270 | SAMPLE EXAMINATION 2 Question7 Use the reaction above to answer the questi
ID: 1041453 • Letter: 2
Question
270 | SAMPLE EXAMINATION 2 Question7 Use the reaction above to answer the questions (a) and (b) below. A student adds some zinc dust to an aqueous solution of CusSO, in an insulated calorimeter, swirls the solution vigorously and observes that the temperature of the solution rises. The student then places a clean metallic Cu strip in a beaker containing 0.1 M Cuso, solution and a cleaned Zn strip in a beaker, of 0.1 M Zn(NO,), solution. A wet piece of twine soaked in KNO, is draped between the beakers so that the two ends of the twinc are submerged, one in the Cuso, solution and one in the Zn(NO,), solution. The metal strips are joined with wires leading to a voltmeter. The student observes the voltage reading to be a positive valuc. for the first reaction, what data are needed here to make the The student wishes to calculate AH calculation? (a) (b) Is the sign of AG for the second experiment positive or negative? Explain.Explanation / Answer
(a) The student wishes to calculate ?H for the first reaction. ?H denotes the enthalpy change of a reaction and the enthalpy change is related to the heat absorbed by the system. We can say that heat is absorbed by the system since the temperature of the system rises. The heat absorbed by the system is given as
Heat absorbed = (mass of the system)*(specific heat capacity of the system)*(rise in temperature of the system).
?H = -(heat absorbed)
In order to obtain ?H, we must be able to measure the mass of the system. Otherwise, we can measure the volume of the system, but we must know the density of the system so that we can easily calculate the mass of the system as mass = volume*density. The specific heat capacity of an aqueous system is taken to be equal to that of water, i.e, 4.184 J/g.°C. A thermometer must be available to measure the rise in temperature of the system.
(b) The half reactions are:
Zn2+ (aq) --------> Zn (s); E0 = -0.76 V
Cu2+ (aq) --------> Cu (s); E0 = 0.34 V
The balanced chemical reaction for the redox process is
Cu2+ + Zn <======> Cu + Zn2+
The standard cell potential is given as
E0cell = E0(Cu2+/Cu) – E0(Zn2+/Zn) = (0.34 V) – (-0.76 V) = 1.10 V.
Since the reaction is under non-standard conditions, we have
Ecell = E0cell – (0.0592 V/2)*log [Zn2+]/[Cu2+] = (1.10 V) – (0.0296 V)*log (0.1 M)/(0.1 M)
= (1.10 V) – (0.0296 V)*log (1) = 1.10 V (since log 1 = 0)
Therefore, ?G = -n*F*Ecell
where n = 2 is the number of electrons involved in the redox process.
Since Ecell is positive, the sign of ?G is negative, i.e, ?G < 0 and hence, the reaction is spontaneous.
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