Question

Draw a diagram for a salt bridge cell for each of the following reactions. Label the anode and cathode, and indicate the direction of current flow throughout the circuit. (a) \(\mathrm{Zn}(s)+\mathrm{Cd}^{2+}(a q) \longrightarrow \mathrm{Zn}^{2+}(a q)+\mathrm{Cd}(s)\) (b) \(2 \mathrm{AuCl}_{4}^{-}(a q)+3 \mathrm{Cu}(s) \longrightarrow 2 \mathrm{Au}(s)+8 \mathrm{Cl}^{-}(a q)+3 \mathrm{Cu}^{2+}(a q)\) (c) \(\mathrm{Fe}(s)+\mathrm{Cu}(\mathrm{OH})_{2}(s) \longrightarrow \mathrm{Cu}(s)+\mathrm{Fe}(\mathrm{OH})_{2}(s)\)

Short answer

Question: In each of the given reactions, identify the anode and cathode, and indicate the direction of current flow throughout the circuit. (a) \(\mathrm{Zn}(s)+\mathrm{Cd}^{2+}(a q) \longrightarrow \mathrm{Zn}^{2+}(a q)+\mathrm{Cd}(s)\) (b) \(2 \mathrm{AuCl}_{4}^{-}(a q)+3 \mathrm{Cu}(s) \longrightarrow 2 \mathrm{Au}(s)+8 \mathrm{Cl}^{-}(a q)+3 \mathrm{Cu}^{2+}(a q)\) (c) \(\mathrm{Fe}(s)+\mathrm{Cu}(\mathrm{OH})_{2}(s) \longrightarrow \mathrm{Cu}(s)+\mathrm{Fe}(\mathrm{OH})_{2}(s)\) Answer: (a) Anode: Zn, Cathode: Cd\(^{2+}\)/Cd, Current flow: left to right (b) Anode: Cu, Cathode: Au\(^{3+}\)/Au, Current flow: left to right (c) Anode: Fe, Cathode: Cu\(^{2+}\)/Cu, Current flow: left to right

Step-by-step solution

Identify the species at the anode and cathode

Since zinc is being oxidized, it loses electrons. Therefore, it acts as the anode. Conversely, cadmium ions are being reduced to form cadmium, so it acts as the cathode.

Draw the salt bridge cell diagram

In this cell, Zn is undergoing oxidation, and Cd\(^{2+}\) undergoing reduction. Place Zn as the left electrode and Cd\(^{2+}\)/Cd as the right electrode. Connect both by a wire and place a salt bridge between them. Label Zn as the anode and Cd\(^{2+}\)/Cd as the cathode.

Indicate the direction of current flow

In a voltaic cell, the electrons flow from the anode to the cathode through the external circuit. Therefore, indicate the current flow from left to right. (b) \(2 \mathrm{AuCl}_{4}^{-}(a q)+3 \mathrm{Cu}(s) \longrightarrow 2 \mathrm{Au}(s)+8 \mathrm{Cl}^{-}(a q)+3 \mathrm{Cu}^{2+}(a q)\)

Identify the species at the anode and cathode

Copper is being oxidized and loses electrons, so it acts as the anode. On the other hand, Au\(^{3+}\) ions (from \(\mathrm{AuCl}_{4}^-\)) are being reduced to form gold, so it acts as the cathode.

Draw the salt bridge cell diagram

Place Cu as the left electrode and Au\(^{3+}\)/Au as the right electrode. Connect both by a wire and place a salt bridge between them. Label Cu as the anode and Au\(^{3+}\)/Au as the cathode.

Indicate the direction of current flow

In a voltaic cell, the electrons flow from the anode to the cathode through the external circuit. Indicate the current flow from left to right. (c) \(\mathrm{Fe}(s)+\mathrm{Cu}(\mathrm{OH})_{2}(s) \longrightarrow \mathrm{Cu}(s)+\mathrm{Fe}(\mathrm{OH})_{2}(s)\)

Identify the species at the anode and cathode

Iron is being oxidized and loses electrons, so it acts as the anode. Meanwhile, copper(II) ions (from \(\mathrm{Cu(OH)_2}\)) are being reduced to form copper, so it acts as the cathode.

Draw the salt bridge cell diagram

Place Fe as the left electrode and Cu\(^{2+}\)/Cu as the right electrode. Connect both by a wire and place a salt bridge between them. Label Fe as the anode and Cu\(^{2+}\)/Cu as the cathode.

Indicate the direction of current flow

In a voltaic cell, the electrons flow from the anode to the cathode through the external circuit. Indicate the current flow from left to right.

Key concepts

Galvanic Cells

A galvanic cell, also known as a voltaic cell, is a device that generates electrical energy from a spontaneous chemical reaction. It's composed of two half-cells, each containing an electrode immersed in an electrolyte, joined together by a salt bridge.

Each half-cell is involved in a half-reaction: one electrode undergoes oxidation (loss of electrons), and the other electrode undergoes reduction (gain of electrons). The electrode where oxidation occurs is called the anode, and the electrode where reduction occurs is called the cathode.

The flow of electrons, driven by the cell's spontaneous reaction, is what we harness as electricity. Galvanic cells are the basis for batteries, where chemical energy is converted into electrical energy we use in countless applications.

Electrochemical Reactions

Electrochemical reactions in galvanic cells involve the transfer of electrons from one substance to another. These are typically redox reactions, which consist of two half-reactions: oxidation (the loss of electrons) and reduction (the gain of electrons).

To balance the flow of charge, a salt bridge is often used in galvanic cells. It allows ions to move between the two half-cells, preventing charge buildup and allowing the reaction to continue until the reactants are depleted.

The electrochemical reactions are spontaneous, meaning they occur without external energy. It is this spontaneity that enables galvanic cells to be used as batteries, providing power for numerous devices.

Anode and Cathode Identification

In galvanic cells, it is crucial to identify the anode and cathode correctly. The anode is always the electrode where oxidation (loss of electrons) occurs, while the cathode is the site of reduction (gain of electrons).

An easy mnemonic to remember is 'AnOx, RedCat' – anode is for oxidation, and cathode is for reduction. When identifying the anode and cathode from a chemical reaction, look for the substance that is losing electrons (oxidation) for the anode and the substance that is gaining electrons (reduction) for the cathode.

By convention, the anode of a galvanic cell is marked as negative, and the cathode as positive. This is because electrons flow out of the anode and into the cathode within the external circuit.

Direction of Current Flow

The direction of current flow in a galvanic cell is from the anode to the cathode externally, and from cathode to anode internally within the electrolyte. The flow of current is due to the movement of electrons, which move through the external circuit from the anode, where they are generated by oxidation, to the cathode, where they are consumed by reduction.

It's important to differentiate between electron flow and conventional current flow. While electrons flow from the negative anode to the positive cathode, conventional current flow, as defined by early scientists, is considered to move in the opposite direction, from the positive side (cathode) to the negative side (anode).

The salt bridge is key to maintaining the circuit, as it balances the charge by allowing ions to flow between both sides of the cell, ensuring uninterrupted current flow.