Define electrolysis .

An electrolysis is the breakdown of a compound into its elements using an electric current passed through a molten ionic compound or concentrated aqueous solution of ions.

True or False? During electrolysis, reduction takes place at the anode and oxidation takes place at the cathode.

False. Oxidation takes place at the anode (anions lose electrons). Reduction takes place at the cathode (cations gain electrons). Remember: RED CATS — REDuction at the CAThode.

What determines which cation is discharged at the cathode during electrolysis of an aqueous solution?

The cation with the most positive electrode potential ( E θ ) is discharged, as it is most easily reduced . If concentrations are high enough, the more concentrated ion may also be preferentially discharged.

During electrolysis, .......... ions move to the cathode where they gain electrons, and .......... ions move to the anode where they lose electrons.

During electrolysis, cations ions move to the cathode where they gain electrons, and anions ions move to the anode where they lose electrons.

Give the cathode and anode products when molten aluminium oxide (Al 2 O 3 ) is electrolysed.

Cathode : aluminium (Al) Anode : oxygen gas (O 2 ) Al 3+ ions are reduced at the cathode and O 2- ions are oxidised at the anode.

True or False? When a concentrated aqueous solution of NaCl is electrolysed, chlorine gas forms at the anode rather than oxygen gas.

True. At high Cl - concentrations, chloride ions are preferentially discharged at the anode over hydroxide ions, producing Cl 2 gas rather than O 2 .

An electrolyte is a molten ionic compound or concentrated aqueous solution of ions that conducts electricity and is broken down during electrolysis.

Why is water present in an aqueous electrolyte solution and what ions does it contribute?

Water is a weak electrolyte that partially ionises : H 2 O ⇌ H + + OH - . This adds H + and OH - ions to the solution, which compete with ions from the dissolved salt during electrolysis.

Platinum and graphite are used as electrodes in some electrolysis cells because they are .......... and do not take part in the reaction.

Platinum and graphite are used as electrodes in some electrolysis cells because they are inert and do not take part in the reaction.

Define Faraday's constant .

A Faraday's constant is the amount of electric charge carried by one mole of electrons or one mole of singly charged ions. Its value is 96 500 C mol -1 .

State the relationship between charge ( Q ), current ( I ) and time ( t ) used in electrolysis calculations.

Q = I x t where Q is in coulombs (C), I is in amperes (A) and t is in seconds (s).

True or False? One faraday of electricity is required to deposit one mole of magnesium (Mg 2+ ) at the cathode.

False. Mg 2+ + 2e - → Mg requires two moles of electrons, so two faradays (2 × 96 500 C) are needed to deposit one mole of magnesium.

The Faraday constant F is related to Avogadro's constant L and the charge on one electron e by: F = ..........

The Faraday constant F is related to Avogadro's constant L and the charge on one electron e by: F = L x e

Describe how Avogadro's constant ( L ) can be determined experimentally using electrolysis.

Copper electrodes are weighed before and after electrolysis of CuSO 4 (aq). The charge passed and the mass of copper deposited are measured. The charge per mole of electrons is calculated and divided by the charge on one electron (1.60 × 10 -19 C) to give L .

True or False? Four faradays of electricity are needed to form one mole of oxygen gas from the oxidation of OH - ions at the anode.

True. 4OH - (aq) → O 2 (g) + 2H 2 O (l) + 4e - . Four moles of electrons are released per mole of O 2 , so four faradays are required.

Define faraday (unit) .

A faraday is the unit of quantity of electricity equal to the charge on one mole of electrons: 1 F = 96 500 C mol -1 .

In an electrolysis experiment to determine Avogadro's constant, why is the mass decrease of the anode often used rather than the mass increase of the cathode?

The solid copper deposited at the cathode may not adhere properly to the cathode surface, leading to an underestimate in mass gained. The mass lost by the anode gives a more reliable measurement.

The amount of substance deposited at an electrode during electrolysis is proportional to both the .......... and the .......... of the constant current passed.

The amount of substance deposited at an electrode during electrolysis is proportional to both the quantity of charge and the time of the constant current passed.

Outline the four steps to calculate the mass of a substance deposited at an electrode during electrolysis.

