Chemical Reactions: Concentration Effects & Enthalpy Changes
Aim: To study the effect of concentration on the rate of reaction between sodium thiosulphate and hydrochloric acid. Theory: Law of mass action states that rate of a chemical reaction is directly proportional to the product of active masses (molar concentrations) of reactants. The rate of reaction between sodium thiosulphate and hydrochloric acid depends on the concentration of reactants. As the concentration of reactants increases, the rate of reaction also increases. The balanced chemical reaction is, Na S₂O, (aq) + 2HCl(aq)- S(s) + NaCl(aq) + SO2(g) + H₂O(I) Sulphur formed during the reaction is insoluble and forms milky non-transparent opaque colloidal solution. The time required to produce enough sulphur, so that cross mark on the paper kept below the conical flask cannot be seen, when observed from top and hence rate of reaction be calculated. Apparatus: Burette (50 mL), conical flask (250 mL), stopwatch, plain paper with cross mark, measuring cylinder (10 mL), etc. Chemicals: 0.1 M Na,S,O,, 1 M HCl, distilled water. Result: Rate of reaction (1/t) is directly proportional to concentration of reactant. Aim: To determine the concentration in terms of molarity of KMnO, solution by titrating it against standard solution of oxalic acid. Theory: The substances available in the state of high purity are used to prepare standard solutions by dissolving a fixed/definite mass in a definite volume of distilled water. Oxalic acid is a primary standard substance. Its molar mass is 126 g/mol. A. Preparation of standard solution of oxalic acid Molar mass of oxalic acid (H.CO, 2H₂O) is 126 g/mol For 1000 mL 1 M oxalic acid solution required mass is 126 g of oxalic acid Hence for 100 mL of 0.1 M oxalic acid the required mass is 126×100×0.1 / 1000×1 =1.26 g of oxalic acid Apparatus: 100 mL standard flask, balance, watch glass, beaker, glass rod, etc. Chemicals: Oxalic acid, distilled water. Procedure: 1. Weigh accurately 1.26 g oxalic acid on watch glass. 2. Transfer the weighed oxalic acid to a beaker and wash the watch glass with distilled water and transfer washings to the beaker. Add little distilled water to dissolve it by stirring. 3. Transfer the solution of oxalic acid from beaker to 100 mL standard flask. Wash beaker twice with water and transfer washings to the 100 mL standard flask. Dilute the solution up to the mark on standard flask to make volume 100 mL. B. Determination of molarity of KMnO, solution using standard solution of oxalic acid. Apparatus: Burette, pipette, conical flask, burner, water bath, etc. Chemicals: KMnO, solution, 0.1 M oxalic acid, 2M H₂SO Observations: 1. solution in a burette: KMnO4 solution 2. solution by a pipette: Oxalic acid 0.1M 3. solution in conical flask: Oxalic acid 2M H2SO4 4. Indicator: KMnO4 Self-indicator 5. End point: Colorless to light pink 6. Chemical Equation: 2KMnO4+5H2C2O4+3H2SO4 – K2SO4 + 2MnSO+8H2O+10CO2 Aim: To determine enthalpy of dissolution of copper sulphate (CuSO, SH,O) in water at room temperature. Theory: Heat of solution is the change in enthalpy when one mole of a substance is dissolved in a large amount of solvent so that further dilution does not change the enthalpy. Generally for 1 mole of solute 400 moles of solvent is recommended. Heat of solution is due to ionization or some hydrate formation. Dissolution may be an exothermic or endothermic process. Reaction: CuSO, SH,O(s) + H₂O(1) CuSO (aq); AH = + 11.7 kJ/mol Apparatus: Polythene bottle, cork having two holes, two 100 mL beakers, measuring cylinder. Chemicals: Copper sulphate and distilled water Aim: To determine the enthalpy of neutralisation of strong acid 0.5M HCl and strong base 0.5M NaOH. Theory: Enthalpy of neutralisation is an enthalpy change when one mole of H’ ions of an acid is completely neutralised by one mole of OH ions of a base to form one mole of water. Heat of neutralisation of any strong acid and strong base is almost constant, AH-57.3 kl. It is due to complete dissociation of strong acid and strong base. Reaction: NaOH(aq) + HCl(aq) NaCl(aq) + H,Ο(Α), ΔΗ – -57.3 J In case of weak acid and strong base the enthalpy of neutralisation is less than that of strong acid and strong base as some heat is utilised for complete ionisation of weak acid. Apparatus: Polythene bottle, cork with two holes, two beakers, thermometer (1/10), measuring cylinder, etc. Chemicals: 0.5 M HCl and 0.5 M NaOH. Aim: To prepare a pure sample of Ferrous Ammonium Sulphate (Mohr’s salt). Theory: Mohr’s salt is one of the important laboratory reagents and used as a reducing agent. Chemically Mohr’s salt is Ferrous Ammonium Sulphate (F.A.S.), an example of ‘double salt. It is prepared by dissolving an equimolar mixture of hydrated ferrous sulphate (Feb 8,0) and ammonium sulphate ((NH) SO] in acidified water. Mohr’s salt [FeSO, (NH, SO, 6HO] SO, 7H,O) crystallizes as a faint green-colored solid. Chemical equation: FeSO4. 7H2O +(NH4)2SO4—-FeSO4(NH4)2SO4 6H2O+H2O Apparatus: China dish, beakers, funnel, glass rod, tripod stand, wire gauze, pair of tongs, etc. Chemicals: Ammonium sulphate, ferrous sulphate, dilute sulphuric acid, etc. Observations and Result: 1. Yield of ferrous ammonium sulphate crystals = 4.3 g 2. Color of ferrous ammonium sulphate crystal is = Light Green 3. Shape of ferrous ammonium sulphate crystals = Monoclinic Aim: To prepare a pure sample of p-nitroacetanilide from acetanilide Date: 1 Theory: p-nitroacetanilide prepared by the nitration of acetanilide by using nitrating mixture (cone. HNO, , H_{2}SO_{4}) as a nitrating reagent. The mixture by using nitrating mixtures nitronium ion ({NO_{2}}^{+}) which acts as an electrophile in the reaction. HNO,+2H,SO,NO,H,O+2HSO Nitronium ion attacks the benzene ring containing an anilide group, mainly at the para position to give p-nitroacetanilide as a major product. Apparatus: Test tubes, conical flask, glass rod, funnel, measuring cylinder, beaker, etc. Chemicals: conc. HNO,, conc. H_{2}SO_{4^{3}} glacial acetic acid, acetanilide, ethanol, ice. Observation and Result: 1. Yield of p-nitroacetanilide crystals = 1.7 2. Color of p-nitroacetanilide crystals = Colorless/white