AP Chemistry: 5.4 Elementary Reactions – Exam Style questions with Answer- MCQ

Question

Step 1:  \(H_2O_2+I^−→IO^−+H_2O\)

Step 2:  \(H_2O_2+IO^−→H_2O+O_2+I^−\)

The mechanism for a chemical reaction is shown above. Which of the following statements about the overall reaction and rate laws of the elementary reactions is correct?
A The chemical equation for the overall reaction is  \(2H_2O_2+I^−→ 2H_2O+O_2+I^−\) , and the rate law for elementary step 2 is rate=\(k[H_2O_2][IO^−]\) .
B The chemical equation for the overall reaction is  \(H_2O_2+IO^−→ H_2O+O_2+I^−\) , and the rate law for elementary step 2 is rate=\(k[H_2O_2]^2[IO^−]\) .
C The chemical equation for the overall reaction is \(2H_2O_2→2H_2O+O_2\) , and the rate law for elementary step 1 is rate=\(k[H_2O_2][I^−]\) .
D The chemical equation for the overall reaction is \(2H_2O_2→2H_2O+O_2\) , and the rate law for elementary step 1 is rate=\(k[H_2O_2]^2\).

▶️Answer/Explanation

Ans:C

Once all the elementary steps are added and simplified, the resulting equation is \(2H_2O_2→2H_2O+O_2\). Because a rate law for an elementary step can be based on the stoichiometric coefficients of the reactants in that step, the rate law for elementary step 1 is rate=\(k[H_2O_2][I^−]\), and the rate law for elementary step 2 is rate=\(k[H_2O_2][IO^−]\).

Question

                       Step 1:  \(N_2O_5→NO_2+NO_3\)                            (slow)

                       Step 2:  \(NO_2+NO_3→NO_2+NO+O_2\)             (fast)

                       Step 3:  \(NO+N_2O_5→3NO_2\)                               (fast)

A proposed reaction mechanism for the decomposition of N2O5(g) is shown above. Based on the proposed mechanism, which of the following correctly identifies both the chemical equation and the rate law for the overall reaction?
A The chemical equation for the overall reaction is \(NO+N_2O_5→3NO_2\) , and the rate law is rate=\(k[NO][N_2O_5]\).
B The chemical equation for the overall reaction is \(2N_2O_5(g)→4NO_2(g)+O_2(g)\) , and the rate law is rate=\(k[N_2O_5]\).
C The chemical equation for the overall reaction is \(2N_2O_5(g)→4NO_2(g)+O_2(g)\) , and the rate law is rate=\(k[N_2O_5]^2 \).
D The chemical equation for the overall reaction is \(N_2O_5(g)+NO(g)+NO_3(g)→4NO_2(g)+O_2(g)\) , and the rate law is rate=\(k[N_2O_5][NO][NO_3]\).

▶️Answer/Explanation

Ans:B

When the elementary steps are added and simplified, the net chemical equation for the overall reaction is 2N2O5(g)→4NO2(g)+O2(g) . Because step 1 is the slowest (rate-determining) step, rate=\([N_2O_5]\).

Question

           Step 1: ?                                                                                 (slow)

           Step 2:   \(NO_2(g)+F(g)→NO_2F(g)\)                         (fast)

           Overall: 2  \(NO_2(g)+F_2(g)→2NO_2F(g)\)

The overall reaction represented above is proposed to take place through two elementary steps. Which of the following statements about the chemical equation for step 1 and the rate law for the overall reaction is correct?
A The chemical equation for step 1 is \(NO_2(g)+F_2(g)→NO_2F(g)+F(g)\) , and the rate law for the overall reaction is rate=\(k[NO_2][F_2]\).
B The chemical equation for step 1 is \(NO_2(g)+F(g)→NO_2F(g)+F_2(g)\) , and the rate law for the overall reaction is rate=\(k[NO_2][F]\).
C The chemical equation for step 1 is \(2NO_2(g)+F_2(g)→2NO_2F(g)\) , and the rate law for the overall reaction is rate=\(k[NO_2][F]\).
D The chemical equation for step 1 is \(3NO_2(g)+F_2(g)+F(g)→3NO_2F(g)\), and the rate law for the overall reaction is rate=\(k[NO_2]^2[F_2]\).

▶️Answer/Explanation

Ans: A

The chemical equations for the two elementary steps should add up to give the net equation for the overall reaction. Based on the chemical equations for step 2 and the overall reaction, the chemical equation for step 1 is \(NO_2(g)+F_2(g)→NO_2F(g)+F(g)\). Because step 1 is the slow (rate-determining) step, the rate law for the overall reaction is set by the stoichiometry of step 1 and is equal to rate=\(k[NO_2][F_2]\).

Question

 For the elementary reaction
\(NO_3 + CO \rightarrow NO_2 + CO_2\)
the molecularity of the reaction is __________, and the rate law is rate = __________.
A) 4, \(k[NO_3][CO][NO_2][CO_2]\)
B) 2, \(k[NO_3][CO]/[NO_2][CO_2]\)
C) 4, \(k[NO_2][CO_2]/[NO_3][CO]\)
D) 2, \(k[NO_2][CO_2]\)
E) 2, \(k[NO_3][CO]\)

▶️Answer/Explanation

Ans: E

Question

The kinetics of the reaction below were studied and it was determined that the reaction rate increased by a
factor of 9 when the concentration of B was tripled. The reaction is __________ order in B.
\(A + B \rightarrow P\)
A) zero           B) first           C) second           D) third           E) one-half

▶️Answer/Explanation

Ans: C

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