Part I: Balancing Oxidation Reduction Reactions

1
Part I Balancing Oxidation Reduction Reactions

• Tuesday, July 24th
• CHM 102

2
Redox Reactions

• What do we typically conserve in non-redox
  reaction?
• Now weve got to take into account the number
  electrons transferred!
• If we dont balance a redox reaction properly,
  what were going to end up with is electrons
  being created or destroyed, and that isnt going
  to happen!

3
Redox Reactions

• What weve already said is that when a reduction
  process occurs, it is always paired to an
  oxidation process.
• A redox reaction can be effectively split into
  two half-reactions- the reduction half, and the
  oxidation half.

4
half-reactions

• Take the redox reaction below
• Sn2(aq) 2Fe3(aq) ? Sn4(aq) 2Fe2(aq)
• What is being oxidized? Reduced? Which is the
  reducing agent? Which is the oxidizing agent?
• The reaction can be split into two halves
• Oxidation Sn2(aq) ? Sn4(aq) 2e-
• Reduction Fe3(aq) e- ? Fe2(aq)
• Why did we need two irons per tin?

5
Now we get to balancing!

• WORKSHEET TIME!

6
Part II Volatic Cells, Cell EMF, and
Spontaneity of Redox Reactions

• Tuesday, July 24th
• CHM 102

7
Voltaic Cells

• Voltaic cells contain substances that go through
  a spontaneous redox reaction that releases energy
  that can be used to do work.
• Voltaic cells power your car, cell phone, and
  your everyday electronic gadgetry.

8
Voltaic Cells
9
Why does the salt bridge matter?

• The salt bridge retains electrical neutrality in
  the solutions the electrodes are sitting in.
• For example, Zn2 ions are dissolving off of the
  anode, and sulfate ions from the salt bridge will
  flow into the anode half of the cell to remain
  charge neutrality in the solution.

10
What does the voltmeter measure?

• The voltmeter measures electromotive force, or
  EMF, and is measured in volts.
• Cell EMF is abbreviated further as Ecell.
• A positive potential corresponds to a voltaic
  cell.
• The EMF is the force that drives the electrons
  through the circuit from the anode to the
  cathode.
• Demo Pouring water and Potential.

11
Standard cell potential

• The standard cell potential is the cell EMF
  measured under standard conditions.
• Standard conditions are at 250C where
  concentration of reactants and products in
  solution are 1 M and pressure of gaseous
  reactants and products are 1 atm.
• Standard cell potential is abbreviated as Eocell.

12
Calculating Eocell

• To calculate standard cell potentials, we use
  tables called standard reduction potential
  tables.
• Standard reduction potential is the potential
  associated with a given half-reaction.
• The more positive the reduction potential, the
  easier the substance is to reduce, and the
  stronger the oxidizing agent it is!

13
Why arent oxidation potentials tabulated?

• We could sit down and tabulate the reduction
  potentials, but that would be a huge waste of
  time! The standard oxidation potential is just
  the negative of the standard reduction potential!

14
Lets try one!

• Youve got the copper-zinc cell
• Zn(s) Cu2(aq) ? Zn2(aq) Cu(s)
• Oxid Zn(s) ? Zn2(aq) 2e-
• Red 2e- Cu2(aq) ? Cu(s)
• Eocell Eored(cathode) Eored(anode)
• Eocell (.34) (-0.76)
• Eocell 1.10 V

15
Practice Problem

• Youve got the cell
• In(aq) ? In3(aq) 2e-
• Cl2(l) 2e- ? 2Cl-(aq)
• Which is the oxidation half? Which is the
  reduction half? (Eocell 1.76 V) What is the
  standard reduction
• potential for In3(aq) being
• reduced to In(aq)?

16
Cell potential and Stoichiometry

• Youve got the copper-zinc cell
• Zn(s) Cu2(aq) ? Zn2(aq) Cu(s)
• So our cell potential is 1.10 V for this cell
• What if we had a cell and just tripled all the
  materials in the cell?
• 3Zn(s) 3Cu2(aq) ? 3Zn2(aq) 3Cu(s)
• It wouldnt affect the cell potential! Think of
  our waterfall view of potential. If you add more
  water, it doesnt change how far the water drops!

17
Tying it all together

• A voltaic cell is based on the following 2
  standard half-reactions
• Ag(aq) e- ? Ag(s)
  Eored 0.80 V
• Co2(aq) 2e- ? Co(s)
  Eored -0.277 V
• Which half reaction occurs at the cathode?
• What is the Eocell?