1.3Voltaic Cell
A voltaic cell produces electrical energy from spontaneous chemical redox reactions. When two different metals are immersed in an electrolyte or placed into their respective half-cells connected by a salt bridge or porous pot, electrons flow through an external circuit from the more negative electrode potential metal to the more positive one, generating an electric current.
Learning Objectives
- Explain redox reactions occurring in a simple chemical cell and a Daniell cell.
- Identify the negative terminal (anode) and positive terminal (cathode) based on standard electrode potential (\(E^0\)) values.
- State the direction of electron flow and electric current in a voltaic cell.
- Write half-equations for oxidation and reduction, as well as the overall ionic equation.
- Write cell notations representing voltaic cells accurately.
- Calculate standard cell voltage (\(E^0_{\text{cell}}\)) using standard electrode potentials.
Comparison of Simple Chemical Cell and Daniell Cell Set-ups
How electrodes, electrolytes, electron flow and current flow are arranged in a single beaker versus two half-cells connected by a salt bridge or porous pot.
Brief Explanation
Understand the key ideas of a voltaic cell before checking your understanding.
1. Simple Chemical Cell
A simple chemical cell consists of two different metal plates dipped into an electrolyte and connected with connecting wires. It converts chemical energy to electrical energy through a redox reaction that causes the flow of electrons.
Example: Magnesium, \(\text{Mg}\), has a more negative \(E^0\) value, so it acts as the negative terminal (anode). Oxidation occurs:
\[\text{Mg(s)} \rightarrow \text{Mg}^{2+}\text{(aq)} + 2e^-\]
Copper, \(\text{Cu}\), has a more positive \(E^0\) value, so it acts as the positive terminal (cathode). Reduction occurs:
\[\text{Cu}^{2+}\text{(aq)} + 2e^- \rightarrow \text{Cu(s)}\]
2. Daniell Cell
A Daniell cell is constructed by combining two half-cells with different \(E^0\) values. Zinc metal and copper metal are dipped into their respective ionic salt solutions, connected by a salt bridge or separated by a porous pot.
Zinc electrode (negative terminal / anode): Oxidation
\[\text{Zn(s)} \rightarrow \text{Zn}^{2+}\text{(aq)} + 2e^-\]
Copper electrode (positive terminal / cathode): Reduction
\[\text{Cu}^{2+}\text{(aq)} + 2e^- \rightarrow \text{Cu(s)}\]
Overall ionic equation:
\[\text{Zn(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Zn}^{2+}\text{(aq)} + \text{Cu(s)}\]
| Feature | Simple Chemical Cell | Daniell Cell |
|---|---|---|
| Set-up | Two different metals in one electrolyte | Two half-cells connected by a salt bridge or porous pot |
| Anode | Metal with more negative \(E^0\) | Zinc electrode, \(E^0 = -0.76\text{ V}\) |
| Cathode | Metal with more positive \(E^0\) | Copper electrode, \(E^0 = +0.34\text{ V}\) |
| Electron flow | From anode to cathode through the wire | From zinc electrode to copper electrode |
3. Cell Notation
The cell notation format represents a voltaic cell:
\[\text{Electrode(s)} \mid \text{Electrolyte(aq)} \parallel \text{Electrolyte(aq)} \mid \text{Electrode(s)}\]
Left side: Anode (negative terminal, oxidation half-cell).
Single vertical line (\(\mid\)): Phase boundary between electrode and solution.
Double vertical line (\(\parallel\)): Salt bridge or porous pot.
Right side: Cathode (positive terminal, reduction half-cell).
