electrode potentials and electrochemical cells

• electrochemical cells

  -generate electricity from spontaneous redox reactions

  -instead of electrons being transferred directly from one ion to the other, a cell is build which separates the two redox processes

  -if a rod of metal is dipped into a solution of its own ions an equilibrium is set up
• half cell

  -a single electrode immersed in a solution of its ions

  -position of equm determines the potential difference between strip and solution
• potential difference depends on

  -nature of ions in solution

  -conc of the ions in solution

  -type of electrode used

  -temperature
• electrode potential

  -a value which shows how easily a substance is reduced

  -these are demonstrated by using reversible half equations
• salt bridge

  -has mobile ions that complete the circuit

  -KCl and KNO3 are commonly used as chlorides and nitrides are usually soluble

  -this should ensure that no precipitates form which can effect equm pos of half cell
• standard electrode potential

  The voltage measured under standard conditions when a half cell is connected to a standard hydrogen electrode.
• standard hydrogen electrode

  -half cell used as a reference electrode and consists of - hydrogen gas in equm with H+ ions of 1.00 moldm-1 at 100kPa

  -also contains an inert platinum electrode
• EMF

  Ecell = Ereduction - Eoxidation
• direction of electron flow

  -negative pole to positive pole
• feasibility

  -when the Ecell is positive

  -more +ve value, easier it is to reduce the species of the left half of the equation - reaction will go in the forward direction
• commercial cells

  -Electrochemical cells can be a useful source of energy for commercial use

  -They can be produced to be non-rechargeable, rechargeable or fuel cells

  -type of cell used in commercial application depend on:

  voltage required

  current needed

  size of the cell

  the cost

  -battery = collection of cells
• non-rechargable cells

  -earliest electrochemical cell - simple metal-ion system

  -invented by John Dnaniell in 1836

  -consists of:

  zinc rod immersed in zinc sulfate solution

  copper cylinder filled with copper sulfate solution

  porous pot that separates the copper sulfate from the

  the zinc sulfate

  -zinc acts as a -ve electrode copper is +ve electrode
• zinc-carbon cells

  -most common type of non-rechargeable cell consists:

  -a zinc casing which acts as the -ve electrode

  -a paste of ammonium chloride acting as an electrolyte

  as well as a +ve electrode

  -a carbon rod acting as an electron carrier in the cell
• why use zinc carbon cells

  -cell provides small current and is relatively cheap compared to other cells

  -as cell discharges, zinc casing wears away and corrosive contents of electrolyte paste leak out

  -extra long life cells have a similar chemistry, but supply higher current and use zinc chloride in the paste

  -another variation uses alkaline paste and have longer life but are much more expensive than regular zinc-carbon cells
• rechargeable cells

  -employ chemical reactions which can be reversed by applying a voltage greater than the cell voltage

  -causes e- to be pushed in the opposite direction

  -eg. lead-acid batteries, NiCad cells, lithium cells
• lead acid battery

  -six cells joined in a series

  -lead as -ve electrode and lead(IV) oxide as +ve electrode

  -electrolyte is sulfuric acid

  -in a car battery, six cells will give 12V

  -when car is in motion, generator provides a push of e- that reverses the reaction and generates lead(IV) oxide

  -designed to produce a high current for a short time
• lead acid battery - disadvantage

  -heavy

  -contains toxic materials: lead and lead(IV) oxide

  -sulfuric acid electrolyte is very corrosive
• NiCad cells

  -nickel-cadmium

  -available in many standard sizes and voltages so can replace almost any application of zinc-carbon cell

  -more expensive but can be recharged hundreds of times

  -they are commercially viable

  - +ve electrode consists of cadmium

  - -ve electrode made of nickel(II) hydroxide-oxide system
• NiCad disadvantage

  -cadmium is a toxic metal so disposal of old NiCad cells is an environmental issue

  -have a problem called the memory effect in which they gradually begin to lose their charge after repeated charge cycles when the cell is not fully discharged

  -the cells appear to 'remember' their lower state of charge
• Lithium cells

  -used in laptop or mobile devices

  -lithium is used because it has a very low density and high electrode potential

  -cell consists of: +ve lithium cobalt oxide electrode, -ve carbon electrode, a porous polymer membrane electrolyte

  -polymer electrolyte cannot leak since its not a liquid or paste - advantage over other cells
• lithium ion cells

  -cell consists of a sandwich of different layers of lithium cobalt oxide and carbon

  -when cell is charged and discharged, lithium ions flow between +ve and -ve through the solid electrolyte
• lithium ion cells disadvantages

  -a global storage of lithium is likely to make lithium ion cells unsustainable as the current demand for lithium exceeds the supply

  -if cells are thrown away in landfills, huge amount of lithium become lost to future generations

  -lithium ion cell fires have raised concerns about their safety - lithium is a very reactive element
• fuel cells

  -electrochemical cell in which a fuel donated e- at one electrode and oxygen gains e- at the other

  -becoming common in automotive industry to replace petrol or diesel engines

  -as fuel enters the cell it becomes oxidised - sets up PD within cell
• fuel cell consists of

  -a reaction chamber with separate inlets for hydrogen and oxygen gas

  -an outlet for the product - water

  -an electrolyte of aq NaOH

  -a semi permeable membrane that separates the hydrogen and oxygen gases
• fuel cell benefits

  -water is the only product so environmentally advantageous

  -reaction is same as hydrogen combusting in oxygen, since it takes place at room temp, all the bond energy is converted into electrical energy instead of heat and light

  -no harmful oxides of nitrogen produced

  -have been used on space craft where product can be used as drinking water for astronauts
• fuel cell risks and problems

  -hydrogen is highly flammable and the production and storage of it carries safety hazards

  -very thick walled cylinders and pipes are needed to store hydrogen which has economic impacts

  -relies on a non-renewable, finite source

  -until a cheaper way is found, they will be limited

  -hydrogens energy density per unit volume is low which means larger containers are needed compared to liquid fuel
• Types of cells

  Fuel cells

  
  

  Non-rechargeable

  • zinc carbon cell
  • lithium

  Rechargeable

  • lead-acid
  • Nicad