Lab 4 Flashcards
For any reversible chemical reaction at equilibrium,
the concentration of all reactants and products are
constant.
- characterized by the equilibrium constant, Keq
Keq and what it favours
Keq»_space; 1 then the reaction lies to the right hand side, i.e. the reaction favors the products, whereas if Keq
Β«1 then the equilibrium lies to the left hand side.
what are you doing in this experiment
you will determine the equilibrium constant for the reversible chemical reaction of
iron (III) ions (Fe3+) and thiocyanate ions, SCNβ to form the iron thiocyanate complex, FeSCN2+:
Fe3+(aq) + SCN-(aq) FeSCN2+(aq) (4.3)
The equilibrium constant expression Keq for this reaction is:
πΎππ = [πΉπππΆπ2+]
[πΉπ3+][ππΆπβ]
what will we use for this experiment
use colour intensity to determine concentration.
the first two ions are essentially WHAT in acid solution
colourless in acid solution while the product has
a colour ranging from pale orange to deep red, depending on concentration.
- intensity is proportional to WHAT and how can it be measured
Since the intensity of the
coloured complex is directly proportional to its concentration, the amount of light absorbed by the
solution can be measured with a spectrophotometer. Provided the concentration is not too high then
most solutes obey Beerβs Law:
π΄ = ππβ
Beerβs Law
This law relates the absorbance we measure βAβ
to the light passed through βββ cm of the solution, the
concentration βcβ of the absorbing species and a constant of proportionality βΞ΅β known as the molar
extinction coefficient (units: M-1cm-1) which is characteristic of a particular compound at a particular
wavelength, Ξ», of light.
f a solute absorbs visible light (Ξ» = 400-700 nm), what does it mean for a large value of E then
a large value of Ξ΅ means that
even dilute solutions of the solute exhibit noticeable colour.
Variables are proportionalβ¦which two are
absorbance
(A) is proportional to concentration (c). You will use a cuvette where the path length βββ = 1 cm
what will be done in Part A of the reaction
In Part A of this experiment you will determine a calibration curve from which the concentration of
FeSCN2+ (in Part B) can be determined from the experimental absorbance.
Knowing the concentration
of FeSCN2+ at equilibrium and initial concentrations of reactants,β¦ what can u do
you can then determine the equilibrium
concentration of each reagent from which Keq for formation of FeSCN2+(aq) can be determined
PART A and PART B
Part A: Determining a calibration curve
Part B: Determining the equilibrium constant.
Le Chatalier Principle : lots of Fe3+ ions
equilibrium lies far to the right, i.e. all the SCNβ(aq) has been
converted to the complex ion, FeSCN2+(aq). We therefore assume that the initial number of moles of SCNβ
is all converted to FeSCN2+
knowing the total volume of the solution, we can determine what
an determine [FeSCN2+].
You will generate a calibration curve by plotting absorbance vs [FeSCN2+] for a series of solutions with
differing values for [FeSCN2+].
How to find Keq
determine
the concentrations of the unreacted reagents and the product.
ach [FeSCN2+] can be determined by the
calibration curve, since the absorbance is directly proportional to [FeSCN2+]
K equation
[x]/[A-x][B-x]
Position of equilibrium with regards to temperature
related to the balance between the tendency toward minimum energy and maximum energy dispersal.
what is Gibbs Free energy
Gibbs free energy change is a measure of reaction
spontaneity or the extent to which a reaction proceeds in the forward direction. The relationship may
be expressed as
G=-RTlnKeq
G units are J/mol
K smaller than 1 =
K bigger than 1=
very little product is made to
reach equilibrium. The reaction, thus, is not spontaneous in the forward direction and βG Μ is a relatively
large positive value.
A large negative βG Μ indicates that the reaction goes almost to completion. At
equilibrium, much more product is present than reactants so K»_space; 1
What is the limiting reactant
SCN-
A large value of G means
Not spontaneous in the forward direction
