1.2Standard Electrode Potential
In electrochemical cells, a potential difference develops when a metal is dipped into an aqueous solution of its ions. Because an absolute single electrode potential cannot be measured directly, scientists pair the half-cell with a standard reference known as the Standard Hydrogen Electrode (SHE). By convention, standard reduction potentials (\(E^0\)) allow us to compare the relative ease with which species gain or lose electrons, helping us determine the strength of oxidising and reducing agents and predict whether displacement reactions will occur.
Learning Objectives
- Describe the standard electrode potential.
- Determine oxidising agent and reducing agent based on their value of standard electrode potentials.
Summary of \(E^0\) Scale and Agent Strengths
Relate the sign and value of \(E^0\) to electron-donating or accepting tendency, oxidising/reducing strength, and the side of the half-equation.
Brief Explanation
Understand the key ideas behind standard electrode potential.
Standard Conditions for \(E^0\)
- Concentration of ions in aqueous solutions is \(1.0\text{ mol dm}^{-3}\).
- Gas pressure of \(1\text{ atm}\) or \(101\text{ kPa}\).
- Temperature at \(25\text{ }^\circ\text{C}\) or \(298\text{ K}\).
- Platinum is used as an inert electrode when a half-cell is not a metal electrode.
Standard Hydrogen Electrode (SHE)
Consists of a platinum electrode (\(\text{Pt}\)) coated with platinum(IV) oxide (\(\text{PtO}_2\)), dipped into an acid containing \(1.0\text{ mol dm}^{-3}\) of hydrogen ions (\(\text{H}^+\)), with hydrogen gas (\(\text{H}_2\)) at a pressure of \(1\text{ atm}\) channelled into the acid.
Function of \(\text{PtO}_2\): To increase the surface area of platinum to adsorb hydrogen gas, so that hydrogen molecules are in closer contact with hydrogen ions in the solution.
Half-equation: \(2\text{H}^+(\text{aq}) + 2e^- \rightleftharpoons \text{H}_2(\text{g})\)
Reference value: \(E^0 = 0.00\text{ V}\)
Determining \(E^0\) for the Zinc Half-cell
Dip a zinc electrode into a solution containing \(1.0\text{ mol dm}^{-3}\) \(\text{Zn}^{2+}\) ions. Connect this half-cell to the SHE through a voltmeter and a salt bridge containing saturated potassium nitrate solution.
- Zinc (negative terminal): \(\text{Zn}(\text{s}) \rightarrow \text{Zn}^{2+}(\text{aq}) + 2e^-\). Zinc releases electrons; electrons flow from Zn to Pt through the external wire.
- SHE (positive terminal): \(2\text{H}^+(\text{aq}) + 2e^- \rightarrow \text{H}_2(\text{g})\).
- The voltmeter reads \(0.76\text{ V}\). Since \(E^0_{\text{SHE}}=0.00\text{ V}\) and zinc is the negative terminal, the zinc reduction half-equation is written as \(\text{Zn}^{2+}(\text{aq}) + 2e^- \rightleftharpoons \text{Zn}(\text{s})\), \(E^0=-0.76\text{ V}\).
| Standard Electrode Potential Series | Meaning |
|---|---|
| More negative or less positive \(E^0\) | Species on the right (atoms/ions) have a greater tendency to release electrons. Easier to undergo oxidation. Act as stronger reducing agents. |
| More positive or less negative \(E^0\) | Species on the left (molecules/ions) have a greater tendency to accept electrons. Easier to undergo reduction. Act as stronger oxidising agents. |
Example 1: \(\text{Cu}\) in \(\text{AgNO}_3\) solution
\(\text{Cu}^{2+} + 2e^- \rightleftharpoons \text{Cu}\quad E^0 = +0.34\text{ V}\)
\(\text{Ag}^+ + e^- \rightleftharpoons \text{Ag}\quad E^0 = +0.80\text{ V}\)
\(E^0\) of \(\text{Cu}\) is more negative / less positive than \(E^0\) of \(\text{Ag}\). Therefore, \(\text{Cu}\) is a stronger reducing agent and \(\text{Ag}^+\) is a stronger oxidising agent.
