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Chapter 20: Problem 39

Zinc reacts spontaneously with silver ion. $$ \mathrm{Zn}(s)+2 \mathrm{Ag}^{+}(a q) \longrightarrow \mathrm{Zn}^{2+}(a q)+2 \mathrm{Ag}(s) $$ Describe a voltaic cell using this reaction. What are the halfreactions?

Short Answer

Expert verified
The half-reactions are: Zn(s) 鈫� Zn虏鈦�(aq) + 2e鈦� (oxidation at anode) and 2Ag鈦�(aq) + 2e鈦� 鈫� 2Ag(s) (reduction at cathode). A voltaic cell is composed of these two half-cells with electron flow from zinc to silver.

Step by step solution

01

Identify the oxidation and reduction components

In a spontaneous reaction within a voltaic cell, oxidation and reduction occur simultaneously but in separate compartments called half-cells. Oxidation is the loss of electrons, and reduction is the gain of electrons. In this reaction, Zn(s) is oxidized to Zn虏鈦�(aq), while Ag鈦�(aq) is reduced to Ag(s).
02

Write the oxidation half-reaction

The oxidation half-reaction takes place at the anode, where zinc metal loses electrons. Write the half-reaction: \( \text{Zn}(s) \rightarrow \text{Zn}^{2+}(aq) + 2e^- \). This equation shows that zinc is oxidized as it loses two electrons.
03

Write the reduction half-reaction

The reduction half-reaction takes place at the cathode, where silver ions gain electrons. Write the half-reaction: \( 2\text{Ag}^{+}(aq) + 2e^- \rightarrow 2\text{Ag}(s) \). This indicates that silver ions are reduced as they each gain an electron.
04

Describe the voltaic cell components

A voltaic cell for this reaction consists of two half-cells. One contains a zinc electrode in a zinc sulfate solution (anode), and the other contains a silver electrode in a silver nitrate solution (cathode). The electrodes are connected by a wire to transfer electrons, and the solutions are connected via a salt bridge to maintain ionic balance.
05

Label the flow of electrons and ions

Electrons flow from the zinc anode to the silver cathode through the external circuit. The salt bridge allows ions to flow; sulfate ions move towards the anode, and nitrate ions move towards the cathode. This movement ensures electrical neutrality in both solutions.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Oxidation-Reduction Reactions
Oxidation-reduction reactions, often called redox reactions, are chemical processes in which electrons are transferred between substances. These reactions are fundamental in chemistry because they drive many processes, from burning fuels to generating electricity.
In a redox reaction, two half-processes occur:
  • Oxidation: A substance loses electrons and releases energy. For instance, in the zinc-silver cell reaction, zinc metal (Zn) undergoes oxidation as it loses two electrons to form zinc ions (\( ext{Zn}^{2+} \), aq)
  • Reduction: A substance gains electrons and consumes energy. In our example, silver ions (\( ext{Ag}^{+} \), aq) gain electrons becoming solid silver (Ag, s).
These reactions are coupled because the electrons lost in oxidation must be gained in reduction, ensuring that electrons are conserved.
Half-Reaction Equations
Half-reaction equations break down the full redox reaction into its oxidation and reduction components. This breakdown helps better understand the specific changes occurring at each part of a chemical process.
Let's dive into the half-reaction equations for the voltaic cell involving zinc and silver:
  • Oxidation half-reaction: This occurs at the anode. The half-reaction for zinc is \( ext{Zn}(s) \rightarrow ext{Zn}^{2+}(aq) + 2e^- \). It shows zinc metal is oxidized as it releases two electrons.
  • Reduction half-reaction: At the cathode, silver ions gain electrons. The equation \( 2 ext{Ag}^{+}(aq) + 2e^- \rightarrow 2 ext{Ag}(s) \) represents silver ions being reduced to form metallic silver.
These half-reactions offer a detailed glimpse into how electrons travel from one reactant to another, balancing the overall charge and mass in a chemical cell.
Electrode Components
In a voltaic cell, each half-reaction happens in separate but connected compartments, namely the electrodes. Understanding the role and composition of these components is crucial for grasping their function:
  • Anode: This is where oxidation takes place. In our zinc-silver voltaic cell, the anode consists of a zinc electrode placed in a solution of zinc sulfate. As the reaction progresses, zinc loses electrons, turning into zinc ions and dissolving in the solution.
  • Cathode: This electrode facilitates the reduction process. Here, a silver electrode is used, immersed in a silver nitrate solution. Silver ions are reduced to form solid silver on this electrode as they receive electrons from the anode.
To complete the circuit, a wire connects the anode and cathode, allowing electrons to flow. Additionally, a salt bridge or porous disk permits the movement of ions, maintaining charge balance and allowing the reaction to proceed smoothly without buildup of charge in either cell.

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Most popular questions from this chapter

Determine the emf of the following cell. $$ \mathrm{Pt}\left|\mathrm{H}_{2}(1.0 \mathrm{~atm})\right| \mathrm{H}^{+}(1.0 \mathrm{M}) \| \mathrm{Cl}^{-}(1.0 \mathrm{M}), \mathrm{AgCl}(s) \mid \mathrm{Ag} $$ The cathode is essentially a silver electrode, \(\mathrm{Ag}^{+}(a q) \mid \mathrm{Ag}\). However, the cathode solution is saturated with silver chloride, so that the silver-ion concentration is determined by the solubility product of \(\mathrm{AgCl}\left(K_{s p}=1.8 \times 10^{-10}\right)\).

The electrolysis of water is often done by passing a current through a dilute solution of sulfuric acid. What is the function of the sulfuric acid?

The amount of lactic acid, \(\mathrm{HC}_{3} \mathrm{H}_{5} \mathrm{O}_{3}\), produced in a sample of muscle tissue was analyzed by reaction with hydroxide ion. Hydroxide ion was produced in the sample mixture by electrolysis. The cathode reaction was $$ 2 \mathrm{H}_{2} \mathrm{O}(l)+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2}(g)+2 \mathrm{OH}^{-}(a q) $$ Hydroxide ion reacts with lactic acid as soon as it is produced. The endpoint of the reaction is detected with an acid-base indicator. It required \(115 \mathrm{~s}\) for a current of \(15.6 \mathrm{~mA}\) to reach the endpoint. How many grams of lactic acid (a monoprotic acid) were present in the sample?

A constant current of \(1.50\) amp is passed through an electrolytic cell containing a \(0.10 \mathrm{M}\) solution of \(\mathrm{AgNO}_{3}\) and a silver anode and a platinum cathode until \(2.48 \mathrm{~g}\) of silver is deposited. a. How long does the current flow to obtain this deposit? b. What mass of chromium would be deposited in a similar cell containing \(0.10 M \mathrm{Cr}^{3+}\) if the same amount of current were used?

Consider the reducing agents \(\mathrm{Sn}^{2+}(a q), \mathrm{Cu}(s)\), and \(\mathrm{I}^{-}(a q) .\) Which is strongest? Which is weakest?
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