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Electrochemistry Concepts

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Katrina Soriano

on 14 March 2011

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Transcript of Electrochemistry Concepts

Electrochemistry 1. Electronegativity 2. Oxidation Numbers 3. Redox Reactions 4. Balanced Net Ionic Equations 5. Half-reactions 6. SOA and SRA 7. Spontaneity 8. Electrochemical Cells 9. Standard Hydrogen Electrode 10. Aqueous and Molten Electrolysis 11. Stoichiometry 12. Applications of Electrochemical Cells 1. Electronegativity electronegativity increases electronegativity increases least electronegative most electronegative 2. Oxidation Numbers is either a positive (+) or a negative (-) number corresponding to the apparent charge that an atom in a molecule or ion would have if the electron pairs in covalent bonds belonged entirely to the more electronegative atom. Rules for Assigning Oxidation Numbers is the ability of an atom in a compound to attract electrons. 3. Redox Reactions involve oxidation and reduction of chemicals
Reduction is the gain of electrons Oxidation is the losing of electrons Oxidizing and Reducing Agents The reducing agent (RA) causes the reduction of another chemical in the reaction and in doing so, it is oxidized. The oxidizing agent (OA) is reduced Of the 5 types of reactions, only double displacement reactions DO NOT involve reduction and oxidation Synthesis, Decomposition, Combustion and Single Displacement are all redox reactions. 4. Balanced Net Ionic Equations Spectator Ions - ions that do not change during a reaction Steps: 1. Balance the equation of atoms 2. Write out the dissolved chemical species as they exist in solution. Precipitates, liquids and gases are not written as ions. 3. Remove common aqueous ions. Resulting equation is the net ionic equation. 5. Half-reactions or.. 1. Write the half reactions of the equation. 2. Multiply the half-reaction by a constant that will cancel the electrons in both equations. 3. Combine the half reactions and it will form the balanced net ionic equation. Oxidation Oxidation-Half Reaction The reaction for the process of oxidation where an atom is losing electrons to become more positive. Reduction-Half Reaction The reaction for the process of reduction where an atom is gaining electrons to become more negative. Note: In all redox reactions the total number of electrons lost in oxidation = total number gained in reduction. Loss of Electrons Reducing Agent Gain of Electrons Reduction Oxidizing Agent 6. SOA and SRA Stronger Oxidizing Agent (SOA) Stronger Reducing Agent (SRA) Redox reactions convert the SOA into a WRA and the SRA into a WOA. Making a Redox Table Solubility Rules The strongest OA is on the top left of a redox table. The strongest RA is on the bottom right of a redox table. 7. Sponteinity Refer to the redox table when writing the oxidation and reduction half reactions. Write the reduction half reaction as it is written on the table. Flip the half reaction for oxidation. Change the sign of the standard electrode potential for the oxidation half reaction. Add the electrode potential of the oxidation and reduction half reactions. If the net electromotive force is positive, the reaction is spontaneous. If the net electromotive force is negative, the reaction is not spontaneous. Spontaneity can also be identified through the positioning of the SOA and SRA in the Redox table. If the SRA lies down right of the SOA, then the reaction is spontaneous. Also... 8. Electrochemical Cells Also called Galvanic or Volataic Cells Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s) Cu(s) Zn(s) Zn2+(aq) Zn 2+ Cu2+(aq) Cu2+ Cu(s) is the SRA Oxidation occurs in cathode Cu2+(aq) is produced Zn(s) is the SOA Reduction occurs in anode Zn2+(aq) forms Zn(s) K+ (cations) goes to cathode NO3- (anions) goes to anode Electron flows from anode to cathode Cell Notation 9. Standard Hydrogen Electrode Every standard reduction potential value on the table has been standardized against the SHE. Hydrogen was used as the standard reference electrode to calculate the SRP of other half reactions. The standard reference electrode is assigned a value of zero. Assigning a new standard reference electrode: Identify the current standard reduction potential of the new reference electrode. Add a value to the reduction potential of the new reference electrode that will make the sum equal to zero. Add the same value to all the other standard reduction potentail to create the new values of each. 10. Electrolysis forcing a non-spontaneous reaction to occur with the use of a power supply. The power supply provides enough voltage to force the transfer or movement of electrons. Aqueous Electrolysis Electrons flow from anode to cathode Anode is the positive electrode Cathode is the negative electrode Cations go to cathode Anions go to anode No need for salt bridge Inert electrodes are used Molten Electrolysis No water present Presence of H2O Occurs in high temperatures Ions are in liquid form 11. Stoichiometry Q = It Q - charge in Coulombs, C I - current in amperes, A, or Coulombs per second t- time in seconds Faraday's Constant 96500 C/mol e- or Farad To get a mass of an atom after a reaction given time and current: It x 1 mol e- x mol atom x atomic mass = actual mass
96500 C mol e- 1 mole atom Charge, C Faraday's Constant ratio of mole of atom and electron mass of atom per mole 12. Applications of Electrochemical Cells Electroplating Electrorefining increasing the purity of impure metals protecting, beautifying, making more valuable Metal ions in the solution are moved by an electric field to coat the electrode In electrochmical cells... 1. ...both electrode may be involved in the reaction 2. ...reactions may occure under standard conditions 3. ...one or both reactions may occur completely in solution 4. ...inert electrodes may be involved Inert Electrodes non-reactive 3 common types: 1. Carbon electrode (C(s)) 2. Nichrome (Ni(s) | Cr (s), alloy) 3. Platinum wire (Pt(s)) If a particular combination reacts, it will always react. The opposite combination will never react. If a combination has no reaction, reverse the equation. Identify the atom that looses electrons (SOA) and the atom that gains electrons (SRA). The product of the SRA is the WOA. Comparing the strength of the oxidizing agents, we put the half reaction of the stronger OA on top of the other half reaction. Redox Table
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