Electrolysis

Electrolysis content flashcards

Briefly explain the process of electrolysis

Ionic compound broken into elements by passing electric current through it.

Different ions attracted to respective electrodes.

Electrodes are objects that can conduct electricity.

Explain why electrodes are usually inert in the electrolysis of a solution.

Electrodes made of graphite (carbon), which is unreactive (inert) and conducts electricity. Inert carbon electrodes do not react with products.

Some metal compounds must be extracted by undergoing electrolysis instead of being extracted by carbon.

Explain why some metal compounds must be extracted by electrolysis.

Metal compounds containing metal more reactive than carbon cannot be heated with carbon to displace it.

Define “electrolyte”

Electrolyte is liquid state / solution of ionic compound containing free-moving ions that can carry a charge.

Explain why ionic compounds must be molten or dissolved in solution for electrolysis to occur.

Ionic compounds are arranged in fixed lattice structure held by strong electrostatic attraction - ions cannot move.

In liquid state / dissolved, ions are free-moving & carry a charge.

Give the difference between a cathode and an anode.

Cathode:

• Cations attracted to negative cathode.

Anode:

• Anions attracted to positive anode.

Explain what is meant by ions are discharged.

Ions loses / gains electrons to become neutral atom / molecule.

Explain the electrolysis of molten ionic compounds.

• Heating ionic compound until molten = enough energy to overcome electrostatic attraction

• Therefore ions are free-moving & can carry charge (conducts electricity = electrolysis)
  
  

• Free-moving ions carry charge towards respective electrodes.

Explain the electrolysis of aqueous ionic compounds

Cathode:

• H⁺ & metal ions compete

• Least reactive element discharged.

Anode:

• OH^- & non-metal ions compete

• Hydroxide discharged unless halide present

Give the half-equation describing the discharging of hydroxide ions and hydrogen ions

• 4OH- (aq) = O2 (g) + 2H2O (l) + 4e-

  
  

• 2H+ (aq) + 2e- = H2 (g)

Explain reduction and oxidation

• Reduction:
  Atom / ion gains electrons, causing overall charge to decrease
  
  

• Oxidation:
  Atom / ion loses electrons, causing overall charge to increase

An example of a half-equation is:
Na+ (l) + e- = Na (l)

Describe what this half equation shows.

• Reduction
  
  

• Sodium cations attracted to cathode, where electrons from cathode transferred to ions to become sodium atoms.

An example of a half-equation is:
2Cl- (aq) = Cl2 (g) + 2e-

Describe what this half equation shows.

• Oxidation
  
  

• Chlorine ions attracted to anode, where electrons from chlorine ions transferred to anode to become chlorine atoms.

Gold and platinum can be found in their native forms when mined.

Explain why aluminium is frequently found as an ore instead?

Aluminium is reactive, so will react to form compounds easily which are found in ores.

Aluminium can be mined in an ore known as “bauxite”. Mining bauxite gives aluminium oxide (Al₂O₃).

Aluminium oxide has a very high melting point.

Explain why, in terms of structure and bonding, aluminium oxide’s melting point is very high.

• Aluminium oxide consists of lattice structure

• The lattice structure held by strong electrostatic attraction = a lot of energy required to overcome this
  
  

• Lattice made of Al³⁺ & O^2-

• The magnitudes in charge are large so stronger electrostatic attraction

Aluminium oxide is dissolved in molten cryolite solution before being electrolysed.

Why is aluminium oxide dissolved in molten cryolite solution first?

• Melting requires a lot of money = unprofitable
  
  

• Molten cryolite solution lowers temperature
  
  

• Energy needed reduced, cost lowered, equipment won’t melt

In the electrolysis of aluminium oxide in cryolite solution, the positive anodes must be replaced regularly.

The anodes are made of graphite (carbon), which is usually inert.

Explain why the anode must be replaced frequently.

• Very high temperatures = oxygen gas react with carbon anode
  
  

• Produced CO₂, anode degrades quickly

At lower temperatures, the graphite (carbon) anode is inert, even when oxygen is produced.

In terms of structure and activation energy, explain why this is the case.

• Each carbon atom is covalently bonded with 4 carbon atoms.
  
  

• Covalent bonds are strong, so high activation energy required to break bonds.
  
  

• Lower temperatures = carbon & oxygen atoms have less kinetic energy, so will collide less forcefully & less frequently.
  
  

• Collisions don’t have enough activation energy so carbon bonds don’t break.

Changes in pH can take place during the process of electrolysis.

Give the ions which are involved in pH changes.

• H⁺ ions:
  Acids release H⁺ ions in water
  
  

• OH^- ions:
  Alkalis release OH^- ions in water

Overall pH value is a balance of above.

Changes in pH can take place during the process of electrolysis.

Give two methods of measuring pH change.

• Universal indicator (UI)
  Changes colour depending on pH level.

• pH probe
  Electronic device dipped into solution, providing numerical value.

