Answered: uestion 1: You are calculating ΔH and…

Chemistry

10th Edition

ISBN:9781305957404

Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste

Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste

Chapter1: Chemical Foundations

Section: Chapter Questions

Problem 1RQ: Define and explain the differences between the following terms. a. law and theory b. theory and...

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Calculating Thermodynamic Values from an Equilibrium Constant
Background
As discussed in lecture, the free energy for a reaction can be related to the equilibrium
constant through the formula below.
K = e (-ΔG° / RT)
Therefore if Kc for a reaction is known, Go can be determined, or vice versa. Furthermore, if
you have the value for Go at two different temperatures, you can calculate H and S through
the familiar equation for Gibbs energy below, since you have two unknowns but also two
equations.
G = H – T S
In this lab you will be studying the solubility of borax (Na2B4O5(OH)4*8H2O), a slightly soluble
sodium salt, at two different temperatures. When solid borax is added to water, the
equilibrium below is established.
Na2B4O5(OH)4*8H2O (s) 2 Na+ (aq) + B4O5(OH)42- (aq) + 8 H2O(l)
If you measure the concentrations for those substances that show up in the reaction quotient,
then the Kc for the reaction at that temperature can be calculated. In this lab, the
concentration of borate ion (B4O5(OH)42-) in solution will be measured by titration with standard
hydrochloric acid according to the equation below.
B4O5(OH)42- (aq) + 2 HCl (aq) + 3 H2O (l)  4 H3BO3 (aq) + 2 Cl- (aq)
The concentrations of the other substances that appear in the reaction quotient can be
calculated from the borate concentration using stoichiometry.
Procedure
Preparation of The Borax-Borate Equilibrium Mixtures
NOTE: Volumetric procedures are not required for this step.
Prepare two separate 250 mL Erlenmeyer flask with magnetic stirring bars and add about 5 g of
borax and 100 mL of deionized water to each. Designate one flask as the room temperature
system and the other as the ice-water system. Prepare the ice-water system first, since it will
take more time to equilibrate.
Prepare an ice-water bath in a 1000 mL beaker. Place an Erlenmeyer flask containing borax
solution in the ice bath and stir the mixture gently on a magnetic stirrer for at least 20 minutes.
Shut off the stirrer, place a thermometer in the flask, and allow the undissolved borax to settle
to the bottom. Do not remove it from the ice bath! The temperature should be close to 5C and
the solution should become clear as the solid borax settles. Be sure to record the temperature
of the solution.
While the ice-water system is settling, stir the room temperature mixture gently on a magnetic
stirrer for at least 10 minutes. Shut off the stirrer, place a thermometer in the flask and leave it
undisturbed to allow the excess undissolved borax to settle to the bottom. The solution portion

should become clear as the solid borax settles. Be sure to record the temperature of the
solution.
Measuring the Ksp of Borax at Room Temperature
Record the temperature of the room temperature borax mixture. Without disturbing the solid
at the bottom, carefully decant about 60 mL of the borate solution into a clean and dry beaker.
Accurately pipet three 10.00 mL aliquots of borax solution into three separate clean Erlenmeyer
flasks. Add approximately 20 mL of distilled water and 3 drops of bromothymol blue indicator
to each flask. The solution should turn blue. Titrate each sample with standardized hydrochloric
acid until the solution changes from blue to yellow-green. Be sure to record the molarity of the
HCl solution. Use your best two titrations to evaluate the Ksp of borax at room temperature.
Measuring the Ksp of Borax at Ice Temperature
Repeat the procedure above for the borax solution at ice temperature. (After the decanting is
complete, it is okay if the decanted solution warms up.)

Question 1: You are calculating ΔH and ΔS for what reaction? (Either provide the chemical equation, or describe in words, since the chemical formulas are somewhat complicated to write). For this reaction, ΔS for the system is probably _____ (positive or negative)? If the sample from a lower temperature requires a smaller amount of HCl during the titration, we can conclude that ΔH is _____ (positive or negative)?

Expert Solution

Step 1

In this experiment procedure, we are determining the solubility of borax at ice temperature ( 5 $° C$ ) and room temperature.

Equilibrium reaction when borax is added to water,

$N a_{2} B_{4} O_{5} {(O H)}_{4} . 8 H_{2} O (s) ⇌ 2 N a^{+} (a q) + B_{4} O_{5} {(O H)}_{4}^{2 -} (a q) + 8 H_{2} O (l)$

At Different temperatures, we have different concentrations of B₄O₅(OH)₄^2- (aq). ( due to different solubility of borax )

So Equilibrium constant at ice temperature and at room temperature will be different.

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