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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

Do polymers form restrictedly only if compound is saturated, only?
milani Reply
what is a free fall?
Beyanca Reply
is when The Only Force acting On an Object is Gravitational Force
Madman
Thats right
Beyanca
then Why ask when you Know the answer?
Madman
She's just helping those who forgot it...bro
Thato
guys I need help on Getting ready for a last minute test
Kenelioe Reply
what help you need
Neil
we'll I'm in grade 12 so we doing this topic about upac thing
Kenelioe
on What?
Madman
the organic molecule section
Kenelioe
IUPAC NAMING WHICH FUNCTIONAL GROUP YOU CANNOT NAME?SO I COULD HELP YOU
Madman
ester
Sboniso
you should also look at structural isomers. Its crucial that they might add that one. also try and write down the structural formula of all the given compounds on the table
milani
hi guys i can explain everything in terms of physics and chemistry
Neil Reply
😂😂😂😂😂
Madman
yes
Lucky
yea
Kenelioe
guys 2mrrow I'm writing a test in chemistry I need help
Kenelioe
Hi guys. Can anyone please tell me what a functional group is?
Samukelo
a functional group depends on how many bonds there are between carbon atoms, if there are single bonds all the way it's an alkane, if there is a presence of at least one double bond it's a Alkene and if there's at least one triple bond it's an alkyne.
Olive
which quantity is measured in Watt?
Saara Reply
work
Absai
explain for me absai
Sboniso
is Power Not Work work is Measured In Joules it has Energy
Madman
thanks
Saara
Pleasure
Madman
what is galvanic cell ?
Oratile Reply
is an electrochemical cell that derives electrical energy from spontaneous redox reactions taking place within the cell.
Hope
how to understand alkane names
Sboniso Reply
suffix Ane in the IUPAC NAMING
Madman
elaborate a little more plz
Kenelioe
Alkanes are Saturated Meaning they don't have Multiple Bonds between Carbon Atoms and When you are Naming THEM THEY MUST END WITH (-ANE ) GET ME?
Madman
What are hydrocarbons
Phelo Reply
are organic Compounds with only Carbon and Hydrogen Atoms
Madman
whats the difference between aldehydes and ketones
Angel Reply
definition of chain parent
Lea Reply
what is wave?
Agness Reply
when butane burns in an excess of oxygen ,the products are
Slay
CO2 and H2O
GARY
Carbon dioxide and Water...
Thato
what is hydrocarbon
Ntswaki Reply
Hydrocarbon is a compostion of mainy carbon and hydrogen
Tsion
hydrocarbon compound made up of carbon and hydrogen only
Thembelani
25 element of physics
musah Reply
an object will remain at rest or move at a constant velocity unless acted upon a net force
Lebogang Reply
thank you
Thabiso
law of inertia
Joan
an object resisting the change in velocity.
Thabiso
newton first law of motion
Tshepo
An object will remain at rest, unless an unbalanced force acts on it.
Anga
or uniform motion
Thato
What is a molecule
Thabiso Reply
a molecule is a simplest structure unite of an elements
Else
thank you
Thabiso
Plz remind me the 1st Newton's law
Thabiso

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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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