1 Chapter 11 Electrochemistry oRedox Reaction Review oGalvanic Cells oThermo of Electrochemistry oNernst Equation oBatteries / Fuel Cells oElectrolytic Cells 1 Big Idea:Electron transfer in a chemical reaction is both material and concentration specific. If the process is spontaneous the transfer of electrons can be used to produce a current and drive electrical devices. Chapter 11: Electrochemistry Redox Reaction Review Assigning Oxidation Numbers The oxidation number (ON) of an element uncombined with another element is zero: Na(s) ON = 0, and H2(g) ON = 0. For monatomic ions, the charge is the ON: Na+ON = +1. The ONs of elements in group 1 equal 1 (ex. Lithium ON = +1) ONs of elements in group 2 equal 2 (ex. Magnesium ON = +2), when the atoms are in a compound. The ON of fluorine is always -1 in compounds. The ON of the other elements in group 7 usually equal -1 when the atoms are in a compound. The ON of oxygen is usually -2 in compounds. Exceptions are fluorine compounds and peroxide (a compound that contains an O-O single bond). Hydrogen's ON is +1 when combined with non metals and -1 when combined with metals. The sum of the ON’s of all the atoms in a species is equal to its total charge. 2 Chapter 11: Electrochemistry Redox Reaction Review Assigning Oxidation Numbers 3 NaCl Element Oxidation Number Reason Na Cl Fe2(SO4)3 Element Oxidation Number Reason SO4 O S Fe Br2 Element Oxidation Number Reason Br Chapter 11: Electrochemistry Redox Reaction Review Determining Which Element is Oxidized and Which is Reduced Step 1: Assign oxidation numbers. Step 2: Use oxidation numbers and ‘OIL RIG’ to identify which element is oxidized and which is reduced. 4 Note: If the question asks which “substance is oxidized”; instead of giving the element that is oxidized give the compound that the element is in. Chapter 11: Electrochemistry Redox Reaction Review Oxidizing Agent: A species that removes electrons from a species being oxidized in a redox reaction. Reducing Agent: The species that supplies electrons to a substance being reduced in a redox reaction. 5 Note: Oxidizing agent is the species being reduced. Note: Reducing agent is the species being oxidized. Chapter 11: Electrochemistry Redox Reaction Review Balancing Redox Reactions in Acidic Conditions Step 1: Write unbalanced half reactions. Step 2: Balance half reactions except for O and H. Step 3: Balance O by using H2O. Step 4: Balance H by using H+. Step 5: Balance electrons in each half reaction. Step 6: Multiply half reactions by an integer so that number of electrons match, then add the reactions together. 6
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