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By the end of this section, you will be able to:
  • Describe the ways in which an equilibrium system can be stressed
  • Predict the response of a stressed equilibrium using Le Châtelier’s principle

As we saw in the previous section, reactions proceed in both directions (reactants go to products and products go to reactants). We can tell a reaction is at equilibrium if the reaction quotient ( Q ) is equal to the equilibrium constant ( K ). We next address what happens when a system at equilibrium is disturbed so that Q is no longer equal to K . If a system at equilibrium is subjected to a perturbance or stress    (such as a change in concentration) the position of equilibrium    changes. Since this stress affects the concentrations of the reactants and the products, the value of Q will no longer equal the value of K . To re-establish equilibrium, the system will either shift toward the products (if Q< K) or the reactants (if Q> K ) until Q returns to the same value as K .

This process is described by Le Châtelier's principle    : When a chemical system at equilibrium is disturbed, it returns to equilibrium by counteracting the disturbance. As described in the previous paragraph, the disturbance causes a change in Q ; the reaction will shift to re-establish Q = K .

Predicting the direction of a reversible reaction

Le Châtelier's principle can be used to predict changes in equilibrium concentrations when a system that is at equilibrium is subjected to a stress. However, if we have a mixture of reactants and products that have not yet reached equilibrium, the changes necessary to reach equilibrium may not be so obvious. In such a case, we can compare the values of Q and K for the system to predict the changes.

Effect of change in concentration on equilibrium

A chemical system at equilibrium can be temporarily shifted out of equilibrium by adding or removing one or more of the reactants or products. The concentrations of both reactants and products then undergo additional changes to return the system to equilibrium.

The stress on the system in [link] is the reduction of the equilibrium concentration of SCN (lowering the concentration of one of the reactants would cause Q to be larger than K ). As a consequence, Le Châtelier's principle leads us to predict that the concentration of Fe(SCN) 2+ should decrease, increasing the concentration of SCN part way back to its original concentration, and increasing the concentration of Fe 3+ above its initial equilibrium concentration.

Three capped test tubes held vertically in clamps are shown in pictures labeled, “a,” “b,” and “c.” The test tube in picture a is half filled with a clear, orange liquid. The test tube in picture b is half filled with a dark, burgundy liquid. The test tube in picture c is half filled with a slightly cloudy, orange liquid.
(a) The test tube contains 0.1 M Fe 3+ . (b) Thiocyanate ion has been added to solution in (a), forming the red Fe(SCN) 2+ ion. Fe 3+ ( a q ) + SCN ( a q ) Fe ( SCN ) 2+ ( a q ) . (c) Silver nitrate has been added to the solution in (b), precipitating some of the SCN as the white solid AgSCN. Ag + ( a q ) + SCN ( a q ) AgSCN ( s ) . The decrease in the SCN concentration shifts the first equilibrium in the solution to the left, decreasing the concentration (and lightening color) of the Fe(SCN) 2+ . (credit: modification of work by Mark Ott)

The effect of a change in concentration on a system at equilibrium is illustrated further by the equilibrium of this chemical reaction:

Questions & Answers

pauli's exclusion is based on what?
avdhesh Reply
What is greatest modification made in dalton's atomic theory?
Ngwesse Reply
Types of electrolytes
Treasure Reply
Strong, weak and non-electrolytes
Grace
welcome
Alieu
thanks what's this platform all about
Nnamdi
list 6 subatomic particles and their mass, speed and charges
Dubem Reply
combination of acid and base
Ayibiro Reply
that salt
Talhatu
calculate the mass in gram of NaOH present in 250cm3 of 0.1mol/dm3 of its solution
Omego Reply
The mass is 1.0grams. First you multiply the molecular weight and molarity which is 39.997g/mol x 0.1mol/dm3= 3.9997g/dm3. Then you convert dm3 to cm3. 1dm3 =1000cm3. In this case you would divide 3.9997 by 1000 which would give you 3.9997*10^-3 g/cm3. To get the mass you multiply 3.9997*10^-3 and
Kokana
250cm3 and get the mass as .999925, with significant figures the answer is 1.0 grams
Kokana
nitrogen, phosphorus, arsenic, antimony and Bismuth
faith Reply
What is d electronic configuration of for group 5
Miracle Reply
Can I know d electronic configuration of for group 5 elements
Miracle
2:5, 2:8:5, 2:8:8:5,...
Maxime
Thanks
Miracle
Pls what are d names of elements found in group 5
Miracle
define define. define
Muh Reply
what is enthalpy
Ayilaran Reply
total heat contents of the system is called enthalpy, it is state function.
Sajid
background of chemistry
Banji Reply
what is the hybridisation of carbon in formic acid?
Maham Reply
sp2 hybridization
Johnson
what is the first element
Josh Reply
HYDROGEN
Liklai
Element that has positive charge and its non metal Name the element
Liklai
helium
oga
sulphur
oga
hydrogen
Banji
account for the properties of organic compounds
mercy Reply
properties of organic compounds
mercy
what's the difference between molecules and compounds
Amha Reply
A compound can be a molecule however compounds must contain more than one element. For example ozone, O3 is a molecule but not a compound.
Justin
what is che? nd what is mistry?
Mukhtar
Practice Key Terms 3

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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