Exercises
Exercise 1. Methanol, CH3OH, has a boiling point of 64.7 °C and a standard enthalpy of vaporization of 37.4 kJ/mol. What is the standard entropy of vaporization in J/(mol⋅K)?
In this example, the system is at equilibrium, therefore \(\displaystyle ΔS=\frac{ΔH}{T}\)
64.7°C = 337.85 K
\(\displaystyle ΔS=\frac{37.4\;kJ/mol}{337.85\;K}=0.111\;kJ/(mol⋅K)=\mathbf{111\;J/(mol⋅K)}\)
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Exercise 2. Pentane, C5H12, has an enthalpy of fusion equal to 8.4 kJ/mol and an entropy of fusion equal to 58.5 J/(mol⋅K). What is the melting point of pentane?
In this example, the system is at equilibrium, therefore \(\displaystyle ΔT=\frac{ΔH}{ΔS}\)
58.5 J/(mol⋅K) = 0.0585 kJ/(mol⋅K).
\(T=\displaystyle\frac{8.4\;kJ/mol}{0.0585\;kJ/(mol⋅K)}\;=\;143.6\;K\;=\;\mathbf{-129.6°C}\)
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Exercise 3. Explain why ΔHvap is always greater than ΔHfus.
The heat of vaporization is always greater than the heat of fusion. This is because the energy is being put into completely disrupting the particle attractions during vaporization while during melting, heat is being put into increasing the distance between particles. It will take much more energy to completely disrupt the particle attractions.
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Exercise 4. For the following processes indicate the sign of ΔH and ΔS. Indicate the type of process.
b) CO2 (s) → CO2 (g) ΔS > 0. This is a chemical process.