Electrolysis and Fuel Cells: Key Concepts

Electrolysis Definition and Basics

  • Electrolysis: Breakdown of an ionic compound in molten or aqueous state by electricity.

  • Covalent compounds lack free charged particles; ionic compounds in solid state lack free ions, so they do not undergo electrolysis.

  • Electrolyte: Molten or aqueous ionic substance that conducts electricity.

  • Electrode: Rod (metal or graphite) conducting electricity into or out of the electrolyte.

  • Anode: Positive electrode; attracts anions. Site of oxidation (loss of electrons).

  • Cathode: Negative electrode; attracts cations. Site of reduction (gain of electrons).

  • Anion: Negatively charged ion; attracted to anode.

  • Cation: Positively charged ion; attracted to cathode.

Charge Transfer

  • External Circuit: Electrons are charge carriers, flowing from the negative terminal to the cathode.

  • Electrolyte: Ions are charge carriers, moving towards electrodes.

  • Electrons do not pass through the solution.

  • At cathode, cations gain electrons (reduction).

  • At anode, anions lose electrons (oxidation).

Electrolysis of Molten Ionic Compounds

  • Cathode Product: Always the metal from the cation. E.g., Pb2+(l)+2e−→Pb(l)\text{Pb}^{2+}(\text{l}) + 2\text{e}^- \rightarrow \text{Pb}(\text{l})

  • Anode Product: Always the non-metal from the anion. E.g., 2Br−(l)→Br2(g)+2e−\text{2Br}^-(\text{l}) \rightarrow \text{Br}_2(\text{g}) + 2\text{e}^-

Electrolysis of Aqueous Solutions

  • Water (H2O\text{H}_2\text{O}) ions (H+\text{H}^+ and \text{OH}^-$), along with compound ions, are present.

  • At Anode (Positive Electrode):

