Question
\( Zn(s)+2HCl(aq)→ZnCl_2(aq)+H_2(g)\)
When the reaction represented above proceeds, heat is produced. Which of the following best describes the reaction?
A It is a combustion reaction because heat is produced by the reaction.
B It is a double replacement reaction because 2Cl atoms are added to Zn.
C It is an acid-base reaction because HCl is an acid that is capable of exchanging \(H^+\).
D It is an oxidation-reduction reaction because zinc is oxidized and hydrogen is reduced.
▶️Answer/Explanation
Ans:D
It is an oxidation-reduction reaction because a Zn atom is oxidized and loses two electrons while two \(H^+\) ions are reduced as each gains an electron.
Question
Which of the following best describes the process represented above that takes place when \(NH_3\) is added to water?
A It is a single replacement reaction in which an electron pair on N is replaced with an H atom.
B It is an acid-base reaction in which a proton is exchanged from \(H_2O\) to \(NH_3\) .
C It is a precipitation reaction in which \(NH_4OH\) , an insoluble solid, is produced.
D It is an oxidation-reduction reaction in which the oxidation number of N changes from −3 to −4 .
▶️Answer/Explanation
Ans:B
It is an acid-base reaction in which \(H_2O\) donates \(H^+\) to \(NH_3\), forming \(NH_4^+\) and \(OH^−\) ions in solution.
Question
\(2Mg(s)+SiCl_4(l)→2MgCl_2(s)+Si(s)\)
Which of the following statements about the reaction represented above is correct?
A It is an oxidation-reduction reaction, and Mg is oxidized.
B It is an oxidation-reduction reaction, and electrons are transferred from \(SiCl_4\) to Mg
C It is an oxidation-reduction reaction, and the oxidation number of Cl changes from +4 to +2 .
D It is not an oxidation-reduction reaction because none of the oxidation numbers change.
▶️Answer/Explanation
Ans:A
In an oxidation-reduction reaction electrons are transferred from one species to another. Mg is oxidized; it loses electrons, as indicated by its oxidation number changing from 0 to +2.
Question
__________ electrons appear in the following half-reaction when it is balanced.
\(S_4O_6^{2-} \rightarrow 2S_2O_3^{2-}\)
A) 4 B) 1 C) 2 D) 6 E) 3
▶️Answer/Explanation
Ans: C
Question
Which of the following best describes the role of the spark from the spark plug in an automobile engine?
(A) The spark decreases the energy of activation for the slow step.
(B) The spark increases the concentration of the volatile reactant.
(C) The spark supplies some of the energy of activation forthe combustion reaction.
(D) The spark provides a more favorable activated complex for the combustion reaction.
(E) The spark provides the heat of vaporization for the volatile hydrocarbon.
▶️Answer/Explanation
Ans: C