Cbse Chemistry Notes for 11 and 12

subsidiary Textual Material in Chemistry for mob XI & yr XII 1 Acknowledgements CBSE ADVISORS Shri Vineet Joshi, I. A. S. , Chairman, CBSE Shri Shashi Bhushan, Director (Acad. ), CBSE CONVENOR & EDITOR Prof. A. K. Bakhshi Department of Chemistry, University of Delhi. DEVELOPMENT squad Prof. A. K. Bakhshi Department of Chemistry, University of Delhi. Dr. Anju Srivastava Hindu College, University of Delhi. Dr. Vimal Rarh S. G. T. B. Khalsa College, University of Delhi. Dr. Geetika Bh whatsoevera Hindu College, University of Delhi. Ms. Anupama Sharma Modern School, Vasant Vihar, New Delhi. MEMBER COORDINATOR Dr.Srijata Das, fostering Officer, CBSE 2 TABLE OF CONTENTS Class XI Unit 5 States of Matter 5. 7. 1 Kinetic Energy and Molecular Speeds 5. 7. 2 Maxwell-Boltzmann dispersal of molecular zip ups Unit 6 Thermodynamics 6. 6. 1. Second jurisprudence of Thermodynamics 6. 8. Third Law of Thermodynamics Unit 7 chemical equilibrium 7. 12. 1 pH of Buffer Solutions Class XII Un it 16 Chemistry in E rattlingday aliveness 16. 4. 2. 1 Antioxidants 3 Unit 5 States of Matter 5. 7. 1 energising ENERGY AND MOLECULAR SPEEDS As you digest studied in the foregoing section the molecules of a spatter be eternally in front and argon colliding with each other and with the walls of the container.Due to these collisions the revivifys and the kinetic energies of the individual molecules keep on changing. However at a accustomed(p) temperature, the average kinetic heartiness of the gas molecules remains unbroken. If at a expose temperature, n1 molecules d avouch fixity v1, n2 molecules have hurry v2, n3 molecules have accelerate v3, and so on. accordingly, the total kinetic energy (EK) of the gas at this temperature is tending(p) by where m is the masses of the molecule. The corresponding average kinetic energy ( If the following term Then the average kinetic energy is presumptuousness up by where c is given by 4 ) of the gas will beThis c is cog nize as go down- close-foursqu be secureness. As the name implies, to calculate c, jump take the squ atomic number 18s of the individual fastnesss, whence their compressed and finally the squ be root of the mean. It can be shown that c is link to temperature by The average kinetic energy depends only on haughty temperature and is related to absolute temperature by the looking at where k = Boltzmann incessant = 1. 38 x 10-23 J K-1 In the contingency of gases, one withal talks of two other speeds, namely, average speed and close- seeming speed. The average speed ( ) at a given temperature is the arithmetic mean of the speeds of various molecules of the gas. . e, where n1 molecules have speed v1, n2 molecules have speed v2, n3 molecules have speed v3, and so on. The family between average speed and temperature T is given by The most presumable speed ( ) of a gas at a given temperature is the speed possessed by the maximum number of molecules at that temperature. cont radictory average speed and root mean square speed, the most probable speed cannot be expressed in terms of the individual molecular speeds. The most probable speed ( ) is related to absolute temperature (T) by the expression 5 Relationship between different types of molecular speedsThe three types of molecular speeds, namely, most probable speed ( ), average speed ( ) and root mean square speed (c) of a gas at a given temperature are related to each other as follows For a grumpy gas, at a particular temperature, It follows from the above relationships that Example write in code the root mean square, average and most probable speeds of oxygen molecules at 27 oC. Solution Given data Molar mass of oxygen, M = 32 g mol-1 = 0. 032 kg mol-1 Temperature, t = 27 oC T = (27 + 273) K = three hundred K 6 Expressions to be used Root mean square speed, reasonable speed, Most probable speed, f wreakual calculationsRoot mean square speed, Average speed, Most probable speed, 5. 7. 2 MAXWELL-B OLTZMANN DISTRIBUTION OF MOLECULAR SPEEDS The speeds of the individual molecules of a gas do not remain constant. They keep on changing due to inter molecular collisions and due to collisions with the walls of the container. However, at a given temperature, the distribution of molecular speeds remains constant. This distribution of molecular speeds is given by Maxwell-Boltzmann distribution practice of legality. 7 For a given gas, at a given temperature, the distribution curve of molecular speeds is shown in phase 5. 7. 1 icon 5. 7. 