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30 Cards in this Set
- Front
- Back
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Essence of Redox Reactions
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Redox reactions involve the the transfer of electrons
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Oxidation
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Loss of electrons
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Reduction
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Gain of electrons
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Oxidizing Agent
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An agent that cause oxidation which means that it gets reduced
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Reducing Agent
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An agent that causes reduction which means that it gets oxidized
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Oxidation Numbers
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These numbers allow you to keep track of the transfer of electrons during a chemical reaction and determine which species gets reduced and which gets oxidized
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Seven Rule for Oxidation Number
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1) Oxidation number of free element is 0
2) Oxidation number of monoatomic ion is equal to the charge of the ion 3) Alkali Metals have an oxidation number of +1. Alkaline Earth Metals have an oxidation number of +2. 4) Halogens have an oxidation number of -1 5) Oxidation number of hydrogen is -1 with atom that is less electronegative. Otherwise the oxidation number is +1. 6) Oxidation number of oxygen is -2. Exception to this are when oxygen is with atoms that are more electronegative. When oxygen is with a peroxide it charge will be -1. 7) The sum of oxidation number will be 0 in a neutral atom and equal to the charge of an atom in a charged compound |
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Convention for writing out formulas
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The cation is written first and the anion is written second. So if you have HCl you know that the hydrogen is going to be +1 and in NaH the charge of the H atom will be -1.
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Process of finding Oxidation number
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--You are going to get a compound for which you will be able to find the oxidation number for one of the compound but you wont know the number for the other one. The reason is that it will be some kind of random element for which you have no rule
--Multiply the charge of the oxidation number on that atom you do know by the number of atoms that are present. Then find the value of the other atoms by setting up an equation. --Once you have the number for both reactants and products, you can tell if a species have been oxidized or reduced. |
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Balancing Redox Reactions 1
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Separate each of the half reaction into the individual reactions. This means that you simply isolate each element and make it it's own reaction
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Balancing Redox Reactions 2
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Balance each of the reaction. this means two things. first you need to balance the actual elements. If you have 2 chlorines on one side, you need to place two chlories on the other side. It also means that you need to balance the oxygen atom. if you have oxygen on one side, then on the other side you need to place the same amount of WATER molecules. Oxygen is balanced by water molecules
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Balancing Redox Reactions 3
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--If there is a charge, then you need to balance this charge. this does not mean that you need to make it so that the charge will be 0. this means that if you have a charge on one side of the reaction, then you need to place the correct amount of electrons on the other side of the reaction so that the charge will be the same. As in if you have a negative 2 charge on one side then you need to place 2 electrons on the other side of the reaction to balance. For this step, you need to look for the charge and work from there.
--In the case where you needed to add waters to balance out the oxygen, you need to place the same number of hydrogens on the side opposite the water to balance out the number of hydrogens. This is a simple point. If you have hydrogens on one side of the reaction then you need to have hydrogens on the other side of the reaction as well. The hydrogens that you add will have a + charge so you need to factor that in when you are doing the balancing of charge. |
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Balancing Redox Reactions 4
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Multiply each of the half reactions by different numbers so that there will be an equal amount of electrons for each of the reactions and that the electrons will cancel out.
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What is an electrochemical cell?
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It is a cell in which redox reactions occur
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Three types of electrochemical cells?
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Galvanic Cells, Concentration cells, Voltaic Cells
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What occurs at the anode/ cathode?
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Oxidation occurs at the anode and reduction occurs at the cathode
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Galvanic Cells and Gibbs Free Energy
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They have spontaneous reactions. These are usually the batteries that you use in your house like in the case of a flashlight. They use spontaneous reactions which means that they will have a negative gibbs free energy.
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Potential Energy and Batteries
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Batteries convert potential energy in kinetic energy
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Shorthand notation for reaction in electrochemical cell
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anode | anode solution || cathode |cathode solution
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What is the purpose of a salt bridge?
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The salt bridge is used to make sure that charge does not build up on one side of the cell. When the reaction in a cell is proceeding, negative charge will build up by the cathode (since that is where reduction is taking place). In order so that the charge would balance out, a salt bridge is used so that the charge dissipates and the reaction can continue.
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Function of Galvanic Cell
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A cathode is place in a cathode solution and an anode is placed in an anode solution. Electrons flow from the anode to the cathode. At the anode, oxidation occurs in which the compound used for the anode (in this case Zn) will lose electrons and gain positive charge. At the cathode, the compound used as the cathode solution (in this case aqueous Cu) will pick up electrons and solidify and thus plate on the cathode
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Reaction of a Galvanic Cells
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Zn (s) → Zn +2 (aq) + 2e- (anode)
Cu+2 (aq) +2e- → Cu (s) (cathode) As you can see from the equation, electrons will flow from the anode to the cathode. Only at the cathode will you get plating because the Cu in solution is solidify on the cathode plate. |
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Electrolytic Cells Gibbs Free Energy
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In these cells the gibbs function is positive which means that the the reaction of an electrolytic cell is non spontaneous and needs energy input from an outside source for the reaction to occur.
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Electrolytic Cells Function
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An electrolytic cell separates molten NaCl to form liquid Na and Cl gas. It does this by separating the NaCl into Na+ and Cl-. These two charged species are then exposed to a cathode and an anode. An anions are attracted to the anode and cations are attracted to the cathode. When Cl- meets with the anode it gets oxidized into Cl gas and when the Na+ meets with the cathode it gets reduced to liquid sodium
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Faraday's Constant
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(1.6 x 10^-19 C/e-) {charge of an electrons}(6.022 x 10^23 e-/mol e-) =
96,487 C/Mol e- |
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Concentration Cells
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These cells have a negative gibbs free energy which means that the reaction is spontaneous. These type of cells work by a difference in concentration between the cells. The cells are made of the same material. Electrons are driven from one end to the other through the difference in the concentration between the cells.
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Anode and Cathode in Electrolytic Cells
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In electrolytic cells, the anode is positive and the cathode is negative. In a galvanic cell, the anode is negative and the cathode is positive. However, at both, reduction occurs at the cathode and oxidation occurs at the anode.
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How can you tell when an element will be oxidized or reduced?
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This is based on a comparison to the the standard hydrogen electrode. For each element the reduction potential is taken to determine the tendency with which is acquires electrons. This is taken under standard conditions which are 298 k and 1 atm. Key: THE HIGHER THE THE REDUCTION POTENTIAL THE GREATER THE TENDENCY FOR REDUCTION.
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EMF of Galvanic Cell vs EMF of Elctrolytic Cell
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The EMF of a galvanic is positive while the EMF of an electrolytic cell is negative
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How do you find the EMF of a cell?
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You subtract the EMF of the cathode from the EMF of the anode.
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