Solubility and 34 Pre-lab Thermodynamics Questions Before beginning this experim

Question

Solubility and 34 Pre-lab Thermodynamics Questions Before beginning this experiment in the laboratory, you should be able to answer the following questions. 1. What do the symbols P, MP, . R. T, and K stand for in the following equations? 2. Write out the equilibrium expression for dissolution of Ca(OH2(s). By adding HCl, will it increase, de- crease, or have no effect on the solubility of Ca(OH)2(s)? Why? How does your answer come into play if you had excess solid Ca(OH)2 in your solution to be titrated? How do the concentrations of HCI, OH-, and Ca2+ relate to each other at the endpoint of an acid-base titra- tion? How is this information used to determine Kp and AG? 3. A saturated solution of manganese(II) hydroxide was prepared, and an acid-base titration was performed to determine its Kp at 25 °C. The endpoint was reached when 50.00 mL. of the manganese(II) hydroxide solution was titrated with 3.42 mL of 0.0010 M HCI solution. What is the Kip of manganese(II) hydroxide? 4. 5. Make a sketch of the graph for the function y In x. For what values of x is y positive? What values of x give a negative result for y? Now consider Go_RT1nK. For what values of K is 0° positive? what values of K give a negative result for ? what does this tell you about the spontaneity of a reaction? 533 Copyright © 2015 Pearson Education, Inc.

Explanation / Answer

1) G0 denotes the free energy change of the reaction. H0 and S0 denote the enthalpy change and the entropy change of the reaction respectively. R is the universal gas constant having a value of 8.314 J/mol.K and T is the absolute temperature of the reaction. K denotes the equilibrium constant for the reaction.

2) Write down the dissociation of Ca(OH)2 as below.

Ca(OH)2 (s) <======> Ca2+ (aq) + 2 OH- (aq)

Ksp = [Ca2+][OH-]2

HCl is a strong electrolyte and dissociates completely in aqueous solution as

HCl (aq) --------> H+ (aq) + Cl- (aq)

H+ (aq) + OH- (aq) ---------> H2O (l)

The proton from HCl combines with OH- to form H2O which remains undissociated. Therefore, [OH-] drops. Ksp is a thermodynamic equilibrium constant and hence remain constant at a particular temperature. Since [OH-] falls on addition of HCl, [Ca2+] must increase proportionately to maintain Ksp. This is possible only when more Ca(OH)2 dissolves in solution, i.e, the solubility of Ca(OH)2 increases on addition of HCl.

The solution is a saturated solution of Ca(OH)2. Adding excess Ca(OH)2 will not alter the solubility of Ca(OH)2 in water, since the solubility depends only on the temperature of the experiment.

3) Write out the balanced chemical equation for the reaction of Ca(OH)2 (which gives Ca2+ and OH- in solution) and HCl.

Ca(OH)2 (s) <=====> Ca(OH)2 (aq) <======> Ca2+ (aq) + 2 OH- (aq) + 2 HCl (aq) -------->

Ca2+ (aq) + 2 OH- (aq) + 2 H+ (aq) + 2 Cl- (aq) ---------> Ca2+ (aq) + 2 Cl- (aq) + 2 H2O (l)

As per the balanced stoichiometric equation above, we must have

1 mole Ca2+ = 2 moles of OH- = 2 moles of HCl = 2 moles of H2O

The solubility of Ca(OH)2 is equal to the molar solubility of Ca2+ and half the molar solubility of OH-.

Since the volume of the solution stays constant, we must have

[Ca2+] = 2*[OH-] = 2*[HCl]

Therefore, if we know the concentrations of the above species, we can easily calculate Ksp. Ksp is related to the free energy change of the reaction, G0 as

G0 = -R*T*ln Ksp

Consequently, if we know Ksp , we can easily determine G0.

4) Write down the dissociation of Mg(OH)2 as below.

Mg(OH)2 (s) --------> Mg2+ (aq) + 2 OH- (aq)

Ksp = [Mg2+][OH-]2

Mg(OH)2 reacts with HCl as below.

Mg(OH)2 + 2 HCl (aq) -------> MgCl2 (aq) + H2O (l)

As per the stoichiometric equation,

1 mole Mg(OH)2 = 2 moles HCl.

Millimoles of HCl used in the titration = (3.42 mL)*(0.00100 M)*(1 mol/L/1 M) = 0.00342 mmole.

Millimoles of Mg(OH)2 titrated = (0.00342 mmole HCl)*(1 mole Mg(OH)2/2 mole HCl) = 0.00171 mmole.

Concentration of Mg(OH)2 = millimoles of Mg(OH)2/volume of Mg(OH)2 = (0.00171 mmole)/(50.00 mL) = 3.42*10-5 M.

Due to the nature of dissociation, we must have [Ca2+] = 3.42*10-5 M and [OH-] = 2*3.42*10-5 M = 6.84*10-5 M.

Plug in the expression for Ksp.

Ksp = [Mg2+][OH-]2 = (3.42*10-5)*(6.84*10-5)2 = 1.6001*10-13 1.60*10-13 (ans).

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