Oxidation-Reduction Reactions PDF

4.9. Oxidation–Reduction reactions in which electrons transfer from one reactant to the other. A fundamental definition of oxidation is the loss of electrons, A fundamental definition of reduction is the gain of electrons. When sodium reacts with chlorine, electrons transfer from the sodium t...

4.9. Oxidation–Reduction reactions in which electrons transfer from one reactant to the other. A fundamental definition of oxidation is the loss of electrons, A fundamental definition of reduction is the gain of electrons. When sodium reacts with chlorine, electrons transfer from the sodium to the chlorine, resulting in the formation of sodium chloride. In this redox reaction, sodium is oxidized and chlorine is reduced. Oxidation–Reduction Reaction The hydrogen in the balloon reacts with oxygen upon ignition to form gaseous water (which is dispersed in the flame).  Combustion Reactions Reactions in which O2(g) is a reactant are called combustion reactions Combustion reactions release lots of energy Combustion reactions are a subclass of oxidation- reduction reactions 2 C8H18(g) + 25 O2(g) 16 CO2(g) + 18 H2O(g) 4  Reactions of Metals with Nonmetals consider the following reactions: 4 Na(s) + O2(g) → 2 Na2O(s) 2 Na(s) + Cl2(g) → 2 NaCl(s) the reaction involves a metal reacting with a nonmetal in addition, both reactions involve the conversion of free elements into ions 4 Na(s) + O2(g) → 2 Na+2O– (s) 2 Na(s) + Cl2(g) → 2 Na+Cl–(s) 5  Oxidation and Reduction in order to convert a free element into an ion, the atoms must gain or lose electrons reactions where electrons are transferred from one atom to another are Oxidation and Reduction reactions atoms that lose electrons are being oxidized, atoms that gain electrons are being reduced 2 Na(s) + Cl2(g) → 2 Na+Cl–(s) Na → Na+ + 1 e– oxidation Cl2 + 2 e– → 2 Cl– reduction Agent losing electron, oxidation reaction, reducing agent Agent gaining electron, reduction reaction, oxidizing agent 6 The oxidation number is a positive or negative number that is assigned to an atom to indicate its degree of oxidation or reduction. In oxidation- reduction processes, the driving force for chemical change is in the exchange of electrons between chemical species. A series of rules have been developed to help us.  Oxidation states for reactions that are not metal + nonmetal, or do not involve O2, we need a method for determining how the electrons are transferred chemists assign a number to each element in a reaction called an oxidation state that allows them to determine the electron8 flow in the reaction  Rules for Assigning Oxidation States Rules are in order of priority 1. free elements have an oxidation state = 0 – Na = 0 and Cl2 = 0 in 2 Na(s) + Cl2(g) 2. monatomic ions have an oxidation state equal to their charge – Na = +1 and Cl = -1 in NaCl 3. (a) the sum of the oxidation states of all the atoms in a compound is 0 – Na = +1 and Cl = -1 in NaCl, (+1) + (-1) = 0 (b) the sum of the oxidation states of all the atoms in a polyatomic ion equals the charge on the ion N = +5 and O = -2 in NO3–, (+5) + 3(-2) = -1 9 Assign oxidation numbers to the atoms in each substance. 1.Cl2 2.GeO2 3.CaCl2 Solution 1.Cl2 is the elemental form of chlorine. Rule 1 states each atom has an oxidation number of 0. 1.By rule 3, oxygen is normally assigned an oxidation number of −2. For the sum of the oxidation numbers to equal the charge on the species (zero), the Ge atom is assigned an oxidation number of +4. 1.The Ca2+ ion has an oxidation number of +2 by rule 2. Also according to rule 2, each chlorine atom is assigned an oxidation number of −1. 10  Another Definition oxidation occurs when an atom’s oxidation state increases during a reaction reduction occurs when an atom’s oxidation state decreases during a reaction Sn4+ + Ca → Sn2+ + Ca2+ +4 0 +2 +2 Ca is oxidized, Sn4+ is reduced Ca is the reducing agent, Sn4+ is the oxidizing agent F2 + S → SF4 0 0 +4−1 S is oxidized, F is reduced S is the reducing agent, F2 is the oxidizing agent  Another Definition oxidation occurs when an atom’s oxidation state increases during a reaction reduction occurs when an atom’s oxidation state decreases during a reaction CH4 + 2 O2 → CO2 + 2 H2O -4 +1 0 4 –2 +1 -2 oxidation reduction  Example: Assign oxidation states, determine the element oxidized and reduced, and determine the oxidizing agent and reducing agent in the following reactions: Reducing Oxidizing agent agent Fe + MnO4− + 4 H+ → Fe3+ + MnO2 + 2 H2O 0 +7 −2 +1 +3 +4 −2 +1 −2 Reduction Oxidation