Reaction occurs: \(\text{Cu}(\text{s}) + 2\text{AgNO}_3(\text{aq}) \rightarrow \text{Cu}(\text{NO}_3)_2(\text{aq}) + 2\text{Ag}(\text{s})\)
Example 2: \(\text{Cu}\) in \(\text{Mg}(\text{NO}_3)_2\) solution
\(\text{Mg}^{2+} + 2e^- \rightleftharpoons \text{Mg}\quad E^0 = -2.38\text{ V}\)
\(\text{Cu}^{2+} + 2e^- \rightleftharpoons \text{Cu}\quad E^0 = +0.34\text{ V}\)
\(E^0\) of \(\text{Cu}\) is more positive than \(E^0\) of \(\text{Mg}\); \(\text{Cu}\) is a weaker reducing agent and does not undergo oxidation. \(\text{Mg}^{2+}\) is a weak oxidising agent that cannot accept electrons from \(\text{Cu}\).
Displacement reaction does not occur.
Try These Questions First
Answer mentally, then check the explanation.
State four standard conditions required to measure standard electrode potential, \(E^0\).
What is the reference value assigned to the standard hydrogen electrode potential, and what is the role of platinum(IV) oxide coating on the platinum electrode?
Given \(E^0(\text{Zn}^{2+}/\text{Zn}) = -0.76\text{ V}\) and \(E^0(\text{Cu}^{2+}/\text{Cu}) = +0.34\text{ V}\), explain whether a displacement reaction occurs when zinc metal, \(\text{Zn}\), is placed in copper(II) sulfate solution.
Common Mistakes
Avoid these errors when working with standard electrode potentials.
Common Misconceptions
Compare each misconception with the correct fact.
An electrode potential of a single half-cell can be measured alone using a single probe or voltmeter.
An absolute single electrode potential cannot be measured directly; it can only be determined by measuring the potential difference between two half-cells connected together.
A negative \(E^0\) value means the reaction cannot take place.
The negative sign simply indicates that the electrode forms the negative terminal when coupled with the standard hydrogen electrode; it reflects a stronger tendency to donate electrons than hydrogen.
Summary
Key facts to remember.
- Electrode potential: Potential difference established between a metal and its aqueous ions at equilibrium.
- SHE: Reference standard with \(E^0 = 0.00\text{ V}\) using \(\text{Pt}\) coated with \(\text{PtO}_2\), \(1.0\text{ mol dm}^{-3}\text{ H}^+\), and \(\text{H}_2\) gas at \(1\text{ atm}\).
- Standard conditions: \(1.0\text{ mol dm}^{-3}\) ion concentration, \(1\text{ atm}\) / \(101\text{ kPa}\), \(25\text{ }^\circ\text{C}\) / \(298\text{ K}\), and inert \(\text{Pt}\) where necessary.
- \(E^0\) series: More negative \(E^0\) = stronger reducing agent (right side, releases electrons); more positive \(E^0\) = stronger oxidising agent (left side, gains electrons).
- Feasibility: A displacement reaction occurs spontaneously when a species with a more negative \(E^0\) donates electrons to a species with a more positive \(E^0\).
Quick Activity
Complete all parts before checking your score.
A. Objective Quiz
Which of the following is a standard condition used to measure the standard electrode potential, \(E^0\)?
The standard reduction potentials for two half-cells are given below:
\(\text{Mg}^{2+}(\text{aq}) + 2e^- \rightleftharpoons \text{Mg}(\text{s})\quad E^0 = -2.38\text{ V}\)
\(\text{Ag}^+(\text{aq}) + e^- \rightleftharpoons \text{Ag}(\text{s})\quad E^0 = +0.80\text{ V}\)
Which statement is correct?
B. Fill in the Blanks
By international consensus, the standard electrode potential of the standard hydrogen electrode is assigned a value of ______ V.
In the standard hydrogen electrode, the platinum electrode is coated with ______ to increase its surface area for adsorbing hydrogen gas.
C. Match the Substance or Property with Its Role
Drag each choice on desktop. On mobile, tap a choice and then tap the matching box.
Answer Choices
Species with more positive \(E^0\) Species with more negative \(E^0\) Salt bridge Negative sign of \(E^0\) in zinc half-cellRole / Description
Keywords
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