for Adi or exothermic. k N The va up positive, NO2 is So endo new wi L. This mug

Question

for Adi or exothermic. k N The va up positive, NO2 is So endo new wi L. This mugu s mar it ousor Woot wishto of ruuasing 3. Assuming that the determined values for AdisHo and AdisSo are temperature independent, is the dissolution of KNO3 spontaneous at all temperatures? Justify your answer. 4. Ammonium nitrate (NH4NO3) is a highly soluble salt for which the dissolution is endothermic. What can be deduced about Adisso for this compound? Magnesium sulfate (MgSO4) is a highly soluble salt for which the solubility decreases with increasing temperature. What can be deduced about AdisGo, AdisH, and AdisS for this

Explanation / Answer

3. We know, Go = Ho - T So

Given, Ho = 21896.58 J/mol,   So = 93.1 J/mol.K

Thus, for KNO3 : Go = 21896.58 J/mol   -   T * (93.1 J/mol.K )        ......equation 1

Go should be negative for a reaction to be spontaneous. Now look at the equation carefully. If at the higher temperature the second term in the equation 1 is bigger than the first term then Go will be negative and reaction will be spontaneous. But at lower temperature the first term is bigger than the second term and then Go will be positive and the reaction will not occur spontaneously.

This threshold temperature can be calculated. For the reaction to be spontaneous:

T * (93.1 J/mol.K ) > 21896.58 J/mol  

or, T > 235.19 K.

So the reaction to be spontaneous the temperature of the reaction must be above 235.19 K.

4. Ammonium nitrate is highly soluble at room temperature (298 K). This indicates that Godis is highly negative.

Go = Ho - T So

The reaction is endothermic, so Ho > 0. As Godis is highly negative, the second term in the equation is much larger than the first term. This can only happen when So has a high positive value.

5. MgSO4 is highly soluble, it indicates that Godis is highly negative.

Go = Ho - T So

Solubility decreases with temperature means with increasing temperature value of Godis becomes more positive (i.e., less negative).

There are four possibilities regarding the values of Ho ,So:

(i) Ho = +Ve,   So = +ve

(ii) Ho = +Ve,   So = -ve

(iii) Ho = -Ve,   So = +ve

(iv) Ho = -Ve,   So = -ve

First three possibilities can be ruled out because in the first three cases value of Godis do not become more positive with increasing temperature.

When Ho = -Ve,   So = -ve : The equation becomes-

Go = Ho - T So = (-Ve) + T(+Ve) ; here when value of T is increased, value of Godis becomes more positive.

Thus for dissolution of MgSO4, Godis < 0;      Ho < 0   and ,   So < 0.

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