An electrolyte is an ionic compound in the molten state or dissolved in aqueous solution that can conduct electricity and undergoes decomposition when an electric current is passed through it.

Why can solid sodium chloride not conduct electricity, but molten sodium chloride can? Name the particles that carry the charge.

In the solid state, ions are held in fixed positions in the lattice and cannot move. When molten or dissolved in aqueous solution , the ions become free to move and carry charge. Writing 'electrons' as the moving particle is a common error that loses marks.

The anode is the ________ electrode. Positive ions are called ________ and move towards the ________ during electrolysis.

The anode is the positive electrode. Positive ions are called cations and move towards the cathode during electrolysis.

True or False? Covalent compounds can undergo electrolysis because they dissolve in water.

False. Covalent compounds have no ions: even when dissolved, they do not produce ions, so there are no charge carriers and electrolysis cannot occur.

During electrolysis of copper bromide solution, why do copper ions move to the cathode? What happens when they arrive?

Copper ions are positively charged and are therefore attracted to the negative electrode (cathode). At the cathode, the copper ions are discharged by gaining electrons and form copper atoms, seen as a pink or orange solid deposit.

During electrolysis, positive ions (cations) move towards the ________ and negative ions (anions) move towards the ________ . Electrons in the external circuit flow from the ________ to the ________ .

During electrolysis, positive ions (cations) move towards the cathode and negative ions (anions) move towards the anode . Electrons in the external circuit flow from the anode to the cathode .

In the electrolysis of a molten binary ionic compound, which type of element is always produced at the cathode and which at the anode ?

The metal is always produced at the cathode (negative electrode): metal cations are attracted there and gain electrons. The non-metal is always produced at the anode (positive electrode): non-metal anions are attracted there and lose electrons.

What are the products at each electrode during the electrolysis of molten lead(II) bromide? State one observation at each electrode.

Cathode: lead metal (grey solid deposits on the electrode). Anode: bromine gas is produced (brown gas; bubbling observed). The products are elements, not ions: write 'bromine', not 'bromide'; the ionic form is never accepted.

At the cathode, ions ________ electrons: this is ________ . At the anode, ions ________ electrons: this is ________ .

At the cathode, ions gain electrons: this is reduction . At the anode, ions lose electrons: this is oxidation .

True or False? During the electrolysis of molten potassium bromide, the product named at the positive electrode (anode) is 'bromide'.

False. The product is the element bromine (Br 2 ), not the ion 'bromide'. Students frequently lose marks by naming the ion rather than the element. 'Chloride' and 'bromide' are never accepted as electrode products in AQA mark schemes.

Why must the electrodes used in electrolysis be inert ? Give two acceptable materials.

Inert electrodes do not react with the electrolyte or the products of electrolysis. If reactive electrodes were used, they would take part in side reactions and contaminate the products. Acceptable materials: graphite and platinum .

In the electrolysis of molten lead(II) bromide: At the cathode, ________ ions gain electrons to form ________ metal. At the anode, ________ ions lose electrons to form ________ gas.

In the electrolysis of molten lead(II) bromide: At the cathode, Pb 2+ ions gain electrons to form lead metal. At the anode, Br - ions lose electrons to form bromine gas.

Why is aluminium extracted by electrolysis rather than by reduction with carbon?

Aluminium is more reactive than carbon , so carbon cannot reduce its ions. Saying 'too reactive' is too vague: the comparison with carbon must be stated explicitly. Metals below carbon in the reactivity series are extracted more cheaply by heating with carbon .

What is cryolite and why is it used in the extraction of aluminium?

Cryolite is a substance mixed with aluminium oxide in the electrolytic cell. It lowers the melting point of the mixture significantly: aluminium oxide alone melts above 2000°C, making direct melting impractical and extremely expensive. Cryolite does not interfere with the electrolysis reaction.

Metals ________ than carbon in the reactivity series are extracted by ________ . Metals ________ than carbon are extracted more cheaply by heating with ________ .

Metals more reactive/higher than carbon in the reactivity series are extracted by electrolysis . Metals less reactive/lower than carbon are extracted more cheaply by heating with carbon .

True or False? The graphite anodes used in the electrolytic extraction of aluminium are inert and do not need to be replaced.

False. Oxygen produced at the anode reacts with the carbon (graphite) electrode to form carbon dioxide, causing the anode to burn away. The anodes must be continually replaced. Stating only that CO 2 forms without first saying oxygen is produced is the most common mark-scheme error.

