Question

Consider the molecular view of an electrochemical cell involving the overall reaction: $$ \mathrm{Zn}(s)+\mathrm{Ni}^{2+}(a q) \longrightarrow \mathrm{Zn}^{2+}(a q)+\mathrm{Ni}(s) $$ Draw a similar sketch showing how the cell might appear after it has generated a substantial amount of electrical current.

Short answer

After generating a substantial amount of current, the sketch should show a decrease in the mass of Zn at the anode and an increase in Zn concentration in the solution. Concurrently, there should be an increase in the mass of Ni deposited at the cathode and a decrease in Ni ion concentration in its solution.

Step-by-step solution

Understanding the Electrochemical Cell Reaction

Begin by understanding the given reaction \(\mathrm{Zn}(s) + \mathrm{Ni}^{2+}(aq) \longrightarrow \mathrm{Zn}^{2+}(aq) + \mathrm{Ni}(s)\).This represents a redox reaction where zinc metal is oxidized to zinc ions while nickel ions are reduced to nickel metal. The solid zinc (\(\mathrm{Zn}(s)\)) serves as the anode and the nickel ion (\(\mathrm{Ni}^{2+}(aq)\)) serves as the cathode in the electrochemical cell.

Drawing the Initial State of the Cell

Initially, the electrochemical cell has solid zinc wire or plate connected to the negative terminal and a nickel salt solution where \(\mathrm{Ni}^{2+}\) ions are present. The positive terminal is connected to a nickel electrode.At the anode, the reaction \(\mathrm{Zn}(s) \rightarrow \mathrm{Zn}^{2+}(aq) + 2e^-\) occurs, and at the cathode, the reaction \(\mathrm{Ni}^{2+}(aq) + 2e^- \rightarrow \mathrm{Ni}(s)\) happens.

Sketching the Cell after Generating Electrical Current

After some time, Zn at the anode will be depleted since the Zn is giving up electrons and going into solution as \(\mathrm{Zn}^{2+}\). The amount of solid Zn will decrease and the \(\mathrm{Zn}^{2+}\) concentration in solution will increase. At the cathode, solid Ni will be deposited out of solution as nickel ions gain electrons, so the amount of solid Ni will increase, and the \(\mathrm{Ni}^{2+}\) concentration will decrease.To illustrate, you could sketch less solid Zn at the anode, increased \(\mathrm{Zn}^{2+}\) in the surrounding solution, more solid Ni deposited at the cathode, and decreased \(\mathrm{Ni}^{2+}\) in the cathode's vicinity.

Key concepts

Redox Reaction

A redox reaction, short for reduction-oxidation reaction, is a chemical process involving the transfer of electrons between two species. It consists of two half-reactions: oxidation, where a substance loses electrons, and reduction, where a substance gains electrons.

For example, during the electrochemical cell reaction \( \mathrm{Zn}(s) + \mathrm{Ni}^{2+}(aq) \longrightarrow \mathrm{Zn}^{2+}(aq) + \mathrm{Ni}(s) \), zinc metal (\(\mathrm{Zn}(s)\)) loses two electrons to form zinc ions (\(\mathrm{Zn}^{2+}(aq)\)) while in the same reaction, nickel ions (\(\mathrm{Ni}^{2+}(aq)\)) gain two electrons to turn into nickel metal (\(\mathrm{Ni}(s)\)).

To visualize the steps in this redox process:\

• Oxidation at the anode: \(\mathrm{Zn}(s) \rightarrow \mathrm{Zn}^{2+}(aq) + 2e^-\)
• Reduction at the cathode: \(\mathrm{Ni}^{2+}(aq) + 2e^- \rightarrow \mathrm{Ni}(s)\)

These half-reactions show how electrons are transferred from zinc to nickel ions, highlighting the core of a redox reaction.

Oxidation and Reduction

In every redox process, oxidation and reduction occur simultaneously. Oxidation describes the loss of electrons, whereas reduction describes the gain of electrons. Each half-reaction in a redox process can be described separately as either an oxidation or a reduction process. The substance that loses electrons is called the 'reducing agent' since it causes something else to be reduced, and the substance that gains electrons is the 'oxidizing agent'.

Let's apply these concepts to the given electrochemical cell reaction: \( \mathrm{Zn}(s) \longrightarrow \mathrm{Zn}^{2+}(aq) + 2e^- \) demonstrates the oxidation process where zinc solid is transformed into zinc ions along with the release of electrons. Conversely, \( \mathrm{Ni}^{2+}(aq) + 2e^- \longrightarrow \mathrm{Ni}(s) \) is the reduction process where nickel ions accept electrons to form nickel metal.

By identifying the changes in oxidation states, students can better understand how the electrons are transferred and which species are oxidized or reduced during the electrochemical reaction.

Galvanic Cells

Galvanic cells, also called voltaic cells, are devices that convert chemical energy into electrical energy through spontaneous redox reactions. The cell comprises two half-cells, each containing an electrode and an electrolyte. The anode is where oxidation occurs, and the cathode is where reduction takes place. An external circuit connects the two electrodes, allowing electron flow from the anode to the cathode while the ionic conductive salt bridge completes the circuit internally.

In our exercise, zinc serves as the anode and nickel as the cathode. As the cell operates, zinc dissolves from the anode, and nickel is deposited at the cathode. This leads to a flow of electrons through the external circuit from zinc to nickel, powering any electrical devices connected to the cell. Over time, visible changes can include reduced zinc mass at the anode and increased nickel deposition at the cathode.

Understanding galvanic cells is crucial for comprehending how batteries work, and thus, represents a fundamental concept in both chemistry and many technological applications.