These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. Lecture 33 Standard Cell Potential - Electrical current is the rate of electron flow and is measured in amperes (A), the flow of one coulomb per second: current (A)=charge(C)/Time(s) - Rate of electron flow through a wire is analogous to the rate of water moving through a stream - Water flows downhill in response to a difference in gravitational potential energy - The driving force for electrical current is a difference in potential energy - Electrons flow from the electrode in which they have higher potential energy to the electrode in which they have lower potential energy - The difference in potential energy between the two electrodes is called potential difference - In a voltaic/galvanic cell electrons flow spontaneously from the anode to the cathode (from –to +) because of this difference in potential energy - The SI unit of potential difference in volt (V) wich is equal to one joule per coulomb 1V=1JC-1 - The larger the potential difference between the electrodes, the stronger the potential for electron flow - Potential difference is also called the electromotive force because it gives rise to the force that results in the motion of electrons - In a voltaic/galvanic cell the potential difference between the two electrodes is called the cell potential or cell emf - The cell potential depends on the relative tendencies of the reactants to undergo oxidation and reduction The Standard Hydrogen Electrode - Standard cell potential or standard emf is the cell potential when the reactants and products are in their standard states - The electrode potential in a half-cell cannot be measured directly - Can only measure the overall potential that occurs when two half-cells are combined in a whole electrochemical cell - We measure all other electrode potentials relative to the standard hydrogen electrode which is arbitrarily assigned a potential of zero - Standard hydrogen electrode is a half cell where hydrogen ions are reduced to hydrogen gas under standard conditions - Electron travel from the anode to the cathode - We define E cellas the difference in voltage between the cathode and the anode - Ecell=Ecathode-Eanode CHE 141 1st Edition- Ecell is positive for spontaneous reactions and negative for nonspontaneous reactions Standard Reduction Potentials - By convention, standard electrode potentials are all listed for reduction half-reactions so are often called standard reduction potential - The more positive the E the greater the tendency to undergo reduction - The more negative the E, the greater the potential energy of an electron at the electrode and the greater the tendency to undergo oxidation - An extensive property depends on the amount of a substance - An intensive property is independent of the amount of a substance - Standard reduction potential is an intensive property - E does not change when the quantities of reactants and products change Predicting Spontaneous Direction of Redox Reactions - The half reaction with the more positive electrode potential attracts electrons more strongly and undergoes reduction o The more positive the E the better oxidizing agent - The half reaction with the more negative electrode potential repels electrons more strongly and undergoes oxidation o The more negative the E the better reducing agent - Write both reduction half equations with most negative E on top - Starting from the products of the half equation on top, draw a counter clockwise arrow - This will remind you that the top half-reaction runs in reverse as it is an oxidation - Oxidation occurs at the anode thus the half reaction on top with the most negative E is the anode - The half equation written on the bottom is a reduction so occurs at the cathode Relationship Between delta G, Ecell and K - Ecell>0(positive) - Spontaneous redox reaction when reactants and products are in their standard states - Delta G<0(negative) - Reaction is spontaneous in direction written under standard conditions - Ecell and deltaG must be related - Since Ecell and delta G are related Ecell and K must also be related - For a spontaneous redox reaction: delta G<0, Ecell>0, K>1 - For a nonspontaneous redox reaction: delta G>0, Ecell<0, K<1 - E=potential energy difference (J)/charge (c)=wmax/c: wmax=-CEcell- C is the number of electrons flowing through circuit - Charge on a single electron=1.602x10^-19 - Charge on 1 mole of electrons=1.602x10^-19 (6.022x10^23)=9.65x10^4 - Charge that flows in an electrochemical reaction is given by=C=nF - Where n=number of moles of electrons from the balanced chemical equation and F=Faradays constant - Wmax=-nFEcell - Delta G=wmax: delta
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