The electrolysis of an aqueous solution, NaCl (aq), is investigated by a student.

Predict what will happen to the starting NaCl solution in terms of pH change, explaining why.

Cathode:

• Only H⁺ ions discharged.

• H⁺ ions leave electrolyte

Anode:

• Only Cl^- ions discharged.

• OH^- ions stay in electrolyte.

pH will rise & become alkaline.

The instructions below are a method for a student investigating the electrolysis of aqueous solutions:

1. (Measure volume of electrolyte)
   Use a _________ cylinder to pour 50cm³ of one aqueous solution into a ______.
   
   

2. (Prepare the circuit)
   Use two clean ________ rods. Attach the graphite rods to a 4V, DC _____ supply, turned off, by using crocodile _____ and wires.
   
   

3. (Add electrodes)
   Place the __________ in the solution. Ensure they are NOT touching.
   
   

4. (Start the experiment)
   Turn on the power supply for a maximum of _ minutes.
   
   

5. (Record and repeat)
   At each electrode, observe and record any observations. After, repeat 1-5 for the other _________.
   
   

Fill in the blanks.

1. (Measure volume of electrolyte)
   Use a MEASURING cylinder to pour 50cm³ of one aqueous solution into a BEAKER.
   
   

2. (Prepare the circuit)
   Use two clean GRAPHITE rods. Attach the graphite rods to a 4V, direct current POWER supply, turned off, by using crocodile CLIPS and wires.
   
   

3. (Add electrodes)
   Place the ELECTRODES in the solution. Ensure they are not touching.
   
   

4. (Start the experiment)
   Turn on the power supply for a maximum of 5 minutes.
   
   

5. (Record and repeat)
   At each electrode, observe and record any observations. After, repeat 1-5 for the other SOLUTIONS.

Short-circuiting is a phenomenon that occurs when a circuit has very low resistance.

If electrolysis were to occur at a higher voltage, this can allow a dangerously large current to flow.

What are the risks of short circuiting?

• Electric shock

• Wires melt

• Fire

• Risk of burns

Give the elements formed at each electrode in the electrolysis of:

• Copper (II) Chloride

• Copper (II) Sulfate

Cathode:

• For both, copper metal produced.
  
  

Anode:

• Copper chloride, chlorine produced

• Copper sulfate, oxygen (& water) produced

Give the elements formed at each electrode in the electrolysis of:

• Sodium Chloride

• Sodium Sulfate

Cathode:

• Hydrogen produced

Anode:

• Sodium chloride, chlorine produced

• Sodium sulfate, oxygen (& water) produced

Describe the observations of the production of chlorine gas in electrolysis.

Then, give the positive test for chlorine.

Observation:
Effervescence of pale green bubbles. Pungent odour.

Test:
Hold damp litmus paper where gas collects. Litmus paper flashes red, then bleaches white.

Describe the observations for the production of oxygen gas in electrolysis.

Then, give the positive test for oxygen.

Observation:
Effervescence of transparent bubbles.

Test:
Collect gas in test tube, then insert glowing splint. Splint relights.

Describe the observations for the production of hydrogen gas in electrolysis.

Then, give the positive test for hydrogen.

Observation:
Vigorous effervescence of transparent bubbles.

Test:
Collect gas in test tube, holding burning splint at tube mouth. Splint burns rapidly with squeaky pop.

Describe the observations for the production of copper metal in electrolysis.

Orange-brown solid coating

Explain why the production of hydrogen can be seen to show a more vigorous observation than oxygen.

• 4 moles of hydroxide ions produces 1 mole of oxygen
  
  

• 4 moles of hydrogen ions produces 2 moles of hydrogen

A student investigating the electrolysis of aqueous solutions should take some safety precautions.

Give a risk assessment of 2 risks.

• Chlorine gas causes respiratory damage. Use a ventilated room, stop power supply.
  
  

• General chemical hazards causes irritation. Wear goggles.

Copper electrorefining is the process in which ________ copper electrodes are used instead of unreactive graphite electrodes.

The copper electrodes are placed inside copper (II) _______ solution.

Fill in the gaps.

Copper electrorefining is the process in which REACTIVE copper electrodes are used instead of unreactive graphite electrodes.

The copper electrodes are placed inside copper (II) SULFATE solution.

In copper electrorefining, the anode is an impure “block” of a copper, containing valuable metals.

Explain how electrolysis separates the copper from the metals in the anode.

• Power supply pulls electrons from copper

• Copper atoms become copper ions and dissolves in solution

• Anode wears away so precious metals fall to bottom

In copper electrorefining, the copper ions will carry a charge towards the cathode to be oxidised.

A student says that because of this, the copper sulfate solution will change colour from blue to clear.

Explain why the student is wrong.

Concentration of copper ions moving into solution equal to concentration leaving.

The cathode in copper electrorefining is initially made of a thin sheet of pure copper.

Describe how the mass of the cathode changes as electrolysis takes place.

Mass increases as more copper layers coat cathode.