Noah Vinter
The vapor pressure of a liquid is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The chemical identities of the molecules in a liquid determine the types and strengths of intermolecular attractions possible, which in turn determine the vapor pressure. Liquids with strong intermolecular attractions are likely to have smaller vapor pressures, and the reverse is true for weaker intermolecular attractions. Water exhibits extensive hydrogen bonding, providing stronger intermolecular attractions and a lower vapor pressure than ethanol. When water is added to ethanol, the mixture exhibits stronger intermolecular attractions than either of the pure components, resulting in a lower vapor pressure.
| Characteristics | Values |
|---|---|
| Water exhibits stronger intermolecular attractions | Fewer molecules escape from the liquid |
| Water has lower vapor pressure than ethanol | Ethanol has a lower vapor pressure than diethyl ether |
| Water's extensive hydrogen bonding | Stronger intermolecular attractions |
| Mixtures with stronger intermolecular attractions between constituents | Lower vapor pressure than expected |
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What You'll Learn
- Water has stronger intermolecular attractions than alcohol
- Water's extensive hydrogen bonding keeps molecules from escaping
- Water lowers the alcohol's vapour pressure more than ethanol
- The mixture's vapour pressure is lower than that of its pure components
- Intermolecular forces affect the vapour pressure of a substance
Water has stronger intermolecular attractions than alcohol
The vapor pressure of a liquid is the pressure exerted by the vapour in equilibrium with the liquid in a closed container at a given temperature. When a liquid vaporizes, gas molecules escape from the liquid. The rate of vaporization depends on the temperature and the intermolecular forces of the liquid. At higher temperatures, more molecules have sufficient energy to escape from the liquid. Liquids with weaker intermolecular forces have higher vapour pressures as it presents less of a barrier to vaporization.
Water and alcohol are both liquids that experience intermolecular forces, but water has stronger intermolecular attractions than alcohol. This is due to the extensive hydrogen bonding capability of water molecules. Each water molecule (H2O) has a highly electronegative oxygen atom, creating a partial negative charge, and two slightly positive hydrogen atoms due to the electronegativity of oxygen. This results in hydrogen bonding, where the slightly positive hydrogen atoms of one molecule are attracted to the slightly negative oxygen atom of another molecule.
The ability of water to form hydrogen bonds effectively results in a lower evaporation rate compared to alcohol. Water molecules are strongly attracted to each other, which makes it more difficult for them to escape from the liquid phase. This is why water remains a liquid at higher temperatures compared to ethanol, which transitions to the gas phase faster. For example, small insects can walk on the surface of water without sinking, while alcohol does not exhibit this property as effectively due to its weaker intermolecular forces.
The difference in intermolecular forces between water and alcohol also affects their boiling points. Water has a higher boiling point than alcohol because it requires more energy to break the hydrogen bonds between water molecules. This is why adding water to alcohol lowers the vapor pressure of the mixture. The strong intermolecular forces of water impede the vaporization of the solution, resulting in a lower overall vapour pressure.
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Water's extensive hydrogen bonding keeps molecules from escaping
The addition of water to alcohol lowers the vapour pressure of the latter. This is due to the extensive hydrogen bonding in water, which keeps the molecules from escaping.
Vapour pressure is the pressure exerted by the vapour in equilibrium with a liquid in a closed container at a given temperature. When a liquid vaporises in a closed container, gas molecules move randomly and occasionally collide with the surface of the condensed phase. These collisions may result in the molecules re-entering the condensed phase, a process known as condensation. The rate of condensation becomes equal to the rate of vaporisation, and the amount of liquid and vapour in the container remains unchanged.
The chemical identities of the molecules in a liquid determine the types and strengths of intermolecular attractions. Relatively strong intermolecular attractive forces impede vaporisation and favour the "recapture" of gas-phase molecules, resulting in a relatively low vapour pressure. Water exhibits extensive hydrogen bonding, which provides stronger intermolecular attractions and fewer molecules escaping the liquid, leading to a lower vapour pressure.
Hydrogen bonds form when hydrogen atoms are covalently bonded to nitrogen, oxygen, or fluorine in compounds such as water (H2O). In these molecules, the hydrogen atoms are positively charged and can form hydrogen bonds with the negatively charged parts of other molecules. These attractions are weaker than true ionic or covalent bonds but strong enough to result in interesting properties. Water molecules readily stick together, forming chains that move along vessels through capillary action and sticking to surfaces, allowing for quicker travel.
The cohesion of water molecules creates surface tension, and the hydrogen bonds between water molecules at the surface act as a barrier. This prevents objects from breaking through, similar to the linked hands in a game of Red Rover, where players hold hands to prevent someone from running through. However, a heavier object or one that is not carefully placed on the water's surface can break the surface tension.