Give the three-step explanation for why the graphite anodes in aluminium extraction must be regularly replaced.

Oxygen is produced at the anode. The oxygen reacts with the carbon (graphite) electrode to form carbon dioxide . The electrode burns away and must be continually replaced .

In the extraction of aluminium: The ore ________ is purified to Al 2 O 3 It is then dissolved in ________ to lower the melting point. At the cathode: ________ + 3e⁻ → ________

In the extraction of aluminium: The ore bauxite is purified to Al 2 O 3 It is then dissolved in cryolite to lower the melting point. At the cathode: Al 3+ + 3e⁻ → Al

State two rules that determine which ions are discharged during the electrolysis of an aqueous solution.

Cathode (reactivity rule): The less reactive positive ion is discharged. If the metal is more reactive than hydrogen, H⁺ is discharged (H 2 produced); if less reactive, the metal is produced. Anode (halide preference): If halide ions are present, the halogen is produced. If not, OH⁻ ions are oxidised and O 2 is produced.

Why do aqueous solutions always contain H⁺ and OH⁻ ions, even if none were added directly?

Water molecules break down to produce H + and OH - ions: H 2 O ⇌ H + + OH - This equilibrium always exists in water, so these ions are present in all aqueous electrolytes and must be considered when predicting products.

In the electrolysis of copper chloride solution: At the cathode, ________ is produced because copper is ________ reactive than hydrogen. At the anode, ________ is produced because chloride ions are present.

In the electrolysis of copper chloride solution: At the cathode, copper is produced because copper is less reactive than hydrogen. At the anode, chlorine is produced because chloride ions are present.

Predict the products at each electrode in the electrolysis of sodium chloride solution . Briefly explain each choice.

Cathode: hydrogen gas. Na + ions are more reactive than H + , so H + is discharged: 2H + + 2e - → H 2 . Anode: chlorine gas (not 'chloride') Cl⁻ halide ions are present and discharged in preference to OH - : 2Cl - → Cl 2 + 2e -

Gas at the cathode that burns with a ________ when a lit splint is applied is ________ . Gas at the anode that relights a glowing splint is ________ . Gas at the anode that turns damp blue litmus red and then bleaches it white is ________ .

Gas at the cathode that burns with a pop when a lit splint is applied is hydrogen . Gas at the anode that relights a glowing splint is oxygen . Gas at the anode that turns damp blue litmus red and then bleaches it white is chlorine .

During the electrolysis of copper chloride solution , what is observed at the positive electrode and the negative electrode ?

Positive electrode (anode): fizzing / bubbles / effervescence as chlorine gas is produced. Negative electrode (cathode): a pink or orange solid forms as copper is deposited.

Give the gas test for hydrogen , oxygen , chlorine . State the exact equipment used in each test.

Hydrogen: lit (burning) splint → squeaky pop. Oxygen: glowing splint → relights. Chlorine: damp blue litmus paper → turns red, then bleaches white.

A common practical error is reading the ________ volume in the measuring cylinder rather than the ________ volume. The ratio of gas volumes H 2 :O 2 from electrolysing potassium sulfate solution is ________ , which matches the ratio of atoms in H 2 O.

A common practical error is reading the solution volume in the measuring cylinder rather than the gas volume. The ratio of gas volumes H 2 :O 2 from electrolysing potassium sulfate solution is 2:1 , which matches the ratio of atoms in H 2 O.

True or False? Stating 'use the same amount of solution' is a valid description of a control variable in an electrolysis investigation.

False. 'Amount' is insufficient and often rejected by mark schemes. You must state the concentration or volume of the solution. Similarly, the power supply must be described as 'same voltage / current', not just 'same electricity'.

Give two reasons why the mass of copper deposited at the negative electrode during an electrolysis experiment might differ from the theoretical value.

Possible reasons include: The electrode was not dry when weighed (inflates mass) Copper fell off the electrode (reduces mass) Error in timing (affects amount of charge passed) Concentration, volume, voltage, or current was different between repeats Impurities in the solution.

For the chlorine test, the litmus paper must be ________ ; it first turns ________ and then is ________ . For the oxygen test, a ________ splint is used (NOT a lit splint) and it ________ .

For the chlorine test, the litmus paper must be damp ; it first turns red and then is bleached white . For the oxygen test, a glowing splint is used (NOT a lit splint) and it relights .

