18.10
Électrolyse
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Q1: How does electrolysis differ from a voltaic cell?
Electrolysis uses external electrical energy to drive nonspontaneous redox reactions, while voltaic cells convert spontaneous chemical reactions into electrical energy. In electrolysis, an external power source like a battery supplies electrons to the cathode and draws them from the anode. This reverses the natural electron flow, enabling separation of ionic compounds into their constituent elements.
Q2: What determines which ions are oxidized or reduced during electrolysis?
Standard electrode potentials predict which substance reacts during electrolysis. Anions with lower electrode potentials are oxidized at the anode, while cations with more positive electrode potentials are reduced at the cathode. In aqueous solutions, water may also participate, competing with dissolved ions based on their relative electrode potentials.
Q3: Why does electrolysis of aqueous potassium bromide produce different products than molten potassium bromide?
In molten potassium bromide, only bromide and potassium ions are present. In aqueous solution, water molecules can also be oxidized or reduced. Bromide oxidation to bromine is favored at the anode due to lower electrode potential, while water reduction to hydrogen gas is favored at the cathode, producing different products than the molten salt.
Q4: How is electrical current related to the amount of substance produced in electrolysis?
Electrical current measures the rate of electron flow in amperes (coulombs per second). The total charge transferred equals current multiplied by time. Using stoichiometry of half-reactions and Faraday's constant (96,485 coulombs per mole of electrons), you can calculate the moles of substance produced or consumed during electrolysis.
Q5: What is the role of the anode and cathode in an electrolytic cell?
The anode is connected to the positive terminal of the battery and is where oxidation occurs. The cathode is connected to the negative terminal and is where reduction occurs. The external power source draws electrons from the anode and drives them toward the cathode, forcing nonspontaneous redox reactions to proceed.
Q6: How much potassium metal is produced from electrolysis of molten potassium bromide?
One mole of electrons produces one mole of potassium atoms through reduction at the cathode. Since one mole of electrons equals 96,485 coulombs of charge, passing this amount of charge through the cell produces approximately 39 grams of solid potassium metal.
Q7: What are practical applications of electrolysis?
Electrolysis is used to separate ionic compounds into constituent elements, refine metallic ores, manufacture commodity chemicals, and electroplate metallic coatings onto objects like jewelry and utensils. Battery recharging also uses electrolysis, driving the discharge reaction in reverse to restore the battery's composition and voltage.