    • If concentrated halide ions (\text{Cl}^-,,\text{Br}^-,,\text{I}^-)arepresent:Halogenproduced.E.g.,) are present: Halogen produced. E.g.,\text{2Cl}^-(\text{aq}) \rightarrow \text{Cl}_2(\text{g}) + 2\text{e}^-</p></li><li><p>Ifnohalideions,ordilutehalideions:Oxygenproducedfrom</p></li><li><p>If no halide ions, or dilute halide ions: Oxygen produced from\text{OH}^-ions.E.g.,ions. E.g.,\text{4OH}^-(\text{aq}) \rightarrow \text{O}2(\text{g}) + 2\text{H}2\text{O}(\text{l}) + 4\text{e}^-</p></li></ul></li><li><p><strong>AtCathode(NegativeElectrode)</strong>:</p><ul><li><p>Ifmetalismorereactivethanhydrogen(aboveHinreactivityseries):Hydrogengas(</p></li></ul></li><li><p><strong>At Cathode (Negative Electrode)</strong>:</p><ul><li><p>If metal is more reactive than hydrogen (above H in reactivity series): Hydrogen gas (\text{H}^+)produced.E.g.,) produced. E.g.,\text{2H}^+(\text{aq}) + 2\text{e}^- \rightarrow \text{H}_2(\text{g})</p></li><li><p>Ifmetalislessreactivethanhydrogen(belowHinreactivityseries):Metalproduced.E.g.,</p></li><li><p>If metal is less reactive than hydrogen (below H in reactivity series): Metal produced. E.g.,\text{Cu}^{2+}(\text{aq}) + 2\text{e}^- \rightarrow \text{Cu}(\text{s})</p></li></ul></li></ul><h5id="f628e68d−cf12−4e06−b0f8−a9317f7ca2f4"data−toc−id="f628e68d−cf12−4e06−b0f8−a9317f7ca2f4"collapsed="false"seolevelmigrated="true">ElectrolysisofAqueousCopper(II)Sulfate(</p></li></ul></li></ul><h5 id="f628e68d-cf12-4e06-b0f8-a9317f7ca2f4" data-toc-id="f628e68d-cf12-4e06-b0f8-a9317f7ca2f4" collapsed="false" seolevelmigrated="true">Electrolysis of Aqueous Copper(II) Sulfate (\text{CuSO}_4)</h5><ul><li><p><strong>UsingInertElectrodes(e.g.,carbon/graphite)</strong>:</p><ul><li><p>Cathode:Coppermetaldeposits.Observation:Reddish−brownsolidforms.)</h5><ul><li><p><strong>Using Inert Electrodes (e.g., carbon/graphite)</strong>:</p><ul><li><p>Cathode: Copper metal deposits. Observation: Reddish-brown solid forms.\text{Cu}^{2+}(\text{aq}) + 2\text{e}^- \rightarrow \text{Cu}(\text{s})</p></li><li><p>Anode:Oxygengasproduced.Observation:Colorlessgasbubbles.</p></li><li><p>Anode: Oxygen gas produced. Observation: Colorless gas bubbles.\text{4OH}^-(\text{aq}) \rightarrow \text{O}2(\text{g}) + 2\text{H}2\text{O}(\text{l}) + 4\text{e}^-</p></li><li><p>Solution:Becomeslessblue(dueto</p></li><li><p>Solution: Becomes less blue (due to\text{Cu}^{2+}depletion)andslightlymoreacidic(depletion) and slightly more acidic (\text{OH}^-depletion).</p></li></ul></li><li><p><strong>UsingCopperElectrodes</strong>:</p><ul><li><p>Cathode:Coppermetaldeposits.Observation:Reddish−brownsolidforms.</p></li><li><p>Anode:Copperelectrodeitselfdissolves.Observation:Anodethins.depletion).</p></li></ul></li><li><p><strong>Using Copper Electrodes</strong>:</p><ul><li><p>Cathode: Copper metal deposits. Observation: Reddish-brown solid forms.</p></li><li><p>Anode: Copper electrode itself dissolves. Observation: Anode thins.\text{Cu}(\text{s}) \rightarrow \text{Cu}^{2+}(\text{aq}) + 2\text{e}^-</p></li><li><p>Solution:Bluecolorremainsunchanged(constant</p></li><li><p>Solution: Blue color remains unchanged (constant\text{Cu}^{2+}concentration).</p></li></ul></li></ul><h5id="45065854−a366−4269−83ac−ff116a60d609"data−toc−id="45065854−a366−4269−83ac−ff116a60d609"collapsed="false"seolevelmigrated="true">Electroplating</h5><ul><li><p><strong>Definition</strong>:Coatingametalsurfacewithalayerofadifferentmetal.</p></li><li><p><strong>Purpose</strong>:Improveappearance,enhancecorrosionresistance(e.g.,chromium,nickel,zincplating)ortopreventdamage.</p></li><li><p><strong>Setup</strong>:Objecttobeplatedisthecathode;pureplatingmetalistheanode;electrolyteisasolublesaltsolutionoftheplatingmetal.</p></li><li><p><strong>Reactions(e.g.,SilverPlating)</strong>:</p><ul><li><p>Cathode(concentration).</p></li></ul></li></ul><h5 id="45065854-a366-4269-83ac-ff116a60d609" data-toc-id="45065854-a366-4269-83ac-ff116a60d609" collapsed="false" seolevelmigrated="true">Electroplating</h5><ul><li><p><strong>Definition</strong>: Coating a metal surface with a layer of a different metal.</p></li><li><p><strong>Purpose</strong>: Improve appearance, enhance corrosion resistance (e.g., chromium, nickel, zinc plating) or to prevent damage.</p></li><li><p><strong>Setup</strong>: Object to be plated is the cathode; pure plating metal is the anode; electrolyte is a soluble salt solution of the plating metal.</p></li><li><p><strong>Reactions (e.g., Silver Plating)</strong>:</p><ul><li><p>Cathode (\text{spoon}):):\text{Ag}^+(\text{aq}) + \text{e}^- \rightarrow \text{Ag}(\text{s})</p></li><li><p>Anode(</p></li><li><p>Anode (\text{pure silver}):):\text{Ag}(\text{s}) \rightarrow \text{Ag}^+(\text{aq}) + \text{e}^-</p></li></ul></li></ul><h5id="ec1a6cd1−bc89−4ecd−aa0e−37256629d660"data−toc−id="ec1a6cd1−bc89−4ecd−aa0e−37256629d660"collapsed="false"seolevelmigrated="true">Hydrogen−OxygenFuelCells</h5><ul><li><p><strong>Function</strong>:Produceelectricityusinghydrogen(</p></li></ul></li></ul><h5 id="ec1a6cd1-bc89-4ecd-aa0e-37256629d660" data-toc-id="ec1a6cd1-bc89-4ecd-aa0e-37256629d660" collapsed="false" seolevelmigrated="true">Hydrogen-Oxygen Fuel Cells</h5><ul><li><p><strong>Function</strong>: Produce electricity using hydrogen (\text{H}2)andoxygen() and oxygen (\text{O}2).</p></li><li><p><strong>OverallReaction</strong>:).</p></li><li><p><strong>Overall Reaction</strong>:\text{2H}2(\text{g}) + \text{O}2(\text{g}) \rightarrow 2\text{H}_2\text{O}(\text{l})$$

    • Only Chemical Product: Water.

    • Advantages: Non-polluting (water product), continuous electricity generation if fuel supplied, no external charging.

    • Disadvantages: Difficult hydrogen storage (gas), lack of refueling infrastructure, less durable than traditional engines, high cost.