Question

2

(a) ΔrG° for the following reaction at 398 K is 2.6 kJ mol−1:

H2(g) + I2(g) → 2 HI(g)

If a gas mixture containing: 2.00 bar of H2, 2.00 bar of iodine vapour, and 0.500 bar of hydrogen iodide, is sealed in a container at 398 K, is the reaction at equilibrium? [4]

(b) Sketch a graph to show how the conductivity of a solution varies with concentration depending on whether the solute is a strong or weak electrolyte. Include the equation which allows the limiting molar conductivity to be obtained from the graph.

Explain why the graphs are different. [4]

The following questions are concerned with electrochemical cells.

(c) If I2 and Br2 are added to a solution containing I– and Br– , what reaction will occur if the concentration of each species is 1M? Explain your answer. The standard reduction potentials of iodine and bromine are given below:

I2 + 2 e– → 2 I– E° = +0.54 V

Br2 + 2 e– → 2 Br– E° = +1.09 V [4]

(d)

Cu(s) | Cu2+(aq, 0.1 mol dm–3 ) || Ag+ (aq, 1.0 mol dm–3 ) | Ag(s)

Ag+ (aq) + e– → Ag(s) E° = +0.80 V

Cu2+(aq) + 2 e– → Cu(s) E° = +0.34 V

(i) Calculate E°cell under standard conditions. [2]

(ii) Calculate Ecell for the concentrations given in the cell diagram above. [4]

(iii) Calculate ΔG for the above cell. [2]

Answer 1

Ans A: In the given reaction the change in number of reactant and product is zero. As according to the LEE CHATLIER PRINCIPLE , the increase in pressure will nothave any effect and the reaction is said to be in equilibrium. If the number of product is more than number of reactant then increase in pressure will lead to the backword reaction . If number of reactant molecules are more than the product then the increase in pressure will lead to the forward reaction

HENCE THE GIVEN REACTION IS IN EQUILIBRIUM.

b: molar conductance of electrolyte increases with the increase in dilution i.e. decrease in concentration.this was attributed to the increase indegree of dissociation of electrolyte.We defined degree of dissociation as the fraction of the total electrolyte in the solution which exist in theform of its ions. In other words we can say that on dilution same amount of electrolyte is capable of furnishing a large number of ions.Itb can be pointed out that increase in number of ions on dolution is much less then the increase in volume of the solution. Therefore the number of ions per unit volume decreases although the molar conductance increases on prugressive dilution.

THE VARIATION OF MOLAR CONDUCTANCE WITH DILUTION IN SOME COMMON IS SHOWN IN GRAPH .

As can be seen in various strong electrolytes , there is tedency for molar conductance to approach certain limiting valuewhen the concentration approaches zero. THE MOLAR CONDUCTNCE AT THAT POINT IS KNOWN AS MOLAR CONDUCTANCE AT ZERO CONCENTRATION OR AT INFINITE DILUTION.

It may be clear that be infinite dilution is meant a solution so dilute that it has maximum orlimiting molar comductance which does not ncrease on further dilution . This value in strong electrolyte can be obtain by extrapolating the molar conductance graph to zero concentration. In the case weak electrolytes ' thereis no indication that limiting value is obtained even when concentration approaches zero. To obtain limiting value in weak electrolytes we have some indirect methods like KOHLRAUSH'S LAW.

Ans: C

Ans : D