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Chapter 20: Problem 77

(a) Suppose that an alkaline battery was manufactured using cadmium metal rather than zinc. What effect would this have on the cell emf? (b) What environmental advantage is provided by the use of nickel-metal hydride batteries over nickel-cadmium batteries?

Short Answer

Expert verified
Substituting cadmium for zinc in an alkaline battery would result in a lower cell emf, due to the higher standard reduction potential of cadmium (\(-0.403 V\)) compared to zinc (\(-0.763 V\)). The environmental advantage of using nickel-metal hydride (Ni-MH) batteries over nickel-cadmium (Ni-Cd) batteries is the less toxic nature of their components, reducing the environmental impact related to cadmium pollution and health risks when properly managed and disposed of.

Step by step solution

01

(a) Background on Alkaline Batteries)

Alkaline batteries are based on the electrochemical reaction that occurs between zinc (Zn) and manganese dioxide (MnO2) in an alkaline medium, typically containing potassium hydroxide (KOH). In this context, zinc acts as the anode, where its oxidation occurs, while the manganese dioxide serves as the cathode where the reduction half-reaction takes place.
02

(a) Finding the Standard Reduction Potentials)

To analyze the effect of substituting cadmium (Cd) for zinc as the anode, we must compare their standard reduction potentials, which is a measure of their tendency to gain electrons and undergo reduction. The standard reduction potential of cadmium is approximately \(-0.403 V\), while that of zinc is \(-0.763 V\).
03

(a) Comparing Standard Reduction Potentials)

Since the standard reduction potential of zinc \(-0.763 V\) is lower than that of cadmium \(-0.403 V\), this means that zinc is more prone to oxidation than cadmium. As a result, if an alkaline battery is manufactured using cadmium metal rather than zinc, it will have a lower cell emf because the difference between the cathode and anode potentials will be smaller, leading to a weaker driving force for the oxidation-reduction reaction.
04

(b) Environmental Impact of Nickel-Cadmium and Nickel-Metal Hydride Batteries)

The environmental advantage of using nickel-metal hydride (Ni-MH) batteries over nickel-cadmium (Ni-Cd) batteries is mainly due to the less toxic nature of nickel-metal hydride batteries components. Cadmium is a known toxic heavy metal that poses environmental and health risks when not properly managed, especially when nickel-cadmium batteries are not properly disposed of at the end of their life cycle. Cadmium can leach into soil and water, entering the food chain and causing significant harm to humans and wildlife. In contrast, nickel-metal hydride batteries contain less toxic metals and offer similar performance characteristics to nickel-cadmium batteries, making them a more environmentally friendly choice for many applications. By using nickel-metal hydride batteries, the environmental impact related to cadmium, such as pollution and health risks, can be reduced.

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Most popular questions from this chapter

Using the standard reduction potentials listed in Appendix E, calculate the equilibrium constant for each of the following reactions at \(298 \mathrm{~K}\) : (a) $\mathrm{Cu}(s)+2 \mathrm{Ag}^{+}(a q) \longrightarrow \mathrm{Cu}^{2+}(a q)+2 \mathrm{Ag}(s)$ (b) $3 \mathrm{Ce}^{4+}(a q)+\mathrm{Bi}(s)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 3 \mathrm{Ce}^{3+}(a q)+ \mathrm{BiO}^{+}(a q)+2 \mathrm{H}^{+}(a q)$ (c) $\mathrm{N}_{2} \mathrm{H}_{5}^{+}(a q)+4 \mathrm{Fe}(\mathrm{CN})_{6}^{3-}(a q) \longrightarrow \mathrm{N}_{2}(g)+ 5 \mathrm{H}^{+}(a q)+4 \mathrm{Fe}(\mathrm{CN})_{6}^{4-}(a q)$

A cell has a standard cell potential of \(+0.257 \mathrm{~V}\) at $298 \mathrm{~K}$. What is the value of the equilibrium constant for the reaction \((\mathbf{a})\) if \(n=1 ?(\mathbf{b})\) if \(n=2 ?(\mathbf{c})\) if \(n=3 ?\)

(a) Write the anode and cathode reactions that cause the corrosion of iron metal to aqueous iron(II). \((\mathbf{b})\) Write the balanced half-reactions involved in the air oxidation of \(\mathrm{Fe}^{2+}(a q)\) to $\mathrm{Fe}_{2} \mathrm{O}_{3} \cdot 3 \mathrm{H}_{2} \mathrm{O}(s)$.

Using standard reduction potentials (Appendix E), calculate the standard emf for each of the following reactions: (a) $\mathrm{Cl}_{2}(g)+2 \mathrm{I}^{-}(a q) \longrightarrow 2 \mathrm{Cl}^{-}(a q)+\mathrm{I}_{2}(s)$ (b) $\mathrm{Ni}(s)+2 \mathrm{Ce}^{4+}(a q) \longrightarrow \mathrm{Ni}^{2+}(a q)+2 \mathrm{Ce}^{3+}(a q)$ (c) $\mathrm{Fe}(s)+2 \mathrm{Fe}^{3+}(a q) \longrightarrow 3 \mathrm{Fe}^{2+}(a q)$ (d) $2 \mathrm{NO}_{3}^{-}(a q)+8 \mathrm{H}^{+}(a q)+3 \mathrm{Cu}(s) \longrightarrow 2 \mathrm{NO}(g)+ 4 \mathrm{H}_{2} \mathrm{O}(l)+3 \mathrm{Cu}^{2+}(a q)$

(a) Write the reactions for the discharge and charge of a nickel-cadmium (nicad) rechargeable battery. (b) Given the following reduction potentials, calculate the standard emf of the cell: $$ \begin{aligned} \mathrm{Cd}(\mathrm{OH})_{2}(s)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cd}(s)+2 \mathrm{OH}^{-}(a q) & \\ E_{\mathrm{red}}^{\circ} &=-0.76 \mathrm{~V} \\ \mathrm{NiO}(\mathrm{OH})(s)+\mathrm{H}_{2} \mathrm{O}(l)+\mathrm{e}^{-} \longrightarrow \mathrm{Ni}(\mathrm{OH})_{2}(s)+\mathrm{OH}^{-}(a q) \\ E_{\mathrm{red}}^{\circ} &=+0.49 \mathrm{~V} \end{aligned} $$ (c) A typical nicad voltaic cell generates an emf of \(+1.30 \mathrm{~V}\). Why is there a difference between this value and the one you calculated in part (b)? (d) Calculate the equilibrium constant for the overall nicad reaction based on this typical emf value.
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