7._Chapter_21_electro_post

7._Chapter_21_electro_post - Electrochemistry Overall...

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Chapter18 Electrochemistry 1 Overall reaction Zn(s) + Cu ( aq) Zn 2+ (aq) + Cu(s) Half reactions Oxidation – loss of electrons Zn(s) Zn 2+ (aq) + 2e 2+ Electrochemistry
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Chapter18 Electrochemistry 2 Daniel Cell If direct contact between the copper ions and zinc metal is avoided, this reaction can be made to do useful work. In voltaic cells direct contact is avoided by physically separating the oxidation and reduction half-reactions. The electrons are forced to flow through an external wire from the zinc half-cell to the copper half- cell. This flow of electrons through an external wire can be used to produce work. A practical voltaic cell is shown below. At the anode, zinc metal is oxidized to zinc ions. Electrons then travel through the external wire to the cathode where copper ions are reduced to copper metal. As the reaction proceeds, Zn 2+ ions are produced at the anode and Cu 2+ ions are consumed at the cathode. To maintain electrical neutrality, sulfate ions must flow through the salt bridge from the half-cell on the left to the half-cell on the right to balance the flow of electrons through the external wire from the anode to cathode.
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Chapter18 Electrochemistry 3 loss of electrons, Oxidation, Anode, -ve Zn(s) Zn 2+ (aq) + 2e (electrons produced) LEO goes (oxidation and anode start with vowels) 2+
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Chapter18 Electrochemistry 4 Cell Notation for Galvanic or voltaic cell 2+ Cell notation Oxidation Reduction
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Chapter18 Electrochemistry 5 The driving force that pushes the negatively charged electrons away form the anode and pulls them toward the cathode is an electrical potential called the electromotive force (emf) or the cell potential (E) or the cell voltage. Cell potential = W = work done (joules) Q charge transferred (coulombs) ( G) Free energy (Joule, J) measure of the spontaneity of a reaction carried out at a constant temperature. If G is negative the reaction is spontaneous If G is positive the reaction is nonspontaneous If G is zero the reaction mixture is at equilibrium G α -nE +ve measurement means a reaction will be spontaneous n = moles of electrons transferred in the reaction E = cell potential (J) G = -nFE F = Faraday constant the electrical charge on one mole of electrons F = 96,500 C/mole e - Where C is the electric charge (coulomb) When 1 C of charge moves between two electrodes that differ in electrical potential by 1 volt, 1 joule of energy is released by the cell and can be used to do electrical work. Volt is potential energy per unit charge V = 1J/1C The cell potential, measured by connecting a voltmeter between the two electrodes, is a measure of the driving force behind this reaction
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Chapter18 Electrochemistry 6 Standard Reduction Potentials Just as a chemical reaction can conceptually be broken into two half-reactions the cell potential can be thought to be composed of two half-cell potentials. The reaction between zinc metal and acid, for example, results from the combination of two half-reactions: Zn(s) Zn 2+ (aq) + 2e - (oxidation) By arbitrarily defining the potential for the half-reaction;
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7._Chapter_21_electro_post - Electrochemistry Overall...

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