The normal boiling point of ethanol (c2h5oh) is 78.3 °c and its molar enthalpy of vaporization is 38.56 kj/mol. what is the change in entropy in the system in j/k when 42.2 grams of ethanol at 1 atm condenses to a liquid at the normal boiling point?

Respuesta :

when the entropy change of vaporization
       
 = enthalpy of vaporization/boiling point temperature 

when we have the enthalpy of vaporization = 38560 J/mol

and the boiling point temperature in Kelvin = 78.3 + 273 = 351.3 K

by substitution:

∴the entropy change of vaporization = 38560J/mol/351.3K

                                                               = 109.76 J/K/mol

and when the liquid has lesser entropy than the gas and we here convert from

gas to liquid so, the change in entropy = -109.76 J/K/mol

now, we need the moles of C2H5OH = mass/molar mass when the molar

mass of C2H5OH = 46 g/mol and mass = 42.2 g 

∴ moles of C2H5OH = 42.2 g / 46 g/mol = 0.92 moles

when 1 mol of C2H5OH turns in liquid entropy change →-109.76 J/K/mol

∴ 0.92 mol of C2H5OH → X

∴ X entropy change when 0.92 mol = -109.76 *0.92 mol / 1 mol

                                                             = 84.64 J/K


The change in the randomness of the system is the entropy change. The entropy change after condensation at the standard boiling point is 84.64 J/K.

What is the entropy change?

When a system undergoes the addition or deletion of the reactant and the products, then the disorder of the system is known as entropy change.

Given,

Enthalpy of vaporization = 38560 J/mol

Boiling point temperature = 351.3 K

[tex]\begin{aligned}\text{Entropy change of vaporization} &= \dfrac{\text{enthalpy of vaporization}}{\text{boiling point temperature}}\\\\&= \dfrac{38560}{351.3}\\\\&=109.76 \;\rm J/K/mol\end{aligned}[/tex]

Here, liquid has less entropy than gas hence the change in entropy is  -109.76 J/K/mol.

Moles of ethanol is calculated as:

[tex]\begin{aligned}\rm moles &= \dfrac{\rm mass}{\rm molar\; mass}\\\\&= \dfrac{42.2}{46}\\\\&= 0.92 \;\rm moles\end{aligned}[/tex]

If 1 mole of ethanol has an entropy change of -109.76 J/K/mol. Then, 0.92 moles will have,

[tex]\dfrac{-109.76 \times 0.92}{1} = 84.64\;\rm J/K[/tex]

Therefore, 84.64 J/K is the entropy change.

Learn more about entropy change here:

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