PRACTICAL NUMBER 5 THE EFFECT OF CHANGES IN VARIOUS CONDITIONS ON EQUILIBRIUM POSITION

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Dorothy Wood
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1 PRACTICAL NUMBER 5 THE EFFECT OF CHANGES IN VARIOUS CONDITIONS ON EQUILIBRIUM POSITION INTRODUCTION In this exercise you will examine the effect of a change in some of the conditions which determine the position of the same equilibrium that you studied in the last practical: (aq) +SCN (aq) (aq) pale yellow colorless reddish brown (1) Thesystemcontains onlyonestronglycolouredspecies,, betterwrittenwithsquarebrackets: [FeSCN] 2+. A change in the equilibrium position is therefore readily detected as a change in the colour of the system. Consider what happens when a change is made to one of the conditions governing an equilibrium. These conditions include such factors as the concentration of the participating species, the temperature, and the pressure. Whichever one is considered, it is important to distinguish clearly between the change itself and the effect that the change has on the equilibrium. For example, in system (1) above, suppose Fe(NO 3 ) 3 is added to the mixture. In solution this consists of separate and NO 3 ions. The nitrate ion may be ignored, since does not participate in the equilibrium. The immediate effect is a sudden rise in the concentration of, which moves the system far away from equilibrium. At this stage the concentration quotient: Q = [FeSCN2+ ] [Fe 2+ ][SCN ] (2) is temporarily quite different from the equilibrium constant. After this, the concentrations of the and fall back to new equilibrium values, and the concentration of the increases, so that Q once more becomes equal to K. The system is said to relax back to equilibrium; but not the original equilibrium position, a new one. The fall in concentration of and is the result of their reacting together to form more in accordance with Le Chatelier s principle. This may be shown in graphical form: New equilibrium position Change imposed The initial change in concentration of is considered as taking no time whilst the subsequent relaxation back to equilibrium is considered to be relatively gradual for the purposes of this exercise. Note that, after the initial increase, the concentration of decreases to the same extent as that of the and the concentration of the rises to an equal extent. This is a consequence of the stoichiometry of the reaction. Since the is an intense reddish-brown in colour, an increase in its concentration will manifest itself by a darkening of the colour of the system and, conversely, a decrease in the concentration will be seen as a lightening of the colour. Thus the final colour of the solution will be darker than the initial colour. In many chemical systems the reactions are so rapid that the response of the system will not be distinguishable from the change in the condition controlling the equilibrium. This is in fact the case with system (1) and it is therefore necessary to exercise the imagination a little to separate 1

2 the two stages. The imposed change is considered, for the purposes of this exercise, to take place instantaneously, giving a vertical line on the graph, whilst relaxation back to the new equilibrium is considered to be much slower, giving a curve on the graph. METHOD Half-fill 12 identical semi-micro test tubes to the same level with the equilibrium mixture of, and [FeSCN] 2+ (aq) provided. Retain one of these tubes as a colour standard and perform the following experiments on the others. Record the concentrations of all the solutions used. 1) Add 1 drop of (aq) to one of the tubes. The change in the volume can be neglected. Record your observations in the results table. Deduce the form of the appropriate graph and complete it. Note that we can consider that initially only the concentration of the is changed (instantaneously) and that the system responds by reacting more slowly, changing the concentrations of all the species. Continue as set out below, recording your results in the same manner in each case. 2) Add 1 drop of KSCN(aq) to another sample of the equilibrium mixture. 3) Add 1 drop of NaF(aq) to another sample. F reacts with the to form the colourless [FeF 6 ] 3 (hexafluoroferrate(iii) ion). It is this removal of free from the solution which should be considered. ) Add 1 drop of NaH 2 PO (aq) (sodium dihydrogenphosphate) to another sample. The colourless complex ion [Fe(PO ) 2 ] 3 (diphosphatoferrate(iii)) is formed. 5) Add 1 drop of Na 2 C 2 O (aq) (sodium oxalate) to a furthertube. Once again a colourless complex ion is formed. This time it is the [Fe(C 2 O ) 3 ] 3 (trioxalatoferrate(iii)) ion. 6) Add 1 drop of the 1M sodium hydroxide solution to a further test tube. Allow any precipitate to settle before examining the solution as before. Your earlier study of qualitative analysis will tell you what the precipitate is, and what effect its formation has. 7) Add 1 drop of the Hg(NO 3 ) 2 (aq) to another tube. This time the colourless [HgSCN] + (thiocyanatomercury(ii)) ion is formed, removing from solution. 8) Add 1 drop of AgNO 3 (aq) to a further test tube. A white precipitate of AgSCN(s) (silver thiocyanate) should be observed. Allow this to settle before observing the solution. 9) To the next tube add a volume of water, equal to the volume of solution it already contains, and observe the result. 1) Examine the effect of a change in temperature on the equilibrium position by warming the tenth test tube in a beaker of hot water. OBSERVATIONS AND DEDUCTIONS Calculate the concentrations of, and [FeSCN] 2+ in the stock solution (an equilibrium mixture) using K c = 891 (at 25 C), given that it is made up by mixing 2.cm 3 of.1m Fe(NO 3 ) 3 with 2.cm 3 of.1m KSCN. Concentration of complex ion = moldm 3 Concentration of stock solution = moldm 3 Concentration of stock solution = moldm 3 Complete the results table, Table 1, paying particular attention to the column headed initial change in condition. Confine yourself to changes which directly affect equilibrium (1), such as an increase or decrease in the concentration of or, or an increase in temperature etc. 2

