1
Q
batteries
A
- batteries provide power to operate mobile devices or devices that are located far from electrical power lines.
2
Q
battery packs
A
- battery packs are used to operate camera traps set up in remote locations to capture photographs on animals that hide from humans.
- a sensor detects an animals presence, triggering the camera.
- the energy that the battery provides depends on redox reactions.
3
Q
chapter 9
A
- a zinc strip reacts with a solution containing copper(II) ions ot form zinc ions and metallic copper.
- the redox reaction is spontaneous, it occurs by itself, without an ongoing input of energy.
–> the reaction is exothermic, releasing heat in the form of energy.
4
Q
very first battery
A
- the very first battery contained both of these metals
- in 1800, alessandro volta, put a layer of saltwater-soaked paper between a copper disk and zinc disk
- the copper disk became positively charged the zinc disk became negatively charged
- if he attatched wires to them and connected the wires the electrons would flow from the zinc disk to the copper disk.
- he knew nothing about the chemical reaction that was happening, just that it was producing electrical energy.
5
Q
electric current & electrochemistry
A
- a flow of electric charges, such as electrons, is called an electric current.
–> a directional motion of electric charges. - the concepts of redox reactions and electric current form the basis of the field electrochemistry: the study of the processes that involve converted electrical energy to chemical energy, and vice versa.
6
Q
galvanis research
A
- volta wanted to research on this after reading a research paper done by Luigi Galvani.
- galvani was dissecting a frog, he touched a brass pin with a steel scalpel and noticed that the frog muscle twitches.
- his students were able to reproduce this reaction whenever they would touch different metals while also touching the frog muscle.
7
Q
frog muscle
A
- the living tissue was necessary to cause the reaction between the two metals.
- volta showed that it was just the metals generating electrical energy.
- scientists continued to study and develop these electrical energy producing cells, they were called “voltaic cells” or “galvanic cells”
8
Q
A galvanic cell
A
- a galvanic cell is a device that uses redox reactions such as zinc+copper to convert potential chemical energy to electrical energy.
- the key to this reactions operation is making sure that the reactants in the redox reaction dont touch.
–> volta accomplished this by placing soaked saltwater paper between the disks. - the coppper and zinc ions could move into the moist medium but they would not escape the disks.
9
Q
how did the electrons flow (disks)
A
- connecting a wire to both disks made electrons flow from one disk to another.
- the electrons flowed throughan external circuit: a circuit outside the reaction vessel where the redox reactions are occuring.
- the energy carried by the electric current in the wires would be used for other purposes such as lighting up a light bulb.
10
Q
The Daniell cell
A
- the first galvanic cell was develped by John Frederic Daniell.
- A Daniell cell consists of a zinc strip in a zinc sulfate solution and a copper strip in a copper(II) sulfate solution.
- the two solutions are connected by a salt bridge, a u-shaped glass tube filled with pottassium chloride.
–> the ends are plugged with porous material such as cotton or glass wool, the ions can diffuse through, but it prevents the solution from mixing.
11
Q
the wires in the daniell cells
A
- a wire is connected to the zinc and copper strips, the other end of the wire can be connected to any electrical device, such as a voltmeter.
- in the Daniell cell or any galvanic cell, the metal strips act as the electrical conductors.
- conductors that carry electrons into and out of a cell are called electrodes.
12
Q
electrolytes
A
- the solutions in a galvanic cell contain electrolytes, such as the zinc sulfate and copper(II) sulfate solutions in the daniell cell.
- electrolytes are subtances that conduct an electric current when dissolved in water in the form of moving ions.
13
Q
oxidation and reduction in the galvanic cell
A
- in any galvanic cell, the oxidation happens at the electrode called the anode and the reduction happens at the electrode called the cathode.
- ex: the zinc electrode is the anode because thats where the zinc atoms undergo oxidation.
the copper electrode is the cathode because thats where the copper ions undergo reduction.
14
Q
what would happen if the zinc atoms in the zinc electrode were in direct contact with the copper(II) ions in the copper (II) sulfate solution?
A
- zinc atoms would spontaneously transfer electrons.
- In the daniell cell, the zinc strip and copper(II) ions stay separted so they cannot escape the metal conductors.
14
Q
in the daniell cell, what ions can move through? what is the only path for the electrons to travel through?
A
- the only ions that can move through the solution are K+, Cl-, Zn2+, and Cu2+.
- the only path for the electrons to travel from the zinc electrodes to the copper(II) ions are through the conductors that make the external circuit.
–> copper(II) ions accept electrons and form atoms, which deposit to the copper electrode.
15
Q
what happens after the copper atoms are deposited to the cathode?
A
- since the solution lost some positive charges, positive pottasium ions diffuse from the salt bridge to the copper(II) sulfate solution to replace the lost positive charges.
- at the same time, the zinc ions that formed when the electrons moved out of the zinc electrode go into the solution.
–> these additional zinc ions make the solution positive, so negative chlorine ions are released from the salt bridge to neutralize the solution.
16
Q
the solution in the salt bridge
A
- the electrolyte solution in the salt bridge is selected so it does not interfere with the reaction.
- there is a complete cirucit around which the current is flowing, in some of the places of that circuit the moving charges are electrons, in some of the places the moving charges are ions.
- the current flows until the concentration of the solutions and the changes in the electrodes are so great that the process can no longer continue.
17
Q
last bit of Daniell Cell
A
- when a zinc strip is in a copper(II) sulfate solution, the energy is converted into heat.
- in a galvanic cell the chemical energy is converted into electrical energy.
