phasechange1 (Points: 8.34) The normal melting point for phosphoric acid is the

Question

phasechange1

(Points: 8.34)

The normal melting point for phosphoric acid is the temperatureat which the following process reaches equilibrium (at 1.0 atm).H3PO4(s) ® H3PO4(l)Use the following thermodynamic data to determine thistemperature.

Substance

DH°f/(kJ/mol)

S°/(J/K · mol)

H3PO4(s)

-1284.4

110.5

H3PO4(l)

-1271.7

150.8

1. ) 347 K

2. ) 305 K

3. ) 286 K

4. ) 315 K

phasechange1

(Points: 8.34)

The normal melting point for phosphoric acid is the temperatureat which the following process reaches equilibrium (at 1.0 atm).H3PO4(s) ® H3PO4(l)Use the following thermodynamic data to determine thistemperature.

Substance

DH°f/(kJ/mol)

S°/(J/K · mol)

H3PO4(s)

-1284.4

110.5

H3PO4(l)

-1271.7

150.8

1. ) 347 K

2. ) 305 K

3. ) 286 K

4. ) 315 K

Explanation / Answer

Here we know from thermodynamics that: G = H - TS So if we are melting a compound and trying to find the temperaturewe may equate: G = H(l) - TS(l) G = H(s) - TS(s) Thus: H(s) - TS(s) =H(l) - TS(l) Now we plug in values (making sure units match): -1284.4 kJ/mol - (T x 0.1105 kJ/Kmol) = -1271.7 kJ/mol - (T x0.1508 kJ/Kmol) Now solve for T and we get: T = 315 K Hope this helped :)

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