ELECD418: Studying Electrochemical Cells and Reduction Potenial Pre-Laboratory A
ID: 1046940 • Letter: E
Question
ELECD418: Studying Electrochemical Cells and Reduction Potenial Pre-Laboratory Assignment 1. Briefly explain why it is important to wash your hands after you complete this experiment, and before you leave the laboratory 2. Briefly define the following terms as they relate to this experiment. (1) reference electrode (2) oxidation half-reaction (3) salt bridge (4) cell potential 3. Briefly distinguish between the reaction that occurs at the anode and the reaction that occurs at the cathode of a galvanic cell.Explanation / Answer
1. Hand washing is a primary safeguard against inadvertent exposure to toxic chemicals or biological agents. Always wash your hands before leaving the laboratory, even though you use gloves. Wash your hands after removing soiled protective clothing, before leaving the laboratory, and before eating, drinking, smoking, or using a rest room. Wash your hands periodically during the day at intervals dictated by the nature of your work. Wash with soap and running water, with hands held downward to flush the contamination off the hands. Turn the tap off with a clean paper towel to prevent recontamination, and dry your hands with clean towels.
2.
a) In potentiometry, two electrodes are generally used - the indicator electrode and the reference electrode. The indicator electrode possesses some characteristic that allows it to selectively respond to changes in the activity of the analyte being measured. For the measured potential to have meaning in this context, the reference electrode must be constructed so that its composition is fixed and its response is stable over time, with observed changes in measured potential due solely to changes in analyte concentration.
b) Fundamentally, redox reactions are a family of reactions that are concerned with the transfer of electrons between species. Like acid-base reactions, redox reactions are a matched set -- you don't have an oxidation reaction without a reduction reaction happening at the same time. Oxidation refers to the loss of electrons, while reduction refers to the gain of electrons. Each reaction by itself is called a "half-reaction", simply because we need two (2) half-reactions to form a whole reaction. In notating redox reactions, chemists typically write out the electrons explicitly:
Cu (s) ----> Cu2+ + 2 e-
This half-reaction says that we have solid copper (with no charge) being oxidized (losing electrons) to form a copper ion with a plus 2 charge.
c) In chemistry, a salt bridge is simply a combination of two noncovalent interactions i.e. electrostatic interactions and hydrogen bonding. Since, noncovalent interactions are relatively weak interactions, formation of salt bridges provide an additional level of stabilizing interactions that can add important contribution to the overall stability of a molecule. Salt bridges are commonly observed in proteins where they provide stability to the entropically unfavorable folded conformation of proteins.
In electrochemistry, salt bridge is defined as a device that is used to provide electrical contact between two solutions i.e. A tube used to connect the oxidation and reduction half-cells of a galvanic cell (voltaic cell), a type of electrochemical cell.
d) The cell potential, Ecell, is the measure of the potential difference between two half cells in an electrochemical cell. The potential difference is caused by the ability of electrons to flow from one half cell to the other. Electrons are able to move between electrodes because the chemical reaction is a redox reaction. A redox reaction occurs when a certain substance is oxidized, while another is reduced. During oxidation, the substance loses one or more electrons, and thus becomes positively charged. Conversely, during reduction, the substance gains electrons and becomes negatively charged. This relates to the measurement of the cell potential because the difference between the potential for the reducing agent to become oxidized and the oxidizing agent to become reduced will determine the cell potential. The cell potential (Ecell) is measured in voltage (V), which allows us to give a certain value to the cell potential.
3. When an electrode is oxidized in a solution, it is called an anode and when an electrode is reduced in solution. it is called a cathode.
When a redox reaction takes place, electrons are transferred from one species to the other. If the reaction is spontaneous, energy is released, which can be used to do work. Consider the reaction of a solid copper (Cu(s)) in a silver nitrate solution (AgNO3(s)).
2Ag+(aq)+Cu(s)?Cu2+(aq)+2Ag(s)
The AgNO3(s) dissociates in water to produce Ag+(aq) ions and NO3?(aq) ions. The NO3-(aq) ions can be ignored since they are spectator ions and do not participate in the reaction. In this reaction, a copper electrode is placed into a solution containing silver ions. The Ag+(aq) will readily oxidize Cu(s) resulting in Cu2+(aq), while reducing itself to Ag(s).
Anode (where oxidation occurs): Cu(s) ? Cu2+(aq) + 2e?
Cathode (where reduction occurs): Ag+(aq) + e? ? Ag(s)
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