Question

Thermodynamics of Dissolution of Substance

Reaction Eqn of borax and HCL

B4O5(OH)4 2-(aq) + 2HCl + 3H2O <_> 4H3BO3 (aq) + 2Cl-

Temp	TRIAL ONE Molarity of HCL	Volume Borax 1	Initial Buret Reading	Final Buret Reading	TRIAL TWO Molarity of HCL	Volume Borax 2	Initial Buret Reading	Final Buret Reading
(°C)	(M)	(mL)	(mL)	(mL)	(M)	(mL)	(mL)	(mL)

25

0.25

5

18

20.45

0.25

5

21

23.5

calculate

Moles of HCL	Moles of Borate	Concentration of Borate	Ksp	Average Ksp	ΔG	ΔH	ΔS
(moles)	(moles)	(M)			(Kj/mol)	(Kj/mol)	(J/mol k)

** I understand that for ΔH ΔS, you need to plot ln(Ksp) vs 1/T and determine their value from that graph.

Values for enthalpy and entropy of the dissolution of borax in water are 110 kJ/mol and 380 J/mol*K respectively. Do your values agree? Calculate your percent error.

You assumed that H and S are constant over the temperature range studied. Are they?

Answer 1

For trial 1:
moles of HCl = 0.25 mol/L * (20.45-18 / 1000)L = 6.125*10^-4 mol
moles of borate = 1 mole ( stoichiometry)
concentration of Borate = [6.125*10^-4 mol HCl *(1mol borate/2 mol HCl)] / (5/1000)L
concentration of Borate = 0.06125 M
K_sp = 4 [Borate ion]³ = 4*[0.06125]³ = 9.1913*10^-4
For trial 2:
moles of HCl = 0.25 mol/L * (23.5-21 / 1000) = 6.25*10^-4 mol
conc of borate = [6.25*10^-4 mol HCl *(1mol borate/2 mol HCl)] / (5/1000)L ] = 0.0625M
K_sp = 4 [0.0625]³ = 9.7656*10^-4
Average K_sp = (9.1913+9.7656 / 2) *10^-4 = 9.4785*10^-4
ln K_sp = (-ΔH/R) *1/T + (ΔS/R)
plot lnK_sp Vs 1/T and find slope and y-intercept of that line ( R=8.314 kJ/mol)
slope = -ΔH/R ; intercept = ΔS/R
percent error in enthalpy= [ | exp value - 110| / exp value ] *100
similarly you can find percent error for entropy as well
Over the temperature range the values are not constant, they keep changing over a negligible range
almost.
[To reduce the complexity and to find a solution such assumptions are made]