1.Maxwell Boltzmann distribution curve You can see from the distribution curve that it is the darn of molecular speed vs the fraction of molecules. The important features of this distribution curve are (i) The fraction of molecules having too low or too high speeds is actually downhearted. (ii) There is a certain speed for which the fraction of molecules is maximum. This speed is cognize as most probable speed. (iii) The most probable speed of a gas, is the speed possessed by the maximum number of molecules of the gas at a given temperature and it corresponds to the peak of the curve. Since the most probable speed ( xpression ) is related to absolute temperature T by the It means that at the same temperature, ignitor gases shall move faster than heavier gases as is evident from the distribution curves of chlorine and northward gases in Figure 5. 7. 2. 8 Figure 5. 7. 2. Maxwell Boltzmann distribution curves for chlorine and nitrogen gases at 300K Effect of temperature In Figure 5. 7. 3. are given the Maxwell Boltzmann distribution curves of a gas at three different temperatures T1, T2 and T3 such that T1>T2>T3. Figure 5. 7. 3. Maxwell Boltzmann distribution curves for a gas at three different temperatures 9It can be seen from these distribution curves that as the temperature increases 1. The consummate curve shifts towards right. 2. The most probable speed increases. 3. The fraction of molecules having most probable speed decreases. 4. The curve becomes broader in the middle range indicating that more molecules have speeds near to the most probable speed. 5. The fraction of molecules having higher speeds increases. 6. The fraction of molecules having demoralise speed decreases. It may be noted that the total area low each of the curves remains the same as the sum of fractions of molecules remains unreduced on changing the temperature. 0 EXERCISE 1. Calculate (i) root mean square speed (ii) average speed and (iii) most probable speed of CO2 molecules at 700 K . 2. A sample of a gas contains 15 molecules with a speed of 3 m s-1, 25 molecules with a speed of 5 m s-1 and 30 molecules with a speed of 8 m s-1. Calculate root mean square speed of these molecules. 3. Calculate the temperature at which the average speed of oxygen catchs that of hydrogen at 20 K. 4. Calculate the temperatures at which the root mean square speed, average speed and the most probable speed of oxygen gas are all equal to 1500 m s-1. Answers 1. i. 629. 92, m s-1 ii. 80. 50 m s-1 iii. 514. 33 m s-1 2. 6. 187 m s-1 3. 320 K 4. T for root mean square speed= 2886 K, T for the average speed = 3399 K, T for most probable speed = 4330 K 11 12 Unit 6 Thermodynamics 6. 6. 1 jiffy LAW OF THERMODYNAMICS You have already studied in the previous sections that the first law of thermodynamics deals only with the law of conservation of energy and it gives no sulfur about the criteria for feasibility of a given physical or chemical ticker parade. This law is always obeyed whether a given process occurs on its own or is made to occur with the help of an external agency.In any case the first law is valid in some(prenominal) the forward and bring up direction of the process. You have also learnt about the archetype of entropy which is a measure of the randomness or the disorder in a system. This concept helps in explaining the spontaneity of the irreversible processes. For an irreversible process, the entropy of the system and environs taken to maintainher i. e. of the universe increases, while for a process at equilibrium it remains constant. This conclusion is of great importance as it helps us to predict whether a given process can take locating impromptu or not.Since all processes in nature occur naturally i. e. irreversibly, it follows that the entropy of the universe is increasing continuously. This important statement i. e. in any spontaneous process, the entropy of the universe (system and surroundings) always increases is also known as the second law of thermodynamics. ?SUniverse = ? Ssystem + ? Ssurroundings Specifically it means that If ? SUniverse > 0, the response is spontaneous If ? SUniverse < 0, the reaction is non-spontaneous If ? SUniverse = 0, the reaction is at equilibriumSince chemists are generally more interested in the system (reaction mixture) than the surroundings, it is therefore more convenient to restate the second law in terms of the thermodynamic p roperties of the system, without regard to the surroundings. As already depict in the previous section, this is possible through the Gibbs free energy G which for a system is defined as G = H TS. In terms of this thermodynamic property, the second law of thermodynamics can be restated as in any spontaneous process at 13 constant temperature and wedge, the free energy of the system (? Gsystem) always decreases.According to this definition If ? Gsystem < 0, the reaction is spontaneous If ? Gsystem > 0, the reaction is non-spontaneous If ?Gsystem = 0, the reaction is at equilibrium 6. 