How To Write Equilibrium Expression

EQUILIBRIUM CONSTANTS: 1000_c This folio explains what is meant by an equilibrium abiding, introducing equilibrium constants expressed in terms of concentrations, K_c. It assumes that you lot are familiar with the concept of a dynamic equilibrium, and know what is meant by the terms "homogeneous" and "heterogeneous" as applied to chemical reactions.
	Important:If you aren't sure virtually dynamic equilibria it is important that you follow this link earlier you continue. If yous aren't sure what homogeneous and heterogeneous mean, you would find it useful to follow this link and read the beginning of the folio that you volition find (actually on catalysis). Utilise the Back button on your browser to render to this page.
We need to look at two different types of equilibria (homogeneous and heterogeneous) separately, because the equilibrium constants are defined differently. A homogeneous equilibrium has everything present in the same phase. The usual examples include reactions where everything is a gas, or everything is present in the aforementioned solution. A heterogeneous equilibrium has things present in more than 1 stage. The usual examples include reactions involving solids and gases, or solids and liquids. K_c in homogeneous equilibria This is the more straightforward case. It applies where everything in the equilibrium mixture is present as a gas, or everything is nowadays in the same solution. A good example of a gaseous homogeneous equilibrium is the conversion of sulphur dioxide to sulphur trioxide at the heart of the Contact Process: A commonly used liquid case is the esterification reaction betwixt an organic acrid and an booze - for instance: Writing an expression for 1000_c Nosotros are going to look at a general case with the equation: No state symbols have been given, but they will be all (thou), or all (l), or all (aq) if the reaction was between substances in solution in water. If you allow this reaction to accomplish equilibrium and then measure the equilibrium concentrations of everything, you lot can combine these concentrations into an expression known as an *equilibrium constant* . The equilibrium constant always has the same value (provided you don't modify the temperature), irrespective of the amounts of A, B, C and D you lot started with. It is also unaffected by a change in force per unit area or whether or not you lot are using a goad. Compare this with the chemic equation for the equilibrium. The convention is that the substances on the right-hand side of the equation are written at the top of the K_c expression, and those on the left-hand side at the bottom. The indices (the powers that you lot have to enhance the concentrations to - for case, squared or cubed or whatever) are just the numbers that appear in the equation.
	Annotation:If you accept run into orders of reaction, don't confuse this with the powers that appear in the rate equation for a reaction. Those powers (the social club of the reaction with respect to each of the reactants) are experimentally determined. They don't necessarily have whatever direct connectedness with the numbers that appear in the equation. Y'all may encounter attempts to derive the expression for M_c by writing rate equations for the forward and back reactions. These attempts brand the primal mistake of obtaining the rate equation from the chemical equation. That's Wrong! You could only practice that for the very simple cases where the reaction took place in a single stride represented by the given equation. Georgio Karam contacted me in July 2015 pointing out that y'all tin in fact derive the K_c expression for more complicated reactions by looking at each step of the reaction individually and then combining the results. Yous will find his commodity by following this link. If you lot know near orders of reaction and rate equations, y'all will detect this very easy to follow. The fact that this is possible doesn't, however, seem to me to justify using the derivation in general terms of aA plus bB going to cC and dD, and assuming that the numbers in front of the substances in the overall equation represent orders of reaction. That is dangerously misleading.
Some specific examples *The esterification reaction equilibrium* A typical equation might be: At that place is merely one molecule of everything shown in the equation. That means that all the powers in the equilibrium constant expression are "i". Y'all don't demand to write those into the K_c expression. As long as you lot proceed the temperature the same, whatever proportions of acid and alcohol you mix together, in one case equilibrium is reached, One thousand_c e'er has the aforementioned value. At room temperature, this value is approximately 4 for this reaction. *The equilibrium in the hydrolysis of esters* This is the reverse of the last reaction: The K_c expression is: If you compare this with the previous example, you will run into that all that has happened is that the expression has turned upside-downward. Its value at room temperature volition be approximately 1/iv (0.25). It is really important to write down the equilibrium reaction whenever you talk about an equilibrium constant. That is the only fashion that you tin can be certain that you have got the expression the right mode upwards - with the right-hand substances on the summit and the left-paw ones at the bottom. *The Contact Process equilibrium* You will recall that the equation for this is: This time the K_c expression volition include some visible powers: Although everything is nowadays as a gas, you all the same measure out concentrations in mol dm^-iii. There is another equilibrium abiding called K_p which is more frequently used for gases. You volition notice a link to that at the bottom of the page. *The Haber Procedure equilibrium* The equation for this is: . . . and the Thou_c expression is: K_c in heterogeneous equilibria Typical examples of a heterogeneous equilibrium include: The equilibrium established if steam is in contact with ruby-red hot carbon. Hither we have gases in contact with a solid. If you milkshake copper with silver nitrate solution, you get this equilibrium involving solids and aqueous ions: Writing an expression for K_c for a heterogeneous equilibrium The of import difference this time is that y'all don't include any term for a solid in the equilibrium expression. Taking another look at the two examples above, and calculation a third one: *The equilibrium produced on heating carbon with steam* Everything is exactly the same every bit earlier in the equilibrium constant expression, except that y'all leave out the solid carbon. *The equilibrium produced between copper and silver ions* Both the copper on the left-hand side and the silver on the right are solids. Both are left out of the equilibrium abiding expression. *The equilibrium produced on heating calcium carbonate* This equilibrium is only established if the calcium carbonate is heated in a closed system, preventing the carbon dioxide from escaping. The just thing in this equilibrium which isn't a solid is the carbon dioxide. That is all that is left in the equilibrium constant expression. Calculations involving Thou_c There are all sorts of calculations yous might be expected to do which are centred effectually equilibrium constants. You might exist expected to calculate a value for K_c including its units (which vary from case to case). Alternatively you might have to calculate equilibrium concentrations from a given value of Yard_c and given starting concentrations. This is simply too huge a topic to be able to deal with satisfactorily on the internet. Information technology isn't the best medium for learning how to do chemistry calculations. It is much easier to do this from a carefully structured volume giving you lots of worked examples and lots of problems to try yourself. If you have constitute this site useful, you might like to accept a expect at my book on chemistry calculations. Information technology covers equilibrium constant calculations starting with the almost niggling cases, and gradually getting harder - upwardly to the moderately hard examples which may be asked in a UK A' level exam.
	Notation:If you are interested in my chemistry calculations volume you might like to follow this link.
Questions to test your understanding If this is the first set of questions you have done, please read the introductory folio before you start. You will need to apply the Dorsum Push button on your browser to come dorsum here afterwards. questions on G_c answers Where would you like to go now? To await at Thousand_p . . . To the equilibrium menu . . . To the Physical Chemistry carte . . . To Main Carte . . . © Jim Clark 2002 (last modified July 2015)