Unit 17: Electrochemistry
How separating an oxidation from its reduction forces electrons through a wire instead of straight across, and what running that trick backward buys you.
9 topics · 33 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
17.1 Balancing Redox Reactions by the Half-Reaction Method
- How it worksHow does splitting a redox reaction into separate oxidation and reduction half-reactions make each one easier to balance on its own?
- Worked exampleFind the balanced redox equation for the reaction between MnO4- and Fe2+ in acidic solution using the half-reaction method.
- Common mistakeWhy can't a redox equation ever be balanced without also balancing the electrons transferred between its two half-reactions?
Your turn1 of 2
Balancing a complex redox equation like MnO4- reacting with Fe2+ can feel overwhelming all at once. How does splitting it into separate oxidation and reduction half-reactions make it easier?17.2 Galvanic Cells and Cell Notation
- How it worksHow does physically separating the oxidation and reduction half-reactions force electrons to flow through an external wire?
- How it worksWalk me through how a salt bridge completes the circuit in a galvanic cell without letting the two solutions mix directly.
- Common mistakeWhy doesn't a galvanic cell run without a salt bridge or some other way to maintain charge balance?
- Going furtherWhat happens to a galvanic cell's operation once its salt bridge is removed or blocked?
Your turn1 of 2
A galvanic cell is set up with two half-cells connected by a wire, but the salt bridge (or another way to maintain charge balance) is left out. Why won't the cell actually run?17.2 Standard Reduction Potentials
- How it worksHow does comparing two half-reactions' standard reduction potentials tell you which one runs as a reduction and which as an oxidation?
- Worked exampleFind the standard cell potential for a galvanic cell built from Zn2+/Zn and Cu2+/Cu half-cells.
- Common mistakeWhy isn't a more negative standard reduction potential a sign of a stronger oxidizing agent?
Your turn1 of 2
A galvanic cell is built from a cathode half-cell with a standard reduction potential of 0.757 V and an anode half-cell with a standard reduction potential of -0.676 V. Find the standard cell potential (E cell = E cathode minus E anode).17.2 Cell Potential and Spontaneity
- How it worksHow does the sign of E cell connect directly to the sign of delta G for the same reaction?
- Worked exampleFind whether a reaction with a calculated standard cell potential of negative 0.46 V is spontaneous under standard conditions.
- Common mistakeWhy isn't a battery with a small cell potential automatically a weak battery in terms of the total energy it can deliver?
- Ties togetherHow does the relationship between cell potential and delta G connect back to the same thermodynamics that predicts any other reaction's spontaneity?
Your turn1 of 2
Find delta G naught, in kilojoules per mole, for a reaction with a standard cell potential of 0 V and n = 3 electrons transferred (delta G naught equals negative n times Faraday's constant times E cell; F = 96485 C/mol).17.3 The Nernst Equation
- How it worksHow does the Nernst equation adjust a cell's potential away from standard conditions as reactant and product concentrations change?
- Worked exampleFind the cell potential of a concentration cell built from two Cu2+/Cu half-cells at 1.0 M and 0.010 M.
- Common mistakeWhy isn't a cell's potential fixed forever at its standard value once the reaction starts running?
- Going furtherWhat happens to a cell's potential as the reaction proceeds and Q approaches K, right up until the battery goes dead?
Your turn1 of 2
Find the cell potential of a concentration cell built from two Cu2+/Cu half-cells (n = 2) at 1.54 M (concentrated) and 0.0653 M (dilute), using the Nernst equation E = negative (0.0592 divided by n) times log(Q), where Q = dilute concentration divided by concentrated concentration (E naught cell = 0 for a concentration cell).17.2 Relating Cell Potential to Gibbs Free Energy and K
- How it worksHow does the equation delta G equals negative nFE cell connect electrochemistry directly to the same free energy that governs every other reaction?
- Worked exampleFind the equilibrium constant for a redox reaction with a standard cell potential of 0.40 V and n = 2 at 298 K.
- Common mistakeWhy isn't the number of electrons transferred, n, something you can leave out of the delta G equals negative nFE cell equation?
Your turn1 of 2
Find the equilibrium constant for a redox reaction with a standard cell potential of 0.727 V and n = 3 at 298 K (delta G naught equals negative nFE cell, and also equals negative RT ln K; F = 96485 C/mol, R = 8.314 J per mol per K).17.5 Batteries and Practical Galvanic Cells
- How it worksHow does a real battery like the lead-acid cell package multiple galvanic cells together to reach a usable voltage?
- How it worksWalk me through the difference between a primary battery that's discarded and a secondary battery that's recharged.
- Common mistakeWhy isn't recharging a battery simply running the exact same reaction forward again from scratch?
- Going furtherWhat happens inside a rechargeable battery, chemically, when an external power source forces current backward through it?
Your turn1 of 2
Recharging a rechargeable (secondary) battery restores its ability to discharge again. Why isn't recharging simply running the exact same reaction forward again from scratch?17.6 Electrolytic Cells and Electrolysis
- How it worksHow does forcing an electric current through a nonspontaneous reaction drive electrolysis, the reverse of what a galvanic cell does?
- Worked exampleFind the mass of copper deposited when a 2.00 A current runs through a copper sulfate solution for 1.00 hour.
- Common mistakeWhy isn't an electrolytic cell just a galvanic cell that happens to be running backward on its own?
- Ties togetherHow does electrolysis's need for outside electrical energy connect back to it being the nonspontaneous, positive delta G counterpart of a galvanic cell?
Your turn1 of 2
Find the mass of copper (molar mass 63.55 g/mol; Cu2+ requires 2 mol electrons per mol of copper deposited) deposited when a 3.63 A current runs through a copper sulfate solution for 2.03 hours (1 Faraday = 96485 C/mol electrons).17.7 Corrosion and Its Prevention
- How it worksHow does rusting work as an unwanted galvanic cell, with iron oxidizing at one spot and oxygen being reduced at another?
- How it worksWalk me through how attaching a sacrificial block of zinc to a ship's hull protects the iron from corroding.
- Common mistakeWhy isn't painting a metal surface a permanent solution to corrosion the way a sacrificial anode can be?
- Going furtherWhat happens to the rate of corrosion once a more easily oxidized metal is placed in electrical contact with iron?
Your turn1 of 2
Shipbuilders attach blocks of zinc metal to a ship's iron hull. How does this sacrificial zinc block actually protect the iron from corroding?