How much energy must be removed from a 94.4 g sample of benzene (molar mass= 78.11 g/mol) at 322.0 K to solidify the sample and lower the temperature to 205.0 K? The following physical data may be useful. ΔHvap = 33.9 kJ/mol ΔHfus = 9.8 kJ/mol Cliq = 1.73 J/g°C Cgas = 1.06 J/g°C Csol = 1.51 J/g°C Tmelting = 279.0 K Tboiling = 353.0 K

Respuesta :

Answer : The energy removed must be, 29.4 kJ

Explanation :

The process involved in this problem are :

[tex](1):C_6H_6(l)(322K)\rightarrow C_6H_6(l)(279K)\\\\(2):C_6H_6(l)(279K)\rightarrow C_6H_6(s)(279K)\\\\(3):C_6H_6(s)(279K)\rightarrow C_6H_6(s)(205K)[/tex]

The expression used will be:  

[tex]Q=[m\times c_{p,l}\times (T_{final}-T_{initial})]+[m\times \Delta H_{fusion}]+[m\times c_{p,s}\times (T_{final}-T_{initial})][/tex]

where,

[tex]Q[/tex] = heat released for the reaction = ?

m = mass of benzene = 94.4 g

[tex]c_{p,s}[/tex] = specific heat of solid benzene = [tex]1.51J/g^oC=1.51J/g.K[/tex]

[tex]c_{p,l}[/tex] = specific heat of liquid benzene = [tex]1.73J/g^oC=1.73J/g.K[/tex]

[tex]\Delta H_{fusion}[/tex] = enthalpy change for fusion = [tex]-9.8kJ/mol=-\frac{9.8\times 1000J/mol}{78g/mol}=-125.6J/g[/tex]

Now put all the given values in the above expression, we get:

[tex]Q=[94.4g\times 1.73J/g.K\times (279-322)K]+[94.4g\times -125.6J/g]+[94.4g\times 1.51J/g.K\times (205-279)K][/tex]

[tex]Q=-29427.312J=-29.4kJ[/tex]

Negative sign indicates that the heat is removed from the system.

Therefore, the energy removed must be, 29.4 kJ

Answer:

  • [tex]-29.48 KJ[/tex] energy must be removed

Explanation:

benzene boiling point = [tex]80^oC = 353.1K[/tex]

benzene freezing point = [tex]5.42^oC = 278.42K[/tex]

No of moles of benzene = [tex]\frac{94.48}{78.11} = 1.2095[/tex]

Energy released,

[tex]q = m*G*\delta T\\\\q = 94.4 * 1.73 * (278.42-322)\\\\q = -7117J[/tex]

Energy released = [tex]1.2095 * (-9.8*10^3)[/tex]

[tex]= -11853.1J[/tex]

Energy released,

[tex]q = n*C*\delta T\\\\q = 1.2095*118.4*(278.42-205)\\\\q = -10511.23J[/tex]

Total heat released

[tex]= -7117 -11853.1 -10511.23\\\\= -29481.3J\\\\= -29.48 KJ[/tex]

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