Electrochemistry: Common Mistakes to Avoid for AP Chemistry
Electrochemical experiments fall into two broad categories. Some experiments are concerned with standard cell voltages, while other experiments use the Nernst equation directly or indirectly. Experiment 21 uses these concepts.
Measurements of the cell potential are essential and require a voltmeter (potentiometer). These measurements may be taken from different combinations of half-cells, or from measurements before and after changes of some aspect of the cell were made.
Using measurements of different half-cell combinations, a set of "standard" reduction potentials may be constructed. This set will be similar to a table of standard reduction potentials. The solutions used in the half-cells must be of known concentration. These solutions are produced by weighing reagents and diluting to volume. The measurements will require a balance and a volumetric flask. It is also possible to produce known concentrations by diluting solutions. This method requires a pipette and a volumetric flask. Review the Solutions and Colligative Properties chapter for solution techniques.
Experiments involving the Nernst equation are primarily concerned with concentrations. One or more of the concentrations in the Q portion of the Nernst equation are calculated by measuring the nonstandard cell potential and comparing this to the standard cell potential. Remember, you calculate the concentration from a measured voltage. Once the concentration is determined, it may be combined with other concentrations and used to calculate an equilibrium constant.
Common Mistakes to Avoid
- Be sure your units cancel to give the unit wanted in your final answer.
- Be sure to round your answer off to the correct number of significant figures.
- Remember that oxidation is the loss of electrons and reduction the gain, and that in redox reactions the same number of electrons is lost and gained.
- Be sure that what you determine to be the oxidizing and reducing agents are reactant species.
- When diagramming an electrochemical cell, be sure the electrons go from anode to cathode.
- Be sure that for a galvanic cell, the cell potential is greater than 0.
- Be sure to label the anode as (–) and the cathode (+) in a galvanic cell.
- In cell notation, be sure to write anode, anode compartment, salt bridge, cathode compartment, cathode in this specific order.
- When using a multiplier to equalize electron loss and gain in reduction half-cell potentials, do not use the multiplier on the voltage of the half-cell.
- When using the Nernst equation, be sure to use the correct form of the reaction quotient, products over reactants, and each concentration raised to the power the same as the coefficient in the balanced reaction.
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