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Chemical equilibrium

Having looked at factors that affect the rate of a reaction, we now need to ask some important questions. Does a reaction always proceed in the same direction or can it be reversible? In other words, is it always true that a reaction proceeds from reactants to products , or is it possible that sometimes, the reaction will reverse and the products will be changed back into the reactants ? And does a reaction always run its full course so that all the reactants are used up, or can a reaction reach a point where reactants are still present, but there does not seem to be any further change taking place in the reaction? The following demonstration might help to explain this.

Demonstration : liquid-vapour phase equilibrium

Apparatus and materials:

2 beakers; water; bell jar

Method:

  1. Half fill two beakers with water and mark the level of the water in each case.
  2. Cover one of the beakers with a bell jar.
  3. Leave the beakers and, over the course of a day or two, observe how the water level in the two beakers changes. What do you notice? Note: You could speed up this demonstration by placing the two beakers over a bunsen burner to heat the water. In this case, it may be easier to cover the second beaker with a glass cover.

Observations:

You should notice that in the beaker that is uncovered, the water level drops quickly because of evaporation. In the beaker that is covered, there is an initial drop in the water level, but after a while evaporation appears to stop and the water level in this beaker is higher than that in the one that is open. Note that the diagram below shows the situation ate time=0.

Discussion:

In the first beaker, liquid water becomes water vapour as a result of evaporation and the water level drops. In the second beaker, evaporation also takes place. However, in this case, the vapour comes into contact with the surface of the bell jar and it cools and condenses to form liquid water again. This water is returned to the beaker. Once condensation has begun, the rate at which water is lost from the beaker will start to decrease. At some point, the rate of evaporation will be equal to the rate of condensation above the beaker, and there will be no change in the water level in the beaker. This can be represented as follows:

l i q u i d v a p o u r

In this example, the reaction (in this case, a change in the phase of water) can proceed in either direction. In one direction there is a change in phase from liquid to vapour. But the reverse can also take place, when vapour condenses to form water again.

In a closed system it is possible for reactions to be reversible, such as in the demonstration above. In a closed system, it is also possible for a chemical reaction to reach equilibrium . We will discuss these concepts in more detail.

Open and closed systems

An open system is one in which matter or energy can flow into or out of the system. In the liquid-vapour demonstration we used, the first beaker was an example of an open system because the beaker could be heated or cooled (a change in energy ), and water vapour (the matter ) could evaporate from the beaker.

Questions & Answers

how to calculate avarage power?
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Aobakwe
4. A builder of mass 75 kg is carrying bricks up a flight of stairs. There are 40 steps, each 25 cm high and he takes 10 s to climb the stairs, carrying 15 kg of bricks at a time. Calculate the total power output of the builder during his climb at constant speed.
Asanda Reply
1stly describe Hydrocarbon for me please
Magadi Reply
Compounds that consists of carbon and hydrogen atoms only.
Learner
Compounds that consists of carbon and hydrogen atoms only.
Learner
How can I understand work energy and power
Kholofelo Reply
bu watching videos n practicing
Moagi
what is power
Sydwell Reply
power is the rate at which work is done
Yonnela
how can I understand reactions?
Thabile
how to calculate kinetic frictional force on inclined?which do equation you use when you have only have the coefficient of kinetic friction ?
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when calculating KE,how do we make velocity the subject of the formula?
Evelyn Reply
Explain projectile
Rivers Reply
a projectile is an object that is only experiencing force of gravity
Nomfanelo
what is a polymerisation
Dube Reply
Which of the two alcohols has the low vapour pressure ,salicylic acid and methanol
Ayeza Reply
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which has the highest density between salicylic acid and methanol
Zimbali
methanol
Modicks
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Zimbali
what is the kinetic molecular theory of magnesium sulphate
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How to draw a freebody diagram for an object in free fall
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how does the polarity of a molecule relates to it's properties?
Aliah Reply
How do we identify alkynes?
Lil Reply
we identify them with tripple bond
Dimakatso
by checking the triple bond
Ikaneng
what is a functional group?
Akhona Reply

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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