Exam 4

• Oxidation-reduction (REDOX) reactions

  Definitions of oxidation, reduction, oxidizing and reducing reagents; oxidation numbers
• Balancing redox reactions using the half-reaction method

  1. Balance mass and charge for each half-reaction

  2. Treat electrons as products/reactants for oxidation/reduction

  3. Combine the two half-reactions to get the overall redox reaction

  4. Account for H+/H2O/OH- as reactants/products

  5. Balance acidic reactions by adding H+ to "mop up" O atoms as H2O

  6. Balance basic reactions by treating as acidic and then adding OH- to neutralize H+
• Voltaic (galvanic) electrochemical cell

  • Two half-cells connected, spontaneous redox reaction produces electric current
  • Circuit completed by salt bridge, allows ion movement but no mixing of solutions
  • Each half-cell contains an electrode in a solution of corresponding ions, equilibrium between solution and surface
  • Reduction occurs at cathode, oxidation occurs at anode
  • Cell potential (Ecell) is driving force, measured in volts (V)
  • Cell represented with anode on left, cathode on right, phase boundaries and salt bridge
• Standard reduction potentials (E°)

  • Quoted relative to standard hydrogen electrode (E=0.000V)
  • All half-reactions written as reductions, most negative E° value becomes oxidation
  • E°cell = E°cathode - E°anode
  • Reaction is spontaneous if E° is positive (or E>0 for non-standard conditions)
  • Reversing a reaction changes the sign of E°
• Relationship between E°, G°, and K

  • G° = -nFE°
  • E° > 0 when G° < 0
  • E° = (RT/nF) lnK
• Nernst Equation


  • Ecell = E°cell - (0.0591/n) logQ
  • Relates non-standard cell potential to standard cell potential
  • Can calculate E for non-standard conditions or concentrations (Q)
• Concentration cell

  • Identical half-cells with different solute concentrations
  • Electrode reactions cause dilute solution to become more concentrated and vice versa
  • E°cell = 0
• Electrolysis

  • External power source drives redox process in non-spontaneous direction
  • Products predicted by considering lowest applied external voltage
• Quantitative electrolysis

  1. Consider passage of current (charge per unit time)

  2. Determine moles of electrons delivered using Faraday constant

  3. Stoichiometric calculation using balanced half-reaction equation
• Corrosion of Fe and other metals

  • Caused by exposure to atmospheric O2 and H2O
  • Can be minimized by protection from air/moisture or providing electrons from another metal
• Properties of subatomic particles

  • Mass
  • Charge
• Nuclei can spontaneously emit subatomic particles and/or electromagnetic radiation - this is radioactivity
• Representing radioactive decay using a nuclear equation

  Mass number and atomic number must balance on both sides
• Principal types of radioactive decay

  • Alpha (α) emission
  • Beta (β-) emission
  • Gamma (γ) emission
  • Electron capture - relies on the reactants side instead of products
  • Positron (β+) emission
• Alpha (α) emission


  • Nucleus emits an alpha particle (4He nucleus)
  • A decreases by 4 units
  • Z decreases by 2 units
• Beta (β-) emission


  • Nucleus emits an electron
  • A is unchanged
  • Z increases by 1 unit
• Gamma (γ) emission


  • Nucleus emits a photon of gamma radiation
  • No change in A or Z
• Electron capture

  • Z decreases by 1 unit
  • No change in A
• Positron (β+) emission


  • Z decreases by 1 unit
  • No change in A
• The "daughter" nuclide formed by decay of a radioactive "parent" nuclide can be predicted by knowing the nature of the process and the identity of the radiation produced and then balancing the appropriate nuclear equation
• Nuclear stability

  • Stable nuclei lie within a "valley of stability" on a plot of neutron number (N) versus Z
  • Unstable nuclei outside this area will decay in predictable ways to move closer to the area of stability
  • No stable nuclei above Z = 83
• Radioactive decay follows first order kinetics

  • Decay rate is proportional to the number of atoms present (N)
  • Rate = kN
  • ln(Nt/N0) = -kt
  • Half life t½ = 0.693/k
• E = mc² - this equation relates mass and energy, which are jointly conserved but can be converted into each other
• Mass defect - some mass is "lost" when a nucleus is formed from protons+neutrons, this becomes the binding energy that holds the nucleus together
• Nuclear fission

  1. Breakup of a heavy nucleus into lighter fragments

  2. Initiated by neutrons

  3. Often more neutrons are released than are put in, which can rapidly propagate a chain reaction
• Nuclear fusion

  1. Combining lighter nuclei into a heavier one

  2. Requires a lot more energy to initiate than fission because two positively charged nuclei are being forced together
• Energy changes in fission/fusion reactions can be calculated using exact nuclear masses and E = mc²