Entropy Calculations: Phase Changes - Video Tutorials & Practice Problems
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1
concept
Entropy of Liquids
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Now similar to the change in the entropy of surroundings. We now talk about the entropy of liquids and they have formulas that are used to calculate either the change in entropy or the change in entropy. Now, we're gonna say that the below formulas are being used during phase changes and the phase changes that we care about are vaporization and fusion. It's because these two phase changes deal with liquid in some way in vaporization. We're talking about going from a liquid to a gas and infusion, which is another name for melting. We're going from a solid to a liquid in both phase changes. A liquid is involved. Now, here we're gonna say that these formulas, they connect entropy and entropy together. In the first one, we're going to say that the change in the entropy of vaporization equals the change in the entropy of vaporization divided by our boiling point of the liquid. Here we're gonna say that the change in entropy of effusion equals the change in entropy effusion divided by the melting point temperature. So here change in delta S Vape and delta S fusion, they are typically units of jewels per Kelvin and entropy of vaporization and entropy of fusion are typically in units of jewels or kilojoules. And as always, when we talk about temperature, even in this case of boiling point temperature and melting point temperature, it has to be in units of Kelvin. So just remember that these formulas are a way of connecting entropy and entropy together whether they deal with vaporization or with fusion.
2
example
Entropy Calculations: Phase Changes Example
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2m
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Here, we're told that Methanol has a normal melting point of 64.7 °C and an entropy effusion of 9.36 joules per Kelvin determine its entropy of fusion. All right. So we're talking about fusion, we know that the original formula that connects entropy of fusion and entropy of fusion is delta S fusion equals delta H fusion divided by R melting point temperature. We need to isolate this variable here. So multiply both sides by temperature. So now we're going to say that the entropy of fusion equals the change in the entropy of fusion times my melting point temperature. Here, we plug in 9.36 jewels per Kelvin because the entropy of you who uses Kelvin, that is an indication we need to convert temperature into Kelvin. So add 2 73.15 to these degrees Celsius. When we do that, we get 337.15 Kelvin, which is what we plug here. Kelvin's cancel out and we get 3155.7 to 4 jewels. So we could say this is our answer. So 315556 joules or if we decide to convert it into kilojoules 3.156 kilojoules. So either one could represent our empathy of fusion. And here we're talking about this per mole. So if you wanted to, you could say joules per mole or kilo joules per mole. OK. Same difference here. We're assuming we're dealing with a mole of a substance. So here this would be our piece of fusion.
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Problem
Problem
Calculate entropy of vaporization of 8.4 g of acetic acid (CH3COOH) with a boiling point of 118 °C, ∆Hvap = 23.7 kJ/mol.
A
201 J/K
B
60.3 J/K
C
28.1 J/K
D
8.48 J/K
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