CHE 141 Dr. Sullivan Lecture 32Outline- Redox Revisited- Balancing Redox Reactions- Electrochemical Cells- Writing Cell DiagramsRedox Revisited- Redox reactions: is the sum of two half-reactions, one of which is the oxidation component: M(s)M+(aq)+e- and the other the reduction component: M+(aq)+e-M(s)- Oxidation corresponds to an increase in oxidation state- Reduction corresponds to a decrease in oxidation state- A reduction half-reaction can be reversed to obtain the corresponding oxidation half-reaction and vice versa- A substance that is easily oxidized (readily loses electrons) is a reducing agent- In any redox reaction, the reducing agent is always itself oxidized: M(s)M+(aq)+e-- A substance that is easily reduced (readily gains electrons) is an oxidizing agent- In a redox reaction the oxidizing agent is always itself reduced: M+(aq)+e-M(s)Balancing Redox ReactionsThese 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.- 1. Assign the oxidation states to all atoms and identify the substances being oxidized and reduced- 2. Separate the overall reaction into two half-reactions: one for oxidation and one for reduction- 3. Balance each half reaction with respect to mass- 4. Balance each half-reaction with respect to charge- 5. Make the number of electrons in both half-reactions equal by multiplying one or both half-reactions by a small whole number- 6. Add the two half-reactions together, cancelling electrons and other species as necessary- The balance mass step is slightly more complex for reactions occurring in acidic or basic solutions- In acidic solutionso Balance all elements other than H and Oo Balance O by adding H2Oo Balance H by adding H+- In basic solutionso Balance all elements other than H and Oo Balance O by adding H2Oo Balance H by adding H+o Neutralize H+ by adding enough OH- to neutralize each H+. Add the same number of OH- ions to each side of the equationElectrochemical Cells- Electric current is the flow of electric charge- Redox reactions involve transfer of electrons from one substance to another thus can be used to generate an electric current- An electrochemical cell is a device which couples electrical current and redox reactions- Voltaic (or galvanic) cell produces electrical current from a spontaneous chemical reaction- Electrolytic cell consumes electrical current to drive a nonspontaneous chemical reaction- A electrochemical cell consists of two half-cells the oxidation half-reaction occurs in one and the reduction half-reaction in the other- A half-cell contains a conductive electrode and a surrounding conductive electrolyte- The surfaces where the exchange of electrons occur are called electrodes- The electrode where oxidation occurs is called the anode- The electrode where reduction occurs is called the cathode- In a voltaic/galvanic cell, electrons flow from the anode to the cathode (from – to +)- As electrons flow out of the anode, positive ions from in the oxidation half-cell, building up a positive charge in solution- As electrons flow into the cathode, positive ions are reduced in the reduction half-cell, thus a negative charge builds up in the solution- A salt bridge provides a pathway through which counter ions can flow between the two-half-cell solutions and neutralize the charge build upWriting Cell Diagrams- The cell diagram shows how the components of an electrochemical cell are connected- Use double vertical lines to represent the salt bridge- The anode is written on the left side of the double lines- The cathode is written on the right side of the double lines- Work inward from the electrode towards the salt bridge using vertical lines to separate different states of matter on the same side- Use commas to separate same states of matter on the same side- If known, use the concentrations of dissolved species in place of any symbols, or the partial pressures of any gases instead of any symbols- Sometimes we have a half-reaction when there is no solid species to act as an electrode sothen use a platinum electrode which provides a chemically inert surface for electron transfer to occur, we include platinum electrodes in cell
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