Cell notation for a Daniell cell:
\[\text{Zn(s)} \mid \text{Zn}^{2+}\text{(aq, 1.0 mol dm}^{-3}\text{)} \parallel \text{Cu}^{2+}\text{(aq, 1.0 mol dm}^{-3}\text{)} \mid \text{Cu(s)}\]
4. Determination of Cell Voltage (\(E^0_{\text{cell}}\))
Formula:
\[E^0_{\text{cell}} = E^0_{\text{(cathode)}} - E^0_{\text{(anode)}}\]
or
\[E^0_{\text{cell}} = E^0_{\text{(positive terminal)}} - E^0_{\text{(negative terminal)}}\]
Calculation for Daniell cell:
\[E^0_{\text{cell}} = (+0.34) - (-0.76) = +1.10\text{ V}\]
5. Experiment 1A: Determining the Voltage of a Voltaic Cell
Aim: To determine the voltage of a voltaic cell by using different pairs of metals.
Problem Statement: How can different pairs of metals dipped into electrolytes affect the voltage of the voltaic cell?
Hypothesis: The greater the difference of the value of standard electrode potential of pairs of metals, the greater the voltage reading.
Variables:
- Manipulated variable: Different pairs of metals.
- Responding variable: The voltage reading of the cell.
- Fixed variable: Volume and concentration of electrolytes.
Operational Definition of Chemical Cell: Voltmeter shows a reading when different types of pairs of metals are dipped into electrolytes.
Try to Answer First
Answer mentally, then check the explanation.
What is the function of the anode in a voltaic cell, and what type of reaction takes place at this electrode?
In a Daniell cell, in which direction do electrons flow through the connecting wire?
Given that \(E^0\) of \(\text{Zn}^{2+}/\text{Zn} = -0.76\text{ V}\) and \(E^0\) of \(\text{Cu}^{2+}/\text{Cu} = +0.34\text{ V}\), calculate the theoretical voltage of a Daniell cell.
Common Mistakes
Avoid these errors when working with voltaic cells.
Concept Misunderstandings
Compare each misconception with the correct fact.
The anode in all electrochemical cells is always positive.
In a voltaic cell, the anode is the negative terminal because it releases electrons into the external circuit via oxidation. (In electrolytic cells, the polarity convention is reversed).
The salt bridge or porous pot carries electrons between the two beakers.
Electrons travel only through the external metal wire; the salt bridge or porous pot allows the movement of ions to complete the circuit and maintain electrical neutrality.
Summary
Key facts to remember.
- A voltaic cell converts chemical energy into electrical energy via spontaneous redox reactions.
- Negative terminal (Anode): Metal with more negative \(E^0\), where oxidation occurs.
- Positive terminal (Cathode): Metal with more positive \(E^0\), where reduction occurs.
- Electrons flow from anode to cathode; electric current flows from cathode to anode.
- Daniell cell uses \(\text{Zn}/\text{Zn}^{2+}\) and \(\text{Cu}/\text{Cu}^{2+}\) half-cells connected by a salt bridge or porous pot.
- Cell notation: \(\text{Electrode(s)} \mid \text{Electrolyte(aq)} \parallel \text{Electrolyte(aq)} \mid \text{Electrode(s)}\).
- Cell voltage formula: \(E^0_{\text{cell}} = E^0_{\text{(cathode)}} - E^0_{\text{(anode)}}\).
Short Activity
Complete all parts before checking your score.
A. Objective Quiz
In a simple voltaic cell using magnesium and copper electrodes dipped in copper(II) chloride solution, why does magnesium act as the negative terminal?
Which of the following is the correct cell notation for a standard Daniell cell?
B. Fill in the Blanks
In a voltaic cell, electrons flow through the external connecting wire from the negative terminal to the ________ terminal.
The standard voltage of a Daniell cell with \(E^0(\text{Cu}^{2+}/\text{Cu}) = +0.34\text{ V}\) and \(E^0(\text{Zn}^{2+}/\text{Zn}) = -0.76\text{ V}\) is ________ V.
C. Match the Term with the Description
Drag each term on desktop. On mobile, tap a term and then tap the matching box.
Answer Choices
Anode Cathode Salt bridge Flow of currentTerm Matching
Keywords
Tap or hover over a term to read its meaning.
