CHEM 1C Chapter Notes - Chapter 20: Strong Electrolyte, Electromotive Force, Voltage

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Published on 13 Oct 2017
School
UC-Irvine
Department
Chemistry
Course
CHEM 1C
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Chapter 20: Electrochemistry
20.1 Lightening and Batteries
§ Electrons flow away from negative charge and toward positive charge.
20.2 Balancing Oxidation-Reduction Equations
20.3 Voltaic (or Galvanic) Cells: Generating Electricity from Spontaneous Chemical
Reactions
§ Electrical current is the flow of electric charge. Electrons flowing through a wire or ions
flowing through a solution both constitute electrical current. Since redox reactions involve
the transfer of electrons from one substance to another, they have the potential to generate
electrical current
§ The generation of electricity through redox reactions is normally carried out in a device
called an electrochemical cell
o A voltaic (or galvanic) cell, is an electrochemical cell that produces electrical cur-
rent from a spontaneous chemical reaction
o electrolytic cell, consumes electrical current to drive a nonspontaneous chemical
reaction
§ The strips act as electrodes, conductive surfaces through which electrons can enter or
leave the half-cells
o Each metal strip reaches equilibrium with its ions in solution according to these
half-reactions:
§ Zn(s) à Zn2+(aq) + 2 e-
§ Cu(s) à Cu2+(aq) + 2 e-
o However, the position of these equilibria—which depends on the potential energy
of the electrons in each metal—is not the same for both metals. The electrons in
zinc have a higher potential energy and therefore zinc has a greater tendency to
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ionize than copper, so the zinc half-reaction lies further to the right. As a result, the
zinc electrode becomes negatively charged relative to the copper electrode.
§ A large potential difference corresponds to a large difference in potential energy between
the two electrodes and therefore a strong tendency for electron flow (analogous to a
steeply descending stream- bed). Potential difference, because it gives rise to the force that
results in the motion of electrons, is also referred to as electromotive force (emf ). In a
voltaic cell, the potential difference between the two electrodes is the cell potential (Ecell)
or cell emf
§ Anode, oxidation, negative sign
§ Cathode, reduction, positive sign
§ Electrons flow from anode to cathode
§ As electrons flow out of the anode, positive ions (Zn2 + in the preceding example) form in
the oxidation half-cell, resulting in a buildup of positive charge in the solution. As electrons
flow into the cathode, positive ions (Cu2 + in the preceding example) are reduced at the
reduction half-cell, resulting in a buildup of negative charge in the solution.
o If the movement of electrons from anode to cathode were the only flow of charge,
the buildup of the opposite charge in the solution would stop electron flow almost
immediately
o This is why we have a salt bridge, which contains a strong electrolyte such as
KNO3 and connects the two half-cells
o The salt bridge allows a flow of ions that neutralizes the charge buildup in the
solution. The negative ions within the salt bridge flow to neutralize the
accumulation of positive charge at the anode, and the positive ions flow to
neutralize the accumulation of negative charge at the cathode. In other words,
the salt bridge completes the circuit, allowing electrical current to flow.
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Electrochemical Cell Notation
§ for some redox reactions, the reactants and products of one or both of the half-reactions
may be in the same phase. In these cases (which we explain further next), we separate the
reactants and products from each other with a comma in the line diagram. Such cells use
an inert electrode, such as platinum (Pt) or graphite, as the anode or cathode (or both)
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Document Summary

Electrons flow away from negative charge and toward positive charge. 20. 3 voltaic (or galvanic) cells: generating electricity from spontaneous chemical. Electrical current is the flow of electric charge. Electrons flowing through a wire or ions flowing through a solution both constitute electrical current. Since redox reactions involve the transfer of electrons from one substance to another, they have the potential to generate electrical current. The strips act as electrodes, conductive surfaces through which electrons can enter or leave the half-cells: each metal strip reaches equilibrium with its ions in solution according to these half-reactions: Cu(s) cu2+(aq) + 2 e: however, the position of these equilibria which depends on the potential energy of the electrons in each metal is not the same for both metals. The electrons in zinc have a higher potential energy and therefore zinc has a greater tendency to ionize than copper, so the zinc half-reaction lies further to the right.