CH 302 1st Edition Lecture 23Outline of Last Lecture I. Worksheet (Readiness Assessment Quiz)Outline of Current Lecture I. ElectrochemistryII. Redox ReactionsIII. Redox NumbersIV. Balancing Redox ReactionsCurrent LectureElectrochemistry- Electrochemistry deals with oxidation/reduction reactions. - These reaction are associated with transfer of electrons between chemical species. - For many reactions, we can separate the two halves of the reaction oxidation and reduction and have them take place in physically different locations. - Then, rather than having the electrons transferred directly between the two species, we can force them to flow through an external circuit.- A key idea in electrochemistry is that the electrical potential (voltage) is related to the free energy of the reaction. - This allows us to both measure the free energy as well as control the free energy by applying an external potential. - In this way, we can make non-spontaneous reactions spontaneous by changing the free energy of the reactants and products with an external potential.Redox Reactions- Oxidation and reduction are key concepts in electrochemistry. - Chemistry involving oxidation and reduction is typically referred to as Redox chemistry. - Oxidation involves the loss of electrons from a species and reduction involves the gain ofelectrons. - Because we are never creating new charges out of nothing (aside from nuclear chemistry), we always have oxidation in conjunction with reduction.- Oxidation 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 GainRedox 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 one element. 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
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