Lab 8 Flashcards
the laws of classical thermodynamics apply specifically to
equilibrium conditions of a system, when they are applied to chemical reactions they relate the equilibrium condition and temperature to the thermodynamic quantities: H, S, and G
H,S,G
H=enthalpy
S=entropy
G=free energy
- we can find these after experimentally studying the effect of temperature on the equilibrium condition for a chemical reaction
what does this experiment investigate
- the effect on solubility of the sparingly soluble salt, sodium tetra borate decahydrate Na2B4O7 (10 H2O)
= also called borax
borax is an example of what
a class compound called borates
- contains polyanions composed of trigonal BO3 and/or structural features of the tetraborate anion suggest that it would be more appropriate to represent borax by the formula Na2[B4O5(OH4)] (8 H2O)
where does borax occur naturally
dry lake beds in California
and south western United States
- important sources of this valuable mineral
- common uses: soap, cleaning products, flux for solder, preservative, and the manufacture of glass
sodium tetraborate will dissociate in water to yield
Na2[B2O5(OH)4] (8H2O) = 2Na 2+ + B4O5(OH)4 2- + 8H2O
- form sodium and borate ions
Equation for Ksp
[Na+]^2[B4O5(OH)4 2-]
two sodium ions are produced per
each borate ion in the reaction
- therefore the concentration of Na+ is twice the concentration of borate ion
molar solubility equation
Ksp=4s^3
finding the concentration of borate ion, in any sample at any given temperature, leads directly to a value for
the solubility product at that temperature
- determination of the borate ion concentration is based on the principle that it is a weak base, as it reacts with water to form OH-, a strong base and boric acid (H3BO3) a very weak acid
equation:
B4O5(OH)4 2- + 5H2O = 4H3BO3 + 2OH-
concentration of the borate Ion can be determined by
a sample acid-base titration
- the hydroxide ion in saturated borax solutions is titrated with hydrochloric acid to a bromocresol green end point
EQUATION:
B4O5(OH)4 2- + 2HCl + 3H2O = 4H3BO3 + 2Cl
the titrant
HCl
what does the titrant data allow for
- allows you to calculate the equilibrium constant for the reaction
G =
H - T(s)
and
-RT(ln K)
equilibrium constant is related to enthalpy and entropy through what change of reaction
ln K = -H/RT + S/R
we can estimate ___ and ___ values from the equilibrium constant
H and S
= ln K = a/T + b
where:
a = -H/R
b = S/R
we can assume that DH and DS are
constant over some temperature range
- likely to be the case provided the temperature interval is small
plot of ln K versus 1/T will retain what slope and y-int
m=H/R
b=S/R
when recording titrant values
2 decimal places
what indicator is used
bromocresol green
