Chapter 23 REDOX & ELECTRODE POTENTIALS

• What is meant by the term oxidation number?

  The number of electrons which must be added or removed to get a neutral atom
• RULES OF OXIDATION NUMBERS:

  1. On simple ions, oxidation state is the charge on the ion
  2. Elements have oxidation number 0
  3. Total oxidation states must equal overall charge
  4. In molecules, the more electronegative element will take the negative charge
  5. Hydrogen is nearly always +1 EXCEPT in hydrides, its -1
  6. Oxygen is nearly always -2 EXCEPT in peroxides, its -1
• What happens in redox reactions?

  Electrons are transferred from one species to another
• What does an electrochemical cell control?

  The transfer of electrons between species to produce electrical energy
• What is a half cell?

  A half cell consists of an element with 2 oxidation states in electrical contact with each other
• What is the problem and solution of non-metal/non-metal ion half cells?

  PROBLEM = non-metals can't be used as the electrode because they cannot conduct electricity

  SOLUTION = a platinum inert electrode is used
• What is electrode potential of a half cell?

  Electrode potential indicates a half cells tendency to lose/gain electrons in an equilibrium
• What does a large positive E(0) mean?

  The half equation has a strong tendency to go from left to right
• What does a large negative E(0) mean?

  The half cell has a strong tendency to go from right to left
• How can you measure standard electrode potential?

  Connect a half cell to a standard hydrogen electrode and measure the emf on the voltmeter
• Why does the standard hydrogen electrode have an E(0) of 0V?

  By definition
• What is the standard electrode potential of a half cell E(0)?

  The electromotive force (emf) of a half cell compared with a standard hydrogen half cell measured under standard conditions of 298K, 100kPa (1 atmosphere) and 1 mol/dm3
• Using the SHE, what happens if H+ concentration is increased?

  E(0) becomes more positive as equilibrium shifts right to oppose the change
• Using SHE, what happens if H2 gas pressure is increased?

  E(0) becomes more negative as the equilibrium shifts left to oppse the change
• Using the SHE, what happens if the temperature is increased?

  E(0) becomes more negative as the equilibrium shifts in favour of the endothermic reaction to oppose the change
• What are the 3 types of electrochemical cells?

  Primary cells

  Secondary cells

  Fuel cells
• What are primary cells?

  Primary cells are non-rechargeable batteries that are designed for single use and cannot be recharged.
• Why are primary cells single-use only?

  Once the chemicals are used up, the battery is flat and cell is discarded

  The reactions at the electrodes cannot be reversed
• Where are primary cells used?

  Wall clocks and smoke detectors
• What are secondary cells?

  Rechargeable batteries
• Why are secondary cells rechargeable?

  The electrical energy-producing reaction is reversible and is able to regenerate the chemicals
• Where are secondary cells used in?

  Car batteries (Lead-Acid)

  Radios, torches (Nickel-Cadmium/Nickel-Metal Hydride)

  Mobile phones, laptops (Lithium-ion/Lithium-ion polymer)
• Why are lithium-ion cells used?

  Lithium is a light metal with high density energy

  Its a flexible polymer formed in various shapes and sizes
• What are the downsides to lithium-ion cells?

  They can become unstable at high temperatures and ignite
• How do fuel cells generate electricity?

  Energy from the reaction of fuel with oxygen creates voltage. Fuel and oxygen flow into the fuel cell together and the products flow out with the electrolyte remaining within
• Why are fuel cells good?

  They are able to operate continuously provided they are supplied continuously

  They do not have to be recharged
• What is the most common type of fuel cell?

  Hydrogen fuel cell
• Why are hydrogen cells the most common?

  Water is the only combustion product and no carbon dioxide is produced
• How are cells made?

  Connecting 2 half cells together with different electrode potentials
• How do cells generate electricity?

  One half cell will release electrons and the other half cell will accept the electrons
• What is the purpose of the salt bridge?

  Neutral bridge that connects 2 solutions allowing ions to be transferred between half cells to keep the reaction flowing and prevent build up at the electrodes
• Which direction does current flow in cells?

  From the negative terminal (most negative E(0) value) to the positive terminal (most positive E(0) value)
• What is the standard cell potential?

  The different between the standard electrode potentials of each cell
• How do you calculate the E(0) of the cell?

  E(0) most positive - E(0) most negative
• What is meant by the term cell reaction?

  The overall chemical reaction taking place in the cell (the sum of the reduction and oxidation half equations)
• How do you calculate overall E(0) of the reaction?

  E(0) reaction left to right - E(0) reaction right to left
• What does the overall E(0) reaction value indicate?

  Whether or not a reaction will take place. If E(0) is positive, the reaction will take place. If E(0) is negative, the reaction wont take place
• What are the limitations of using E(0) to predict if a reaction will take place?

  The reaction may not happen under standard conditions (e.g. concentration isn't 1M or temperature isn't 298K

  A high activation energy needed therefore the reaction is slow

  E(0) applies only for aqueous equilibria and not all reactions take place in aqueous solutions
• Electrolyte

  A solution that carries ions and enables the flow of electric charge between the electrodes.