3 Table 1: Results Table. System number Operation Imposed change in conditions Change in colour intensity 1 add 2 add 3 add F add H 2 PO 5 add C 2 O 2 6 add OH 7 add Hg 2+ 8 add Ag + 9 double volume 1 warm solution Complete the sketch graphs in Fig. 1 to Fig. 1 showing the effect of the imposed change on the concentrations of the various species participating in the equilibrium. Imagine for the purposes of this exercise that the imposed change takes place instantaneously at the time already marked on graph as time of change and that the relaxation of the system back to equilibrium is relatively slow. Note that the initial values of concentration, before the equilibrium is disturbed, are marked on the graph for you. Although these are sketch graphs you need to expend considerable time and effort to get them right. Final concentrations must be constant and changes in concentration must agree with the stoichiometry of the reaction. Use a ruler to ensure these things, but do not draw lines which are supposedto becurves with it! Remember that, if the imposed change is an increase in concentration of, for example, the final equilibrium concentration must be higher than the original concentration of. 3

4 Figure 1: Addition of Figure 2: Addition of Figure 3: Addition of F Figure : Addition of H 2 PO Figure 5: Addition of C 2 O 2 Figure 6: Addition of OH Figure 7: Addition of Hg 2+ Figure 8: Addition of Ag +

5 Figure 9: Double the volume Figure 1: Warm the solution PROBLEMS 1) For those systems where a chemical reaction occurs, apart from reaction represented by Eq. (1), write balanced chemical equations. 2) Is equilibrium represented by Eq. (1) endothermic or exothermic according to your experimental results? Explain your conclusion. 3) The additions of fluoride ion, dihydrogenphosphate ion, oxalate ion, and hydroxide ion are essentially identical in their effects on the equilibrium represented by Eq. (1). Similarly the effects of the additions of mercury(ii) ion and silver ion are virtually identical with each other, though different from those of the former ions, from the chemical point of view. Explain these statements. ) The addition of more solvent to equilibrium (1) can be likened to decreasing the total pressure in an equilibrium involving gases. Justify this statement. 5) The chloride ion reacts with according to the equation:- (aq)+cl (aq) [FeCl ] (aq) (3) Write down the concentration quotient (Q) for this equilibrium. Deduce a relationship between this concentration quotient, and that for the following equilibrium:- [FeSCN] 2+ (aq)+cl (aq) [FeCl ] (aq)+scn (aq) () 6) The equilibrium represented by Eq. () above can be considered to occur in four separate stages. In the first stage a single chloride ion is added to an ion: (aq)+cl (aq) [FeCl] 2+ (aq) (5) In the next three stages, three more chloride ions are added successively until [FeCl ] is finally formed. Write down all four equilibria and deduce a relationship between these stepwise equilibrium constants, K 1 through K, and the equilibrium constant for the overall process. i.e. the equilibrium represented by Eq. () 5

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