Vaporization latent heat vs boiling point

In summary, the conversation discusses the concept of using Hess's law to calculate the heat of vaporization of ethanol at its boiling temperature and 1 bar, which is 42.37 kJ/mol according to the table. This involves using the heat capacities of the liquid and vapor, and taking into account that the heat of formation value for ethanol gas is for a hypothetical ideal gas state at 25 C and 1 bar. The calculated value for the boiling temperature is -664K.
  • #1
yecko
Gold Member
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Homework Statement
If the vaporization latent heat of Ethlyl alcohol under the standard conditions is 38.56 kJ/mol, estimate the boiling point of Ethlyl alcohol (in °C) according to the A26.
Relevant Equations
vaporization latent heat = Hf-Hg = Hfg
delta G = delta H - T delta S
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answer is 78oCdelta G = delta H - T delta S
-235,310- (-277,690) = -38.56e3- T (282.59-160.70)
T = -664K

I am not sure if my concept is correct. May anyone help a little bit on that please?
thank you
 
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  • #2
Seems to me you used the g and h columns in reverse. Your delta G is a difference of two h values.
 
  • #3
The heat of formation value for the ethanol gas in the table is for a hypothetical ideal gas state of ethanol at 1 bar and 25 C. This is not an actual thermodynamic equilibrium state of ethanol, since, at 25 C and 1 bar, ethanol is actually a liquid. When they say that the heat of vaporization under standard conditions is 38.56 kJ/mol, what they mean is that the heat of vaporization at the boiling temperature and 1 bar is 38.56 kJ/mol. The hypothetical heat of vaporization from the liquid to the vapor at 25 C and 1 bar is, according to the table, 42.37 kJ/mol. You need to use Hess's law to express the heat of vaporization at the boiling temperature T and 1 bar in terms of the "heat of vaporization" at 25 C and 1 bar. This involves using the heat capacities of the liquid and vapor.
 

FAQ: Vaporization latent heat vs boiling point

1. What is the difference between vaporization latent heat and boiling point?

Vaporization latent heat is the amount of energy required to change a substance from liquid to gas at its boiling point, while boiling point is the temperature at which a substance changes from liquid to gas.

2. Why is vaporization latent heat important?

Vaporization latent heat is important because it is the energy that is required to overcome the intermolecular forces holding a substance together in its liquid state. It is also a key factor in determining the rate at which a substance will evaporate.

3. How does vaporization latent heat relate to the properties of a substance?

The vaporization latent heat of a substance is directly related to its intermolecular forces and the strength of its bonds. Substances with stronger intermolecular forces will have a higher vaporization latent heat, while substances with weaker intermolecular forces will have a lower vaporization latent heat.

4. Can vaporization latent heat be measured?

Yes, vaporization latent heat can be measured using a calorimeter, which is a device that measures the amount of heat absorbed or released during a phase change. This measurement is typically done at the boiling point of a substance.

5. How does vaporization latent heat differ from specific heat?

Vaporization latent heat and specific heat are both measures of the energy required for a substance to change its phase, but they differ in the type of phase change. Vaporization latent heat is specifically for the phase change from liquid to gas, while specific heat is for the change in temperature of a substance. Additionally, specific heat is measured in units of energy per unit mass per degree (J/kgK), while vaporization latent heat is measured in units of energy per unit mass (J/kg).

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