BACKGROUND
The goal of the experiment is to determine the rate law for the reaction of FD&C Red #3 with hypochlorite. The rate law takes the form given by eq 1.
−d[Red#3]=k[Red#3]m [OCl−]n
dt
(1)
The reaction is carried out under pseudo-first-order conditions. Hypochlorite is added in a large excess relative to Red #3, so the factor [OCl−]n behaves as a constant, and a simplified rate law (eq 2) is obtained.
−d[Red#3] = (k[OCl−]n )[Red #3]m = k′[Red #3]m
dt
(2)
It follows that the pseudorate constant, the rate constant that is measured directly, is given by eq 3.
Determination of the order with respect to hypochlorite (n) constitutes the most challenging part of the experiment and requires performing at least three trials with different concentrations of hypochlorite and a constant concentration of the dye. The pseudorate constant (k′) is obtained by plotting ln[Red #3] versus time. To deduce the relationship between k′ and [OCl−], students examine the ratios of their pseudorate constants (Table 1). Because the reaction is first order in hypochlorite, k′ should change proportionately with [OCl−].