Chemistry IGCSE - Electrochemistry

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20 Terms

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Electrolysis

decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current

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Mnemonic for electrodes

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Electrolyte

the molten or aqueous substance that undergoes electrolysis

<p>the molten or aqueous substance that undergoes electrolysis</p>
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Electrolysis of molten lead(II) bromide (PbBr₂)

  • Cathode (–): Pb²⁺ + 2 e⁻ → Pb(l)

    Lead metal (solid) collects at the cathode

  • Anode (+): 2 Br⁻ → Br₂(g) + 2 e⁻

    Reddish brown bromine gas

<ul><li><p><strong>Cathode (–):</strong> Pb²⁺ + 2 e⁻ → Pb(l)</p><p>Lead metal (solid) collects at the cathode</p></li><li><p><strong>Anode (+):</strong> 2 Br⁻ → Br₂(g) + 2 e⁻</p><p>Reddish brown bromine gas </p></li></ul><p></p>
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Electrolysis of concentrated aqueous sodium chloride (NaCl)

  • Cathode (–): 2 H₂O + 2 e⁻ → H₂(g) + 2 OH⁻

    Hydrogen bubbles at cathode

  • Anode (+): 2 Cl⁻ → Cl₂(g) + 2 e⁻

    Green - yellow chlorine gas bubbles at anode

<ul><li><p><strong>Cathode (–):</strong> 2 H₂O + 2 e⁻ → H₂(g) + 2 OH⁻</p><p>Hydrogen bubbles at cathode</p></li><li><p><strong>Anode (+):</strong> 2 Cl⁻ → Cl₂(g) + 2 e⁻</p><p>Green - yellow chlorine gas bubbles at anode</p></li></ul><p></p>
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Electrolysis of dilute sulfuric acid (H₂SO₄)

  • Cathode (–): 2 H₂O + 2 e⁻ → H₂(g) + 2 OH⁻

    Bubbles of hydrogen gas at cathode

  • Anode (+): 2 H₂O → O₂(g) + 4 H⁺ + 4 e⁻

    Oxygen bubbles at anode

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Describe transfer of charge during electrolysis: the movement of electrons in the external circuit

electrons leave battery’s negative side → travel down the external wire → into the cathode (-) → out of the anode (+) → back to the battery’s positive side

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Describe transfer of charge during electrolysis: the loss or gain of electrons at the electrodes

  • Cathode (-) ⟶ + ions from the molten or aqueous electrolyte gain electrons ⟶ reduction

  • Anode (+): - ions give up electrons ⟶ oxidation

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Describe transfer of charge during electrolysis: the movement of ions in the electrolyte

cations (+) move to cathode (-) ⟶ anions (-) move to anode (+) through the electrolyte

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What is produced using aqueous copper (II) sulfate (CuSO₄)

  • Cathode (-): Copper metal ( Cu²⁺ + 2 e⁻ → Cu(s) )

    Reddish brown deposit of copper forms

  • Anode (+): Oxygen gas (4 OH⁻ → O₂(g) + 2 H₂O + 4 e⁻)

    Bubbles of oxygen gas appear

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What is produced using aqueous copper (II) sulfate (CuSO₄) with copper electrodes

  • Cathode (-): Copper metal (Cu²⁺ + 2 e⁻ → Cu(s))

    Cathode stays shiny or gains a fresh copper coating

  • Anode (+): Copper metal is oxidized back to Cu²⁺ (Cu(s) → Cu²⁺ + 2 e⁻)

    Copper anode slowly dissolves → mass decreases → solution stays blue.

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metals or hydrogen are formed at the cathode (-)

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Non metals (OTHER THAN HYDROGEN) are formed at the anode (+)

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Cathode (+) → reduction → (RED CAT)

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Anode (-) → oxidation

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Order of reactivity

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If metal is more reactive than hydrogen (ONLY FOR AQUEOUS SOLUTION) → hydrogen gas discharge at the cathode (-)

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In DILUTE solutions → oxygen produced at the anode (+)

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Hydrogen - oxygen fuel cell

uses hydrogen + oxygen → produce electricity with water as the only chemical product

<p> uses hydrogen + oxygen → produce electricity with water as the only chemical product</p>
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Pros + cons of a hydrogen - oxygen fuel cell

Pros

Cons

  • Only water is produced → it's non polluting

  • Hydrogen is renewable if made using renewable electricity

  • Fuel cells are more efficient than petrol engines

  • Hydrogen is hard to store → it’s a gas → needs special tanks

  • Few refuelling stations for hydrogen compared to petrol

  • Making hydrogen requires electricity → may come from fossil fuels