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1 REDOX REACTIONS REACTIONS Batteries Day 1 Review Oxidation numbers 20.1 The Meaning of Oxidation and Reduction > What Are Oxidation and Reduction? What Are Oxidation and Reduction? What happens to a substance that undergoes oxidation? What happens to a substance that undergoes reduction? 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Electron Transfer Reactions • oxidation-reduction or redox reactions are Electron transfer reactions. • Redox reactions can result in the generation of an electric current. • Therefore, this field of chemistry is often called ELECTROCHEMISTRY. 3 20.1Oxygen The Meaning of Redox and Oxidation and Reduction > When methane (CH4)burns in air, it oxidizes: reacts with oxygen • One oxide of carbon is carbon dioxide, CO2. CH4(g) + 4 2O2(g) → CO2(g) + 2H2O(g) Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.1 The Meaning of Not and all Reduction oxidation Oxidation involves burning. > • metals oxidizes to form compounds such as iron(III) oxide (Fe2O3). 4Fe(s) 5 + 3O2(g) → 2Fe2O3(s) Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. •20.1 Common liquid The Meaning of household bleach contains > a substance that Oxidation Reduction (NaClO), sodiumand hypochlorite releases oxygen, which oxidizes stains to a colorless form. • Hydrogen peroxide (H2O2) also releases oxygen when it decomposes. It is both a bleach and a mild antiseptic that kills bacteria by oxidizing them. 6 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.1 Meaning ofthat undergoes A The substance Oxidation and Reduction oxidation gains > oxygen. A substance that undergoes reduction loses oxygen. Original definition • Reactions that involve the processes of oxidation and reduction are called oxidation-reduction reactions. • Oxidation-reduction reactions are also known as redox reactions. 7 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.1 The Meaning of What Are Oxidation and MODERN REDOXReduction? DEFINITION Oxidation and Reduction > Oxidation Reduction Loss of electrons Gain of electrons • Oxidation is now defined to mean complete or partial loss of electrons • Reduction is now defined to mean complete or partial gain of electrons 8 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What Are Oxidation and Redox Reactions > Reduction? 20.1 The Meaning of Oxidation and Reduction During a reaction between a metal and a nonmetal, electrons are transferred from atoms of the metal to atoms of the nonmetal. metal nonmetal lose gain Mg(s) 9 + S(s) IONIC heat MgS(s) Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. REDOX REACTIONS Cu(s) + 2 Ag+(aq) g Cu2+(aq) + 2 Ag(s) In all reactions if something has been oxidized then something has also been reduced 10 REDOX Reactions: Metals Lose electrons Metals (Cu) are “loser”: oxidized 11 Metals (K) are oxidized HNO3 is the reduced hydrogen is reduced Cu + HNO3 --> Cu2+ + NO2 2 K + 2 H2O --> 2 KOH + H2 Examples of Redox Reactions Metal + acid Zn + HCl Zn = loses electrons H+ = gains electrons Cl- = spectator 12 Examples of Redox Reactions Metal + halogen 2 Al + 3 Br2 ---> 2AlBr3 13 Examples of Redox Reactions Nonmetal (S) + Oxygen Metal (Mg) + Oxygen 14 20.2 Oxidation Numbers > Assigning Oxidation Numbers How do we determine which element has gained and which has lost electrons? oxidation numbers An oxidation number is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction. 15 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.2 Oxidation Numbers > Assigning Oxidation Numbers Assigning Oxidation Numbers What is the general rule for assigning oxidation numbers? 16 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. OXIDATION NUMBERS The electric charge an element APPEARS to have when electrons are counted by some arbitrary rules: 1. An element has ox. no. = 0. Zn O2 I2 S8 2. In simple ions, ox. no. = charge on ion. -1 for Cl+2 for Mg2+ 17 OXIDATION NUMBERS 3. In a compound: O has ox. no. = -2 (except in peroxides: in H2O2, O = -1) 4. In a Molecule: Ox. no. of H = +1 (except when H is associated with a metal as in NaH where it is -1) 5. Algebraic sum of oxidation numbers = 0 for a compound = overall charge for an ion 18 What is the oxidation 20.2 Oxidation Numbers > number of chromium in each compound? Chromium Cr potassium dichromate (K2Cr2O7) is Orange +1 +6 chromium(III) potassium sulfate (CrK(SO4)2) is purple +3 +1 +6 –2 CrK(SO4)2 –2 K2Cr2O7 19 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. OXIDATION NUMBERS NH3 N = ClO- Cl = H3PO4 P = MnO4- Mn = Cr2O72- Cr = C 3H 8 C = Oxidation number of F in HF? 20 20.2 Oxidation Numbers > Oxidation-Number Oxidation-Number Changes in Changes in Chemical Chemical Reactions Reactions How are oxidation and reduction defined in terms of a change in oxidation number? 