AP Chemistry 5.2 Rate Laws

[Maximum number: 3]

Answer the following questions about ascorbic acid (vitamin C).

(a)

Dehydroascorbic acid (DHAsc) can be produced by reacting ascorbic acid with the triiodide ion, $\mathrm{I}_{3}{ }^{-}$ , as represented by the following equation.

\mathrm{HAsc}+\mathrm{I}_{3}^{-} \rightarrow \mathrm{DHAsc}+3 \mathrm{I}^{-}+2 \mathrm{H}^{+}

The student runs three trials of the reaction with different initial concentrations of HAsc and $\mathrm{I}_{3}{ }^{-}$ , producing the following data.

[ 3 ]

(i)

The rate law for the reaction is rate $=k[\mathrm{HAsc}]\left[\mathrm{I}_{3}{ }^{-}\right]$ . Explain how the data in the table support the conclusion that the reaction is first order with respect to [HAsc].

[ 1 ]

C (i) For a correct explanation.

Examples of acceptable responses may include the following:

- $\frac{4.914 \times 10^{-4}}{2.457 \times 10^{-4}}=\left(\frac{0.900}{0.450}\right)^{a}$ , thus a=1

- Comparing trials 1 and 3, the rate doubles when the concentration of ascorbic acid is

doubled and the triiodide ion concentration is constant, indicating that the process is

first order with respect to ascorbic acid.

(ii)

Calculate the value of the rate constant, k, for the reaction. Include units with your answer.

[ 2 ]

(ii) For the correct calculated value:

rate $=k[\mathrm{HAsc}]\left[\mathrm{I}_{3}{ }^{-}\right]$

Using trial 1 data:

$k=\frac{\mathrm{rate}}{[\mathrm{HAsc}]\left[\mathrm{I}_{3}^{-}\right]}=\frac{2.457 \times 10^{-4} \mathrm{M} / \mathrm{s}}{(0.450 \mathrm{M})(1.200 \mathrm{M})}=4.55 \times 10^{-4} \mathrm{M}^{-1} \mathrm{~s}^{-1}$

For the correct units:

$M^{-1} \mathrm{~s}^{-1}$

Point 07

Point 08

Point 09

AP Chemistry 5.2 A Represent Experimental Data with a Consistent Rate Law Expression Questions

Exam points

AP Chemistry 5.2 A Represent Experimental Data with a Consistent Rate Law Expression Questions question 1

Question (a)

Question (i)

Question (ii)