Chem(ch-4)

• Many important chemical processes utilize electricity
• Chemical reaction can be used to produce electricity
• Conversion and storage of electrochemical energy are used in batteries, smart phone, notebooks, car batteries and in fuel cells
• In daily life, protective and decorative electroplating is used in various industries and development of the electric automobiles and parts including other decorative items
• The extraction of reactive metals (Na, Mg, Al) and refining of some metals (Cu) are based on electrochemical Methods
• Conductors

  Substances which conduct or allow the passage of electricity. In a conductor, electric current can flow freely. All metals either solids or liquid state are conductors.
• Conductors


  • solid copper, liquid mercury, graphite (non-metallic solid)
• Semiconductors

  Substances (elements) which have an ability to conduct between conductors and non-conductors.
• Semiconductors

  • silicon, germanium, tin, selenium, gallium arsenide
• Non-conductors or insulators


  Solid substances which do not conduct electricity. Non-metallic solids electric current cannot flow freely except graphide.

  
• Insulators


  • wood, plastic, rubber, Styrofoam
• Electrolytes


  Substances that ionise in solution or in a molten state can conduct electricity. Ionic compounds (salt solution), acid and base.
• Non-electrolytes


  Do not dissociate into ions in solution and cannot conduct electricity. Covalent compounds (sugar solution)
• Electrodes


  Electrical conductors that emits or collects electrons in an electrolytic cell usually made of unreactive metals (Pt), non-metal carbon (graphite).
• Electrodes

  • Oxidation and reduction (redox) reactions take place
  • Inert electrode does not participate in a chemical reaction
  • Anode - Positive ions (cations) discharge, donor of electrons (oxidation)
  • Cathode - Negative ions (anions) discharge, acceptor of electrons (reduction)
• Electrical conductivity

  A fundamental property of a material that quantifies how strongly it conducts electric current. If a substance has free electrons or mobile ions, it conducts electricity.
• Electrical conductivity of metals

  • Metals contain free valence electrons that move freely through the entire metallic lattice
  • Silver (Ag), copper (Cu), gold (Au), Aluminium (Al) have high electrical conductivity
• Electrical conductivity of electrolytes

  • Due to the presence of mobile ions
  • Electrolytes contain electrically charged particles called ions
  • Electrolytes can conduct electricity due to the movement of these ions
  • Non-electrolytes do not contain ions and so they cannot conduct electricity
• Electrochemical cells


  Devices that convert chemical energy into electrical energy or vice versa when a chemical reaction occurs by electrical energy
• Types of electrochemical cells

  • Electrolytic cell
  • Chemical cell
• Electrical wires are coated with plastic or rubber to prevent electric shock
• Copper (Cu) metal is the most commonly used conductor in electrical wiring and electrical circuits because it has high electrical conductivity
• Classify the given substances

  • Electrolytes - table salt solution, molten magnesium chloride
  • Non-electrolytes - sugar solution
  • Conductors - mercury, iron
  • Non-conductors - plastic
• Electrolytic process is the use of electrolysis by which ionic substances are decomposed when an electric current is passed through them
• Electrolytic process is commercially important as a stage in the separation of elements from minerals and ores using an electrolytic cell
• Electrolytic cells


  Cells in which electrical energy is transformed into chemical energy. A container with electrodes immersed in the reaction material (electrolyte) and connected to a source of direct current. Electrolytes conduct electricity due to dissociation and discharge of ions at the respective electrodes. Electron transfer occurs between electrodes and electrolytes.
• Electrolysis of molten sodium chloride using Pt electrode

  1. Na+ ions accept electrons and discharge at the cathode

  2. Cl- ions give up electrons and discharge at the anode

  3. Liquid sodium floats to the top of the melt above the cathode and is drained off into a storage tank

  4. Chlorine gas bubbles out of the melt above the anode
• Electrolysis of aqueous solutions

  • Aqueous salt solutions contain cations and anions from the complete dissociation of salt and some H+ and OH- ions from slight dissociation of water
  • Alkaline and aqueous acid solutions contain H+ and OH- ions, acid radicals and basic radicals
• Selective discharge of ions

  The preferential discharge of these ions depends on the position of the metal or group in the electrochemical series, the concentration of electrolytes and nature of the electrode used
• Electrochemical series (for aqueous solution)

  • K+
  • Ca2+
  • Na+
  • Mg2+
  • Al3+
  • Zn2+
  • Sn2+
  • Pb2+
  • H+
  • Cu2+
  • Ag+
  • SO4(2-)
  • NO3-
  • Cl-
  • Br-
  • I-
  • OH-
• Concentration of the electrolyte

  The concentration of ions in the electrolyte. As the concentration of an ion increases, the tendency of the ion to discharge from the solution also increases.
• Nature of the electrode

  The discharge of ions during electrolysis can be influenced by the type of electrodes used. Some are chemically inert (e.g. graphite, platinum), others are reactive and may influence the ionic discharge (e.g. Cu, Hg, Ag).
• Electrolysis of aqueous sodium chloride solution using graphite electrodes

  1. H+ ions are discharged at the cathode, producing H2 gas

  2. OH- ions are discharged at the anode, producing O2 gas
• Electrolysis of saturated aqueous sodium chloride solution (brine) using platinum electrodes

  1. H+ ions are discharged at the cathode, producing H2 gas

  2. Cl- ions are discharged at the anode, producing Cl2 gas
• Electrolysis of aqueous copper(II) sulphate solution using platinum electrodes

  1. Cu2+ ions are discharged at the cathode, depositing Cu

  2. OH- ions are discharged at the anode, producing O2 gas
• Electrolysis of aqueous copper(II) sulphate solution using copper electrodes

  1. Cu2+ ions are discharged at the cathode, depositing Cu

  2. Cu anode dissolves, increasing the concentration of Cu2+ ions in the solution
• Cu (s)


  Solid copper
• Reaction at the anode

  4OH −(aq) → 2H2O (l) + O2 (g) + 4ē
• The colour of the solution is pale and more acidic due to H+ ions left in the solution
• Electrolysis of aqueous copper (II) sulphate solution using copper electrodes

  Cu (s) deposits on the cathode, mass of cathode increase. Cu anode dissolves and the mass of anode decreases. No colour change in solution