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Bellwork: Complete and balance the following equations. Identify the
oxidizing and reducing agents.
1. MnO4- (aq) + CH3OH (aq) Mn 2+ (aq) + HCO2H (aq) (acidic solution)
2. Pb(OH)4 2- (aq) + ClO- (aq) PbO2 (s) + Cl – (aq) (Basic Solution)
Electrochemistry Terminology #1
Oxidation – A process in which an element attains a more positive oxidation state
Na(s) Na+ + e-
Reduction – A process in which an element attains a more negative oxidation state
Cl2 + 2e- 2Cl-
Electrochemistry Terminology #2
Gain Electrons = Reduction
An old memory device for oxidation and reduction goes like this…
LEO says GER
Lose Electrons = Oxidation
Electrochemistry Terminology #3
Oxidizing agent
The substance that is reduced is the oxidizing agent
Reducing agent
The substance that is oxidized is the reducing agent
Electrochemical Cells
GALVANIC / VOLTAIC
• Spontaneous chemical reaction
• Often used as batteries
ELECTROLYTIC
• Nonspontaneous chemical reaction
• Driven by outside power source
BOTH types above are considered electrochemical cells!
Electrochemistry Terminology #4
Anode
The electrode where oxidation occurs
Cathode
The electrode where reduction occurs
Memory device:
Reduction at the
Cathode
GALVANIC / VOLTAIC CELLS Galvanic / Voltaic cells have the following parts:
• Anode: electrode where oxidation occurs; may appear to shrink with time
• Cathode: electrode where reduction occurs; may appear to grow with time
• Inert electrodes (optional): Usually platinum (Pt) of graphite; used when gases are involved or when species are oxidized or reduced but remain ions (i.e. Fe2+ Fe3+)
• Salt Bridge: maintains electrical neutrality in a galvanic/voltaic cell by allowing non-reactive ions to migrate into and out of cells; may be replaced with a porous cup
GALVANIC / VOLTAIC CELLS Other Aspects:
• Electrons ALWAYS flow FROM anode TO cathode in
galvanic/voltaic cells!
• Standard Cell Notation: anode | solution (conc) || solution (conc) | cathode
• Voltmeter: high input impedance device used to measure the
potential difference between the cathode and anode in an electrochemical cell without allowing current to flow; this quantity is also known as cell emf or cell potential (Ecell) and is often measured in volts (joule/coulomb, J/C)
No current flows here…circuit is not complete. There’s a massive resistance in the air between the cells.
In this system, the salt bridge supplies ions to complete the circuit. They flow as needed to counteract the flow of electrons. Cations flow to the cathode cell and anions flow to the anode cell.
Standard Reduction Potential
• Every half-reaction cell has a potential, which is measured against a standard cell
• The standard of choice is often the Standard Hydrogen electrode (SHE), which is made of a piece of platinum that is in contact with H2 gas at 1 atm and a 1M solution of H+ ions
• The SHE is assigned a value of 0.0V
• Standard reduction Potentials are tabled at Standard Conditions (duh!)
• Standard Conditions: 1 atm for gases, 1M for solutions, and 298K for all species
• We again use the symbol “naught” ( ) to indicate standard conditions, much like in thermochemistry, i.e. potential at standard conditions is E
• Values are always tabled as reductions, whether or not the species is undergoing reduction in the cell!
Standard Reduction Potential
NOTE: REACTION OCCURING ABOVE HAS ZINC BEING OXIDIZED AND H+ BEING REDUCED WITH A POSITIVE POTENTIAL…THIS MEANS ZINC WILL HAVE A NEGATIVE REDUCTION POTENTIAL!!
Measuring Standard Electrode Potential
Potentials are measured against a hydrogen ion reduction reaction, which is arbitrarily assigned a potential of zero volts.
A Closer Look…
The oxidation-reduction reaction is spontaneous. A solution containing K2Cr2O7 and H2SO4 is poured into one beaker, and a solution of KI is poured into another. A salt bridge is used to join the beakers. A metallic conductor that will not react with either solution (such as platinum foil) is suspended in each solution, and the two conductors are connected with wires through a voltmeter or some other device to detect an electric current. The resultant voltaic cell generates an electric current. Indicate the reaction occurring at the anode, the reaction at the cathode, the direction of electron migration, the direction of ion migration, and the signs of the electrodes.
Standard Reduction Potential There are some generalities that can be drawn from these values… • Elements with the most positive reduction potentials are easily
reduced (generally nonmetals)
• Elements with the least positive reduction potentials are easily oxidized (generally metals)
• The standard potentials can also be used to determine the relative strength of oxidizing and reducing agents
• Can be used as an activity series (This is why AP doesn’t give you the activity series as you are accustomed to seeing it)
• If you are trying to produce a Galvanic/Voltaic cell…MORE POSITIVE REDUCTION POTENTIAL GETS REDUCED!
Calculating Standard Cell Potential for Voltaic Cells
1. Decide which element is oxidized or reduced; MORE POSITIVE REDUCTION POTENTIAL GETS REDUCED!
2. Assign the oxidized species as the Anode and the reduced species as the cathode.
3. Balance the RedOx Equation using the half reactions from the table; DO NOT MULTIPLY POTENTIALS UNDER ANY CIRCUMSTANCE!
4. 𝐸𝑜𝑐𝑒𝑙𝑙 = 𝐸𝑜
𝑐𝑎𝑡ℎ𝑜𝑑𝑒 − 𝐸𝑜𝑎𝑛𝑜𝑑𝑒
5. IF Ecell is POSITIVE, the process is SPONTANEOUS
6. If Ecell is NEGATIVE, the process is NONSPONTANEOUS
Table of Reduction Potentials
Measured against
the Standard Hydrogen Electrode
Answer: -.40V
EXAMPLE A voltaic cell is based on a Co2+/Co half cell and a AgCl/Ag half-cell. (a) Write the line notation for this cell; (b) What is the standard cell potential?
EXAMPLE A voltaic cell is based on a Co2+/Co half cell and a AgCl/Ag half-cell. (a) Write the line notation for this cell; (b) What is the standard cell potential?
