galvanicelectrolytic need power source two electrodes produces electrical current anode (-) cathode (+) anode (+) cathode (-) salt bridge vessel conductive medium Comparison of Electrochemical Cells E ° cell > 0. E ° cell < 0.
Electron transfer reactions are oxidation-reduction or redox reactions. Results in the generation of an electric current (electricity) or be caused by imposing an electric current. Therefore, this field of chemistry is often called ELECTROCHEMISTRY.
oxidation: loss of electrons reduction: gain of electrons LEO the lion says GER! GER!
Batteries Corrosion Industrial production of chemicals such as Cl 2, NaOH, F 2 and Al Biological redox reactions The heme group
One ½ cell rxn. occurs in each compartment. Zn Zn 2+ + 2e – in the anode. Cu 2+ + 2e – Cu in cathode. But not without a connection. Zn Zn 2+ Cu 2+ Zn + Cu 2+ Zn 2+ + Cu SO 4 2– Cu Anode=Oxidation Cathode=Reduction
But even with a connection of the electrodes, no current flows. We need to allow neutrality in the solutions with a salt bridge to shift counterions. Zn Zn 2+ Cu 2+ Zn + Cu 2+ Zn 2+ + Cu SO 4 2– Cu 2e –
Cell Potential Cell Potential or Electromotive Force (emf): The “pull” or driving force on the electrons.
The voltage generated by the Zn/Cu galvanic cell is +1.1V under standard conditions. Standard conditions are: T = 25°C and P = 1 bar for gases. Solids and liquids are pure. Solutions are 1 M in all species. E ° cell is sum of ½ cell E ° values.
All ½ cells are catalogued as reduction reactions & assigned reduction potentials, E °. The lower reduction potential ½ rxn is reversed to become the oxidation. E ° oxidation = – E ° reduction That makes spontaneous E ° cell > 0. But E ° red can’t be found w/o E ° ox !
We need a standard electrode to make measurements against! The Standard Hydrogen Electrode (SHE) 2H + (aq) + 2e – H 2 (1 bar) E ° 0 V 1 bar H 2 flows over a Pt electrode, and the full E ° cell is assigned to the other electrode. E ° SHE = 0 V. E.g., standard calomel electrode: Hg 2 Cl 2 (s) + 2e – 2 Hg( l ) + Cl – E ° SCE = +0.27V a more physically convenient reference.
Shorthand for a complete redox cell is of the form: Anode | anodic soln. || cathodic soln. | Cathode So making a cell of Cu corrosion, Cu | Cu 2+ || NO 3 –, NO(g), H + |Pt where all ions should be suffixed (aq) and both metals should have (s).
Primary Battery : can not be recharged e.g. Mercury Battery Secondary Battery: rechargeable (storage batteries) e.g. Ni-Cad Battery Fuel Cell: reactants supplied from an external source e.g. H2/O2 fuel cells.
MERCURY BATTERY Anode: Zn is reducing agent under basic conditions : Cathode : HgO + H 2 O + 2e- ---> Hg + 2 OH - can not be recharged
It is because the products of the reaction are solids that the Ni-Cd battery can be recharged The solid hydroxides are sticky, and remain in place. If current is applied, the reaction can be driven backwards !
When you charge a battery, you are forcing the electrons backwards (from the + to the -). To do this, you will need a higher voltage backwards than forwards.
But in mercury battery the ZnO is not sticky, and doesn’t remain attached to the electrode. This battery is not rechargeable
H 2 AS A FUEL Cars can use electricity generated by H 2 /O 2 fuel cells. H 2 carried in tanks or generated from hydrocarbons
17-44 Galvanic cells for which the reactants are continuously supplied. anode: 2H 2 + 4OH 4H 2 O + 4e cathode : 4e + O 2 + 2H 2 O 4OH 2H 2 (g) + O 2 (g) 2H 2 O(l) Fuel Cells
Mercury batteries take advantage of the high density of Hg to be quite small: used in watches, hearing aids, calculators, etc. Lithium-iodine batteries are particularly small and lightweight, but also very long-lived Often used in pacemakers, where they can last for 10 years
THE END For more lessons, visit www.chalkbored.com www.chalkbored.com
For a galvanic cell, the electrode at which reduction occurs is called the: A: AnodeB: Cathode final 5/50 Dr. Keck poll
For a galvanic cell, the electrode at which reduction occurs is called the: B: Cathode
For a galvanic cell, the electrode with negative polarity is called the: A: AnodeB: Cathode final 50/50 Dr. Keck poll
For a galvanic cell, the electrode with negative polarity is called the: A: Anode
Which of the following statements is incorrect a. In a galvanic cell, reduction occurs at the anode. b. The cathode is labeled "+" in a voltaic cell. c. Oxidation occurs at the anode in a voltaic cell. d. Electrons flow from the anode to the cathode in all electrochemical cells. 32
a. In a galvanic cell, reduction occurs at the anode.
Consider the following notation for an electrochemical cell Zn|Zn 2+ (1M)||Fe 3+ (1M), Fe 2+ (1M)|Pt What is the balanced equation for the cell reaction? a. Zn(s) + 2Fe 3+ (aq) → 2Fe 2+ (aq) + Zn 2+ (aq) b. Zn 2+ (aq) + 2Fe 2+ (aq) → Zn(s) + 2Fe 3+ (aq) c. Zn(s) + 2Fe 2+ (aq) → 2Fe 3+ (aq) + Zn 2+ (aq) d. Zn(s) + Fe 3+ (aq) → Fe 2+ (aq) + Zn 2+ (aq) e. Zn(s) + Fe 2+ (aq) → Fe(s) + Zn 2+ (aq) 34
What is the oxidation state of nitrogen in HNO 3 ? A: +3B: +4 C: +5D: -5 final50/50 Dr. Keck poll
What is the oxidation state of nitrogen in HNO 3 ? C: +5
38 Consider the following electrode potentials: Mg 2+ + 2e – Mg E° = –2.37 V V 2+ + 2e – V E° = –1.18 V Cu 2+ + e – Cu + E° = 0.15 V Which one of the reactions below will proceed spontaneously from left to right? a. Mg 2+ + V V 2+ + Mg b. Mg 2+ + 2Cu + 2Cu 2+ + Mg c. V 2+ + 2Cu + V +2 + Cu 2+ d. V + 2Cu 2+ V 2+ + 2Cu + e. none of these