Write the half-equation at the electrode. Determine the charge (in C) needed to deposit one mole using Faraday's constant . Calculate the charge transferred: Q = I x t . Use simple proportion with the molar mass to find the mass deposited.

The charge Q transferred during electrolysis is calculated using Q = .......... where I is in amperes and t is in .......... .

The charge Q transferred during electrolysis is calculated using Q = I × t where I is in amperes and t is in seconds .

True or False? To deposit one mole of silver (Ag + + e - → Ag) requires 193 000 C of electricity.

False. Ag + + e - → Ag involves only one mole of electrons, so only one faraday (96 500 C) is required to deposit one mole of silver.

How many coulombs are needed to deposit one mole of magnesium from Mg 2+ ions at the cathode?

Mg 2+ + 2e - → Mg requires 2 moles of electrons. Charge = 2 × 96 500 = 193 000 C

Define electrolysis calculation .

An electrolysis calculation is a method that uses the charge passed ( Q = I × t ), Faraday's constant and the half-equation to find the mass or volume of substance produced or deposited at an electrode.

True or False? At room temperature, one mole of any gas produced at an electrode during electrolysis occupies 24.0 dm 3 .

True. The molar volume of a gas at room temperature is 24.0 dm 3 mol -1 . This value is used when calculating volumes of gas liberated at electrodes.

A current of 0.50 A flows for 10 minutes through molten CaCl 2 . How many coulombs are transferred?

Q = I × t Q = 0.50 × (10 × 60) Q = 300 C

To produce one mole of O 2 gas at the anode (4OH - → O 2 + 2H 2 O + 4e - ), .......... C of electricity is required.

To produce one mole of O 2 gas at the anode (4OH - → O 2 + 2H 2 O + 4e - ), 386 000 C of electricity is required.

Why is the time converted to seconds (not minutes) when using Q = I × t in electrolysis calculations?

Because current is measured in amperes (A = C s -1 ) and the resulting charge Q must be in coulombs (C). Using minutes would give an incorrect value of Q and an incorrect mass or volume.

A LevelChemistryCambridge (CIE)Flashcards24. ElectrochemistryElectrolysis

Electrolysis (Cambridge (CIE) A Level Chemistry): Flashcards

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Define electrolysis.

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Cards in this collection (27)