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Water lowers the alcohol's vapour pressure more than ethanol
The vapor pressure of a liquid is influenced by the strength of the intermolecular forces within it. Liquids with strong intermolecular forces have a lower vapor pressure, as the molecules are held more tightly within the liquid. Conversely, liquids with weaker intermolecular forces have higher vapor pressures, as their molecules are more easily able to escape into the gas phase.
Ethanol exhibits hydrogen bonding, resulting in stronger intermolecular forces. This means that ethanol has a lower vapor pressure than some other substances, such as diethyl ether. However, water has even stronger intermolecular forces due to its extensive hydrogen bonding. As a result, water exhibits a lower vapor pressure than ethanol.
When water is added to alcohol, the mixture has stronger intermolecular forces than pure alcohol. This is because water molecules can form hydrogen bonds with the alcohol molecules, in addition to with other water molecules. As a result, the vapor pressure of the mixture is lower than that of pure alcohol.
The decrease in vapor pressure upon adding water can be observed in an azeotrope of 95% ethanol and water. This mixture has a higher vapor pressure than predicted by Raoult's law, causing it to boil at a lower temperature than either of its pure components. This deviation from Raoult's law is further evidence of the stronger intermolecular forces in the mixture compared to the individual components.
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The mixture's vapour pressure is lower than that of its pure components
The vapour pressure of a mixture can be different from that of its pure components, and in the case of water and alcohol, the mixture's vapour pressure is lower. This phenomenon can be explained by the concept of intermolecular forces and interactions.
Vapor pressure is the pressure exerted by a substance in its gaseous state, which is in equilibrium with its liquid or solid form at a given temperature. It indicates the substance's tendency to evaporate, with a higher vapour pressure indicating a stronger tendency to change into a gas. The key factor influencing vapour pressure is the strength of intermolecular forces within the substance and between its molecules and the surrounding environment.
Water and alcohol (ethanol) are both capable of hydrogen bonding, which results in stronger intermolecular forces. In a mixture of water and alcohol, these molecules interact with each other through hydrogen bonding, creating an even stronger attraction between them. This means that the molecules are "held in" the liquid more firmly and are less likely to escape into the gas phase. As a result, the vapour pressure of the mixture is lower than that of its individual components.
The concept of negative deviations in vapour pressure supports this explanation. When the vapour pressure of a mixture is lower than expected based on the properties of its pure components, it indicates stronger intermolecular attractions between the constituents of the mixture. This is exactly what happens when water and alcohol are mixed, leading to a vapour pressure lower than that of pure water or pure alcohol.
Additionally, the size of the molecules and the dispersion forces they experience play a role. Water is much smaller than alcohol molecules and exhibits weaker dispersion forces. However, its extensive hydrogen bonding capabilities result in stronger intermolecular attractions overall. This further contributes to the lower vapour pressure of the water-alcohol mixture.
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Intermolecular forces affect the vapour pressure of a substance
The strength of intermolecular forces directly affects a substance's vapour pressure. Vapour pressure is the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. It indicates a liquid's tendency to evaporate. When the temperature of a liquid increases, the attractive interactions between liquid molecules become less significant compared to the entropy of those molecules in the gas phase, increasing the vapour pressure.
Substances with strong intermolecular forces will have lower vapour pressures because these forces impede vapourisation and favour the "recapture" of gas-phase molecules when they collide with the liquid surface. Conversely, weak intermolecular forces present less of a barrier to vapourisation and reduce the likelihood of gas recapture, resulting in relatively high vapour pressures.
For example, ethanol exhibits stronger intermolecular forces (IMF) due to hydrogen bonding, which means fewer molecules escape from the liquid at any given temperature, resulting in a lower vapour pressure than diethyl ether. Water, despite being smaller than ethanol, also has stronger IMFs due to extensive hydrogen bonding, resulting in a lower vapour pressure than ethanol or diethyl ether.
When two substances with different vapour pressures are mixed, the resulting vapour pressure can deviate from the expected value. If the vapour pressure is lower than expected, it indicates stronger intermolecular attractions between the constituents of the mixture than in the pure components, meaning the molecules are "held in" the liquid more strongly when a second molecule is present. An example is a mixture of trichloromethane (chloroform) and 2-propanone (acetone), which boils above the boiling point of either pure component due to stronger intermolecular attractions in the mixture.
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Frequently asked questions
Water exhibits extensive hydrogen bonding, which provides stronger intermolecular attractions. This results in fewer alcohol molecules escaping as vapour, and therefore a lower vapour pressure.
Vapour pressure is the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. It indicates a liquid's tendency to evaporate.
As temperature increases, the attractive interactions between liquid molecules become less significant compared to the entropy of those molecules in the gas phase, increasing the vapour pressure.
