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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

What is rightful definition of element
angela Reply
an element is the group of (vertical columns) of the periodic table exhibit similar chemical behaviour.
is alkanes a saturated hydrocarbon?
Faith Reply
it's saturated cos it has single bonds
yh....because they don't undergo additional reactions which hydrogen and other atoms can add across the carbon-carbon or triple bond
and me...I'm I wrong?
how does metal looses electron
Sammy Reply
By oxidation and reduction
by oxidation loss
An acid is a proton donor.
Eric Reply
what is an acid
Amara Reply
an acid is a substance when dissolved in water produces hydrogen ion or hydroxonium ion
is a substance which dissolves in water to produce hydrogen ions as the only positively charged ions
what is ionic bonding
Kylian Reply
It involves the transferring of electron from a metal to a non mental
that's right
bonding between a metal and a non metal
calculate the hydrogen ion concentration of the solution when pH=5
Adamu Reply
answer pls
no ideas
What is thermodynamics
prince Reply
what is the meaning this word twentieth
Is the branch of physics that deal with heat and temperature and their relation to work, energy and properties of matter
There are no topics on hydrocarbons
Zina Reply
I don't understand
they are there please check under organic chemistry in the contents.
its not making sense to me I still don't understand
megan Reply
How and why
we need diagram for easy going and understand
How can we easily differentiate between the 5 gas laws
Favour Reply
first amd foremist me i only know 3 gas laws, so please list them here
the gas laws i know include pressure law boyles law charles law i differentiate these with this formular big take classy pork pigs. viral i read thus as 1 big classy pigs take pork viral big.....take means in boyles law,temp is constant clasy.....prk mns in chrls lw, press cons
classy.....pork means in charles law pressure is constant pigs.....viral means, in pressure law volume is kept constsng ,the rest is the same boyles states that vol of a gas is inversely prop to volume keeping temp constant charles law, state vol of a gas is directly prop to temp keepn pressure cns
how many carbon is present in alkene
Alase Reply
it's the carbon to carbon being double bonded to each other that makes it an alkene, not the amount of carbon itself. ex: C=C, C=C=C. both are alkenes.
I need more light on alkene
an alkene is a hydrocarbon, you can find this under the sub topic of organic chemistry in this textbook, check it out please,but before you know about alkenes you should first know about alkanes, alkanes are saturated and dnt easily react while alkenes have double bonds and can react .
other usefullness of hydrogen apart from this, it is colourless, odourless and tasteless
Bukunmi Reply
it is neutral to litmus paper, it is insoluble in water
the enthalpy of a system
Nasirat Reply
changing in heat of a system which can be lost or gained
is the energy change that occurs when molar quantities of reactants as stated in a chemical equation completely react
enthalpy of reaction is the energy change that occurs when molar quantities of reactants as stated in a chemical equation completely react
what is the unit of pressure
Tim Reply
what is pressure measured in?
millimeter mercury ,mmHg or ATM
it is atm...in lower case not upper
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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