Question

which one has the higher standard molar enthalpy at 25C?

1) NO2(g) or CH3CH2CH3(g) - NO2 has higher mass by 2g but CH3CH2CH2 is more complex so i'm not sure which one it is

2) Xe or CO2 - Xe has much higher mass but CO2 is more complex ?

2) H2O2 or H2S - both have very similar masses ??

Please explain your answer. thanks

Answer 1

Answer)

I think you are asking about entropy not enthalpy right? because we can not tell enthalpy of molecule .... only we can tell the enthalpy change.

Standard molar Entropy = is the measure of randomness of 1 mole of substance at standard conditions.

Randomness is directly proportional to degree of freedom present in molecule.

Degree of free is directly proportional to number of atoms present in molecule

Degree of freedom (DOF) = 3N (where n is the number of atoms present in molecule.

1) NO2(g)

Number of atoms = 3

Degree of freedom(DOF) = 3 x 3 = 9

CH3CH2CH3(g)

Number of atoms = 11

Degree of freedom(DOF) = 3 x 11 = 33

CH3CH2CH3(g) has 33 DOF while NO2(g) has only 9 DOF. So CH3CH2CH3(g) has much more randomness than  NO2(g)

That's why CH3CH2CH3(g) has the higher standard molar entropy.

2) Xe

Number of atoms = 1

Degree of freedom(DOF) = 3 x 1 = 3

CO2

Number of atoms = 3

Degree of freedom(DOF) = 3 x 3 = 9

CO2 has 9 DOF while Xe has only 3 DOF. So CO2 has much more randomness than Xe

That's why CO2 has the higher standard molar entropy.

3) H2O2

Number of atoms = 4

Degree of freedom(DOF) = 3 x 4 = 12

H2S

Number of atoms = 3

Degree of freedom(DOF) = 3 x 3 = 9

H2O2 has 12 DOF while H2S has only 9 DOF. So H2O2 has much more randomness than H2S

That's why H2O2  has the higher standard molar entropy.

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