Question

Useful information: C=4.184 gºl.°C for liquid H2O, molar mass of H2O is 18.015 g/mole. 1. Many compounds can exist in more than one solid form. These different forms may have very different physical properties as a result of how the molecules are arranged in the solid. An example that we are all familiar with is the different allotropic forms of C (diamond and graphite). Another example of this phenomenon is PbO, which exists in a yellow and a red form. S (J.K.mol') 68.70 Substance PbO (s, yellow) PbO (s, red) PbO2 (s) O2(g) 03 (g) AH (kJ mol ) AG' (kJ-mol!) -217.32 -187.89 -218.99 -188.93 -277.40 -217.33 66.50 68.6 205.138 238.93 +142.70 +163.20 L Calculate AG for the conversion of yellow PbO to red Pbo. Which is the more stable form under standard conditions?

Answer 1

The reaction occuring will be

PbO(s,yellow) -------------------- PbO(s,red)

Delta G(rxn) = Delta G(products) - Delta G(reactants) = Delta G(PbO(s,red)) - Delta G(PbO(s,yellow)) = -188.93 + 187.89 = -1.04 kJ/mol

Since the Delta G of the reaction is negative, implies this reaction is favored.

Hence, Red color is more favored under standard conditions.

Note - Post any doubts/queries in comments section.