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Flashcards in Electrolysis Deck (42)

What is an electric current?

A flow of electrons or ions


What is an electrolyte?

A substance that conducts electricity and is decomposed by it


Why do ionic compounds conduct electricity only when molten or in a solution?

A solid does not have any mobile electrons and the ions are locked tightly in a lattice and aren't free to move so does not conduct electricity. As soon as the solid melts the ions do become free to move around and it is this movement that enables the electrons to flow in the external circuit.


What is electrolysis?

Electrolysis is a chemical change caused by passing an electronic current through a compound which is either molten or in a solution.


Why does reduction take place at the negative electrode?

Because here the positively charged ions are gaining electrons from the negatively charged ions and are reduced


Why does oxidation take place at the positive electrode?

Because here the negatively charged ions are giving away electrons to the positively charged ions so are oxidised


What happens at the cathode?
H+ vs M+ where M is a metal

If the metal is higher than hydrogen in the reactivity series then H+ is discharged and H2 is formed


What happens at the anode?
OH- vs other negative ions

OH- will be discharged unless the negative ion present is a halide eg. Cl-, Br-, I-


What is the half equation for OH-?

4OH- = 2H2O + O2 + 4e-


What is the half equation for hydrogen?

2H+ + 2e- = H2


What is the half equation for bromine? (Or other halides)

2Br- = Br2 + 2e-


What is the half equation for copper?

Cu2+ + 2e- = Cu NOT 2Cu!!!


What are the products of the electrolysis of sodium chloride?

Sodium hydroxide


What is the point of the diaphragm in the electrolysis of sodium chloride?

It separates the products (Cl2 from H2 and NaOH) and prevents unwanted reactions


What unwanted reactions does the diaphragm prevent?

H2 + Cl2 = 2HCl (very explosive and exothermic)
2NaOH + Cl2 = NaCl + H2O + NaOCl (BLEACH) and (we don't want NaCl again)


What are the uses of sodium hydroxide?

Manufacture of soap, paper and bleach


What are the uses of chlorine?

Making bleach
Sterilising water to make it safe to drink


What are the two electrodes placed in in the purification of copper?

The two electrodes are placed in a solution of copper sulphate


What are the two electrodes made of in the the purification of copper?

The positive electrode is made of impure copper which is to be purified
The negative electrode is made of pure copper


How does the mass of the electrodes change during the purification of copper?

Positive electrode decreases mass
Negative electrode increases mass


What happens during the purification of copper?

Copper ions produced at the anode travel in solution to the cathode, producing pure copper. The pure copper cathode increases greatly in mass, while the anode decreases in mass - sometimes more than the increase because impurities are also lost The impurities left behind at the anode form a sludge beneath it.


What is the electrolysis of dilute sulphuric acid basically doing?

Decomposition of water


Why is sulphuric acid placed in the water?

Because then that means the the hydrogen and oxygen atoms will go the electrodes and the SO42- ions will stay behind


What is the charge carrier in the copper wire?



What is the charge carrier in the electrodes

Delocalised electrons


What are the charge carriers in the solution?



What are the purpose of ions in an electrolyte?

Charge carriers


Is a covalent compound an electrolyte?

No because covalent compounds don't contain ions and all electrolytes contain ions.


How can you remember which electrode is which?

PANC - positive anode, negative cathode


Explain how electrolysis is set up and explain what is happening:
(helps you understand)

Have some sort of power supply with a voltage and connect each end to an electrode; one is positive, one is negative. Put this in a molten solution or one dissolved in water. The power source will pump electrons away from the left-hand electrode to the right-hand electrode. The excess of negatively charged electrons on the right-hand electrode makes it negatively charged. The left-hand electrode is positive because it is short of electrodes. There is a limit to how many electrodes the the power source can squeeze into the negative electrode (cathode) because of the repulsion by electrons already there.


Describe the process of electrolysis on molten lead bromide:

The positive lead (II) ions are attracted to the cathode. When they get there each lead (II) ion picks up two electrons from the electrode and forms neutral lead atoms. These fall to the bottom of the container as molten lead. This leaves space in the electrode that more electrons can move into.

Bromide ions are attracted to the positive anode. When they get there, the extra electron which makes the bromide ion negatively charged moves onto the electrode because this electrode is short of electrons. This turns each bromide ion into a bromine atom. These join in pairs to make bromine molecules.


What does discharging an ion mean?

When the ions are discharged at the electrodes it simply means that it loses its charge - either giving up electron(s) to the electrode or receiving electrodes from it.


What type of reaction is electrolysis?

Redox because it involves the loss and gain of electrons


What are positive/negative ions called?

Positive ions are cations because they are attracted to the cathode and negative ions are anions because they are attracted to the anode.


Why can't some ionic substances be electrolysed when molten?

some break up into simpler substances before their melting point and others have such high melting points that it isnt possible to do so in a lab.


What is the equation for the reversible reaction of water into hydrogen and hydroxide ions?

H2O(l) ⇌ H+(aq) + OH-(aq)


What would happen during the electrolysis of copper (II) sulfate during electrolysis?

Copper and hydrogen would both be attracted to the anode but copper is below hydrogen in the reactivity series so it will be discharged. The sulfate ions and hydroxide ions would be attracted to the cathode but as the sufate ions are not halides they would not be discharged. Instead you get oxygen from the discharge of hydroxide in the water.


What would happen after long time if you left the electrolysis of copper sulfate?

After a long time eventually all the copper ions would be used up so the solution would turn from blue to colourless. Just the hydrogen ions and sulfate ions would be left and os the solution turns into dilute sulfuric acid. Then the electrolysis would continue for the sulfuric acid


Describe the electrolysis of sulfuric acid using carbon electrodes:

In this case the only positive ions arriving at the anode is hydrogen ions from the acid and the water, these are discharged to give hydrogen gas. Sulfate ions and hydroxide ions arrive and as they are not halide ions they are not discharged. Twice as much hydrogen is produced


What will happen with non-electrolytes?

No current will flow and even if the circuit is closed, if there is a bulb in the circuit it will not turn on as the non electrolyte will not conduct electricity nor be decomposed by it.


Describe an experiment to differentiate between an electrolyte and a non-electrolyte:

Attach one electrodes to the negative terminal of a power supply. Connect the positive terminal to a bulb and then attack the other side of the bulb to an electrode. Place both electrodes into the solution that you would like to test, they must not touch each other. If the bulb lights up the substance is an electrolyte, if it doesnt it is not.


Why don't covalent compounds conduct electricity?

Because there are no charged particles free to move