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The equilibrium constant

Equilibrium constant

The equilibrium constant (K c ), relates to a chemical reaction at equilibrium. It can be calculated if the equilibrium concentration of each reactant and product in a reaction at equilibrium is known.

Calculating the equilibrium constant

Consider the following generalised reaction which takes place in a closed container at a constant temperature :

A + B C + D

We know from "Factors affecting reaction rates" that the rate of the forward reaction is directly proportional to the concentration of the reactants. In other words, as the concentration of the reactants increases, so does the rate of the forward reaction. This can be shown using the following equation:

Rate of forward reaction [A][B]

or

Rate of forward reaction = k 1 [A][B]

Similarly, the rate of the reverse reaction is directly proportional to the concentration of the products. This can be shown using the following equation:

Rate of reverse reaction [C][D]

or

Rate of reverse reaction = k 2 [C][D]

At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction. This can be shown using the following equation:

k 1 [ A ] [ B ] = k 2 [ C ] [ D ]

or

k 1 k 2 = [ C ] [ D ] [ A ] [ B ]

or, if the constants k 1 and k 2 are simplified to a single constant, the equation becomes:

k c = [ C ] [ D ] [ A ] [ B ]

A more general form of the equation for a reaction at chemical equilibrium is:

a A + b B c C + d D

where A and B are reactants, C and D are products and a, b, c, and d are the coefficients of the respective reactants and products. A more general formula for calculating the equilibrium constant is therefore:

K c = [ C ] c [ D ] d [ A ] a [ B ] b

It is important to note that if a reactant or a product in a chemical reaction is in either the liquid or solid phase, the concentration stays constant during the reaction. Therefore, these values can be left out of the equation to calculate K c . For example, in the following reaction:

C ( s ) + H 2 O ( g ) C O ( g ) + H 2 ( g )

K c = [ C O ] [ H 2 ] [ H 2 O ]
  1. The constant K c is affected by temperature and so, if the values of K c are being compared for different reactions, it is important that all the reactions have taken place at the same temperature.
  2. K c values do not have units. If you look at the equation, the units all cancel each other out.

The meaning of k c Values

The formula for K c has the concentration of the products in the numerator and the concentration of reactants in the denominator. So a high K c value means that the concentration of products is high and the reaction has a high yield. We can also say that the equilibrium lies far to the right. The opposite is true for a low K c value. A low K c value means that, at equilibrium, there are more reactants than products and therefore the yield is low. The equilibrium for the reaction lies far to the left.

Calculations made easy

When you are busy with calculations that involve the equilibrium constant, the following tips may help:

  1. Make sure that you always read the question carefully to be sure of what you are being asked to calculate. If the equilibrium constant is involved, make sure that the concentrations you use are the concentrations at equilibrium , and not the concentrations or quantities that are present at some other time in the reaction.
  2. When you are doing more complicated calculations, it sometimes helps to draw up a table like the one below and fill in the mole values that you know or those you can calculate. This will give you a clear picture of what is happening in the reaction and will make sure that you use the right values in your calculations.
    Reactant 1 Reactant 2 Product 1
    Start of reaction
    Used up
    Produced
    Equilibrium

Questions & Answers

The phenomenon that describes the change in the frequency
Neo Reply
Doppler effect
sinelizwi
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mudodzwa Reply
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Anelisiwe Reply
final energy-initial energy of substances
Chantel
what is the difference between ethene and ethane
Joy Reply
Ethane is a saturated organic molecule while ethene is unsaturated Ethane has 2 carbon atoms and 6 hydrogen atoms, while ethene has 2 Carbons and 4 hydrogens Ethane has single bond between the 2 Carbon atoms while ethene has double bond
Rito
How do you know that the reaction is in acid medium or basic medium
Miss Reply
What formular is used when source and Observer are moving towards to each other
Esihle Reply
put a + on top n - on the bottom
Sanele
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Ansiwe Reply
doppler effect is the perceived change in frequency of a sound caused by either the source or the listener
Qaqamba
moving relative to each other
Qaqamba
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Sheron Reply
It does this by lowering the activation energy.
Sheron
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Nambile Reply
reaction that releases energy
Beauty
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Joseph
Catalyst is to speed up the reaction
CELO
catalyst is a substance added into a solution to speed up the process
Joseph
As it speeds up the reaction by lowering the activation energy, it does not take part in the reactants or products formed
Rito
reaction that release energy
Qaqamba
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Given
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mudodzwa
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Nambile Reply
reaction that gives off heat to the surroundings
Thabile
formula for calculating the heat of reaction
Anelisiwe
I need help in electrostatic
Muthuphei Reply
in exactly in electrostatic
Beauty
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factors affecting the rate of reaction
thabo Reply
hi
Xiluva
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Mashweme
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The first thing you can do is to understand ohm's law and the difference between a series and parallel connection concerning current, resistance and potential difference
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Hi Xiluva,not sure if this will be helpful but to understand electricity better I suggest you download Siyavula physical science books there's both a learner book and a teacher's guide I was struggling too they really did help you might be able to do some exercises after reading siyavula material
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zama Reply
ep=mgh
mudodzwa
Ep=mgh
Maximurm
What is polymer
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chemical compound with molecules bonded together in long ,repeating chains
Bukani
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Monica Reply
A force is a pull or push and it can be applied in rocket launchers moving cars & stoping (breaking them).
Duncan
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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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