In electrolysis, which electrode causes oxidation and which causes reduction ? Define each in terms of electrons.

Anode (positive electrode): causes oxidation : negative ions lose electrons and are discharged. Cathode (negative electrode): causes reduction : positive ions gain electrons and are discharged. OIL RIG: Oxidation Is Loss, Reduction Is Gain.

Write the half equation for the formation of chlorine at the anode. State two common errors students make when writing anode half equations.

2Cl⁻ → Cl 2 + 2e⁻ (also accepted: 2Cl⁻ – 2e⁻ → Cl 2 ; multiples allowed). Common errors include: Writing Cl instead of Cl 2 (chlorine is diatomic) Missing the negative sign on the electron, which fails to balance the charges. Forgetting to balance the Cl atoms and electrons

Complete the cathode half equations: Cu 2+ + __ e - → ____ 2H + + __ e - → ____ Al 3+ + __ e - → ____

Cu 2+ + 2 e - → Cu 2H + + 2 e - → H 2 Al 3+ + 3 e - → Al

True or False? To write the cathode half equation for the production of aluminium, you start with neutral aluminium (Al) on the left side of the equation.

False. The starting material must be the Al 3+ ion , not neutral aluminium: Al 3+ + 3e - → Al . Starting with Al is a common exam error.

How do you verify that a half equation is correctly balanced? Illustrate using 2Cl⁻ → Cl 2 + 2e⁻ .

Check two things: Atoms balanced: 2 Cl on the left, 2 Cl in Cl 2 on the right. Charges balanced: Left = 2 × (−1) = −2; right = 2 electrons × (−1) = −2. Both sides equal −2, so the equation is balanced.

Complete the anode half equations: 2Cl - → ____ + __ e - 4OH - → ____ + ____ + 4e -

2Cl - → Cl 2 + 2 e - 4OH - → O 2 + 2H 2 O + 4e -

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Electrolysis (AQA GCSE Combined Science: Trilogy: Chemistry): Flashcards

Exam code: 8464

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FrontThe Process of Electrolysis
Define electrolyte.

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Cards in this collection (35)

