Tuesday, June 4, 2019

The Study Of Solubility Equilibrium

The Study Of Solubility EquilibriumThe first part of this stir aims to tick off the solvability product constant of potassium hydrogen tartrate in water supply, and thereby determine how the enthalpy and entropy change of the dis reply response, according to the equilibrium KHC4H4O6 (s) K+ (aq) + HC4H4O6- (aq) (1), changes with temperature. The befriend part of this experiment aims to examine the common ion effect through determination of the Ksp of KHC4H4O6 in potassium nitrate (KNO3) dissolvent for varying K+ concentrations.ResultsI.Titrating 25.0cm3 of KHC4H4O6 solution with 0.07415M NaOH solution with phenolphthalein indicator gave the fol starting results To determine Ksp at 302.15KAmount of NaOH utilize = clean volume of NaOH utilise 0.07415M= 13.35 10-3 0.07415= 9.899 10-4 breakwater= Amount of HC4H4O6- moveedTotal volume of the solution = Average volume of NaOH used + 25.0 cm3 of HC4H4O6- solution= 13.35 + 25.0 = 38.35 cm3Since K+ = HC4H4O6-, Ksp = K+HC4H4O6 - = HC4H4O6-2 = 0.25812 = 6.662-10-4Table 1 Solubility product constant of potassium hydrogen tartrate at various temperaturesTemperature / KAverage VNaOH used / cm3Amount of NaOH used / counterspyAmount of HC4H4O6- reacted / molHC4H4O6- / mol dm-3Solubility of HC4H4O6-/ mol dm-3Kspln Ksp1/T / K-1282.56.650.00049310.0049310.015580.015580.0002427-8.3240.0035402929.250.00068690.0068690.020030.020030.0004010-7.8210.00342530213.350.00098990.0098990.025810.025810.0006663-7.3140.003311311.518.350.0013610.013610.031390.031390.0009852-6.9230.003210322.526.650.0019760.019760.038260.038260.001464-6.5270.003101Given that Greaction = Hreaction TSreaction - (2) and Greaction = RT ln K - (3), combining the two equations and rearranging gives us the linear function- (4)Using the data obtained in this experiment to plot this linear function gives Graph 1 shown belowGraph 1 Linear curve of ln Ksp against 1/TEquation of line-ln Ksp = 4113(1/T) + 6.264=- 4113Hreaction=- (- 4113 8.314)=+34 195 J m ol-1 (4s.f.) =+34.195 kJ mol-1Uncertainty (standard difference of opinion)= 105.3 8.314 = 875.5Hreaction= + 34 195 875.5 J mol-1=6.264Sreaction=6.264 8.314=+52.08 J K-1 mol-1 (4s.f.)Uncertainty (standard deviation)= 0.3497 8.314 = 2.907Sreaction= + 52.08 2.907 J K-1 mol-1Greaction = + 34 195 T (+ 52.08) J mol-1Solubility of HC4H4O6- and Ksp increases with increasing temperature, and a positive Hr quantify shows that the dissolution of KHC4H4O6 is an endothermic process. Since the order of Sr is smaller than that of Hr, Gr is forever positive in the temperature range of 282.5K to 322.5K, as carried out in this experiment. This indicates that the dissolution of KHC4H4O6 is always non-spontaneous for this temperature range, and hence KHC4H4O6 is a sparingly soluble salt.II.Titration of KHC4H4O6 in KNO3 solutions of different concentrations with 0.07413M NaOH solution with phenolphthalein indicator at room temperature to observe the common ion effect gave the following resu lts To determine the Ksp and solubility of HC4H4O6- at room temperature when KNO3 = 0.01MAmount of K+ from KNO3 = 0.01 70 10-3 = 7.00-10-4 molAmount of NaOH used = 11.45 10-3 0.07413 = 8.508 10-4 mol= Amount of HC4H4O6- reactedTotal amount of K+ = (7.00-10-4) + (8.508 10-4) = 1.55 10-3 molTotal volume = 25.0 + 11.45 = 36.45 cm3K+total =Ksp =Table 2 Solubility product constant of potassium hydrogen tartrate at various potassium nitrate concentrationsT / KKNO3 / MAverage volume of NaOH used / cm3Amount of NaOH / molAmount of HC4H4O6- reacted / molTotal amount of K+ / molK+total / mol dm-3Solubility of HC4H4O6- / mol dm-3Ksp3010.0111.450.00085090.00085090.0015510.042550.023340.00099323010.0210.350.00076910.00076910.0021690.061360.021760.0013353020.039.500.00070590.00070590.0028060.081330.020460.0016643020.048.600.00063910.00063910.0034390.10240.019020.001947Graph 2 Graph of solubility of KHC4H4O6 (M) against K+total (M)The data obtained in this part of the experiment shows that solubility of HC4H4O6- decreases with increasing total K+ concentration for a given(p) temperature (302K). This is due to the common ion effect the heading of the common ion K+ suppresses the dissociation of KHC4H4O6 according to (1), since K+ concentration is greater than the equilibrium KHC4H4O6 concentration. The greater the K+ concentration, the lesser the extent of dissociation of KHC4H4O6, and hence the less soluble HC4H4O6- is in water.DiscussionThe solubility of a substance is the amount of the substance dissolved in 1 L of its saturated solution for a given temperature. Ksp on the other hand, is the product of the ion concentrations raised to their respective powers on the