chemistry 3rd term

• ELECTROCHEMICAL CELLS

  Electrochemical cell is a device that converts chemical energy to electrical energy.

  SOME PARAMETERS ASSOCIATED WITH ELECTROCHEMICAL CELL
• Electrochemical cell


  A device that converts chemical energy to electrical energy
• Parameters associated with electrochemical cell

  • Standard hydrogen electrode
  • Standard electrode potential
  • Salt bridge
• Standard hydrogen electrode


  The reference standard by which the standard electrode potential of a metal ions/metal system is measured against. The standard hydrogen electrode potential has been assigned a value of zero.
• Standard electrode potential (Eo)


  The potential difference set up between the metal and one-molar solution of its ions at 25oC (arbitrarily taking the standard electrode potential of hydrogen ions/hydrogen gas system as zero volts)
• Salt bridge

  A piece of paper soaked in potassium chloride solution or ammonium chloride solution. Its functions are to complete the electric circuit and enable the movement of ions from one half-cell to another so as to maintain electrical neutrality in the solution.
• Ionic theory


  Used to explain the behavior of electrolytes when electric current is passed through their solution
• Ionization


  When an electrolyte is melted or dissolved in water, some if not all of the molecules of the substance dissociate into freely-moving charge ions i.e. positive ions (called cations) and negative ions (anions)
• Current flow in electrolytes

  The free ions lose their random movement, the positive ion becomes attracted to the negative electrode (called the cathode) while the negative ions moves toward the positive electrode (called anode)
• Electrolysis


  The process whereby chemical decomposition occurs when an electric current is passed through an electrolyte
• Electrodes


  • Conductors in the form of wires rods or plates through which an electric current enters or leaves the electrolyte
  • Cathode (negative electrode)
  • Anode (positive electrode)
• Electrolyte


  A compound, in a molten or aqueous state which can conduct electricity and is decomposed in the process
• Types of electrolytes


  • Strong electrolytes (ionize completely, conduct large electric current)
  • Weak electrolytes (partially ionized, conduct electricity poorly)
• Examples of electrolytes


  • NaCl
  • CuCl2
  • CUSO4
  • HCl
  • H2SO4
• Non-electrolytes


  Do not conduct electricity since they do not ionize
• Electrolytic cell


  An assembly of two electrodes in an electrolyte used for the electrolysis of a substance
• Electrolytic cell


  Converts electrical energy to chemical energy, electric current is required for the reaction to occur, non-spontaneous process, cathode is negative electrode, anode is positive electrode, electrodes in same compartment, no salt bridge needed, only one electrode used
• Electrochemical cell


  Converts chemical energy to electrical energy, chemical reaction produces electrical energy, spontaneous process, cathode is positive electrode, anode is negative electrode, electrodes in separate compartments, salt bridge required, two electrodes used
• PRINCIPLES OF ELECTROLYSIS

  Cations and anions

  An electrolyte composed of positive and negative ions.

  During electrolysis, electrons move from the negative pole of the battery to the cathode, making

  it to be electron-rich and negatively charged. Hence, positive ions migrate to the cathode; and are called cations. The anode becomes electron-poor and positively charged. Hence, negatively

  charged ions migrate to the anode, and are called anions.
• CATIONS

  If two cations are present in

  an electrolyte the cation

  below the other one in the

  electrochemical series will be

  preferentially discharged.

  ANIONS

  If two anions are present in

  electrolyte, the anions that is

  above the other one in the

  electrochemical series will be

  preferentially discharged
• Inert electrodes are the electrodes that will tamper with the electrolysis. Examples are

  carbon (graphite) and platinum.
• At the cathode: Both Na+ ions and H+ ions migrate to the cathode where H+ ions are preferentially discharged because they are below Na+ ions in the electrochemical series. At the cathode: Both Na+ ions and H+

  ions migrate to the cathode where H+ ions are preferentially

  discharged because they are below Na+ ions in the electrochemical series.
• CONCENTRATION OF IONS IN THE ELECTROLYTE

  At the anode: Both, Cl- and OH- will migrate to the anode where Cl ions will be preferentially discharged because they are more concentrated. (i.e. they are more in concentration inside the

  electrolyte).

  At the cathode: Both the Na+ and H+ will migrate to the cathode where H+ will be preferentially discharged. This is because even though Na+ ions are more in concentration in the electrolyte, the distance between Na+ and H+ in the electrochemical series is very far.
• 3. EFFECT OF THE NATURE OF ELECTRODE

  At the anode: Both SO4 2- and OH- will migrate to the anode but none of them will be discharged.

  The reason is that, the anode (which is a copper metal) has the nature as the metallic ions (copper

  (II) ions) in the electrolyte.

  At the cathode: Both the Cu2+ ions and H+ ions will migrate to the cathode where Cu2+ ions will be preferentially discharged.
• Electrochemical series


  Also known as reactivity series, an arrangement of the standard electrolyte potentials of metal ions/metal half cells and those of non-metal/non-metal half cells in order of starting from the most negative to the most positive
• Electrochemical series


  • Strongly electropositive series
  • Strongly electronegative elements (usually non-metals and oxidizing agents)
• Electrochemical series


  • Strongly electropositive series at the top
  • Strongly electronegative elements (usually non-metals and oxidizing agents) at the bottom
• Allotropy


  The existence of an element in two or more different forms in the same physical state
• Allotropes of sulphur


  • Rhombic (or α-sulphur)
  • Monoclinic or prismatic (β-sulphur)
  • Amorphous sulphur (S-sulphur)
  • Plastic sulphur
• Rhombic sulphur (α-sulphur)


  • Free sulphur exists as allotrope at below 96°C
  • Has octahedral structure made up of S8 molecules
  • Colour is brightly yellow
  • Melting point of 113°C
  • Density of 2.08g/cm3
• Monoclinic sulphur (β-sulphur)


  • The only stable allotrope between 96°C and 119°C
  • Consists of long, thin and needle shaped
  • Colour is amber
  • Changes to rhombic sulphur crystals at room temperature
  • Melting point of 119°C
  • Density of 1.98g/cm3
• Amorphous sulphur


  • Pale-yellow in colour
  • Has no regular crystalline shape
• Plastic sulphur


  Prepared by heating yellow sulphur until it boils in a test tube
• Properties of sulphur


  • Yellow solid existing in crystalline or amorphous form
  • Non-metallic and exhibits allotropy
  • Sublimes to give flowers of sulphur
  • Non-conductor of heat and electricity