Define electrolyte.
An electrolyte is an ionic compound in the molten state or dissolved in aqueous solution that can conduct electricity and undergoes decomposition when an electric current is passed through it.
Why can solid sodium chloride not conduct electricity, but molten sodium chloride can? Name the particles that carry the charge.
In the solid state, ions are held in fixed positions in the lattice and cannot move. When molten or dissolved in aqueous solution, the ions become free to move and carry charge.
Writing 'electrons' as the moving particle is a common error that loses marks.
The anode is the ________ electrode. Positive ions are called ________ and move towards the ________ during electrolysis.
The anode is the positive electrode. Positive ions are called cations and move towards the cathode during electrolysis.
True or False?
Covalent compounds can undergo electrolysis because they dissolve in water.
False.
Covalent compounds have no ions: even when dissolved, they do not produce ions, so there are no charge carriers and electrolysis cannot occur.
During electrolysis of copper bromide solution, why do copper ions move to the cathode? What happens when they arrive?
Copper ions are positively charged and are therefore attracted to the negative electrode (cathode).
At the cathode, the copper ions are discharged by gaining electrons and form copper atoms, seen as a pink or orange solid deposit.
During electrolysis, positive ions (cations) move towards the ________ and negative ions (anions) move towards the ________.
Electrons in the external circuit flow from the ________ to the ________.
During electrolysis, positive ions (cations) move towards the cathode and negative ions (anions) move towards the anode.
Electrons in the external circuit flow from the anode to the cathode.
In the electrolysis of a molten binary ionic compound, which type of element is always produced at the cathode and which at the anode?
The metal is always produced at the cathode (negative electrode): metal cations are attracted there and gain electrons.
The non-metal is always produced at the anode (positive electrode): non-metal anions are attracted there and lose electrons.
What are the products at each electrode during the electrolysis of molten lead(II) bromide? State one observation at each electrode.
Cathode: lead metal (grey solid deposits on the electrode).
Anode: bromine gas is produced (brown gas; bubbling observed).
The products are elements, not ions: write 'bromine', not 'bromide'; the ionic form is never accepted.
At the cathode, ions ________ electrons: this is ________.
At the anode, ions ________ electrons: this is ________.
At the cathode, ions gain electrons: this is reduction.
At the anode, ions lose electrons: this is oxidation.
True or False?
During the electrolysis of molten potassium bromide, the product named at the positive electrode (anode) is 'bromide'.
False.
The product is the element bromine (Br₂), not the ion 'bromide'. Students frequently lose marks by naming the ion rather than the element. 'Chloride' and 'bromide' are never accepted as electrode products in AQA mark schemes.
Why must the electrodes used in electrolysis be inert? Give two acceptable materials.
Inert electrodes do not react with the electrolyte or the products of electrolysis. If reactive electrodes were used, they would take part in side reactions and contaminate the products. Acceptable materials: graphite and platinum.
In the electrolysis of molten lead(II) bromide:
At the cathode, ________ ions gain electrons to form ________ metal.
At the anode, ________ ions lose electrons to form ________ gas.
In the electrolysis of molten lead(II) bromide:
At the cathode, Pb²⁺ ions gain electrons to form lead metal.
At the anode, Br^- ions lose electrons to form bromine gas.
Why is aluminium extracted by electrolysis rather than by reduction with carbon?
Aluminium is more reactive than carbon, so carbon cannot reduce its ions.
Saying 'too reactive' is too vague: the comparison with carbon must be stated explicitly. Metals below carbon in the reactivity series are extracted more cheaply by heating with carbon.
What is cryolite and why is it used in the extraction of aluminium?
Cryolite is a substance mixed with aluminium oxide in the electrolytic cell. It lowers the melting point of the mixture significantly: aluminium oxide alone melts above 2000°C, making direct melting impractical and extremely expensive. Cryolite does not interfere with the electrolysis reaction.
Metals ________ than carbon in the reactivity series are extracted by ________.
Metals ________ than carbon are extracted more cheaply by heating with ________.
Metals more reactive/higher than carbon in the reactivity series are extracted by electrolysis.
Metals less reactive/lower than carbon are extracted more cheaply by heating with carbon.
True or False?
The graphite anodes used in the electrolytic extraction of aluminium are inert and do not need to be replaced.
False.
Oxygen produced at the anode reacts with the carbon (graphite) electrode to form carbon dioxide, causing the anode to burn away. The anodes must be continually replaced. Stating only that CO₂ forms without first saying oxygen is produced is the most common mark-scheme error.
Give the three-step explanation for why the graphite anodes in aluminium extraction must be regularly replaced.
1. Oxygen is produced at the anode.
2. The oxygen reacts with the carbon (graphite) electrode to form carbon dioxide.
3. The electrode burns away and must be continually replaced.
In the extraction of aluminium:
The ore ________ is purified to Al₂O₃
It is then dissolved in ________ to lower the melting point.
At the cathode:
________ + 3e⁻ → ________
In the extraction of aluminium:
The ore bauxite is purified to Al₂O₃
It is then dissolved in cryolite to lower the melting point.