Define electrolysis.
An electrolysis is the breakdown of a compound into its elements using an electric current passed through a molten ionic compound or concentrated aqueous solution of ions.
True or False?
During electrolysis, reduction takes place at the anode and oxidation takes place at the cathode.
False.
Oxidation takes place at the anode (anions lose electrons). Reduction takes place at the cathode (cations gain electrons). Remember: RED CATS — REDuction at the CAThode.
What determines which cation is discharged at the cathode during electrolysis of an aqueous solution?
The cation with the most positive electrode potential (E^θ) is discharged, as it is most easily reduced. If concentrations are high enough, the more concentrated ion may also be preferentially discharged.
During electrolysis, .......... ions move to the cathode where they gain electrons, and .......... ions move to the anode where they lose electrons.
During electrolysis, cations ions move to the cathode where they gain electrons, and anions ions move to the anode where they lose electrons.
Give the cathode and anode products when molten aluminium oxide (Al₂O₃) is electrolysed.
Cathode: aluminium (Al)
Anode: oxygen gas (O₂)
Al³⁺ ions are reduced at the cathode and O^2- ions are oxidised at the anode.
True or False?
When a concentrated aqueous solution of NaCl is electrolysed, chlorine gas forms at the anode rather than oxygen gas.
True.
At high Cl^- concentrations, chloride ions are preferentially discharged at the anode over hydroxide ions, producing Cl₂ gas rather than O₂.
Define electrolyte.
An electrolyte is a molten ionic compound or concentrated aqueous solution of ions that conducts electricity and is broken down during electrolysis.
Why is water present in an aqueous electrolyte solution and what ions does it contribute?
Water is a weak electrolyte that partially ionises: H₂O ⇌ H⁺ + OH^-. This adds H⁺ and OH^- ions to the solution, which compete with ions from the dissolved salt during electrolysis.
Platinum and graphite are used as electrodes in some electrolysis cells because they are .......... and do not take part in the reaction.
Platinum and graphite are used as electrodes in some electrolysis cells because they are inert and do not take part in the reaction.
Define Faraday's constant.
A Faraday's constant is the amount of electric charge carried by one mole of electrons or one mole of singly charged ions. Its value is 96 500 C mol^-1.
State the relationship between charge (Q), current (I) and time (t) used in electrolysis calculations.
Q = I x t
where Q is in coulombs (C), I is in amperes (A) and t is in seconds (s).
True or False?
One faraday of electricity is required to deposit one mole of magnesium (Mg²⁺) at the cathode.
False.
Mg²⁺ + 2e^- → Mg requires two moles of electrons, so two faradays (2 × 96 500 C) are needed to deposit one mole of magnesium.
The Faraday constant F is related to Avogadro's constant L and the charge on one electron e by: F = ..........
The Faraday constant F is related to Avogadro's constant L and the charge on one electron e by: F = L x e
Describe how Avogadro's constant (L) can be determined experimentally using electrolysis.
Copper electrodes are weighed before and after electrolysis of CuSO₄ (aq). The charge passed and the mass of copper deposited are measured. The charge per mole of electrons is calculated and divided by the charge on one electron (1.60 × 10^-19 C) to give L.
True or False?
Four faradays of electricity are needed to form one mole of oxygen gas from the oxidation of OH^- ions at the anode.
True.
4OH^- (aq) → O₂ (g) + 2H₂O (l) + 4e^-. Four moles of electrons are released per mole of O₂, so four faradays are required.
Define faraday (unit).
A faraday is the unit of quantity of electricity equal to the charge on one mole of electrons: 1 F = 96 500 C mol^-1.
In an electrolysis experiment to determine Avogadro's constant, why is the mass decrease of the anode often used rather than the mass increase of the cathode?
The solid copper deposited at the cathode may not adhere properly to the cathode surface, leading to an underestimate in mass gained. The mass lost by the anode gives a more reliable measurement.
The amount of substance deposited at an electrode during electrolysis is proportional to both the .......... and the .......... of the constant current passed.
The amount of substance deposited at an electrode during electrolysis is proportional to both the quantity of charge and the time of the constant current passed.
Outline the four steps to calculate the mass of a substance deposited at an electrode during electrolysis.
1. Write the half-equation at the electrode.
2. Determine the charge (in C) needed to deposit one mole using Faraday's constant.
3. Calculate the charge transferred: Q = I x t.
4. Use simple proportion with the molar mass to find the mass deposited.
The charge Q transferred during electrolysis is calculated using Q = .......... where I is in amperes and t is in .......... .
The charge Q transferred during electrolysis is calculated using Q = I × t where I is in amperes and t is in seconds.
True or False?
To deposit one mole of silver (Ag⁺ + e^- → Ag) requires 193 000 C of electricity.
False.
Ag⁺ + e^- → Ag involves only one mole of electrons, so only one faraday (96 500 C) is required to deposit one mole of silver.
How many coulombs are needed to deposit one mole of magnesium from Mg²⁺ ions at the cathode?
Mg²⁺ + 2e^- → Mg requires 2 moles of electrons.
Charge = 2 × 96 500 = 193 000 C
Define electrolysis calculation.
An electrolysis calculation is a method that uses the charge passed (Q = I × t), Faraday's constant and the half-equation to find the mass or volume of substance produced or deposited at an electrode.
True or False?
At room temperature, one mole of any gas produced at an electrode during electrolysis occupies 24.0 dm³.
True.
The molar volume of a gas at room temperature is 24.0 dm³ mol^-1. This value is used when calculating volumes of gas liberated at electrodes.
A current of 0.50 A flows for 10 minutes through molten CaCl₂. How many coulombs are transferred?
Q = I × t
Q = 0.50 × (10 × 60)
Q = 300 C
To produce one mole of O₂ gas at the anode (4OH^- → O₂ + 2H₂O + 4e^-), .......... C of electricity is required.
To produce one mole of O₂ gas at the anode (4OH^- → O₂ + 2H₂O + 4e^-), 386 000 C of electricity is required.
Why is the time converted to seconds (not minutes) when using Q = I × t in electrolysis calculations?
Because current is measured in amperes (A = C s^-1) and the resulting charge Q must be in coulombs (C). Using minutes would give an incorrect value of Q and an incorrect mass or volume.

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