Determination of K, ΔG˚, ΔH˚, and ΔS˚
Borax Na2B4O7·10H2O(Sodium Tetraborate
Decahydrate )
Quiz Question # 4
Look at following equation to write down your Ksp:
)(8)()()(2)(10 22
4542742 lOHaqOHOBaqNasOHOBNa
First things first…
Safety: Put bags away Goggles Lab Jacket
Turn in Lab Reports
LAB!
Borax Equilibrium
)(8)()()(2)(10 22
4542742 lOHaqOHOBaqNasOHOBNa
OHOBNa
OHOHOBNaK
2742
82
2454
2
10
)(
822
454
2)( OHOHOBNaK
2454
2)(OHOBNaK sp
Let’s Simplify!!
Stoichiometry tells us that for each Na2+ there are two B4O5(OH)42-:
2454 )(2 OHOBNa
2454
2)(OHOBNaK sp
2454
22454 )()(2 OHOBOHOBK sp
32454 )(4 OHOBK sp
Today’s Rxn
Actual reaction for today:
ClaqOHBOHHClaqOHOB 2)()(432)()( 322
454
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:
spKRTG ln
STHG
STHKRT sp ln
Rearrange to a linear form (so you can compare with a graph)
by dividing both sides by (-RT)
RT
ST
RT
HK sp
ln and T cancels out
R
S
TR
HK sp
1
ln
y = 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
Trial 1
Name [HCl] TempVolume Borax Solution Used
Volume HCl used
(M) (°C) (mL) (mL)Jack and Jill 0.250 33 5.2 5.9
Formal Lab Report – due in two weeks
Table 1, 2 – for each trial solve for:
Table 4 – average the trials to find (for each temp):
Temp (K) mol HCl used
mol borate ion
[Borax] (M) Ksp lnKsp
ΔG (kJ/mol
)
Temp (K) Ksp lnKspΔG
(kJ/mol)
1/T
Data Tables (con’t)
Temp (K) ΔG (kJ/mol)
Table 5 – use ΔH and ΔS to solve for ΔG for each Temp
Graph: Make using Table 4 data
Calculations
Find molarity of borax (mol/L) [Borax] Find moles borate (use moles HCl) Find volume of solution (borate + HCl)
Find KspKsp = 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
R
S
TR
HK sp
1
ln
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 dSused 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
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