vapor pressure, atm and boiling point

[ssh18]

If ammonia is a gas at room temperature, why is its vapor pressure greater than atmospheric pressure (760 torr)? I thought things boiled when vapor pressure equaled atm. So if ammonia is gas (it's boiled) why is it's vapor pressure greater than and not equal to atm??

---

Members don't see this ad.

 

[BTC]

At vapor pressure, liquid and gas are at equilibrium. If actual pressure is lower than vapor pressure, the reagent will be in gas phase because the pressure is not strong enough to force the gas into a liquid state.

 

[werd]

a liquid will evaporate until the pressure of its gas form above it is equal to its vapor pressure at that temperature. if water has a vapor pressure of 35mmHg at 25C, it will evaporate until there is water vapor exerting 35mmHg of pressure above it. if there is not enough water to achieve this, it will become all gas; if there is enough water you will have a liquid/gas equillibrium. so, vapor pressure is the pressure of the gas form the compound is trying to achieve.

at a compound's boiling point, the vapor pressure is equal to atmospheric pressure. that is, we expect it to become all gas if the if the pressure is atmospheric pressure. there is nothing unexpected about having vapor pressures above atmospheric pressure - this just means that the temperature is above the compound's boiling temperature. it also implies the pressure you would need to exert on the gas before you started to get some liquid ammonia back. alternatively, if you placed liquid ammonia in a closed bottle, you would expect it to evaporate in the bottle until the pressure from ammonia in the bottle is equal to its vapor pressure... this pressure may be greater than atmospheric pressure... just hope that the bottle holds .

 

[andafoo]

If ammonia is a gas at room temperature, why is its vapor pressure greater than atmospheric pressure (760 torr)? I thought things boiled when vapor pressure equaled atm. So if ammonia is gas (it's boiled) why is it's vapor pressure greater than and not equal to atm??

Clarification ...

you seem to think that ammonia has a vapor pressure of 760 (atmospheric) @ room temp (25 C).

what you've probably done is taken the specific example of water and applied it to ammonia. Each chemical compound has a unique liq/vap equilibrium behavior, meaning at different temperatures they have different vapor pressures.

So for water it is correct to say @ 25C water will be at VLE when P = 760 mmHg.

But for ammonia that is different.


As for your question:

Assume that both ammonia and water are at the same temperature 25 C, seeing that ammonia is a vapor at RT and water is a liquid at RT, that must mean that the vapor pressure of ammonia is higher than that of waters', which happens to be atmospheric.

Another way to think about it is that more pressure must be applied to the gaseous ammonia @ 25 C in order for you to achieve a liquid state.

 

[TawMus]

Clarification ...

you seem to think that ammonia has a vapor pressure of 760 (atmospheric) @ room temp (25 C).

what you've probably done is taken the specific example of water and applied it to ammonia. Each chemical compound has a unique liq/vap equilibrium behavior, meaning at different temperatures they have different vapor pressures.

So for water it is correct to say @ 25C water will be at VLE when P = 760 mmHg.

But for ammonia that is different.

As for your question:

Assume that both ammonia and water are at the same temperature 25 C, seeing that ammonia is a vapor at RT and water is a liquid at RT, that must mean that the vapor pressure of ammonia is higher than that of waters', which happens to be atmospheric.

Another way to think about it is that more pressure must be applied to the gaseous ammonia @ 25 C in order for you to achieve a liquid state.

VP for ammonia would be lower than water at RT, because ammonia is gas at RT and water is liquid at RT right? Because for ammonia to be in the gas phase, it must have exceeded the VLE at a lower temperature than water. Or else it would be in a different state.