Q.ii. What is a salt bridge ?

Reduction is the gain of electrons; in the Daniell cell it is the $Cu^{2+}$ ion that gains electrons and deposits as copper metal.

Identifying the half-reactions

The overall cell reaction is: $Zn(s) + Cu^{2+}(aq) \rightarrow Zn^{2+}(aq) + Cu(s)$.

Splitting it: zinc metal is losing electrons to become $Zn^{2+}$ (oxidation), while $Cu^{2+}$ ions are gaining electrons to become copper metal (reduction).

Reduction half-reaction

$Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$

This occurs at the copper electrode, which acts as the cathode of the cell.

Oxidation is the loss of electrons; in the Daniell cell the zinc electrode loses electrons and dissolves into solution as $Zn^{2+}$.

Identifying the half-reaction

From the overall reaction $Zn(s)+Cu^{2+}(aq)\rightarrow Zn^{2+}(aq)+Cu(s)$, zinc metal is converted to $Zn^{2+}$ ions, which requires the loss of two electrons per Zn atom — this is oxidation.

Oxidation half-reaction

$Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$

This occurs at the zinc electrode, which acts as the anode of the cell (and is the negative terminal, since it is the source of electrons flowing through the external circuit).

The anode is, by definition, the electrode at which oxidation takes place — in the Daniell cell this is the zinc electrode.

Why zinc is the anode

In any galvanic cell, the anode is defined as the electrode where oxidation occurs. Since zinc metal loses electrons and goes into solution as $Zn^{2+}$ (as shown by the overall reaction $Zn(s)+Cu^{2+}(aq)\rightarrow Zn^{2+}(aq)+Cu(s)$), the zinc electrode is the site of oxidation.

Half-cell reaction at the anode

$Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$

The electrons released here travel through the external wire to the copper electrode, and in a galvanic cell the anode is also the negative electrode.

The cathode is, by definition, the electrode at which reduction takes place — in the Daniell cell this is the copper electrode.

Why copper is the cathode

In any galvanic cell, the cathode is defined as the electrode where reduction occurs. Since $Cu^{2+}$ ions gain electrons and are deposited as copper metal (from the overall reaction $Zn(s)+Cu^{2+}(aq)\rightarrow Zn^{2+}(aq)+Cu(s)$), the copper electrode is the site of reduction.

Half-cell reaction at the cathode

$Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$

The electrons arriving here (via the external wire, from the zinc anode) are consumed by the $Cu^{2+}$ ions. In a galvanic cell, the cathode is also the positive electrode.

A galvanic cell is an electrochemical device that generates electricity directly from a spontaneous chemical (redox) reaction by keeping oxidation and reduction physically apart.

Definition

A galvanic cell (also called a voltaic cell) is an electrochemical device in which the chemical energy released by a spontaneous ($\Delta G < 0$) redox reaction is converted directly into electrical energy. This is achieved by physically separating the oxidation half-reaction and the reduction half-reaction into two different half-cells (each containing an electrode dipped in an electrolyte), connected externally by a wire (through which electrons — and hence current — flow) and internally by a salt bridge (which completes the circuit and maintains electrical neutrality by allowing ion flow).

Example

The Daniell cell is the classic example: a zinc electrode in $ZnSO_4$ solution (anode, oxidation) connected via a salt bridge to a copper electrode in $CuSO_4$ solution (cathode, reduction), overall driven by $Zn(s)+Cu^{2+}(aq)\rightarrow Zn^{2+}(aq)+Cu(s)$, which spontaneously pumps electrons from the zinc rod, through the external circuit, to the copper rod.