Processes in Electrochemical Cells Revision Notes for Grade 12 NSC Matric Physical Sciences

Processes in Electrochemical Cells

Understanding half-cells and half-reactions

Electrochemical cells work through the interaction of two separate components called half-cells. Understanding how these half-cells function is essential for mastering electrochemical processes.

What is a half-cell?

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Half-cell: A half-cell consists of a metal electrode placed in a solution containing ions of the same metal.

For example, a copper half-cell contains a copper electrode immersed in a copper sulphate solution containing Cu²⁺ ions. Similarly, a zinc half-cell contains a zinc electrode in a zinc sulphate solution with Zn²⁺ ions.

How galvanic cells are constructed

A complete galvanic cell combines two different half-cells:

One half-cell contains the anode and its corresponding electrolyte
The other half-cell contains the cathode and its corresponding electrolyte
The half-cells are connected by a salt bridge
The electrodes are connected through an external circuit

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The salt bridge is crucial as it allows ion movement to maintain electrical neutrality while preventing the solutions from mixing directly.

Half-reactions in electrochemical cells

Each half-cell undergoes a specific type of reaction called a half-reaction. These reactions involve the transfer of electrons and determine the overall behaviour of the cell.

Reactions at the copper electrode (cathode)

At the copper electrode, there is an increase in mass because Cu²⁺ ions from the solution gain electrons and are deposited as solid copper atoms:

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

Since electrons are gained by the copper ions, this is the reduction half-reaction. The electrode where reduction occurs is called the cathode.

Reactions at the zinc electrode (anode)

At the zinc electrode, there is a decrease in mass because solid zinc atoms lose electrons and go into solution as Zn²⁺ ions:

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

Since electrons are lost by the zinc atoms, this is the oxidation half-reaction. The electrode where oxidation occurs is called the anode.

Key principles for identifying reactions

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Remember these essential rules:

Oxidation occurs at the anode (loss of electrons)
Reduction occurs at the cathode (gain of electrons)
The anode becomes negatively charged (due to excess electrons)
The cathode becomes positively charged (due to electron deficiency)

Combining half-reactions to find overall reactions

To determine the overall reaction in an electrochemical cell, you combine the two half-reactions from both half-cells.

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Step-by-step process:

Identify the oxidation and reduction half-reactions
Balance the electrons (ensure equal numbers are lost and gained)
Add the half-reactions together
Cancel out the electrons to get the overall equation

Example calculation

For a zinc-copper cell:

Oxidation half-reaction: $\text{Zn}(s) \rightarrow \text{Zn}^{2+}(aq) + 2e^-$
Reduction half-reaction: $\text{Cu}^{2+}(aq) + 2e^- \rightarrow \text{Cu}(s)$
Overall reaction: $\text{Zn}(s) + \text{Cu}^{2+}(aq) \rightarrow \text{Zn}^{2+}(aq) + \text{Cu}(s)$

Standard cell notation

Standard cell notation provides a shorthand way to represent electrochemical cells using specific symbols and conventions.

Notation format

$\text{Anode}|\text{Anode ions}||\text{Cathode ions}|\text{Cathode}$

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Symbol meanings:

| represents a phase boundary (solid/liquid interface)
|| represents the salt bridge
The anode is always written first (on the left)
The cathode is always written second (on the right)

Example notation

For the zinc-copper cell: $\text{Zn}(s)|\text{Zn}^{2+}(aq)||\text{Cu}^{2+}(aq)|\text{Cu}(s)$

Current and electron flow

Understanding the direction of current and electron flow is crucial for electrochemical cell analysis.

Electron flow direction

Electrons flow from the anode to the cathode through the external circuit
This occurs because the anode (where oxidation happens) produces excess electrons
The cathode (where reduction happens) consumes electrons

Conventional current direction

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Key difference: Conventional current flows from the cathode to the anode externally - this is opposite to electron flow direction. Current represents the flow of positive charge.