8. THIRD LAW OF THERMODYNAMICS The troika law of thermodynamics is concerned with determination of entropy. It states that a internality which is unblemishedly vitreous silica clear at absolute zip of temperature has an entropy of nonentity. In a perfect crystal at absolute zero of temperature, each constituent of the crystal (atom, molecule or ion) is at the proper crystal lattice range an d it therefore has the lowest energy.This means that this particular state is of perfect order, i. e, has zero disorder and hence zero entropy. Using zero value of entropy at absolute zero for a perfectly crystalline substance as the reference point (initial state), one can calculate absolute value of entropy of the substance in any state (solid, liquid state or gas) at any temperature by calculating ? S for the process/es in going from the initial state to the state of the substance for which entropy is to be calculated. However there are certain substances which possess certain entropy even at absolute zero.This entropy is known as residual entropy. The origin of residual entropy can be explained on the basis of the disorder which remains at absolute zero in certain crystals composed of AB types of molecules where A and B are similar atoms (as in CO). In such substances, there is a truly little energy difference between AB AB AB and .. AB BA BA AB.. and other arrangements so th at the molecules exact the orientations AB and BA at random in the solid. This gives rise to twain(prenominal) disorder, also known as frozen disorder, and consequently residual entropy.For object lesson in case of CO, the measured residual entropy is 5 J K-1 mol-1 which is close to the value expected for a random structure Figure 1 (B) of the form CO CO OC CO CO OC OC. 14 Figure 1. Alternative molecular arrangements for CO (A) Perfect crystal (B) Actual crystal EXERCISE 1. State Second Law of Thermodynamics. 2. Write the conditions in terms of ? H and ? S when a reaction would be always spontaneous. 3. When ? H > 0 and ? S < 0, a reaction is never spontaneous. Explain. 4. newsmonger on the spontaneity of a reaction at constant temperature and pressure in the following cases i) ? H < 0 and ? S > 0 (ii) ? H > 0 and ? S < 0 (iii) ? H < 0 and ? S < 0 (iv) ? H > 0 and ? S > 0 5. State third law of thermodynamics. 6. Explain the term residual entropy. 15 Unit 7 E quilibrium 7. 12. 1 pH OF BUFFER SOLUTIONS HENDERSON HASSELBALCH EQUATION As you have already learnt in the previous section a pilot burner re issue is a solution which resists flip in its pH on the addition of pocketable numerate of dot or a base. Most of the important pilot burner solutions generally consist of mixtures of weak acids and their seasons or weak bases and their salts.These fender solutions are known as acerb airplane pilots and basic wings respectively. If the pH of the buffer solution is less than 7, it is called an acidic buffer and if the pH of the buffer solution is great than 7 it is called a basic buffer. Examples of some acidic and basic buffers are as given below Acidic buffer Basic buffer 1) Acetic acid and atomic number 11 acetate rayon 1) Ammonium hydrated oxide and ammonium ion ion chloride 2) Formic acid and sodium formate 2) Ammonium hydroxide and ammonium process pH of an acidic buffer A very common practice of an acidic buffer is a mixture of acetic acid and sodium acetate.Acetic acid, being a weak acid, is very just about dissociated, while sodium acetate being a salt is solely dissociated. The mixture olibanum contains acetic acid molecules as well acetate ions and sodium ions. At equilibrium CH3COOH(aq) H+ (aq) + CH3COO-(aq) Taking the negative logarithm on both the sides, we get 16 (7. 39) In general, (7. 40) It may be noted that the stringency of the acetate ions is taken to be almost equal to the assiduousness of the salt because the acetate ions coming from fully dissociated salt (here sodium acetate) suppress the ionization of the weak acid (acetic acid). This equation (7. 