Tuesday 2nd February 2013
Lab Experience n. 4 Voltmeter Experiment
Conductivity between Redox Couples
How to create a battery?
Objective/Task:
Measure the
existing conductivity between two Redox Couples, Cu + CuSO4
and Fe + FeSO4 · 7H2O. Cu+2/Cu
and Fe+2 / Fe
Background information:
It has been experimentally proved that the voltage of a battery depends on a number of factors: temperature, electrodes used and concentrations of all species involved in the redox reaction.
As the potential of the cell shows some variation on the conditions, it is necessary to establish which will be these conditions in order to compare both half–cells of a battery and between batteries. These established conditions are the standard s standard s standard state for electrochemical cells:
- Temperature 25°C
- Gas pressure 1 atm
- Concentration of solutions 1M
- Most stable state for solids
A galvanic cell is a system where a redox reaction occurs but where the half-reactions are physically separated and connected only through a salt bridge (to make the ionic equilibrium possible). The electrons travel along an external circuit from the anode to the cathode. The electrode where the oxidation half-reaction takes place is called anode, and the other electrode, where reduction occurs, is called cathode. This is easily remembered if we associate vowels and consonants:
ANODE-OXIDATION, CATHODE-REDUCTION.
The redox or reduction potential is known as the tendency of chemical species in a redox reaction to obtain electrons. It is the adding up of two semi reactions. There can be assigned to each of the half-cells or electrodes a single potential. It can be measured in millivolts (mV) or volts (V).
Eº Cell = Eº cathode - Eº anode
A high reduction potential is the one that has a high positive numerical value when the reaction of reduction is being carried out. It means that the capacity it has of reduction is very high, and the other specie is likely to be oxidized. (SOTO, Lauro)
Materials:
· Cotton
· U-tube
· Iron
· Iron (II) sulphate heptahydrate - FeSO4 · 7H2O
· Copper
· Copper (II) sulphate – CuSO4
· Spatula
· 2 x 50 mL beaker
· Test tube
· HCl
· Water (H2O)
· Voltmeter
· Stirring rod
Procedure:
(The process of making the salty bridge is in steps 1 and 2)
1. Take the U-tube and fill it more or less to the middle with HCl. Then, add 2 spoons of salt using the spatula and shake the U-tube until the salt is completely dissolved. Next, fill it entirely with water.
2. Place a piece of cotton inside each of the two holes of the U-tube and make sure that the solution doesn’t spill out once it is turned upside down. Use the cotton as stoppers.
(The process of obtaining the Redox couples is in steps 3 to 7)
3. Get the test tube and add 2 spoons of FeSO4·7H2O. Then add 10mL of water and shake it until the salt dissolves completely into the water (and it forms a homogeneous mixture).
4. Pour the mixture into the first beaker, and add more water to the solution.
5. Get the second beaker and add 3 mL of CuSO4. Then add 10 mL of water and use the stirring rod to stir gently the mixturE so that it becomes homogeneous - a solution.
6. Grab the tweezers from one of the cables of the voltmeter and hold the piece of Iron (the screw) with them. Place the screw inside the first solution, but be aware that the tweezers or the cable does not touch it.
7. Grab the tweezers from the other cable of the voltmeter and hold a piece of Copper wire with them. Place the piece of Copper wire inside the second solution, but take the advice of the previous step of the procedure.
8. Turn the U-tube upside down and settle one of its ends inside the first beaker and the other end inside the second beaker. Finally, turn the voltmeter on, pointing the wheel to 2000 m (millivolts) and the end of the arrow points 200 m (millivolts).
Results:
The result of this redox couple is that the conductivity measures 820 millivolts. Experimental value: 0,820 V
Theoretical value:
1. Determine the redox couples:
2. Calculate the standart redox potential in order to determine which species is being reduced and which one is being oxidized:
If we interpretate the data we discover that Copper is in this case the species which tends the most to be reduced, moreover it wasn't difficul to discover that as Iron is more likely to be oxidized rather than to be reduced. We base this statement on the values from the first column, where the higher the value the higher the tendency to be reduced.
Determine the cathode and the anode of the galvanic cell: The iron is the anothe because is the one who is oxidised and the copper is the cathode as it is the species which is being reduced in the reaction.
Now that we have both the experimental value (0,820 V) and the theoretical value (0,786).
Absolute error: Experimental value - Theoretical value
Absolute error: 0,820 - 0,786 = 0,034. It is an error by excess (bigger than the exact one).
