IB Chemistry Power Points Topic 09 Voltaic Cells SLwww.pedagogics.ca
ELECTROCHEMISTRYDeals with chemical changes produced by an electric current and withthe production of electricity by chemical reactionsAll electrochemical reactions involve transfer of electrons and areredox reactionsElectrochemical reactions take place in electrochemical cell (anapparatus that allows a reaction to occur through an externalconductor)
ELECTROCHEMICAL CELLSTwo types:Voltaic cells: produce electrical energyAlso called galvanic cells. In these cells spontaneouschemical reactions generate electrical energy and supply itto an external circuit.Electrolytic cells: require electrical energyThese are cells in which an external electrical source forcesa nonspontaneous reaction to occur.
Electrochemical Terms – all cellsElectrode: A conductor used to establish contact with anonmetallic part of a circuit, such as an electrolyteHalf-cell: a metal electrode in contact with a solution ofits own ionsAnode: The electrode where oxidation takes placeCathode: The electrode where reduction takes place
VOLTAIC CELLS• Cells in which spontaneous reactions produceselectrical energy• The two half-cells are separated so that electrontransfer occurs through an external circuit• Each half-cell contains the oxidized and reducedforms of a species in contact with each other• Half-cells are linked by a piece of wire and a saltbridge
The Zinc-Copper cellComposed of two half-cells:1. A strip of copper immersed in 1 M CuSO42. A strip of zinc immersed in 1 M ZnSO4Experimentally we see:Initial voltage is 1.10 voltsThe mass of the zinc electrode decreasesThe mass of the copper electrode increases[Zn2+] increases and [Cu2+] decreases
Zinc – Copper Cell - notation Salt bridge Zn Zn 2+ (1.0 M) Cu 2+ (1.0 M) CuElectrode Species (with concentrations) in contact with electrodes
A salt bridge (or porous partition) has three functions:1. The salt bridge allows for the flow of ions and therefore electrical contact between the two half-cells2. As a result of electrical contact, the salt bridge maintains the electrical neutrality in each half-cell as ions flow into and out of the salt bridge3. The salt bridge prevents mixing of the electrode solutions
Electrical neutrality in each half cell is important! anions flow into the oxidation half-cell to counter the build-up of positive chargeCurrent flows spontaneously from and vice versanegative to the positive electrode(oxidation electrode to reduction If this did notelectrode) happen, current would stop flowingIn voltaic cells, voltage drops as thereaction proceeds. When voltage = 0,the reaction is at equilibrium
Voltaic Cells - Summary Cathode:Anode: reductionoxidation positivenegative Voltaic cells: Electrochemical cells in which a spontaneous redox reaction can be harnessed to produce an electric current.