Borax
Na2B4O7·10H2O
(Sodium Tetraborate
Decahydrate )
Determination of
K, ΔG˚, ΔH˚, and ΔS˚
Quiz Question # 4

Look at following equation to write down
your Ksp:

2
Na2 B4O7 10H 2O(s)  2Na (aq)  B4O5 (OH )4 (aq)  8H 2O(l )
First things first…

Safety:
Put bags away
 Goggles
 Lab Jacket

Turn in Lab
Reports
 LAB!

Borax Equilibrium
2
Na2 B4O7 10H 2O(s)  2 Na (aq)  B4O5 (OH ) 4 (aq)  8H 2O(l )

Na  B O (OH ) H O
K
2
 2
4
5
8
4
Na2 B4O7 10H 2O

K  Na
2
 B O (OH ) H O
2
 2
4

K sp  Na
5
8
4
2
 B O (OH ) 
2
 2
4
5
4
Let’s Simplify!!
Stoichiometry tells us that for each Na2+ there are two B4O5(OH)42-:
Na   2B O (OH ) 
2

4
4
  B O (OH ) 
K sp  Na 

5
2
2
4
 K sp  2 B4 O5 (OH ) 4

2
5
4
 B O (OH ) 
2
2
4
 K sp  4 B4O5 (OH ) 4
5

2 3
4
Today’s Rxn
Actual reaction for today:
2
B4O5 (OH ) 4 (aq)  2HCl  3H 2O  4B(OH ) 3 (aq)  2Cl 
The borate ion titrated with HCl yields the above.
Calculate moles borate ion to get Ksp.
Gibbs Free Energy (ΔG), Enthalpy
(ΔH), and Entropy (ΔS)
Once you have Ksp, determine the Gibbs free energy:
G   RT ln K sp
G  H  TS
  RT ln K sp  H  TS
Rearrange to a linear form (so you can compare with a graph)
by dividing both sides by (-RT)
ln K sp 
H TS

RT
RT
 and T cancels out  ln K sp 
y
H  1  S
 
R T  R
= m x + b
Changes in Procedure







Each group does one temp (assigned). 2 trials.
Add 2 scoops of borax and 75 mL of distilled water to a
beaker
Heat the solution (<50 °C) so that the solution is saturated.
Heat slowly! No need to speed up the heating process.
Once slightly above assigned temperature, take off hotplate.
If all the solid has dissolved, add some more borax to beaker.
Pour 5 mL of the solution into a graduated cylinder once the
solution is at the desired temperature. Record volume to
tenths place.
Be sure to record temperature to the tenths place.
Procedure (continued)



Add the 5 mL of solution to an Erlenmeyer with 50
mL distilled water, 5 drops bromocresol green, mix
thoroughly, add a little distilled water to rinse the
graduated cylinder into the Erlenmeyer
(Quantitative Transfer)
Titrate with HCl (clean/rinse buret and fill w/ ~20
mL HCl)
REPEAT! The above steps for two more titrations.

DO NOT try to prepare three samples at once, unless
you can guarantee the temperature of the borax solution
will not change dramatically!

Turn in class:
Pre-lab, filled in data tables, observations
 Type in you and your partner’s data in class
computers on opened excel spreadsheet

Name
[HCl]
Jack and Jill
(M)
0.250
Trial 1
Volume Borax Volume
Temp
Solution Used HCl used
(°C)
(mL)
(mL)
33
5.2
5.9
Formal Lab Report – due in
two weeks

Table 1, 2 – for each trial solve for:
Temp (K)

mol HCl mol borate [Borax]
used
ion
(M)
Ksp
ΔG
lnKsp (kJ/mol
)
Table 4 – average the trials to find (for each temp):
Temp (K)
Ksp
ΔG
lnKsp (kJ/mol
)
1/T
Data Tables (con’t)
Table 5 – use ΔH and ΔS to solve for ΔG for
each Temp
ΔG
Temp (K)
(kJ/mol)
Graph: Make using Table 4
data
Graph 1: 1/T vs. ln Ksp
0.00
0.00315
-1.00
0.0032
0.00325
0.0033
0.00335
0.0034
ln Ksp
-2.00
y = 461.3x - 7.0772
R² = 0.0006
-3.00
-4.00
Series2
-5.00
Linear (Series2)
-6.00
-7.00
-8.00
-9.00
1/T (1/K)
Calculations

Find molarity of borax (mol/L) [Borax]
Find moles borate (use moles HCl)
 Find volume of solution (borate + HCl)


Find Ksp
 Ksp = 4[Borax]
Calculations

Determine dH (kJ/mol) and dS (J/mol K)
from this graph.
note: dH = ΔH
 Use trendline from graph y = mx + b

H
ln K sp 
R
 1  S
 
T  R
Calculations con’t
 Calculate
the values for dG for
each sample using:
dG = -RT ln Ksp
used for table 4
 dG = dH - TdS with your solved
values for dH and dS
used for table 5

Discussion Analysis
 Consider
the following:
 Compare
solving delta G between the
two equations, which yielded better
results?
 Consider the precision of the graphs