G11 Chemistry, LC4, Section 17.2 notes
Section 17.2: Galvanic cells: Electricity from Chemistry
Electrochemical cells
- Batteries are electrochemical cells. In a battery, the two halves of a spontaneous redox reaction
are separated and made to transfer electrons through a wire.
- The lemon is a container for a solution of electrolyte, the lemon juice. Lemon juice is acidic. The
hydrogen ions from partially dissociated citric acid provide the ions for conduction of charge through
the lemon battery. The two dissimilar metal strips are the electrodes at which an oxidation and
reduction reactions take place.
- A potential difference between two substances is a measure of the tendency of electrons to flow
from one to the other.
- The electron pressure at the cathode is kept low by the reduction reaction, and the electrons flow
from a region of high pressure (negative potential at the anode) to a region of low pressure
(positive potential at the cathode). This potential difference between the electrodes in the lemon
battery causes an electrical current to flow.
Potential Difference
In this model of a lemon battery, the level of the electron sea is
raised or lowered by the chemical reactions at the electrode surfaces,
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creating a potential difference across the battery. A spontaneous
oxidation reaction raises the electron pressure (potential) at the
anode, and a spontaneous reduction reaction reduces the pressure at
the cathode. The “sea level” in the lemon juice is uniform throughout
and is intermediate between the levels at the two electrodes.
Because the redox reactions that take place during electrolysis
are not spontaneous, a battery is needed to pump
electrons from an area of low potential to one of high
potential.
Pumping electrons: From high potential (Anode where more negative potential and where
oxidation) to (Cathode where more positive potential and where reduction), but in the
non spontaneous electrolysis pumping electrons from low potential to high
- An electrical potential difference is called voltage and is expressed in units of volts in honor of
Alessandro Volta.
- An electrochemical cell in which an oxidation-reduction reaction occurs spontaneously to produce
a potential difference is called a galvanic cell. They are sometimes called voltaic cells.
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Galvanic cell
Salt bridge: allows ionic conduction to complete the electrical circuit and prevent a buildup of excess
charge at the electrodes.
- In a galvanic cell, chemical energy is converted into electrical energy.
-wires connect the cell to a light source with a low-voltage requirement, called a light-emitting diode or
LED. If the circuit to the cell is complete, the LED lights up, showing that the cell is doing useful work.
With time, the light intensity will fade. Why doesn’t it stay lit indefinitely? Eventually, all of the
magnesium in the anode becomes oxidized. The capacity of the battery has been exceeded, the
magnesium is gone and, if there is no electrode, there can be no cell.
- A galvanic cell that has been packaged as a portable power source is often called a battery.
Better and better batteries
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This table may be used as a general guide to the ease with which a substance will lose electrons. By
examining this table, you can see why copper, gold, and silver are the metals most commonly used in
jewelry. All three are hard to oxidize and are, thus, resistant to corrosion.
- The most powerful batteries combine strong oxidizing agents and strong reducing agents to give the
largest possible potential difference.
To get better battery;
1- The farther apart the two substances are in the table, the greater the potential difference between
the electrodes, and the greater the energy delivered by each electron that flows through the
external wire.
2- several of the cells can be connected in series
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Carbon-Zinc Dry Cell:
This type of battery is often called a dry cell because there is no aqueous electrolyte solution; a
semisolid paste serves that role.
The carbon rod in the center of the cylinder—surrounded by a moist, black paste of manganese(IV) oxide (MnO2) and
carbon black—acts as a cathode.
Ammonium chloride (NH4Cl) and zinc chloride (ZnCl2) serve as electrolytes.
Zinc shell is the anode
-What is meant by alkaline battery ? Alkaline batteries contain potassium hydroxide (KOH) in place of the ammonium
chloride electrolyte, and they maintain a high voltage for a longer period of time.
The major redox reaction taking place in a carbon-zinc dry cell.
1- When electrons leave the casing, zinc metal is oxidized.
2- The oxidation number of manganese is reduced from 4+ to 3+
3- Adding the two half-reactions together gives
Automobile Lead Storage Battery:
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Lead Storage Battery
A lead storage battery is not a dry cell because it contains several connected cells filled with an aqueous
solution of sulfuric acid(H2SO4), which serves as the electrolyte. The electrodes are alternating plates of lead metal(Pb)
and lead(IV) oxide(PbO2). The case surrounding the battery is hard plastic. It holds the cells in place and acts as an
insulator because it does not conduct electricity itself. This helps keep the electricity inside the battery.
The major redox reaction taking place in a Lead storage battery:.
1- The lead metal is oxidized to Pb2+ ions as it releases two electrons at the anode.
2- The Pb4+ ions in lead oxide gain two electrons, forming Pb2+ ions at the cathode.
3- The Pb2+ ions combine with SO42- ions from the dissociated sulfuric acid in the electrolyte solution
to form lead(II) sulfate at each electrode
- Thus, the net reaction that takes place when a lead-acid battery is discharged results in the formation of
lead sulfate at both of the electrodes.
- The reaction that occurs during discharge of a lead-acid battery is spontaneous and requires no energy
input. The reverse reaction, which recharges the battery, is not spontaneous and requires an input of
electricity from the car’s alternator. Current enters the battery and provides energy for the reaction in
which lead sulfate and water are converted into lead(IV) oxide, lead metal, and sulfuric acid.
Aqueous Lithium Battery:
This battery is less toxic and will probably be cheaper to manufacture than the nickel-cadmium batteries
used in most electric cars in operation today.
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How can a lithium battery have an aqueous electrolyte as lithium explodes violently when it comes into
contact with water.?
Two facets of the construction of this new battery keep the lithium metal from reacting with water. First,
the lithium is in the form of individual atoms embedded in a material such as manganese(IV) oxide,
rather than as a solid metal. Second, the electrolyte is full of dissolved lithium salts, so the lithium
ions that are produced travel to the site of reduction without reacting with water.
Very important questions:
1- What is the function of the salt bridge in a galvanic cell?
The salt bridge allows the electrical circuit to be completed by ionic conduction and prevents a buildup of excess charge at the
electrodes.
2- What is the function of the acid in the lead-acid storage battery used in cars?
The acid serves as an electrolyte.
3- What is the difference between an electrolytic cell and a galvanic cell?
An electrolytic cell requires an external source of electricity to power a redox reaction that is not spontaneous. A galvanic cell
requires no power source because the redox reaction is spontaneous.
4- What is a galvanic cell?
A galvanic cell is a chemical system that proceeds spontaneously while producing an electric current.
5-
What happens to the case of a carbon-zinc dry cell as the cell is used to produce an electric current?
The zinc metal in the case is oxidized to Zn2+ ions.
6-
How can you make a non-spontaneous redox reaction take place in a cell?
Energy in the form of electricity is applied to power the reaction.
7- Why is the electrolyte necessary in both galvanic and electrolytic cells?
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The electrolyte completes the circuit for the electric current to flow through.
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