5 6 name partner part b heat of solution trial 1

5-6
NAME: ________________________ PARTNER: ________________________
Part B : Heat of solution
Trial 1 Trial 2
Mass of water
Mass of solid
Initial temperature
Final temperature
 T
Heat change of solution
Heat change of calorimeter
Heat change of the reaction
 H
rxn
per gram of solid
Average
Show calculations for trial 1:
TO HAND IN

6-1
EXPERIMENT 6 : SPECTROPHOTOMETRIC DETERMINATION OF
THE EQUILIBRIUM CONSTANT K
c
Introduction
At any given temperature, a chemical reaction carried out in a closed vessel will reach
equilibrium. When the reactants are mixed together, they will form products until a state is reached
where the amount of reactants and products no longer change. Equilibrium is a dynamic process in
which the concentration of the reactants and products remain constant over time. At this point the
reaction has reached equilibrium and will remain so until the system is altered in some way. For a
given reaction, at a given temperature, the equilibrium state can be characterized by an equilibrium
constant K. For the general reaction:
a A + b B c C + d D (1)
The equilibrium expression is written:
b
eq
a
eq
d
eq
c
eq
c
B A
D C
K
] [ ] [
] [ ] [



The square brackets indicate the concentrations (in moles per liter) of the species at equilibrium .
The magnitude of K expresses the position of equilibrium for the reaction. For example, a
small value of K indicates that the equilibrium lies far to the left (a reactants-favoured reaction)
whereas a large value for K indicates that reaction lies far to the right (a products-favoured
reaction).
Background
The purpose of this experiment is to determine K
C
for the following reaction:
Fe
3+
(aq)
+ SCN

(aq)
FeSCN
2+
(aq)
(3)
Iron (III) ion thiocyanate ion thiocyanoiron ion
In order to calculate K
c
for this reaction, the concentrations of all ions present at
equilibrium ([FeSCN
2+
]
eq
, [SCN

]
eq
and [Fe
3+
]
eq
) need to be determined, according to equation
(2).
FeSCN
2+
ions in solution produce a red-brown coloured solution. This coloured solution
absorbs blue light very well. A spectrophotometer is an instrument capable of measuring this
absorption (see the section on spectrophotometry several pages below). This device is most
commonly used to determine the concentration of a known solute in a given solution by the
application of the Beer-Lambert Law.
According to the Beer-Lambert law the concentration of a solute that absorbs light is
proportional to that absorbance (see Spectrophotometry Overview on the following pages for
(2)

6-2
more details). The relationship can be expressed as A = εlc where A is the absorbance, ε is the
molar extinction coefficient (which depends on the nature of the chemical and the wavelength of
the light used), l is the length of the path light must travel through the solution in centimeters, and
c is the concentration of the given solution. For this experiment, ε and l will be kept constant, so the
Beer-Lambert law can be simplified as follows, where k is a proportionality constant equal to ε x l :
A = kc (4)
The Beer-Lambert relationship must be solved for a given solute using standard solutions.