Equilibrium in a closed vessel and the rate of reaction
الأسبوع 10 · الباب الثالث «الاتزان الكيميائي» (١): مقدمة الباب والاتزان الفيزيائي (التبخر والتكثف) والتفاعلات التامة والعكسية والاتزان الكيميائي حالة ديناميكية ومعدل التفاعل وما يتوقف عليه وطبيعة المواد المتفاعلة · 3 دروس
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Equilibrium in a closed vessel: evaporation and condensation
Chemistry Sec 3 Term 1, Chapter 3: pp. 53–58
GoalExplain how evaporation and condensation reach equal rates in a closed vessel, and what saturated vapour pressure means.
New words
dynamic equilibrium · a state in which two opposite changes go on at equal rates, so nothing seems to changeevaporation and condensation · liquid water turning to vapour, and vapour turning back to liquidsaturated vapour pressure · the largest vapour pressure a liquid can give in a closed vessel at a given temperature
Explanation
An equilibrium system looks still, but it is busy inside. Put a little liquid water in a closed vessel and keep it at a fixed temperature. Molecules at the surface escape into the space above: this is evaporation. Vapour molecules hit the surface and stay: this is condensation. At first there is no vapour, so evaporation is faster. As vapour builds up, more molecules return, so condensation speeds up. Soon the two rates are equal. After that, the amount of liquid and the pressure of the vapour stay constant, although both changes still go on. That is a dynamic equilibrium, and the vapour pressure then is the saturated vapour pressure, the largest vapour pressure at that temperature. It is a physical equilibrium because no new substance forms. The vessel must be closed: in an open dish the vapour escapes, condensation never catches up, and the water dries away.
Worked examples
Minute
0
1
2
3
4
5
evaporation
10
10
10
10
10
10
condensation
0
4
7
9
10
10
Read the table. These invented rates are in made-up units per minute. Evaporation stays at 10. Condensation starts at 0 and rises: 4, 7, 9, 10, 10. The rates are first equal at minute 4. After that both stay at 10, so the vapour pressure is steady even though molecules still cross the surface both ways.
Open or closed? A bowl of water left open on a table dries up in some days, but the water in a closed jar of the same size does not disappear. In the open bowl the vapour moves away into the room, so condensation can never match evaporation. In the closed jar the vapour stays, the two rates become equal, and the liquid level stays steady.
Why “saturated”? At a fixed temperature the vapour above the liquid cannot go on getting denser, because the extra vapour condenses as fast as the liquid evaporates. The pressure at that point is the saturated vapour pressure. At a different fixed temperature the equilibrium is reached at a different pressure, so the value is quoted for a given temperature. The book’s own graphs and heated-vessel example are not used here: we draw our own and give no heating step.
Class activity
Closed-jar watch, with a grown-up. Pour about 1 cm of cool tap water into a clear plastic container and put the lid on tightly. Mark the water level on the outside with a pen. Put a second, open container with the same amount beside it. Leave both on a windowsill, away from heaters and never in an oven. After a day look at the lid of the closed one for droplets and compare the two levels. See what happens; the droplets are condensation, and the closed container loses less water. Write one sentence that explains the difference with the words evaporation and condensation.
Practice
1
In a closed vessel of water at a fixed temperature, what is true of the rates of evaporation and condensation at equilibrium?
They are equal, so the liquid level and the vapour pressure stay constant.
2
Why does a bowl of water dry up if it is open, but not if it is closed?
In the open bowl the vapour escapes, so condensation cannot catch up with evaporation; in a closed vessel an equilibrium forms.
3
Why is the saturated vapour pressure always given for a stated temperature?
Because the equilibrium, and so the largest vapour pressure, is different at different temperatures.
4
Is the water ⇌ vapour equilibrium chemical or physical? Give a reason.
Physical, because no new substance forms; only the state of water changes.
