Chemistry, 5: Electricity and chemistry

• Electrolytes are compounds that can conduct electricty

  They can go under electrolysis
• Electrolysis is the breakdown of ionic compounds when molten or aqueous solution by passage of electricity
• The anode is the positive electrode, it attracts anions which are negative ions.
• The anion is the negative ion
• Cations are positive ions
• Cathodes are negative electrode that attracts cations which are positive ions.
• Electric current flowing is caused by electrons moving in electrons and wires of the circuits and by ions moving in the solution

  If the current does not flow, current contains non-electrolyte
• What are the conditions for electrolysis?


  Substance being electrolysis must contain ions

  Ions must be free to move, molten or dissolved in water

  D.c. Is used with electrodes

  Electrode is connected to power supply accordingly

  Electrode made out of carbon or graphite as it is inert and does not undergo chemical changes during electrolysis
• What is the electrolysis of molten lead (II) bromide?


  Lead (II) bromide (PbBr2) is ironically bonded and contains Pb2+ ions and Br- ions

  When solid is melted and voltage is applied, ions are able to move

  Positive lead ions move to the cathode

  Lead ions accept electrons to form lead atoms

  Pb2+ + 2e- -> Pb

  Negative bromide ions move to the anode

  Bromide ions give up electrons to form bromine ions, then molecules

  2Br-(l) -> Br2(g) + 2e-

  Product: Lead and bromine

  There is a silvery deposits of lead near the bottom, and brown vapour near the anode.
• What is the electrolysis of sodium chloride solution?


  At the cathode:

  Na+ and H+ move to the cathode

  Only the H+ discharges b/c less reactive than Na+

  Na+ remains as ions, turns the solution alkali

  2H+ + 2e- -> H2

  At the anode:

  Cl- and OH- move to the a ode

  If dilute, OH- are discharged | 4OH- -> 2H2O + O2 + 4e-

  If concentrated, C- are discharged | 2Cl- -> Cl2 + 2e-

  Bubbling and effervescence seen at both electrodes

  When dilute sodium chloride solution is electrolysed, solution becomes increasingly alkaline as sodium hydroxide is formed
• What is the electrolysis of copper (II) sulfate solution?


  At the cathode:

  Cu2+ and H+ ions move to the cathode

  Cu2+ ions are discharged

  Cu2+ + 2e- -> Cu

  Red brown coating b/c of copper

  At the anode:

  SO4^2 and OH- ions move to the anode

  OH- ions are discharge

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

  Colourless O2 gas bubbles

  Solution becomes paler blue b/c copper ions are discharged
• What is the electrolysis of copper (II) sulfate solution when using copper electrodes?


  At the cathode:

  Cu2+ gains 2 electrons and is discharged

  Copper atoms are formed, mass of electrodes increases

  Cu2+ + 2e -> Cu

  At the anode:

  Cu atoms loses 2 electrons and Cu2+ ions are formed

  Anode dissolves and loses mass

  Cu -> Cu2+ + 2e-

  Concentration of Cu2+ remains constant as the rate of production and removal is balanced

  Impure copper falls to the bottom

  Copper ions deposited as pure copper on carhode

  Cathode is replaceable by another thin piece of copper
• Metals form at the cathode
• Non-metals form at the anode
• In aq solutions, what forms at the cathode depends on reactivity

  Metals below hydrogen are deposited on the cathode

  Metals above hydrogen are not deposited, and hydrogen gas is deposited on the cathode
• In aq solutions, what forms at the anode depends upon concentration

  If concentrated, it will be the product

  If dilute, it will be oxygen
• What is more reactive than carbon?


  Potassium

  Sodium

  Calcium

  Magnesium

  Aluminium
• How are elements more reactive than carbon extracted?


  Electrolysis
• What is more reactive than hydrogen, but less reactive than carbon?


  Zinc

  Iron

  Tin

  Lead
• How are elements more reactive than hydrogen and less reactive than carbon extracted?


  Reduction
• What elements are less reactive than hydrogen?


  Copper

  Silver

  Gold

  Platinum
• How are elements less reactive than hydrogen extracted?


  Found in native state
• Electroplating uses electrolysis to cost an object with a thin film of metal
• Electroplating improves the appearance of metals, and prevents corrosion

  Electroplating modifies chemical reactivity of the plated object
• In electroplating, the anode is the metal you want to plate object with
• In electroplating, the cathode is the object you want to plate.
• What is the electroplating for silver?

  The anode is silver

  Ag -> Ag+ + e-

  Silver loses an atom, and forms a silver ion

  The cathode is the object

  Ag+ + e- -> Ag

  Silver ions accept an electron and forms a silver atom

  Mass increases at the cathode is equal to the decrease at the anode

  Concentration of silver ions in the silver nitrate electrolyte remains constant
• Aluminium is extracted from the ore bauxite

  Aluminium oxide is extracted from bauxite via purification

  It is insoluble in water and has a high melting point, therefore dissolved in molten cryolite to lower the melting point.
• In aluminium extraction, anodes are carbon and the cathode is a carbon-lined steel case
• In aluminium extraction,

  At the cathode:

  Aluminium is formed

  Al3 + 3e- -> Al

  At the anode:

  Oxygen is formed

  2O2- -> O2 + 4e-

  Overall reaction:

  2Al2O3 -> 4Al + 3O2
• In aluminium extraction:

  • Oxygen reacting with carbon anodes form CO2 that escapes
  • Carbon anode must be replaced as it gets oxidised by the oxygen and eaten away, becoming too small for reaction to occur
  • Replacing it ensures sufficient surface area for reaction to occur, prevents anode from breaking off and contaminating molten aluminum
• Why can aluminium be used for packacing?


  Non-toxic

  Impermeable

  Resistant to corrosion
• Why can aluminium be used for planes?


  High strength-to-weight ratio

  Low density

  Resistant to corrosion

  Alloys are used as it is stronger than pure Aluminium
• Why can Aluminium be used for electrical cables?


  High electrical conductivity

  Low density
• Why can aluminium be used for building materials?

  Malleable

  Low corrosion

  High strength-to-weight ratio
• Why can aluminium be used for kitchenware?


  Shiny appearance

  Non-corrosive
• What does the electrolysis of concentrated sodium chloride solution (brine)?


  Sodium hydroxide

  Hydrogen

  Chlorine
• What is the electrolysis of brine?


  At the cathode:

  Sodium is more reactive than hydrogen

  Hydrogen ions turn mokecuoke

  2H+ + 2e- -> H2 (reduction of H+)

  At the anode:

  Both OH- and Cl- are attracted to the anode

  Only Cl- ions turn into molecule as it is concentrated

  2Cl- -> Cl2 + 2e- (oxidation of Cl-)
• In electrolysis of brine, chlorine and sodium hydroxide must be kept apart by diaphragm cell as it would form bleach if combined.
• What are the uses of sodium hydroxide?


  Bleach

  Water

  Paper

  Soap