dissolution equilibrium equation, and is constant for a given temperature.Since the dissolution of KHC4H4O6 is an endothermic process, when temperature increases, the forward reaction is favoured to absorb the excess heat. This causes Ksp values to increase with increasing temperature, as observed, sin ce concentration of products, i.e. K+ and HC4H4O6-, increases. The heat absorbed is used to overcome solute-solute and solvent-solvent interactions, such that solute-solvent interactions can form during the dissolution process. Also, as a solid dissolves, entropy of the system is increased, since the greater number of liquid particles increases disorderliness. Hence as temperature increases, Gr will be increasingly negative, indicating that the dissolution of KHC4H4O6 gets increasingly spontaneous as temperature increases.In this experiment, Hr and Sr are assumed to be insignificantly dependent on temperature.rH(T2) = rH(T1) + (T2-T1) rCp - (5) (Atkins, 2006)From Kirchhoffs law (5), rH is dependent on temperature, assuming that constant-pressure heat capacities (Cp) is independent of temperature. For the above assumption to attain true, rCp should be insignificant, i.e. (Cp) of the products and reactants should have approximately equal values. Since Cp is affected by how much of a substance there is in the solution the greater the number of particles, the greater the amount of heat energy needed to raise the overall temperature of the solution by 1K and the KHC4H4O6 solution used in titration is saturated, the amount of substance in the solution can be approximated to be the same. Thus, Cp of the products and reactants can be approximated to be the same, and hence rCp is minimum. Similarly, since Sr is dependent on Cp as healthful, we can assume it to be insignificantly dependent on temperature as well.For the above argument to hold true, the KHC4H4O6 solution used in titration must be saturated, and steps to ensure this should be taken one, continual swirling of the solution before filtration to ensure all solid has been dissolved two, maintaining supposed temperature of the solution immediately before filtering, since the saturated solution is filtered in small portions three, apparatus used to contain the filtrate must be dry such that the saturated s olution is not diluted by the presence of any water. General titration techniques were also employed, such as rinsing apparatus with the solutions that they are to contain to ensure no contamination and accurate concentrations, as well as keeping the amount of phenolphthalein indicator, a weak acid, to a minimum, to prevent the lowering of the pH of the solution, which results in more than the required amount of NaOH needed to react with the saturated KHC4H4O6 solution.For Section 1 of this experiment, comparing experimental and literature Ksp values gives the following At approximately 302KLiterature value of solubility of KHC4H4O6 in water = 7.3693 103 kgsalt/kgwater (Lopes, 2001)Literature Ksp value = (7.3693 103 gsalt/mlwater) (188.1772 g mol-1)2= (39.161 10-3 mol L-1)2= 1.534 10-3Experimental Ksp value (Section 1) = 6.663 10-4Mean Ksp value (Section 2) = 1.485 10-3The literature Ksp value in Section 1 of this experiment was 2.302 times higher than that of the experiment al Ksp value at 302K. Besides, since Ksp is only dependent on temperature, Ksp values at the same temperature should be constant and independent of concentrations. The mean Ksp value obtained in the Section 2 of this experiment, however, was 2.229 times greater than that obtained in Section 1 of this experiment, though it only had a 3.300% difference from the literature Ksp value.The abnormally low Ksp value obtained in Section 1 of this experiment indicates less than expected K+ and HC4H4O6- concentrations in the solution, and can stem from either the effect of a deviation from temperature, or from the solution being unsaturated. However, the data obtained in this experiment showed an accurate slue expected of Ksp values for increasing temperature, hence eliminating temperature deviation as a possible source of error. Furthermore, this trend also reflects expected solubility trends, and is sufficient in demonstrating the aims of this experiment.ConclusionThe aims of this experimen t have been met, as shown by the increasing trend of Ksp values for increasing temperature, as well as the decreasing solubility of HC4H4O6- in water for increasing K+ concentration, due to the common ion effect. These combined prove that Ksp is only dependent on temperature, given that care has been taken to ensure a saturated solution when carrying out the experiment.

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