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