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2. What is the oxidation number of Na in Na2O? Since O = -2, the two Na must each be +1. 3. What is the oxidation number of Cl in ClO¯? The O is -2, but since a -1 must be left over, then the Cl is +1. 4. What is the oxidation number for each element in KMnO4? K = +1 because KCl exists. We know the Cl = -1 because HCl exists. O = -2 by definition Mn = +7. There are 4 oxygens for a total of -8, K is +1, so Mn must be the rest. 5. What is the oxidation number of S in SO4 2¯ O = -2. There are four oxygens for -8 total. Since -2 must be left over, the S must = +6. Please note that, if there is no charge indicated on a formula, the total charge is taken to be zero. Practice Problems Find oxidation numbers 1. N in NO3¯ 2. C in CO3 2¯ 3. Cr in CrO4 2¯ N is 5+ because the charge is -1 so -6+5=-1 C is 4+ because the charge is -2 so -6+4= -2 Cr is +6 because the charge is -2 so -8+6=-2 4. Cr in Cr2O7 2¯ 5. Fe in Fe2O3 6. Pb in PbOH + 7. V in VO2 + 8. V in VO 2+ 9. Mn in MnO4¯ 10. Mn in MnO4 2¯ Cr is -8 because the charge is -2 so -16-14=-2 Fe is 3+ because the charge is 0 so 6-6=0 2+ 5+ 4+ 7+ 6+ Notes: Depending on the charge + or - number of the problem, it can vary what the oxidation number of an element can be. Oxygen is normally -2 it all depends on the ion charge. The oxidation number of something like Magnesium can change completely to something its not in charge wise so that it can complete the problem. 162
7. Half Reactions A half-reaction is simply one which shows either reduction OR oxidation, but not both. Here is the example redox reaction used in a different file: Ag + + Cu → Ag + Cu 2+ It has BOTH a reduction and an oxidation in it. That is why we call it a redox reaction, from REDuction and OXidation. What you must be able to do is look at a redox reaction and separate out the two half-reactions in it. To do that, identify the atoms which get reduced and get oxidized. Here are the two half-reactions from the above example: Ag+ → Ag Cu → Cu 2+ The silver is being reduced, its oxidation number going from +1 to zero. The copper's oxidation number went from zero to +2, so it was oxidized in the reaction. In order to figure out the halfreactions, you MUST be able to calculate the oxidation number of an atom. Keep in mind that a half-reaction shows only one of the two behaviors we are studying. A single halfreaction will show ONLY reduction or ONLY oxidation, never both in the same equation. Also, notice that the reaction is read from left to right to determine if it is reduction or oxidation. If you read the reaction in the opposite direction (from right to left) it then becomes the other of our two choices (reduction or oxidation). For example, the silver half-reaction above is a reduction, but in the reverse direction it is an oxidation, going from zero on the right to +1 on the left. There will be times when you want to switch a half-reaction from one of the two types to the other. In that case, rewrite the entire equation and swap sides for everything involved. If I needed the silver half-reaction to be oxidation, I'd write Ag → Ag+ rather than just doing it mentally. The next step is that both half-reactions must be balanced. However, there is a twist. When you learned about balancing equation, you made equal the number of atoms of each element on each side of the arrow. That still applies, but there is one more thing: the total amount of charge on each side of the half-reaction MUST be the same. When you look at the two half-reactions above, you will see they are already balanced for atoms with one Ag on each side and one Cu on each side. So, all we need to do is balance the charge. To do this you add electrons to the more positive side. You add enough to make the total charge on each side become EQUAL. To the silver half-reaction, we add one electron: To the copper half-reaction, we add two electrons: Ag+ + e¯ ---> Ag Cu ---> Cu 2+ + 2e¯ 163
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Honors Chemistry Class Policies and
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Unit 1 (22 days) Chapter 1 Introduc
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Lorenzo Walker Technical High Schoo
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The Learning Goal for this assignme
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RULE #3: To add/subtract in scienti
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For addition and subtraction, look
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Neutron Madness We have already lea
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In the space below, write the unabb
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Create groups for these Scientist a
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The Learning Goal for this Assignme
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Atomic Size Define Atomic Size: The
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Define Electronegativity: Electrone
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Unit 3 Chapter 25 Nuclear Chemistry
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Name ____________________ Juan Roqu
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Ion Charge? What is the charge on f
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C 2 H 6 O Ethanol CH 3 CH 2 O Step
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Linear Molecular Geometry Orbital E
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Trigonal Pyramidal Molecular Geomet
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Trigonal Bi pyramidal Molecular Geo
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Octahedral Molecular Geometry Orbit
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Orbitals Equation Lone Pairs Angle
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Stoichiometry and balanced equation
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