Lab 3 Flashcards
what is Ea
the energy necessary to initiate a chemical reaction
examples:
combustion of paper; the reaction of cellulose with oxygen is spontaneous, but you need to initiate the
combustion by adding activation energy from a lit match.
what will be done in this experiment
investigate the reaction of crystal violet (CV) with sodium hydroxide.
what is CV
Crystal violet is a bright violet organic compound that is often used as a pH indicator since it exhibits different
colours as the pH changes
CV reaction with HO-
Reaction with HOβ yields tris
(4-N,N- dimethylaminophenylmethnol
which
is colourless. Because of the distinct colour change, UV/visible spectroscopy is a good method to follow
the reaction.
Figure 3.1
The reaction of CV to form CVOH in aqueous alkaline solution.
Le-Chat principle
Although this is a reversible reaction, the equilibrium can be driven towards the product side by the
addition of excess hydroxide ions according to Le Chatelierβs Principle. The rate of the forward
reaction (Rf) is then given by
the rate equation
Rf = π[CV]^x[HOβ]^y
- the concentration of NaOH will be much greater than the concentration of crystal
violet. In such cases [HOβ] is approximately constant and we can approximate to:
Rf = πΒ΄[CV]x
order of rxn
This reaction is known to be first order with respect to crystal violet (x = 1).
how to observe progress of reaction
As the reaction proceeds,
the colour of crystal violet slowly disappears and colourless CVOH is formed. In this experiment the
rate of the reaction is obtained by monitoring the change in absorbance of the crystal violet at 565 nm
using a spectrophotometer.
equation for dilute solutions
In dilute solutions, absorbance (Abs) is proportional to the concentration
of crystal violet (c) according to Beerβs Law:
Abs=ecl
ABS=ABSORPTION
βAbsβ 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)
a large value of Ξ΅ means that
even dilute solutions of the solute exhibit noticeable colour.
the
absorbance is proportional to concentration
Reactions are typically monitored using a cuvette with a known path length
βββ = 1 cm
For the first order reaction the
integrated rate law is:
find k: crystal violet (CV) with HOβ ions.
ln[CV]t = ln[CV]0 β πt
Since the absorbance is proportional to concentration [CV]
, then a plot of ln(absorbance) vs time (t)
will give a straight line with the slope = -π.
In the second part of the experiment you will determine the rate of reaction at different temperatures.
by the Arrhenius
equation:
π = π΄π^(βπΈπ/π
π)
convert Arrhenius equation to equation of a line
be converted
to a straight line form, y = mx + b, by taking the natural logarithm of both sides:
ππ π = ππ π^(βπΈπ/π
π) + ln π΄
ln π = βπΈπ/π
π + ln π΄
bc ln e =1
To determine the activation energy for this reaction, Ea, you will monitor
the reaction at different
temperatures while keeping initial concentrations of the reactants the same for each trial. This will give
you a series of rate constants (π) at different temperatures (T). A plot of ln(π) vs (1/T) will be a straight
line with gradient βEa/RT from which you can determine Ea
A well-known approximation in chemistry states that the rate of a reaction often doubles for every β¦.
for every 10
ΜC increase in temperature. You will determine the rate constant at four temperatures; 10, 15, 20 and
25 ΜC, so you should expect the rate constant to approximately double over this range.
reactant solutions
2.5 x 10^β5 M crystal violet (45 mL) *
0.10 M NaOH (45 mL)
Limiting reactant
CV
parameters to be monitored to calculate k
- ln absorption vs time
- ln k vs 1/temperature
small Ea means small k (slow rxn rate): T/F
F