21 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. REDOX REACTIONS 1.increase in oxidation number of same element = OXIDATION decrease in oxidation number of same element = REDUCTION 2. 22 ThisOxidation figure illustrates 20.2 Numbers a> redox reaction that shows what occurs when a shiny iron nail is dipped into a solution of copper(II) sulfate. Cu2+SO4 + Fe → Fe2+SO4 + Cu • The iron reduces Cu2+ ions in solution and is simultaneously oxidized to Fe2+. •The iron becomes coated with metallic copper. 23 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Oxidation-Number Changes in copper wire is placed in a Chemical Reactions solution of silver nitrate? 20.2 Oxidation Numbers > What happens when +1 +5 –2 0 +2 +5 –2 0 2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2Ag(s) 24 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.2 Numbers > UseOxidation changes in oxidation number to identify which atoms are oxidized and which are reduced in the following reaction. 2HNO3(aq) + 3H2S(g) → 2NO(g) + 4H2O(l) + 3S(s) +1 +5 –2 +1 –2 +2 –2 +1 –2 0 2HNO3(aq) + 3H2S(g) → 2NO(g) + 4H2O(l) + 3S(s) Sulfur is oxidized because its oxidation number increases (–2 to 0). Nitrogen is reduced because its oxidation number decreases (+5 to +2). 25 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.2 Oxidation Numbers > Use changes in oxidation number to identify which atoms are oxidized and which are reduced in the following reactions. a. Cl2(g) + 2HBr(aq) → 2HCl(aq) + Br2(l) b. C(s) + O2(g) → CO2(g) 26 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Which of the following are redox reactions? A. NH3 + HCl → NH4Cl B. SO3 + H2O → H2SO4 C. NaOH + HCl → NaCl + H2O D. H2S + NHO3 → H2SO4 + NO2 + H2O 27 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Which of the following are redox reactions? A. NH3 + HCl → NH4Cl B. SO3 + H2O → H2SO4 C. NaOH + HCl → NaCl + H2O D. H2S + NHO3 → H2SO4 + NO2 + H2O 28 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.2 Oxidation Numbers > Identifying Oxidized and Reduced Atoms Use changes in oxidation number to identify which atoms are oxidized and which are reduced in the following reaction. Zn(s) + 2MnO2(s) + 2NH4Cl(aq) → ZnCl2(aq) + Mn2O3(s) + 2NH3(g) + H2O(l) 29 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Identifying Redox Reactions What are the two classes of chemical reactions? 30 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Redox Equationscan > AllDescribing chemical reactions be assigned to one of two classes. One class of chemical reactions is oxidation-reduction (redox) reactions, in which electrons are transferred from one reacting species to another. The other class includes all other reactions, in which no electron transfer occurs. 31 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > How can you tell if this is a redox reaction? N2(g) + O2(g) → 2NO(g) During an electrical storm, oxygen molecules and nitrogen molecules in air react to form nitrogen monoxide. 32 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > single-replacement, synthesis, decomposition, and combustion reactions are redox reactions. • Potassium metal reacts violently with water to produce hydrogen gas (which ignites) and potassium hydroxide. 33 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Identifying Redox Reactions Examples of reactions that are not redox reactions include double-replacement reactions and acid-base reactions. 34 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 35 Review of Terminology for Redox Reactions • OXIDATION—loss of electron(s) by a species; increase in oxidation number. • REDUCTION—gain of electron(s); decrease in oxidation number. • OXIDIZING AGENT—electron acceptor; species is reduced. • REDUCING AGENT—electron donor; species is oxidized. 20.2 Oxidation Numbers > BIG IDEA Reactions Redox reactions are identified by changes in oxidation number. 36 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Oxidation-Number Changes in 20.2 Oxidation Numbers > You can define oxidation and reduction Chemical Reactions in terms of a change in oxidation number. An increase in the oxidation number of an atom or ion indicates oxidation. A decrease in the oxidation number of an atom or ion indicates reduction. 37 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.