CH 302 1st EditionExam 4 Study Guide: Lectures: 23 - 27Be sure you know how to…1. Identify an oxidation – reduction (redox) reaction based on changes in oxidation numbers across the chemical change.2. Identify oxidizing/reducing agents in chemical reaction.3. Balance a net redox reaction using the ½ reaction method in acidic or basic solution.4. Recognize degrees of reactivity based on an activity series table or a standard reduction potential table.5. Apply standard reduction potential data to determine the relative strength of oxidizing/reducing agents.6. Construct an electrochemical cell diagram, including identifying the anode, cathode, directionof electron flow, sign of the electrodes, direction of ion flow in salt bridge, from a redox reaction or from short hand cell notation.7. Describe the standard hydrogen electrode and state it’s function.8. Apply standard reduction potential data to calculate the standard cell potential for an electrochemical cell and from the sign of the potential predict if the cell is voltaic or electrolytic.9. Calculate the cell potential for a nonstandard cell.10. Describe fully the relationship between the free energy and the cell potential.11. Describe fully the relationship between cell potential and the equilibrium constant.12. Explain thermodynamically the operation of a concentration cell, and be able to predict the concentration in the cell based on the cell potential.13. Understand the relationship between charge delivered or produced and the amount of reactant used or product formed for both galvanic and electrolytic cells.Lecture 23ElectrochemistryRedox Numbers- In order to identify Redox chemistry, chemists have developed a system by which we canthink about the "number of electrons" a certain element "has". - This is an idea that simply helps sort out the oxidation and reduction process.- In every chemical species, we can assign an oxidation number (or oxidation state) to each element. - This can be thought of as the number of electrons that "belong" to that element compared to the number of valence electrons the element has. - It is important to note that this is an idea that helps us to think about and classify the chemistry. - Within a compound the electrons don't have any labels and don't belong to anyone oneelement. This is merely an accounting trick for us to use as chemists.- For this we have a series of rules.1. The sum of all the oxidation numbers in a chemical species must equal the charge on the chemical species. Therefore if we are looking at a neutral molecule, when we add upall the oxidation numbers for all the atoms in the molecule, we should get zero. If we have apolyatomic ion with a charge of -2, then the sum of the oxidation numbers of all the atoms should be -2. This is the most important rule. It will help us figure out the oxidation number of any elements that are not "assigned" a number.2. The oxidation number of an atom in its neutral elemental state is zero. Monatomic species have no charge. Diatomic species if they break will end up as neutral atoms. For example, what is the oxidation number of oxygen in O2? Zero.3. The oxidation number for a monatomic ion is its charge. See above.4. Hydrogen is given the oxidation number of +1. Unless it is bonded to a metal in which case it is -1. (Note: H2 is not a compound.) What is the oxidation state of H in H2O? +1. What is the oxidation state of H in CH4? +1. In LiH? -1. In H2. 0.5. Oxygen is given the oxidation number of -2. Unless it is bonded to another in a peroxided bond (oxygen oxygen single bond). Then it is -1. What is the oxidation number of O in H2O? -2. In hydrogen peroxide, H2O2? -1.6. The most electronegative element is assigned its charge as an ion. What is the oxidation state of Mg and Br in MgBr2? Br is -1, Mg is +2. This is essentially the rule for the monatomic ions.Balancing Redox Reactions- Step 1: Determine the oxidation numbers of the elements in the reaction and identify the oxidation and reduction reactions.- Step 2: Identify the oxidation and reduction half reactions and write the equations for each.- Step 3: For each half-reaction, balance all elements except for hydrogen and oxygen. Then balance the oxygen by adding H2O. Then balance the hydrogen using H+. Finally balance the charge in the half-reaction using electrons.- Step 4: Multiply one or both of the balanced half-reaction by whole numbers to equalize the number of the electrons in each half-reaction.- Step 5: Double check that all the elements and charge are balanced.The half reaction method in basic conditions is nearly identical. The steps are the same up through step 5.- Step 6: We now have a balanced equation except it has H+ and we want to get to basic conditions. So now, add enough OH- to each side of the equations to neutralize away any H+. One side of the equation should now have the same number of H+ and OH-. These form water, H2O molecules. Now, cancel out any waters that appear on both sides of the equation.- Step 7: Double check that all elements and charge are balanced.Lecture 24Electrochemical CellsOxidation and Reduction:- If something is oxidized then its oxidation number goes up. Oxidation is the loss of electrons.- If something is reduced then its oxidation number goes down. Reduction is the gain of electrons.- Here are two pictures and ways to help you remember this. Pick one and remember it forever.1. LEO says GER Lose Electrons Oxidation. Gain Electrons Reduction.2. OIL RIG Oxidation Is Loss. Reduction Is GainElectrochemical Cells- In an electrochemical cell, we physically separate the oxidation and reduction chemistry in different "compartments". - The electrons from the oxidation are then run through an external circuit before being used in the reduction reaction. - As this is moving negative charge from one location to another, we need to compensate for this by moving other charges to balance this displacement of charge. - This is accomplished by using a salt bridge that allows the migration of spectator ions to balance the flow of electrons.- Each half of the electrochemical cell has an electrode to which the wire for our external circuit is connected. - The chemistry takes place at the surface of this electrode. The electrode on the oxidation side is called the anode. - The electrode on the reduction side is called the cathode. As a short hand, we
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