0) is called Henderson- Hasselbalch equation. According to this equation, the pH of an acidic buffer consisting of a mixture of a weak acid and its salt depends on the pKa of the acid, and the concentrations of the salt and acid in the mixture. If concentration of the salt is equal to that of the acid, because Buffer actio n of Acidic buffer The buffer action of an acidic buffer consisting of a mixture of acetic acid and sodium acetate can be explained in the following way. If we add small amount of HCl to the buffer solution it will ionise to give H+ ions. These trustingness with CH3COO- ions to form weakly ionized acetic acid.CH3COO-(aq) + H+(aq) ? CH3COOH(aq) + H2O(l) Since H+ ions get neutralised by CH3COO- ions, there will be no form in pH. On the other hand, if we add a few drops of NaOH to the buffer solution, it provides OH- ions which are neutralized by the acetic acid present in the mixture. 17 OH-(aq) + CH3COOH(aq) ? CH3COO-(aq) + H2O(l) Therefore the pH of the solution remains unchanged. pH of a Basic Buffer A very common example of a basic buffer is a mixture of ammonium hydroxide and ammonium chloride. Ammonium hydroxide being a weak base is very slightly dissociated, while ammonium chloride being a salt is completely dissociated.The mixture thus contains ammonium hydroxide molecules a s well as ammonium ions and chloride ions. At equilibrium, NH4OH(aq) NH4+(aq) + OH-(aq) Taking the negative logarithm on both the sides, we get (7. 41) In general, (7. 42) 18 It may be noted that the concentration of the ammonium ions is taken to be almost equal to the concentration of the salt because ammonium ions coming from the fully dissociated salt (here ammonium chloride) suppress the ionization of the weak base ammonium hydroxide. This equation (7. 42) is called Henderson- Hasselblach equation. Further, pH can be calculated easily from pOH using Hence,It may be noted that pKa + pKb = 14 Buffer action of basic buffer The buffer action of a basic buffer consisting of a mixture of ammonium hydroxide and ammonium chloride can be explained in the following way. If a few drops of HCl are added to the buffer solution, it will ionise to give H+ ion which will comply with OH- ions to form H2O. H+(aq) + OH- (aq) ? H2O(l) Therefore, the addition of HCl will not change the pH. On the o ther hand, when a few drops of NaOH solution are added to the buffer solution, it would give OH- ions. These OH- ions combine with NH4+ ions present in the solution to form NH4OH, which is only slightly ionised.OH-(aq) + NH4+ (aq) ? NH4OH(aq) Hence, the addition of a base also will not change the pH. 19 EXERCISE 1. A certain buffer is made by miscellanea sodium formate and formic acid in water. With the help of equations explain how this buffer neutralizes addition of a small amount of an acid or a base. 2. A basic buffer is made by mixing ammonium hydroxide and ammonium nitrate in water. Explain how this buffer resists change in its pH on addition of a small amount of an acid or a base. 3. What would be the pH of a solution obtained by mixing 10 g of acetic acid and 15 g of sodium acetate and making the volume equal to 1 L.Dissociation constant of acetic acid at 25oC is 1. 75 x 10-5. 4. A buffer solution contains 0. 40 mol of ammonium hydroxide and 0. 50 mol of ammonium chloride t o make a buffer solution of 1 L. Calculate the pH of the resulting buffer solution. Dissociation constant of ammonium hydroxide at 25oC is 1. 81 x 10-5. Answers 3. pH = 4. 80 4. 9. 161 20 Class XII Unit 16 Chemistry in day-after-day Life 16. 4. 2. 1. ANTIOXIDANTS Antioxidants are one of the important and necessary food additives. These compounds look into the action of oxygen on food thus reducing its speed of decomposition by oxidation. Hence they help in the preservation of food.These act as sacrificial materials, i. e. these are more reactive towards oxygen than are the materials they are protect. They also reduce the rate of involvement of free radicals in the aging process. Examples The two most familiar antioxidants used are butylated hydroxy toluene (BHT) and butylated hydroxy anisole (BHA) (Figure 1). These phenolic compounds are added to preserve fats in encase food. Oxygen reacts preferentially with BHA or BHT rather than oxidizing fats or oils, thereby protecting them from spoilage. The addition of BHA to butter increases its retentivity life from months to years.Sometimes, BHT and BHA are added in gang with citric or ascorbic acids to produce a more active synergetic effect. Figure 1 Butylated hydroxy toluene (BHT) and butylated hydroxy anisole (BHA) Sulphur dioxide and sulphites ( such as sodium sulphite, sodium bisulphite and sodium metabisulphite) are useful antioxidants for wine-colored and beers, sugars syrups and cut, peeled or dried fruits and vegetables. They prevent or reduce their discoloration. 21 EXERCISE 1. Name one antioxidant commonly used to increase the storage life of butter. 2. Name one antioxidant used in wine and beers. 22a