . pK a is essentially this constant’s log. Similarly, the base dissociation constant is K b , while pK b is constant’s log. The constants for acid and base dissociation are usually expressed in moles per litre (mol/L). Q. What is ‘Solubility Product’ ? A. ‘Solubility Product’ is defined as the product of the concentration of the ions which is in equilibrium with the solid compound in a saturated solution. It is denoted by K sp . 33

34. Q. Explain solubility product constant . A. The solubility product constant [K sp ] is a thermodynamic constant that describes the equilibrium between a solute and its dissolved ions. The constant is a measure of how much solute will dissolve in a given quantity of solvent at equilibrium. It is expressed in units of concentration (mol/L). The value of [K sp ] depends upon the solubility of the substance. The lesser the solubility product, the lesser will be the solubility and vice versa. Contd. 34

35. For example, the equilibrium between Barium sulphate and the saturated solution can be represented by the equation: BaSO 4 [aq] ↔ Ba 2+ [aq] + SO 4 2- [aq] The equilibrium constant can be given as: K = [Ba 2+ ][SO 4 2- ] / [BaSO 4 ], For a pure solid substance, the concentration remains constant. Thus, it can be written as: K sp = K[BaSO 4 ] = [Ba 2+ ][SO 4 2- ] The value of the solubility product depends upon the solubility of the substance. The lesser the solubility product, the lesser will be the solubility and vice versa. 35

36. Q. What is the difference between solubility and solubility product? A. The key difference between solubility and solubility product is that the concentration of a substance in the saturated solution is termed the solubility of that substance , whereas the solubility product describes the mathematical product of the dissolved ion concentrations raised to the power of their stoichiometric coefficients, of that substance . 36

37. Q. What is the common ion effect ? A. The common ion effect is an effect that suppresses the ionization of an electrolyte, when another electrolyte (which contains an ion which is also present in the first electrolyte, i.e. a common ion) is added. It is considered to be a consequence of La Chatelier’s principle (or the equilibrium law). The common ion effect causes the reduction of solubility when adding like ions. The common ion effect is seen when weak and strong electrolytes are mixed. 37

38. Q. Give an example of the common ion effect . A. The example is as follows: Solutions containing both sodium chloride [NaCl] and silver chloride [AgCl] also contain a common ion, Cl - ion. The equilibrium is as follows: AgCl ↔ Ag + Cl The equilibrium will be moved to the left to generate more solid AgCl. As a result, the solubility of AgCl will decrease. 38

39. Q. What is an common ion effect and its application ? A. If to an ionic equilibrium, AB ↔ A + + B - , a salt containing a common ion is added, the equilibrium shifts in the backward direction. This is called common ion effect. The phenomenon is an application of Le-Chatelier’s principle for equilibrium reactions that has become a regular occurrence in chemical analysis and industrial researches. The common ion effect is employed in the purification of substances, treatment of water, salting out in soap formation, and quantitative analysis . 39

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