Chemical Kinetics Lab - Ascorbic Acid Reduction of Iron (III)
The object of this experiment was to determine a rate law and a rate constant. The reaction that was studied was:
2 Fe(CN)6 ^-3 + C6H8O6 = 2 Fe(CN)6 ^-4 + C6H8O6 + 2H ^+
Summary of obtained values:
m = 1
n = 1
k = .632 (rounded to 1)
I'm having trouble with the discussion questions.
The answers to 1 and 2 are supposed to be almost identical.
1.) The normal use of a transfer pipette leaves a small amount of solution in the tip. When pipetting ferricyanide solution in this experiment, this solution was forcibly expelled. Why does this make absolutely no difference (as long as it is done consistently between runs) in the determination of m and n, and a very small one (almost certainly less than the uncertainty) in the value of k?
2.) You made an attempt to determine the best estimate of zero time, i.e., the actual time of mixing. Instead, you simply started taking transmittance readings as soon as possible, effectively calling your first reading zero time. Why is knowing zero time unimportant - at least the way the experiment turned out?
3.) Show that for an isolated second-order reaction, a plot of 1/A vs t is linear, with slope of k'/ab
I am having the same issue
1.) The determination of the rate law (m and n) and the rate constant (k) in this experiment is based on the analysis of the change in the concentration of reactants with time. The expelled solution from the transfer pipette, although small in quantity, will have a consistent concentration and will be present in both the reactant and product side of the balanced equation.
Since the expelled solution is present in both the reactant and product side and is consistent across multiple runs, it cancels out when calculating the rate of reaction. This is because the change in concentration of the expelled solution will occur in both the forward and reverse reactions, resulting in equal contributions to the overall rate of reaction.
The effect of the expelled solution on the rate constant (k) is negligible because its concentration is small relative to the concentrations of the other reactants. The extremely small contribution from the expelled solution would be overshadowed by the uncertainties associated with the experimental measurements. Hence, it has almost no effect on the determination of the rate law (m and n) and a very small effect on the value of the rate constant (k).
2.) Knowing the exact zero time, which is the actual time of mixing, is not crucial in this experiment because the reaction rate is determined by the change in concentration over time. As long as the initial concentrations are consistent and the reaction is monitored from the same time point for all runs, the relative rate of change can still be analyzed accurately.
By starting the transmittance readings as soon as possible, the experimental data begins to be recorded immediately after mixing, effectively assuming that the reaction has initiated instantaneously. This assumption simplifies the analysis of the rate of reaction.
In this case, the actual zero time is unimportant because the experiment turned out to show that the reaction rate is not highly sensitive to the initial conditions or the reaction time immediately after mixing. The observed rates obtained by assuming the first reading as zero time can still be used to determine the rate law and rate constant with reasonable accuracy.
3.) To derive the linear relationship between 1/A (reciprocal of concentration) and time (t) for an isolated second-order reaction, we can start with the integrated rate equation:
1/A = kt + 1/A0
Where:
A = concentration of reactant at time t
A0 = initial concentration of reactant
k = rate constant
For a second-order reaction, the rate law can be represented as:
rate = k[A]^2
Rearranging the rate equation gives:
1/[A] = 1/(A0) + kt
Comparing this equation to the integrated rate equation, we can see that the slope of the linear plot 1/A vs t is equal to k, and the y-intercept is equal to 1/(A0):
slope = k = rate constant
y-intercept = 1/(A0)
Additionally, in the integrated rate equation, the term 1/(A0) corresponds to the reciprocal of the initial concentration, while the term kt corresponds to the slope multiplied by t. Therefore, the linear plot of 1/A vs t for an isolated second-order reaction will have a slope equal to k' (rate constant divided by the concentrations) and a y-intercept equal to 1/(A0 * B0), where B0 is the initial concentration of the other reactant (if present in the reaction).