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Chapter 20: Problem 19

Indicate whether the following balanced equations involve oxidation-reduction. If they do, identify the elements that undergo changes in oxidation number. (a) $2 \mathrm{NO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}_{2}(a q) \longrightarrow 2 \mathrm{HNO}_{3}(a q)$ (b) $\mathrm{FeS}(s)+2 \mathrm{HCl}(a q) \longrightarrow \mathrm{FeCl}_{2}(a q)+\mathrm{H}_{2} \mathrm{~S}(g)$ (c) $\mathrm{Fe}(s)+2 \mathrm{HNO}_{3}(a q) \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)+ 2 \mathrm{NO}_{2}(g)+\mathrm{FeO}(s)$

Short Answer

Expert verified
(a) Equation (a) involves an oxidation-reduction reaction. Nitrogen undergoes oxidation (oxidation number increases from +4 to +5), and oxygen undergoes reduction (oxidation number decreases from -1 to -2). (b) Equation (b) does not involve an oxidation-reduction reaction, as there is no change in oxidation numbers of any element. (c) Equation (c) involves an oxidation-reduction reaction. Iron undergoes oxidation (oxidation number increases from 0 to +2), and nitrogen undergoes reduction (oxidation number decreases from +5 to +4).

Step by step solution

01

(a) Check if equation (a) involves oxidation-reduction

For equation (a), first assign the oxidation numbers to each element in the reactants and products: \(NO_2\): N = +4, O = -2 \(H_2O_2\): H = +1, O = -1 \(HNO_3\): H = +1, N = +5, O = -2 Now compare the oxidation numbers before and after the reaction: - Nitrogen (N) has its oxidation number increased from +4 to +5 - Oxygen (O) has its oxidation number decreased from -1 to -2 Since there is a change in oxidation numbers, equation (a) involves an oxidation-reduction reaction. Nitrogen undergoes oxidation, and oxygen undergoes reduction.
02

(b) Check if equation (b) involves oxidation-reduction

For equation (b), assign oxidation numbers to each element in the reactants and products: \(FeS\): Fe = +2, S = -2 \(HCl\): H = +1, Cl = -1 \(FeCl_2\): Fe = +2, Cl = -1 \(H_2S\): H = +1, S = -2 Now compare the oxidation numbers before and after the reaction: - There is no change in the oxidation numbers of any element. Since there is no change in oxidation numbers, equation (b) does not involve an oxidation-reduction reaction.
03

(c) Check if equation (c) involves oxidation-reduction

For equation (c), assign oxidation numbers to each element in the reactants and products: \(Fe\): Fe = 0 \(HNO_3\): H = +1, N = +5, O = -2 \(H_2O\): H = +1, O = -2 \(NO_2\): N = +4, O = -2 \(FeO\): Fe = +2, O = -2 Now compare the oxidation numbers before and after the reaction: - Iron (Fe) has its oxidation number increased from 0 to +2 - Nitrogen (N) has its oxidation number decreased from +5 to +4 Since there is a change in oxidation numbers, equation (c) involves an oxidation-reduction reaction. Iron undergoes oxidation, and nitrogen undergoes reduction.

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Most popular questions from this chapter

Gold exists in two common positive oxidation states, +1 and +3 . The standard reduction potentials for these oxidation states are $$ \begin{array}{l} \mathrm{Au}^{+}(a q)+\mathrm{e}^{-} \quad \longrightarrow \mathrm{Au}(s) \quad E_{\mathrm{red}}^{\circ}=+1.69 \mathrm{~V} \\ \mathrm{Au}^{3+}(a q)+3 \mathrm{e}^{-} \longrightarrow \mathrm{Au}(s) E_{\mathrm{red}}^{\circ}=+1.50 \mathrm{~V} \end{array} $$ (a) Can you use these data to explain why gold does not tarnish in the air? (b) Suggest several substances that should be strong enough oxidizing agents to oxidize gold metal. (c) Miners obtain gold by soaking gold-containing ores in an aqueous solution of sodium cyanide. A very soluble complex ion of gold forms in the aqueous solution because of the redox reaction $$ \begin{aligned} 4 \mathrm{Au}(s)+8 \mathrm{NaCN}(a q) &+2 \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(g) \\ \longrightarrow & 4 \mathrm{Na}\left[\mathrm{Au}(\mathrm{CN})_{2}\right](a q)+4 \mathrm{NaOH}(a q) \end{aligned} $$ What is being oxidized, and what is being reduced in this reaction? (d) Gold miners then react the basic aqueous product solution from part (c) with \(\mathrm{Zn}\) dust to get gold metal. Write a balanced redox reaction for this process. What is being oxidized, and what is being reduced?

A voltaic cell similar to that shown in Figure 20.5 is constructed. One half- cell consists of an iron strip placed in a solution of \(\mathrm{FeSO}_{4}\), and the other has an aluminum strip placed in a solution of \(\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} .\) The overall cell reaction is $$ 2 \mathrm{Al}(s)+3 \mathrm{Fe}^{2+}(a q) \longrightarrow 3 \mathrm{Fe}(s)+2 \mathrm{Al}^{3+}(a q) $$ (a) What is being oxidized, and what is being reduced? (b) Write the half- reactions that occur in the two half-cells. (c) Which electrode is the anode, and which is the cathode? (d) Indicate the signs of the electrodes. (e) Do electrons flow from the aluminum electrode to the iron electrode or from the iron to the aluminum? (f) In which directions do the cations and anions migrate through the solution? Assume the \(\mathrm{Al}\) is not coated with its oxide.

Complete and balance the following half-reactions. In each case, indicate whether the half-reaction is an oxidation or a reduction. (a) $\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}(a q) \longrightarrow \mathrm{Cr}^{3+}(a q)$ (acidic solution) (b) \(\mathrm{Mn}^{2+}(a q) \longrightarrow \mathrm{MnO}_{4}^{-}(a q)\) (acidic solution) (c) \(\mathrm{I}_{2}(s) \longrightarrow \mathrm{IO}_{3}^{-}(a q)\) (acidic solution) (d) \(\mathrm{S}(s)(a q) \longrightarrow \mathrm{H}_{2} \mathrm{~S}(g)\) (acidic solution) (e) \(\mathrm{NO}_{3}^{-}(a q) \longrightarrow \mathrm{NO}_{2}^{-}(a q)\) (basic solution) (f) \(\mathrm{H}_{2} \mathrm{O}_{2}(a q) \longrightarrow \mathrm{OH}^{-}(a q)\) (basic solution)

(a) A \(\mathrm{Cr}^{3+}(a q)\) solution is electrolyzed, using a current of \(7.60 \mathrm{~A}\). What mass of \(\mathrm{Cr}(s)\) is plated out after 2.00 days? (b) What amperage is required to plate out $0.250 \mathrm{~mol} \mathrm{Cr}\( from a \)\mathrm{Cr}^{3+}\( solution in a period of \)8.00 \mathrm{~h}$ ?

(a) Which electrode of a voltaic cell, the cathode or the anode, corresponds to the higher potential energy for the electrons? (b) What are the units for electrical potential? How does this unit relate to energy expressed in joules?
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