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102_Exp_Thermodynamics - CHEMICAL THERMODYNAMICS The...

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Unformatted text preview: CHEMICAL THERMODYNAMICS The Tendency for Reactions to Occur. Objectives: To study the factors that affect the spontaneity of a chemical reaction. To determine some thermodynamic functions for a chemical reaction. Background Reading: Zumdahl, Sections 16.6 and 17.3.. Introduction: Ire the spontaneity of a chemical change. All these take into account both the enthalpy change of a reaction, AH, and the change 111 the entropy or randomness, A3, of the reaction. One simple measurement is the equilibrium constant, K. Consider the reaction: aA+bB # cC+dD [03]“ [1361‘ [MP [13¢]b K: Where [Ac], [Be], [Ce], [De] are equilibrium concentrations. At any given concentrations, the concentration expression, Q, is: [C]° [13]" [Al‘ [31" Q: comparing K and Q values one can predict the direction K = Q only at equilibrium. By reaction. Usually, measurements of K are not easy to carry out. of the spontaneous potential or electromotive force, EMF, ction will proceed spontaneously potential indicates that the reaction will proceed spontaneously in ur is the free energy change, that the rea the opposite direction. AG. A third criterion of the tendency of a reaction to 000 (1) If we lcnow B“ for example, both K and AG° can be calculated. ely and therefore are often be made quickly and accurat favoured for finding n cell potential. In In this experiment you will first measure-the effect of concentration 0 hemical reaction. From these you the second part you will measure EMF and AH for a- c will calculate AG and AS values; potential (a measure of the tendency of a reaction eously. Therefore, we expect the spontan ring into the cell reaction. to occur) to depend on the concentration of the substance ente EXPERIMENTAL PROCEDURE: 1. 3. 10. 11. 12. Prepare 50 ml of a solution containing 0.10M Fe(NO,)3 and 0.10M Pesou by mixing 17 ml of 0.30M FeSO‘. 17 ml of 0.30M Fe(NQ;), and 15 ml of [-120 (using a graduated cylinder). Introduce 25 ml of this solution into a 150 ml beaker (beaker l) and the rest to a second 150 ml beaker (beaker 2). Carefully clean two carbon electrodes by washing and then rubbing the surface with a filter paper. Place one electrode in eaCh beaker and join the two beakers with a salt bridge (a U—tube filled with saturated KNO, solution, free of air bubbles; and plugged at the two ends with small amounts of cotton). Connect the leads from the voltmeter to the electrodes. Stir the electrodes in the solution and make sure that they are in good contact. Read the voltage. If it differs from zero, either the electrodes are not clean or the contact between the leads and the electrodes is poor. Clean the electrodes and connectvthern to the leads, read the voltage again and report again, until zero voltage is observed. Discard the solution in {gal-cor 1) only. Keep (beaker 2) for step 11 below. Prepare 50ml of a solution which is 0.10M I-"e(I\lO,)3 and 0.001M in FeSO‘ by mixing 17 ml of 0.30M Fe(N03)3, 1.0 ml of 0.050M FeSO)| (from the dispenser in the fume hood), 1.0 mi of 3.0M H2804 (from the dispenser), and 31 ml of water. Introduce 25 ml of this solution in beaker 1. Repeat steps 4,5 and 6 above. Read the voltage and record it. B. FREE ENERGY, ENTHALPY AND ENTROPY The change in free energy (AG), is related to the changes in enthalpy (AH). and entropy (AS), as follows: AG = AH —'— TAS (2) In the following experiment you will determine AG, AH and A8 for the reaction: Mnor + 31%,” + 4H‘ ——-—> Mnoz + 3Fe3’ + 21-120 (A) EXPERIMENT; L PROCEDURE: 1. Wash beaker 1, add to it about 0.5g of manganese dioxide, MnOz, and introduce 25 ml of a ready made mixture of 0.10 M Kill/[n04 and 0.1M HNO,. 2. Reinsert the electrode in beaker 1 and measure the voltage of the cell consisting of the two half cells: Fe” ————> Fe” + e (beaker 2) and MnOf' .+ 4H“ + 3c ———> me, + 2H10 (beaker 1) When the voltage is stabilized record it. 3. Calculate AG for the cell reaction using the relationship: AG (K1) = —— 96.5 nE where E is the observed voltage in volts and n is number of moles of electrons per mole of reactants and products. 4. Measure AH for the reaction as follows: (a) In a 100 ml beaker, place 20ml of 0.20M 5.0ml of water, (all measured by graduated cylinder). (c) Carefully mea solutions), to the nearest 02°C. (d) Add the FeSO. solution to the KMnO‘ solution in the beake record the highest observed temperature. KMnO4, 5.0m} of 6M HNO3 and r with stirring and DATA AND CALCULATIONS ’ om beaker 1 electrode to beake can check I In the above experiment: NAME LAB SEC. DATE DESK NO. A. CONCENTRATION EFFECT DATA AND CALCULATIONS ctrode or vice versa? 2. Which electrode is the anode ? 3. Write the equation for the half-reaction at each electrode. // // f f 4. Write the equation for the overall cell reaction. sland2)we 5. Using the Nemst equation and the concentrations of the two cells (beaker can check the voltage measurement: 0.059 E=—-—-—-—-—-logQ n e concentration expression for the net reaction of the cell. . Where Q is th In the above experiment: [ Fez‘] beaker I Q = [ Fer] beaker 2 Calculate E and compare it with the observed value. Eobserved = EM ( show calculations) = _______________..—__—————-—-—-._____________ _________________._.____._.._____.—-—--————— _____________________,._.___——_——-—————— B. FREE “ENERGY, ENTHALPY AND ENTROPY ' DATA AND CALCULATIONS 1.. Voltage of the cell (V) 2. AG for the cell reaction (K1) 3. .Average temperature of solutions before mixing 4. Maximum temperature after mixing 5. Change in temperature (AT) 6. Total volume of final solution 7. Mass of solution (g) (aesume density=1.0 g/cm’) 8. Specific heat of solution 4.18 J/g.°C 9. Heat absorbed by solution (KJ) 10. Heat absorbed by the beaker (KI) NB. Heat capacity of the beaker (100 ml beaker) 30 11°C (150 ml beaker) 4O 11°C 11. Heat evolved by the reaction (KJ) (calculate the initial moles of m0: and Fe” and 12. Calculate AH per mole of MnOf note carefully which reagent limits the extent of the reaction). "- 13. From the AG and AH values obtained, calculate AS for reaction (A) under the conditions you used. // . // ...
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