- in this example the external cirucit is connected to a voltmeter, you could connect it to a light bulb and convert it into light energy.
18
Q
Types of electrodes
A
- galvanic cells can be made with different electrodes other than zinc and copper of the Daniell cell.
ex: you could have a galvani cell with chromium and silver electrodes. - the electrolyte in the salt bridge would be pottisum nitrate rather than pottasium chloride.
- a small amount of chloride ions could diffuse into the silver nitrate cell and form a precipitate with the silver ions, that is why we would not use potassium chloride.
19
Q
Inert electrodes
A
- the zinc anode, copper cathode, and the silver and chromium electrodes are all metals that can act as electrical conductors.
- some redox reactions involve oxidizing and reducing agents that are not solid metals.
–> Instead they will be dissoled electrolytes or gasses so they cannot be used as electrodes.
20
Q
how can we use oxi. and red. agents that are not solid metals?
A
- To use these oxidizing and reducing agents, you have to use inert electrodes.
- an inert electrode is made from material that is neither a reactant nor a product of the redox reaction, instead it provides a surface where the redox reaction can occur.
21
Q
an example of a inert electrode
A
- an inert electrode can be a platinum electrode.
- the lead electrode can be the anode.
- Lead atoms lose electrons that travel through the electrode while the lead(II) ions dissolve in the solution. The reduction half-reaction involves dissolved iron(III) ions that accept an electron from the platinum inert electrode and become dissolved iron(II) ions. The platinum atoms in the platinum electrode (cathode) remain unchanged.
22
Q
Cell notation
A
- sketching the entire cell helps to understand visually, but not a fast way to communicate.
- a short hand method to represent galvanic cells is cell notation.
- the cell notation of a Daniell cell is
Zn(s) | Zn 2+(aq) || Cu 2+(aq) |Cu(s)
23
Q
How does cell notation work?
A
- the anode is usually on the left and the cathode is on the right.
- each vertical line represents a phase boundary, between the electrode and the solution in a half-cell.
- for ex., in the Daniell cell notation the first line shows tthat the zinc ions are in different phases or states – solid and aqueous.
- the double vertical line represents the salt bridge between the half-cells.
24
cell notation and inert electrodes?
- Inert electrodes do not appear in the chemical equations or half-reactions, however they are included in the cell notation:
Pb(s) | Pb 2+(aq) || Fe 3+(aq), Fe 2+(aq) | Pt(s)
- A comma seperates the formulas Fe3+ and Fe2+ ions involved in the reduction half-reaction to indicate that they are in the same phase (aq).
25
Cell potential
- the oxidation reaction at the anode causes the anode to become negatively charged, the reduction reaction at the cathode causes the cathode to become positively charged.
- when a seperation of charge exists, a force acts on the charged particles, therefore they have potential energy.
26
What is electrical potential difference?
- Between two points, the amount of energy that a unit charge would gain by moving from one point to the other, is called **electrical potential difference.**
- in galvanic cell the two points are the two electrodes.
- a voltmeter measures the potential difference (aka the voltage).
27
the magnitude of the EPD?
- the magnitude of the electrical potential difference between the electrodes in the galvanic cell is an important property, it determines how much energy can be conffered on the moving charges.
- the electrical potential difference between the electrodes is called the **cell potential.**
28
what are the factors that the cell potential depends on?
- the cell potential depends on many factors; nature of oxidizing and reducing agents, concentrations of salt solution in the half-cells, the temperature of the solutions, and the atmospheric pressure.
- there is a set of standard conditions for reporting cell potentials.
29
standard cell potential2.0
- the standard cell potential, symbolized by Ecell, is the potential difference between the electrodes of a galvanic cell when the concentrations of the salt solutions of 1.0mol/L, the atmospheric pressure is 1.0 atm, the electrodes are pure metals, the temperature is 25C, and there is no electric current flowing in the cell.
- when all of these are constant, the only factor affecting the cell potential is the identity for the oxidizing and reducing agents.
30
half-cell potentials
- measure a standard cell potentail for any combination of oxidizing agents and reducing agents. But it would be better to describe half-cell potentials.
- Any type of electrical potential difference always reports a difference between two points, you can define a half-cell potential by choosing a reference point.
31
standard reduction potential
- all of the standard cell potentials for a large number of compounds are found in a table.
- the half-reactions are listed as reduction reactions.
32
how would you write a reduction half-cell potential for a compound that is not reduced by hydrogen half-cell, such as the zinc half cell?
- zinc is oxidized when a zinc half-cell is connected to a hydrogen half-cell.
- the reaction goes spontaneous in a direction opposite to the reduction of zinc.
- therefore you would consider the potential difference to be negative.
- *check notes for the reduction standard half cell potential aka standard reduction potential for zinc and copper*
- another important application of these data is in choosing the salts for a salt bridge in galvanic cells.
33
calculating standard cell potentials
- using this formula we can calculate the standard cell potential for any cell which the standard reduction potentials fo the oxidizing and reducing agents are listed in a table.
- Ecell=Ecathode-Eanode.
34
positive and negative standard cell potentials
- a positive standard cell potential tells you that the reaction will proceed spontaneously, in the direction indicated.
- a reaction will always proceed spontaneously if the reducing agent on the left side is stronger than the reducing agent on the right side of the chemical equation.
- if your standard cell potential is negative, the reaction will not proceed spontaneously in the direction indicated. It proceeds in the reverse direction.
- **"An ox and a red cat**: anode oxidation and a reduction cathode.
Chemistry GRADE 12!
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