Electrolysis (Edexcel IGCSE Chemistry (Modular): Unit 2): Flashcards
Exam code: 4XCH1
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Cards in this collection (35)
What is electric current?
Electric current is the flow of charged particles, such as electrons or ions.
Why do most covalent compounds not conduct electricity?
Most covalent compounds do not conduct electricity because they have no freely moving charged particles to carry the current. They act as insulators.
True or False?
Ionic compounds can conduct electricity in the solid state.
False.
In the solid state, ions are in fixed positions in the lattice and cannot move, so no current can flow.
Define anion.
An anion is a negatively charged ion. During electrolysis, anions migrate towards the anode.
During electrolysis, .......... migrate towards the cathode and .......... migrate towards the anode.
During electrolysis, cations migrate towards the cathode and anions migrate towards the anode.
Define cation.
A cation is a positively charged ion. During electrolysis, cations migrate towards the cathode.
Why can ionic compounds conduct electricity when molten or dissolved in water?
Ionic compounds conduct when molten or dissolved because their ions are free to move and carry charge. In the solid state, ions are locked in fixed positions in the lattice.
True or False?
Covalent substances can be used as electrical insulators in solid, liquid and gaseous form.
True.
Covalent substances have no freely moving charged particles, so they cannot carry current. Examples include plastic cable coatings, silicone oils and sulfur hexafluoride gas.
What carries electric current in a metal compared to a salt solution?
In a metal, it is the electrons that move and carry current. In a salt solution, it is the ions in solution that move towards the electrodes.
What is a binary ionic compound?
A binary ionic compound is a compound made up of exactly two elements joined by ionic bonding. An example is lead(II) bromide.
What is produced at the cathode and anode during the electrolysis of molten lead(II) bromide?
At the cathode, positive lead ions gain electrons to form lead metal, deposited on the electrode. At the anode, negative bromide ions lose electrons to form bromine gas, which is brown.
True or False?
In aqueous electrolysis, halide ions are discharged at the cathode.
False.
Halide ions are negatively charged, so they migrate to the anode. At the anode they lose electrons and form a halogen, such as chlorine.
In the electrolysis of aqueous solutions, water molecules dissociate to produce .......... ions and .......... ions, which can be discharged at the electrodes.
In the electrolysis of aqueous solutions, water molecules dissociate to produce hydrogen ions and hydroxide ions, which can be discharged at the electrodes.
How is it decided whether hydrogen gas or a metal forms at the cathode in aqueous electrolysis?
If the metal ion is above hydrogen in the reactivity series, then hydrogen gas is produced at the cathode. If the metal ion is below hydrogen in the reactivity series, the metal is deposited instead.
Complete the table showing the products of electrolysis for each aqueous solution.
Solution
Cathode product
Anode product
Sodium chloride
Dilute sulfuric acid
Copper(II) sulfate
Complete the table showing the products of electrolysis for each aqueous solution.
Solution
Cathode product
Anode product
Sodium chloride
Hydrogen
Chlorine
Dilute sulfuric acid
Hydrogen
Oxygen
Copper(II) sulfate
Copper
Oxygen
True or False?
Graphite electrodes are used in electrolysis because they are inert and do not interfere with the reaction.
True.
Graphite is chemically inert, meaning it does not react with the electrolyte or the products formed at the electrodes.
What is produced at the anode during aqueous electrolysis when no halide ions are present?
When no halide ions are present, hydroxide (OH–) ions are discharged at the anode. They lose electrons and form oxygen gas.
What is an electrolyte?
An electrolyte is a substance that conducts electricity when molten or dissolved in water, due to the presence of freely moving ions.
What is oxidation in terms of electrons?
Oxidation is the loss of electrons by a substance. At the anode, negative ions lose electrons and are oxidised.
True or False?
At the cathode, positive ions are oxidised.
False.
At the cathode, positive ions gain electrons and are reduced. Oxidation (loss of electrons) occurs at the anode.
What is reduction in terms of electrons?
Reduction is the gain of electrons by a substance. At the cathode, positive ions gain electrons and are reduced.
What happens to negative ions when they reach the anode during electrolysis?
At the anode, negative ions lose electrons and are oxidised. This forms neutral substances that are discharged as products.
Oxidation is the .......... of electrons; reduction is the .......... of electrons.
Oxidation is the loss of electrons; reduction is the gain of electrons.
What is the half-equation at the cathode during electrolysis of molten lead(II) bromide?
The cathode half-equation is Pb2+ + 2e– → Pb. Lead ions gain two electrons and are reduced to form lead metal.
During electrolysis of aqueous sodium chloride, chloride ions lose electrons at the anode to form .......... gas.
During electrolysis of aqueous sodium chloride, chloride ions lose electrons at the anode to form chlorine gas.
Why are water molecules, rather than hydroxide ions, discharged at the anode during the electrolysis of copper(II) sulfate solution?
Copper(II) sulfate solution is slightly acidic, so it does not contain hydroxide ions. Instead, water molecules are discharged at the anode, forming oxygen gas.
True or False?
In a balanced half-equation, the total charge on each side must be equal.
True.
Balancing charges is a key rule when writing half-equations. Count up the total charge on each side to check they are equal.
What is produced at the cathode in the electrolysis of aqueous sodium chloride, and what is the half-equation?
Hydrogen gas is produced at the cathode. The half-equation is 2H+ + 2e– → H2. Hydrogen ions gain electrons and are reduced.
Complete the table showing observations during electrolysis.
Solution
Cathode observation
Anode observation
Sodium chloride
Dilute sulfuric acid
Copper(II) sulfate
Complete the table showing observations during electrolysis.
Solution
Cathode observation
Anode observation
Sodium chloride
Colourless gas (hydrogen — 'pop' with lighted splint)
Gas that bleaches litmus (chlorine)
Dilute sulfuric acid
Colourless gas (hydrogen — 'pop' with lighted splint)
Colourless gas (oxygen — relights glowing splint)
Copper(II) sulfate
Pink-brown deposit (copper)
Colourless gas (oxygen — relights glowing splint)
How do you test for hydrogen gas in an electrolysis experiment?
Hold a lighted splint near a sample of the gas. If it burns with a 'pop', the gas is hydrogen.
True or False?
Chlorine gas bleaches a piece of damp litmus paper.
True.
If the gas produced at the anode bleaches litmus paper, this confirms the gas is chlorine. This is the test used in the electrolysis practical.
How do you test for oxygen gas in an electrolysis experiment?
Insert a glowing splint into a sample of the gas. If the splint relights, the gas is oxygen.
In the electrolysis practical, .......... test tubes are placed over the electrodes to collect any .......... products.
In the electrolysis practical, inverted test tubes are placed over the electrodes to collect any gaseous products.
True or False?
The electrolysis of copper(II) sulfate solution produces chlorine at the anode.
False.
Copper(II) sulfate contains no halide ions, so oxygen is produced at the anode. A pink-brown deposit of copper forms at the cathode.
What observation is made at the cathode during electrolysis of copper(II) sulfate solution?
A pink-brown deposit forms on the cathode. This is copper metal, deposited as copper ions gain electrons and are reduced.
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