electrochemistry experiment 12. oxidation – reduction reactions consider the reaction of copper...
TRANSCRIPT
Oxidation – Reduction Reactions
• Consider the reaction of Copper wire and AgNO3(aq)
AgNO3(aq)
Ag(s)
Cu(s)
Oxidation – Reduction Reactions
• If you leave the reaction a long time the solution goes blue!
• The blue is due to Cu2+(aq)
Oxidation-Reduction Reactions
• So when we mix Ag+(aq) with Cu(s) we get Ag(s) and Cu2+
(aq)
• Ag+(aq) + 1e- Ag(s)
• Cu(s) Cu2+(aq) + 2e-
• The electrons gained by Ag+ must come from the Cu2+
• Can’t have reduction without oxidation (redox)• Each Cu can reduce 2 Ag+
2Ag+(aq) + 2e- 2Ag(s)
Cu(s) Cu2+(aq) + 2e-
2Ag+(aq) + 2e- + Cu(s) 2Ag(s)+ Cu2+
(aq) + 2e-
lose electrons = oxidation
gain electrons = reduction
7
Redox Reactions & Current
• redox reactions involve the transfer of electrons from one substance to another
• therefore, redox reactions have the potential to generate an electric current
• in order to use that current, we need to separate the place where oxidation is occurring from the place that reduction is occurring
Tro, Chemistry: A Molecular Approach
10
Electrochemical Cells
• electrochemistry is the study of redox reactions that produce or require an electric current
• the conversion between chemical energy and electrical energy is carried out in an electrochemical cell
• spontaneous redox reactions take place in a voltaic cell– aka galvanic cells
• nonspontaneous redox reactions can be made to occur in
an electrolytic cell by the addition of electrical energy
Tro, Chemistry: A Molecular Approach
11
Electrochemical Cells
• oxidation and reduction reactions kept separate– half-cells
• electron flow through a wire along with ion flow through a solution constitutes an electric circuit
• requires a conductive solid (metal or graphite) electrode to allow the transfer of electrons– through external circuit
• ion exchange between the two halves of the system– electrolyte
Tro, Chemistry: A Molecular Approach
12
Electrodes
• Anode (donates electrons to the cathode)– electrode where oxidation occurs– anions attracted to it– connected to positive end of battery in electrolytic cell– loses weight in electrolytic cell
• Cathode (attracts electrons from the anode)– electrode where reduction occurs– cations attracted to it– connected to negative end of battery in electrolytic
cell– gains weight in electrolytic cell
• electrode where plating takes place in electroplating
Tro, Chemistry: A Molecular Approach
13
Voltaic Cell
the salt bridge is required to complete the circuit and maintain charge balance
Tro, Chemistry: A Molecular Approach
14
Current and Voltage
• the number of electrons that flow through the system per second is the current– unit = Ampere– 1 A of current = 1 Coulomb of charge flowing by each second– 1 A = 6.242 x 1018 electrons/second– Electrode surface area dictates the number of electrons that
can flow
• the difference in potential energy between the reactants and products is the potential difference (the potential for an electric field to cause an electrical current)– unit = Volt– 1 V of force = 1 J of energy/Coulomb of charge– the voltage needed to drive electrons through the external
circuit– amount of force pushing the electrons through the wire is
called the electromotive force, emf
Tro, Chemistry: A Molecular Approach
15
Cell Potential
• the difference in potential energy between the anode the cathode in a voltaic cell is called the cell potential
• the cell potential depends on the relative ease with which the oxidizing agent is reduced at the cathode and the reducing agent is oxidized at the anode
• the cell potential under standard conditions is called the standard emf, E°cell
– 25°C, 1 atm for gases, 1 M concentration of solution– sum of the cell potentials for the half-reactions
16
Standard Reduction Potential
• a half-reaction with a strong tendency to occur has a large + half-cell potential
• when two half-cells are connected, the electrons will flow so that the half-reaction with the stronger tendency will occur
• we cannot measure the absolute tendency of a half-reaction, we can only measure it relative to another half-reaction
• we select as a standard half-reaction the reduction of H+ to H2 under standard conditions, which we assign a potential difference = 0 V
– standard hydrogen electrode, SHE
Tro, Chemistry: A Molecular Approach
18
Half-Cell Potentials
• SHE reduction potential is defined to be exactly 0 V
• half-reactions with a stronger tendency toward reduction than the SHE have a + value for E°red
• half-reactions with a stronger tendency toward oxidation than the SHE have a + value for E°red
• ΔE°cell = E°oxidation + E°reduction
– E°oxidation = -E°reduction
– when adding E° values for the half-cells, do not multiply the half-cell E° values, even if you need to multiply the half-reactions to balance the equation
• ΔGocell=-nFΔE°cell
Tro, Chemistry: A Molecular Approach
20
Electrochemical Cell Summary
salt bridgee-
anode
Zn (s)--> Zn2+ (aq)+ 2e-
cathode
Cu2+(aq)+ 2e- --> Cu(s)
The differing stability of reactants, (Zn(s), Cu2+(aq)), and products (Zn2+, and Cu(s)), creates a potential energy gradient through which the charges migrate (from high energy to low).
This manifests as a potential difference Ecell, across the electrodes. Where -qEcell is the change in potential energy when an amount of negative charge (-q) passes from the anode to the cathode
The cell potential is related to the free energy of the reaction according to the relation Gcell = -nFEcell
The cell potential can be calculated knowing the standard reduction potentials. These can be used to find Eo
red for the reaction at the cathode, and Eo
ox (= - Eored). Then Eo
cell = Eoox+ Eo
red Zn2+(aq) + 2e- --> Zn(s) -0.76V
Cu2+(aq) + 2e- --> Cu(s) Ered=0.34VZn(s) --> Zn2+(aq) + 2e- Eox= 0.76V
Ecell=1.1 VEcell = 0.76V+0.34V = 1.1V
Tonight
• Construction of Voltaic Cells and Measurement of Cell Potentials
• Use the corresponding 0.1 M metal sulfate of the same metal as the electrode in the half cell
• Construct a salt bridge • Measure the voltage
Trial electrodes Trial electrodes
1 Cu/Zn 4 Zn/Pb
2 Cu/Pb 5 Zn/Ni
3 Cu/Ni 6 Pb/Ni