Vapor pressure of ethanoic acid vs. ethanol

In summary, the vapor pressure of ethanoic acid and ethanol differs due to the presence of a functional group in ethanoic acid that is absent in ethanol. Ethanol has a higher vapor pressure due to weaker intermolecular forces, and the vapor pressure of both compounds increases with temperature. Ethanoic acid has a lower vapor pressure at the same temperature due to stronger intermolecular forces. Other factors that can affect the vapor pressure include surrounding pressure, concentration, and the presence of other substances.
  • #1
i_love_science
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Homework Statement
Ethanoic acid (CH3COOH) has a much lower vapor pressure than ethanol (CH3CH2OH). What is the most reasonable explanation?

A) The polarizability of 2 oxygen atoms increases London forces of attraction in ethanoic acid compared with ethanol.
B) Hydrogen bonding in ethanoic acid is the strongest attractive force and is mainly responsible for the observed data.
C) Ethanol has a -OH group and can hydrogen bond; therefore the London forces must cause the effect.
D) Both ethanol and ethanoic acid have an -OH, so the difference is the dipole of the 2nd oxygen that increases the attractive forces.
Relevant Equations
intermolecular forces
I think the answer is D because both molecules can hydrogen bond with their OH groups, but the C=O bond in ethanoic acid is polar and contributes to dipole-dipole interactions.

The solution says B is the correct answer, because ethanoic acid molecules strongly hydrogen bond so that most molecules are part of dimers. I don't really understand this, and why I'm wrong. Could anyone help me? Thanks.
 
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  • #2
Yes, B is correct. Arrange two molecules so that the OH group of one is close to the C=O group of the other, yin/yang fashion.
 

FAQ: Vapor pressure of ethanoic acid vs. ethanol

What is the difference between the vapor pressure of ethanoic acid and ethanol?

The vapor pressure of a substance refers to the pressure exerted by its gas molecules in equilibrium with its liquid or solid form at a given temperature. In the case of ethanoic acid and ethanol, the main difference lies in their molecular structures. Ethanoic acid has a carboxylic acid functional group, which makes it a polar molecule, while ethanol has a hydroxyl group, making it a less polar molecule. This difference in polarity affects the strength of intermolecular forces between the molecules, resulting in a higher vapor pressure for ethanol compared to ethanoic acid at the same temperature.

How does temperature affect the vapor pressure of ethanoic acid and ethanol?

The vapor pressure of a substance increases as the temperature increases. This is because at higher temperatures, the molecules have more kinetic energy, allowing them to escape from the liquid phase and enter the gas phase more easily. Therefore, both ethanoic acid and ethanol will have higher vapor pressures at higher temperatures compared to lower temperatures. However, the increase in vapor pressure will be greater for ethanol due to its lower intermolecular forces.

Why does ethanoic acid have a lower vapor pressure than ethanol?

As mentioned earlier, the difference in polarity between ethanoic acid and ethanol affects the strength of intermolecular forces between their molecules. Ethanoic acid has stronger intermolecular forces, such as hydrogen bonding, compared to ethanol. These stronger forces hold the molecules together in the liquid phase, making it more difficult for them to escape into the gas phase. As a result, ethanoic acid will have a lower vapor pressure compared to ethanol at the same temperature.

Can the vapor pressure of ethanoic acid and ethanol be equal?

It is possible for the vapor pressure of ethanoic acid and ethanol to be equal, but only at a specific temperature known as the azeotropic point. At this temperature, the liquid mixture of ethanoic acid and ethanol will have the same composition as the vapor above it. This means that the vapor pressure of the liquid and the vapor pressure of the gas phase will be equal, resulting in a constant boiling point. However, this only occurs at a specific ratio of ethanoic acid and ethanol, and any deviation from this ratio will result in different vapor pressures.

How does the vapor pressure of ethanoic acid and ethanol affect their boiling points?

The vapor pressure of a substance is directly related to its boiling point. A higher vapor pressure at a given temperature means that the substance will have a lower boiling point. This is because the vapor pressure is the pressure required to overcome the intermolecular forces and allow the substance to change from a liquid to a gas. Therefore, since ethanol has a higher vapor pressure than ethanoic acid, it will also have a lower boiling point.

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