Question

There are two crystalline forms of PbO , one is yellow and the other isred. The standard enthalpies of formation of these twoforms are - 217.3 and -219.0 kJ per mole respectively. Calculate the enthalpy change for the solid - solid phase transition.
PbO( yellow ) `rarr ` PbO ( red)

Answer 1

Correct Answer - `-1.7 kJ mol^(-1)`
Given `: (i) Pb(s) + (1)/(2) O_(2)(g) rarr PbO ` ( yellow ) , `Delta H = - 217. 3 kJ mol^(-1)`
(ii) `Pb(s) + (1)/(2) O_(2)(g) rarr PbO(` Red)`, Delta H = - 219 .0 kJ mol^(-1)`
Eqn. (ii) - Eqn. (i) gives the required result.