Worked example: Analysing a galvanic cell

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Worked Example: Galvanic Cell Analysis

Question: For the cell $\text{Zn}(s)|\text{Zn}^{2+}(aq)||\text{Ag}^+(aq)|\text{Ag}(s)$ :

Give the anode and cathode half-reactions
Write the overall equation
Determine current direction

Solution:

Step 1: Identify oxidation and reduction reactions

Zinc is the anode (written on left) - oxidation occurs
Silver is the cathode (written on right) - reduction occurs

Step 2: Write the half-reactions

Oxidation at anode: $\text{Zn}(s) \rightarrow \text{Zn}^{2+}(aq) + 2e^-$
Reduction at cathode: $\text{Ag}^+(aq) + e^- \rightarrow \text{Ag}(s)$

Step 3: Balance electrons and combine

The zinc half-reaction produces 2 electrons
The silver half-reaction consumes 1 electron
Multiply silver half-reaction by 2: $2\text{Ag}^+(aq) + 2e^- \rightarrow 2\text{Ag}(s)$
Overall reaction: $\text{Zn}(s) + 2\text{Ag}^+(aq) \rightarrow \text{Zn}^{2+}(aq) + 2\text{Ag}(s)$

Step 4: Determine current direction

Electrons flow from zinc (anode) to silver (cathode)
Conventional current flows from silver (cathode) to zinc (anode)

Worked example: Finding overall reactions from half-reactions

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Worked Example: Combining Half-Reactions

Question: Given these half-reactions:

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

Determine the overall reaction and standard cell notation.

Solution:

Step 1: Identify oxidation and reduction

$\text{Fe}(s) \rightarrow \text{Fe}^{2+}(aq) + 2e^-$ is oxidation (loss of electrons)
$\text{Cu}^{2+}(aq) + 2e^- \rightarrow \text{Cu}(s)$ is reduction (gain of electrons)

Step 2: Check electron balance

Both reactions involve 2 electrons, so they're already balanced

Step 3: Combine equations

$\text{Cu}^{2+}(aq) + 2e^- + \text{Fe}(s) \rightarrow \text{Cu}(s) + \text{Fe}^{2+}(aq) + 2e^-$
Cancel electrons: $\text{Cu}^{2+}(aq) + \text{Fe}(s) \rightarrow \text{Cu}(s) + \text{Fe}^{2+}(aq)$

Step 4: Write standard cell notation

Iron is the anode (oxidation), copper is the cathode (reduction)
Notation: $\text{Fe}(s)|\text{Fe}^{2+}(aq)||\text{Cu}^{2+}(aq)|\text{Cu}(s)$

Comparison between galvanic and electrolytic cells

Both types of electrochemical cells involve electron transfer, but they operate differently:

Galvanic cells

Spontaneous reactions occur naturally
Chemical energy converts to electrical energy
Anode is negative, cathode is positive
Used in batteries and fuel cells

Electrolytic cells

Non-spontaneous reactions require external energy
Electrical energy converts to chemical energy
Anode is positive, cathode is negative
Used in electroplating and electrolysis

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The key difference is that galvanic cells produce electricity from chemical reactions, while electrolytic cells use electricity to drive chemical reactions.

Exam tips for electrochemical processes

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Key Exam Strategies:

Always identify which electrode is the anode and which is the cathode first
Use the mnemonic "OIL RIG" - Oxidation Is Loss, Reduction Is Gain
Remember "Red Cat" - Reduction at Cathode
Check electron balance when combining half-reactions
In standard notation, anode is always written first
Current flows opposite to electron flow

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Key Points to Remember:

Half-cells are the building blocks of electrochemical cells, containing an electrode in a solution of its ions
Oxidation occurs at the anode (electron loss), reduction occurs at the cathode (electron gain)
Overall reactions are found by combining balanced half-reactions and cancelling electrons
Standard cell notation follows the format: Anode|Anode ions||Cathode ions|Cathode
Electron flow goes from anode to cathode, while conventional current flows from cathode to anode

Processes in Electrochemical Cells (Grade 12 NSC Matric Physical Sciences): Revision Notes