At the cathode:
Al³⁺ + 3e⁻ → Al
State two rules that determine which ions are discharged during the electrolysis of an aqueous solution.
Cathode (reactivity rule): The less reactive positive ion is discharged. If the metal is more reactive than hydrogen, H⁺ is discharged (H₂ produced); if less reactive, the metal is produced.
Anode (halide preference): If halide ions are present, the halogen is produced. If not, OH⁻ ions are oxidised and O₂ is produced.
Why do aqueous solutions always contain H⁺ and OH⁻ ions, even if none were added directly?
Water molecules break down to produce H⁺ and OH^- ions:
H₂O ⇌ H⁺ + OH^-
This equilibrium always exists in water, so these ions are present in all aqueous electrolytes and must be considered when predicting products.
In the electrolysis of copper chloride solution:
At the cathode, ________ is produced because copper is ________ reactive than hydrogen.
At the anode, ________ is produced because chloride ions are present.
In the electrolysis of copper chloride solution:
At the cathode, copper is produced because copper is less reactive than hydrogen.
At the anode, chlorine is produced because chloride ions are present.
Predict the products at each electrode in the electrolysis of sodium chloride solution. Briefly explain each choice.
Cathode: hydrogen gas.
Na⁺ ions are more reactive than H⁺, so H⁺ is discharged: 2H⁺ + 2e^- → H₂.
Anode: chlorine gas (not 'chloride')
Cl⁻ halide ions are present and discharged in preference to OH^-: 2Cl^- → Cl₂ + 2e^-
Gas at the cathode that burns with a ________ when a lit splint is applied is ________.
Gas at the anode that relights a glowing splint is ________.
Gas at the anode that turns damp blue litmus red and then bleaches it white is ________.
Gas at the cathode that burns with a pop when a lit splint is applied is hydrogen.
Gas at the anode that relights a glowing splint is oxygen.
Gas at the anode that turns damp blue litmus red and then bleaches it white is chlorine.
During the electrolysis of copper chloride solution, what is observed at the positive electrode and the negative electrode?
Positive electrode (anode): fizzing / bubbles / effervescence as chlorine gas is produced.
Negative electrode (cathode): a pink or orange solid forms as copper is deposited.
Give the gas test for hydrogen, oxygen, chlorine. State the exact equipment used in each test.
Hydrogen: lit (burning) splint → squeaky pop.
Oxygen: glowing splint → relights.
Chlorine: damp blue litmus paper → turns red, then bleaches white.
A common practical error is reading the ________ volume in the measuring cylinder rather than the ________ volume.
The ratio of gas volumes H₂:O₂ from electrolysing potassium sulfate solution is ________, which matches the ratio of atoms in H₂O.
A common practical error is reading the solution volume in the measuring cylinder rather than the gas volume.
The ratio of gas volumes H₂:O₂ from electrolysing potassium sulfate solution is 2:1, which matches the ratio of atoms in H₂O.
True or False?
Stating 'use the same amount of solution' is a valid description of a control variable in an electrolysis investigation.
False.
'Amount' is insufficient and often rejected by mark schemes. You must state the concentration or volume of the solution. Similarly, the power supply must be described as 'same voltage / current', not just 'same electricity'.
Give two reasons why the mass of copper deposited at the negative electrode during an electrolysis experiment might differ from the theoretical value.
Possible reasons include:
- The electrode was not dry when weighed (inflates mass)
- Copper fell off the electrode (reduces mass)
- Error in timing (affects amount of charge passed)
- Concentration, volume, voltage, or current was different between repeats
- Impurities in the solution.
For the chlorine test, the litmus paper must be ________; it first turns ________and then is ________.
For the oxygen test, a ________splint is used (NOT a lit splint) and it ________.
For the chlorine test, the litmus paper must be damp; it first turns red and then is bleached white.
For the oxygen test, a glowing splint is used (NOT a lit splint) and it relights.
In electrolysis, which electrode causes oxidation and which causes reduction? Define each in terms of electrons.
Anode (positive electrode): causes oxidation: negative ions lose electrons and are discharged.
Cathode (negative electrode): causes reduction: positive ions gain electrons and are discharged.
OIL RIG: Oxidation Is Loss, Reduction Is Gain.
Write the half equation for the formation of chlorine at the anode.
State two common errors students make when writing anode half equations.
2Cl⁻ → Cl₂ + 2e⁻ (also accepted: 2Cl⁻ – 2e⁻ → Cl₂; multiples allowed).
Common errors include:
1. Writing Cl instead of Cl₂ (chlorine is diatomic)
2. Missing the negative sign on the electron, which fails to balance the charges.
3. Forgetting to balance the Cl atoms and electrons
Complete the cathode half equations:
Cu²⁺ + __e^- → ____
2H⁺ + __e^- → ____
Al³⁺ + __e^- → ____
Cu²⁺ + 2e^- → Cu
2H⁺ + 2e^- → H₂
Al³⁺ + 3e^- → Al
True or False?
To write the cathode half equation for the production of aluminium, you start with neutral aluminium (Al) on the left side of the equation.
False.
The starting material must be the Al³⁺ ion, not neutral aluminium:
Al³⁺ + 3e^- → Al.
Starting with Al is a common exam error.
How do you verify that a half equation is correctly balanced? Illustrate using 2Cl⁻ → Cl₂ + 2e⁻.
Check two things:
1. Atoms balanced:
  2 Cl on the left, 2 Cl in Cl₂ on the right.
2. Charges balanced:
  Left = 2 × (−1) = −2; right = 2 electrons × (−1) = −2.
  Both sides equal −2, so the equation is balanced.
Complete the anode half equations:
2Cl^- → ____ + __e^-
4OH^- → ____ + ____ + 4e^-
2Cl^- → Cl₂ + 2e^-
4OH^- → O₂ + 2H₂O + 4e^-

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