It has been experimentally proved that the voltage of a battery depends on a number of factors: temperature, electrodes used and concentrations of all species involved in the redox reaction.
As the potential of the cell shows some variation on the conditions, it is necessary to establish which will be these conditions in order to compare both half–cells of a battery and between batteries. These established conditions are the standard s standard s standard state for electrochemical cells:
- Temperature 25°C
- Gas pressure 1 atm
- Concentration of solutions 1M
- Most stable state for solids
A galvanic cell is a system where a redox reaction occurs but where the half-reactions are physically separated and connected only through a salt bridge (to make the ionic equilibrium possible). The electrons travel along an external circuit from the anode to the cathode. The electrode where the oxidation half-reaction takes place is called anode, and the other electrode, where reduction occurs, is called cathode. This is easily remembered if we associate vowels and consonants:
ANODE-OXIDATION, CATHODE-REDUCTION.
The redox or reduction potential is known as the tendency of chemical species in a redox reaction to obtain electrons. It is the adding up of two semi reactions. There can be assigned to each of the half-cells or electrodes a single potential. It can be measured in millivolts (mV) or volts (V).
Eº Cell = Eº cathode - Eº anode
A high reduction potential is the one that has a high positive numerical value when the reaction of reduction is being carried out. It means that the capacity it has of reduction is very high, and the other specie is likely to be oxidized. (SOTO, Lauro)
Materials:
· Cotton
· U-tube
· Iron
· Iron (II) sulphate heptahydrate - FeSO4 · 7H2O
· Copper
· Copper (II) sulphate – CuSO4
· Spatula
· 2 x 50 mL beaker
· Test tube
· HCl
· Water (H2O)
· Voltmeter
· Stirring rod
Procedure:
(The process of making the salty bridge is in steps 1 and 2)
1. Take the U-tube and fill it more or less to the middle with HCl. Then, add 2 spoons of salt using the spatula and shake the U-tube until the salt is completely dissolved. Next, fill it entirely with water.
2. Place a piece of cotton inside each of the two holes of the U-tube and make sure that the solution doesn’t spill out once it is turned upside down. Use the cotton as stoppers.
(The process of obtaining the Redox couples is in steps 3 to 7)
3. Get the test tube and add 2 spoons of FeSO4·7H2O. Then add 10mL of water and shake it until the salt dissolves completely into the water (and it forms a homogeneous mixture).
4. Pour the mixture into the first beaker, and add more water to the solution.
5. Get the second beaker and add 3 mL of CuSO4. Then add 10 mL of water and use the stirring rod to stir gently the mixturE so that it becomes homogeneous - a solution.
6. Grab the tweezers from one of the cables of the voltmeter and hold the piece of Iron (the screw) with them. Place the screw inside the first solution, but be aware that the tweezers or the cable does not touch it.
7. Grab the tweezers from the other cable of the voltmeter and hold a piece of Copper wire with them. Place the piece of Copper wire inside the second solution, but take the advice of the previous step of the procedure.
8. Turn the U-tube upside down and settle one of its ends inside the first beaker and the other end inside the second beaker. Finally, turn the voltmeter on, pointing the wheel to 2000 m (millivolts) and the end of the arrow points 200 m (millivolts).
Results:
The result of this redox couple is that the conductivity measures 820 millivolts. Experimental value: 0,820 V
Theoretical value:
1. Determine the redox couples:
2. Calculate the standart redox potential in order to determine which species is being reduced and which one is being oxidized:
|
Redox couples
|
Cathodic
reduction process
|
E0 (V)
|
|
Cu2+ / Cu
|
Cu2+ + 2e-
= Cu0
|
+ 0.336
|
|
Fe2+/Fe
|
Fe2+
+ 2e- = Fe0
|
-0.44
|
If we interpretate the data we discover that Copper is in this case the species which tends the most to be reduced, moreover it wasn't difficul to discover that as Iron is more likely to be oxidized rather than to be reduced. We base this statement on the values from the first column, where the higher the value the higher the tendency to be reduced.
Determine the cathode and the anode of the galvanic cell: The iron is the anothe because is the one who is oxidised and the copper is the cathode as it is the species which is being reduced in the reaction.
Now that we have both the experimental value (0,820 V) and the theoretical value (0,786).
Absolute error: Experimental value - Theoretical value
Absolute error: 0,820 - 0,786 = 0,034. It is an error by excess (bigger than the exact one).
Relative error = 
References:
Potenciales estándar de reducción. Recovered from http://www.prepafacil.com/enp/Main/PotencialesEstandarDeReduccion (last access date: 9/03/2013)
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