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Science C3 Topic 3
GCSE edexcel further additional chemistry: electrolytic processes
| Question | Answer |
|---|---|
| What is electrolysis? | Electrolysis is the process in which electrical energy from a DC supply decomposes compounds |
| What is an electrolyte and what state must it be in? | An electrolyte is an ionic substance in a molten state or dissolved in water - since the ions can move freely and conduct the charge, electrolysis can occur |
| Which ions migrate to which electrodes? | * Cations are positive ions, so migrate to the negatively charged cathode * Anions are negative ions, so migrate to the positively charged anode |
| What is reduction, which electrode does it occur at? | Reduction is when a positive ion gains electrons, forming a neutral atom - occurs at the cathode (electrons are transferred from the electrode to the cations) |
| What is oxidation, which electrode does it occur at? | Oxidation is when a negative ion loses electrons, forming a neutral atom - occurs at the anode |
| What are half-equations? | Half equations show the reactions that occur at each electrode |
| Write the electrolysis of magnesium bromide and sodium chloride as half equations | NaCl Anode: 2Cl- -> Cl2 + 2e- Cathode: Na+ + e- -> Na MgBr2 Anode: 2Br- -> Br2 + 2e- Cathode: Mg2+ + 2e- -> Mg |
| What are some uses of sodium and how is it produced? | Produced by electrolysis of NaCl. Used in street lamps as the vapour gives out a bright yellow glow when an electric current is passed through. A coolant in nuclear reactors - transfers heat from core to the water which turns to steam + drives generators |
| What happens at the electrodes and what are the half-equations of the electrolysis of copper chloride solution? | •Anode: chlorine ions are discharged (lose electrons) to give chlorine gas •2Cl- (aq) -> Cl2 (g) +2e- •Cathode: copper ions are discharged (gain electrons) •Cu2+ (aq) +2e- -> Cu (g) |
| What happens at the electrodes and what are the half-equations of the electrolysis of copper sulphate solution? | •Anode: hydroxide ions are discharged (lose electrons) to form water and oxygen •4OH- (aq) -> 2H2O (l) + O2 (g) + 4e- •Cathode: copper ions are discharged (gain electrons) •Cu2+ (aq) +2e- -> Cu (g) |
| What happens at the electrodes and what are the half-equations of the electrolysis of sodium sulphate solution? | •Anode: hydroxide ions are discharged (lose electrons) to form water and oxygen •4OH- (aq) -> 2H2O (l) + O2 (g) + 4e- •Cathode: hydrogen ions are discharged (gain electrons) to give hydrogen gas •Cu2+ (aq) +2e- -> Cu (g) |
| What happens at the electrodes, what is left over at the end, and what are the half-equations of the electrolysis of sodium chloride solution? | •Anode: chlorine ions are discharged to give chlorine gas •2H+ (aq) +2e- -> H2 (g) •Cathode: hydrogen ions are discharged to give hydrogen gas •2H+ (aq) +2e- -> H2 (g) •Na+ (aq) and OH- (aq) ions are left forming a sodium hydroxide solution, NaOH (aq) |
| How is electrolysis used in the purification of copper? | Anode = impure copper. Cathode = a sheet of pure copper. Electrolyte is copper sulphate solution. During electrolysis the copper atoms in anode lose 2 electrons & become copper ions- they dissolve in solution, migrate to cathode + deposited as pure copper |
| In the purification of copper, which electrode gains mass and which loses mass? | The anode loses mass and the cathode gains mass |
| What happens to the impurities that were in the copper after it has been purified? | The impurities from the anode are insoluble and form sludge which is collected as it may contain valuable metallic elements |
| What are the half-equations that occur during the purification of copper? | *Anode: Cu (s) -> Cu2+ (aq) + 2e- *Cathode: Cu2+ (aq) + 2e- -> Cu (s) |
| What is electroplating? | Electroplating is covering one metal with a thin layer of another metal, using electrolysis |
| What are the electrodes and the electrolyte when iron is being plated with silver? | * The anode is the metal that will form the plating - silver * The cathode is the object to be plated - iron * The electrolyte contains ions of the metal that will form the plating - e.g. silver nitrate solution |
| How can the layer of plating be thickened? | To form a thicker plating layer, increase the current or carry out electrolysis for a longer time period |
| Why might someone want to electroplate metals? | improves resistance to corrosion, cosmetic purposes (silver or gold plating gives an attractive appearance and is cheaper than making the whole object out of the expensive metals) |
| What is galvanised steel? | Galvanised steel is steel coated with zinc- the zinc protects the iron from rusting and forms a zinc oxide which has a similar colour to zinc, protecting both metals |
| How is sodium produced? | Sodium is produced from the electrolysis of molten sodium chloride |
| Why does sodium have to be produced from a molten form? | Because sodium chloride solution electrolysises to form hydrogen, which is a product of the water, rather than forming sodium, which is a product of the salt. This is because sodium is too reactive. |
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