Electrolysis of Aqueous Solutions

• Aqueous solutions will always have water present (H2O)
• In the electrolysis of aqueous solutions, the water molecules dissociate producing H+ and OH-  ions: H2O ⇌ H+ + OH–

  • These ions are also involved in the process and their chemistry must be considered
  • We now have an electrolyte that contains ions from the compound plus ions from the water
• Which ions get discharged and at which electrode depends on the relative reactivity of the elements involved

  • Concentrated and dilute solutions of the same compound give different products
  • For anions, the more concentrated ion will tend to get discharged over a more dilute ion
• Negatively charged OH- ions and non-metal ions are attracted to the positive electrode
• If halide ions (Cl, Br, I) and OH are present then the halide ion is discharged at the anode, loses electrons and forms a halogen (chlorine, bromine or iodine)
• If no halide ions are present, then OH is discharged at the anode, loses electrons and forms oxygen gas

  • In both cases the other negative ion remains in solution
• The concentration of the solution also affects which ion is discharged:

  • If a concentrated halide solution is being electrolysed, the halogen forms at the anode
  • If a dilute halide solution is being electrolysed, oxygen is formed
• Example: for a concentrated solution of barium chloride, the Cl ions are discharged more readily than the OH ions, so chlorine gas is produced at the anode

  • If the solution is dilute however only the OH ion is discharged and so oxygen would be formed
• Positively charged H+ and metal ions are attracted to the negative electrode but only one will gain electrons

  • Either hydrogen gas or metal will be produced
• If the metal is above hydrogen in the reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode

  • This is because the ions of the more reactive metal will remain in the solution, causing the ions of the least reactive metal to be discharged
  • Therefore, at the cathode, hydrogen gas will be produced unless the positive ions from the ionic compound are less reactive than hydrogen, in which case the metal is produced
• Reactivity Series

  A) potassium

  B) sodium

  C) lithium

  D) calcium

  E) magnesium

  F) aluminium

  G) carbon

  H) zinc

  I) iron

  J) hydrogen

  K) copper

  L) silver

  M) gold

  N) most

  
• Aqueous copper sulfate contains the following ions:

  • Cu2+, SO4^2-, H+ and OH-
• Using graphite electrodes:

  • Cu2+ and H+ will both be attracted to the cathode but the less reactive ion will be discharged
  • In this case, copper is less reactive than hydrogen
  • Copper ions are discharged at the cathode, gain electrons and are reduced to form copper metal
  • The half equation for the reaction at the electrode is: Cu2+   +   2e-   →   Cu
  • SO4^2- and OH are both attracted to the anode
  • OH- ions lose electrons more readily than SO4^2-
  • OH- lose electrons and are oxidised to form oxygen gas
  • The half equation for the reaction at the anode is: 4OH– ⟶ O2   +   2H2O   +   4e–
• Using graphite electrodes:

  A) test tube

  B) positive electrode

  C) beaker

  D) power supply

  E) electrolyte

  F) negative electrode

  
• Using copper electrodes:

  • The cathode increases in mass while the anode decreases
  • This occurs as copper atoms are oxidised at the anode and form ions while copper ions are reduced at the cathode, forming copper atoms
  • The gain in mass by the negative electrode is the same as the loss in mass by the positive electrode
  • Therefore the copper deposited on the negative electrode must be the samecopper ions that are lost from the positive electrode
  • That implies that the concentration of the Cu2+ ions in the solution remains constant
• Using copper electrodes:

  A) direct current

  B) variable resistor

  C) copper cathode

  D) aqueous

  E) copper (II) sulfate

  F) copper anode

  G) ammeter

  
• Products formed for common aqueous solutions

  A) chlorine gas

  B) hydrogen gas

  C) oxygen

  D) hydrogen gas

  E) oxygen gas

  F) lower

  G) discharged

  H) oxygen gas

  I) electrons

  J) hydrogen gas

  
• About 0.00001% of water molecules break down into H+ and OH- ions:

  • H2O → H+ + OH-
  • H+ = hydrogen ions
  • OH- = hydroxide ions
• H+ and OH- ions are present in solutions of ionic compounds in water and so could also be discharged

  • e.g. a solution of soidum chloride in water, there are Na+, Cl-, H+ and OH- ions are present and the H+ and Cl- ions are discharged to form hydrogen and chlorine so the other ions (Na+ and OH-) are left in the solution
• At each electrode, only the ions that are the easiest do discharge are discharged
• Ions lower in the reactivity series are easier to discharge

  • solutions containing ions of low reactivity metals e.g. copper, silver, gold, and platinum have the ions of the low reactivity metal discharged instead of H+ ions because the low reactivity ions are easier to discharge with H+ ions
• Solutions containing ions of higher reactivity metals e.g. sodium, calcium, aluminium, zinc

  • H+ ions are discharged instead of the metal because the H+ ions are easier to discharge than the metal ions
  • when H+ ions are discharged, H2 is formed: 2H+ + 2e- → H2
• If the compound contains halide ions e.g. Cl-, Br- or I-, then the halide ions is discharged instead of the OH- ion

  • Halide ions are discharged instead of OH- ions because halide ions are easier to discharge then OH- ions
• Solutions containing other negative ions. e.g. nitrate ions, sulfate ions, carbonate ions

  • OH- ions are discharged because the OH- ions are easier to discharge than the metal ions
  • when OH- are discharged, O2 is formed: 4OH- - 4e- → O2 + 2H2O
• Ions you need to know:

  • SO3 2-
  • SO4 2-
  • NO 3-
  • CO3 2-
  • HCO3 -
  • NH4 +
  • OH -
• • Electrolysis of solutions

  A) copper

  B) copper

  C) hydrogen

  D) hydrogen

  E) oxygen

  F) chlorine

  G) oxygen

  H) chlorine

  
• Possible products in this experiment

  • all of these reactions bubble

  A) copper

  B) sodium

  C) hydrogen

  D) brown coating

  E) grey coating

  F) bubbles of gas

  G) chlorine

  H) oxygen

  I) bubbles

  J) gas

  K) bleach

  L) damp litmus

  M) bubbles

  N) gas

  O) bleach damp

  P) litmus

  
• Sodium ions are drawn through the membrane where OH- reacts to form NaOH