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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 does cell look like
Charu Reply
What is impulse and how does it = to momentum
Robin Reply
what is physics
Jhon Reply
what is a catanation ?
Rithabele Reply
The ability of a few elements, most especially carbon, to yield chains and rings by forming covalent bonds with atoms of the same element.
Ofentse
different structural formula
Lucas Reply
different type of structural formula
Lucas
primary,secondary and tertiary
Snowy
how does the earth revolute
Kevin Reply
What is the momentum
Given Reply
The product of the object's mass and it's velocity
Mvula
electric field def
FREENCO
what is secondary alcohol?
Raymond
is when Carbon that is bonded with OH is also bonded to 2 carbons of the chain.
Gontse
Electric field is the region of space around a charged object where other charged objects will experience an electrostatic force ( coulomb forces) without coming into contact with the object that is generating the field.
Kagiso
Electric field is the region of space around a charged object where other charged objects will experience an electrostatic force ( coulomb forces) without coming into contact with the object that is generating the field.
Kagiso
how is ester formed
Aubrey Reply
how is n ester formed
Aubrey
Alcohol reacts with a carboxylic acid
Texas
and the reaction is catalysed by sulphuric acid
Vhulenda
An ester is form when an alcohol reacts a carboxylic acid and sulphuric acid is used as a catalyst which therefore eliminates water.
Azola
an athlete with a mass of 70kg runs at a velocity of 45km . determine the athlete's momentum
Lesedi Reply
Is that a velocity or something else
msawenkosi
45km/h i guess
Texas
Change to m/s
Texas
45km/h = 12.5 m/s p=mv =70×12.5 =875 kg.m/s
Thato
what are the measures of the rates of reaction
Lesego Reply
Volume Concentration Temperature Pressure Surface Area
Thato
the principle of superposition of waves
Sfundo Reply
what is work
Kool Reply
is this a group chat
Nobuhle Reply
Hey can y'all define newton's 2nd law
mthebzification
If a resultant force act on an object...the object will accelerate in the direction of a resultant force,the acceleration of the object is directly proportional to the net force and inversely proportional to the mass of the object
mosa
how do you calculate tension force
Bulumko
use the formula Fnet=ma if there is tension connecting two objects
Sboniso
to calculate Tension, usually calculate acceleration first Draw separate free body diagrams for each body. Apply Fnet = ma to calculate Tension
Kevin
Hi people
Paul
when a resultant force acting on an object the object will accelerate in the direction of a force at an acceleration directly proportional to the force and invesly proportional to the mass of the object.
Raymond
Hey people
Kaytee
how does temperature affect the equilibrium position
Blessing Reply
an increased temperature increases the average kinetic energy thus in turn increases the number of effective collisions........
Lwando
so...which reaction is favored between endothermic and exothermic .when temperature is increased..?
Blessing
exothermic reaction because energy is realised to the surroundings as heat and light energy ....graphical so much energy is realised as reactants to form product and because temperature is high rate of reaction is fast which means there is a successful collision
Code
INTEMENDO - INCREASE IN TEMPERATURE FAVOURS ENDOTHERMIC DETEMEXO - DECREASE IN TEMPERATURE FAVOURS EXOTHERMIC
Thato
an object will continue in a state of rest unless it is acted upon an unbalanced force
Junior Reply
Newton's Law 1
Code
First Newton's Law
Azola
Newton's first law
Surprise
newton first law
Thinavhuyo
Newton's first law
Blessing

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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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