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Chemistry 27: Electro-chemistry

Chemistry 27: Electro-chemistry. Christopher Chui. Circuits and Conductivity. A complete metal circuit allows electrons to flow A galvanometer measures electric current Electric current is the flow of charged particles A salt bridge is a U-tube containing ionic substance in solution

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Chemistry 27: Electro-chemistry

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  1. Chemistry 27: Electro-chemistry Christopher Chui Electro-chemistry -- C. Chui

  2. Circuits and Conductivity • A complete metal circuit allows electrons to flow • A galvanometer measures electric current • Electric current is the flow of charged particles • A salt bridge is a U-tube containing ionic substance in solution • The galvanometer registers the current flow between two different metals placed in a salt solution Electro-chemistry -- C. Chui

  3. Conduction and Potential Difference • Metallic conduction or electronic conduction refers to the movement of electrons through a metal • Electric potential difference is measured in volts • The rate of electron flow is measured in amperes • Electrolytes are substances whose solutions conduct electricity • Molten salts and solutions of acids, bases, and salts conduct electricity • Conduction that takes place by the migration of ions is called electrolytic conduction • Positive ions are called cations; negative ions are anions Electro-chemistry -- C. Chui

  4. Electrode Reactions • Positive ions (cations) are attracted to the cathode • At the cathode, positive ions are reduced by gaining electrons: Na+ + e- Na • Negative ions (anions) are attracted to the anode • At the anode, negative ions are oxidized by losing electrons: 2 Cl- Cl2 + 2 e- • In a cell, oxidation and reduction occur as separate half-reactions at the same time • Electrolysis is the use of an electric current to produce a chemical change Electro-chemistry -- C. Chui

  5. Electrolysis of a Salt Solution • In the electrolysis of a concentrated NaCl solution, H2 is produced at the cathode, and Cl2 is produced at the anode • 2 NaCl + 2 H2O  2 NaOH + H2 + Cl2 • Electronic conduction is the movement of electrons through a metal • Electrolytic conduction is the migration of ions through the liquid between the electrodes Electro-chemistry -- C. Chui

  6. Voltaic Cells • A voltaic cell converts chemical energy into electric energy • A voltaic cell consists of zinc metal in a solution of zinc ions connected with a salt bridge and an external circuit to copper metal in a solution of copper ions • Reactions take place at separate electrodes. Electrons flow from the anode to the cathode in the external circuit • When using the shorthand method of representing cells, place the anode on the left Electro-chemistry -- C. Chui

  7. Redox and Standard Electrode Potentials • The hydrogen (reference) half-cell is assigned zero potential: 2 H+(aq) + 2 e- H2 (g) • The hydrogen half-cell consists of a sheet of Pt immersed in 1M solution of H+ ions. H2 gas is bubbled into the solution at a pressure of 101.3 kPa. The H2 molecules are adsorbed on the Pt surface and form the electrode • Potentials vary with temp T, pressure P, and conc. • Half-cell potential tables are useful for predicting the direction of chemical reactions • Electrons flow from donor (anode) to acceptor (cathode) Electro-chemistry -- C. Chui

  8. Cell Potential • Standard reduction potential is an intensive property • A negative voltage for a redox reaction means the reaction is not spontaneous, and the reverse reaction will occur • Magnesium has a higher potential than copper • If a reaction yields a negative voltage, the reverse reaction yields a positive voltage • The standard reduction potential indicates a tendency of the half-reaction to gain electrons. The greater (more +ve) the voltage, the greater the tendency to gain electrons Electro-chemistry -- C. Chui

  9. Electrolysis and Effects of Conditions on Cells and Energy • German chemist, Nernst, found: • E=E0 – (RT)/nF ln(products conc/reactants conc) • R is the gas constant; T in K; F factor from volt to J = 96485; ln is natural log; n is the number of electrons transferred • After simplification, E=E0 - 0.05916/n log(prod/react) Electro-chemistry -- C. Chui

  10. pH Meter, Energy and Electric Cells • The Nernest equation can be applied to half-cells or to an entire cell: E=E0-2.3 RT log K/(nF) • E is a linear function of pH • The pH meter makes use of a hydrogen ion electrode and a reference electrode, or a combination electrode • At equilibrium, E=0, therefore, 2.3 RT log K = nFE0 • DG0 = -nFE0 Electro-chemistry -- C. Chui

  11. Quantitative Electrochemistry • Cell voltage can be used to find 1) Gibbs free energy change; 2) tendency to gain electrons; 3) tendency to lose electrons; 4) equilibrium constant, Keq • A reaction tends to be spontaneous if Keq >>1, E0>0, and DG < 0 • 1 coul is the amount of electricity produced by 1 amp flowing for 1 sec; 96485 coul = 1 mole of electrons • Cu 2+ + 2 e-Cu • 2 moles of electrons will reduce 1 mole of Cu 2+ to Cu • Cr in Cr2O7 2- has an oxidation number of 6+ Electro-chemistry -- C. Chui

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