1 REDOX REACTIONS Day 1 Review Oxidation numbers REACTIONS BatteriesBatteries

20.1 The Meaning of Oxidation and Reduction 2 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What Are Oxidation and Reduction? What happens to a substance that undergoes oxidation? What happens to a substance that undergoes reduction?

3 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.

20.1 The Meaning of Oxidation and Reduction 4 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Oxygen and Redox When methane (CH 4 ) burns in air, it oxidizes: reacts with oxygen One oxide of carbon is carbon dioxide, CO 2. CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O(g)

20.1 The Meaning of Oxidation and Reduction 5 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Not all oxidation involves burning. metals oxidizes to form compounds such as iron(III) oxide (Fe 2 O 3 ). 4Fe(s) + 3O 2 (g) → 2Fe 2 O 3 (s)

20.1 The Meaning of Oxidation and Reduction 6 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Common liquid household bleach contains sodium hypochlorite (NaClO), a substance that releases oxygen, which oxidizes stains to a colorless form. Hydrogen peroxide (H 2 O 2 ) also releases oxygen when it decomposes. It is both a bleach and a mild antiseptic that kills bacteria by oxidizing them.

20.1 The Meaning of Oxidation and Reduction 7 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. A substance that undergoes oxidation gains oxygen. A substance that undergoes reduction loses oxygen. Reactions that involve the processes of oxidation and reduction are called oxidation-reduction reactions. Oxidation-reduction reactions are also known as redox reactions. Original definition

20.1 The Meaning of Oxidation and Reduction 8 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What Are Oxidation and Reduction? MODERN REDOX DEFINITION Oxidation is now defined to mean complete or partial loss of electrons Reduction is now defined to mean complete or partial gain of electrons Oxidation Loss of electrons Reduction Gain of electrons

20.1 The Meaning of Oxidation and Reduction 9 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What Are Oxidation and Reduction? Redox Reactions During a reaction between a metal and a nonmetal, electrons are transferred from atoms of the metal to atoms of the nonmetal. Mg(s) + S(s) MgS(s) heat metal nonmetal lose gain IONIC

10 In all reactions if something has been oxidized then something has also been reduced REDOX REACTIONS Cu(s) + 2 Ag + (aq)  Cu 2+ (aq) + 2 Ag(s)

11 REDOX Reactions: Metals Lose electrons HNO 3 is the reduced 2 K + 2 H 2 O --> 2 KOH + H 2 Metals (K) are oxidized Metals (Cu) are “loser”: oxidized Cu + HNO 3 --> Cu 2+ + NO 2 hydrogen is reduced

12 Examples of Redox Reactions Metal + acid Zn + HCl Zn = loses electrons H + = gains electrons Cl- = spectator

13 Examples of Redox Reactions Metal + halogen 2 Al + 3 Br 2 ---> 2AlBr 3

14 Examples of Redox Reactions Metal (Mg) + Oxygen Nonmetal (S) + Oxygen

20.2 Oxidation Numbers > 15 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Assigning Oxidation Numbers oxidation numbers How do we determine which element has gained and which has lost electrons? An oxidation number is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.

20.2 Oxidation Numbers > 16 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Assigning Oxidation Numbers What is the general rule for assigning oxidation numbers?

17 The electric charge an element APPEARS to have when electrons : are counted by some arbitrary rules : The electric charge an element APPEARS to have when electrons : are counted by some arbitrary rules : OXIDATION NUMBERS 1. An element has ox. no. = 0. Zn O 2 I 2 S 8 2. In simple ions, ox. no. = charge on ion. -1 for Cl - +2 for Mg 2+

18 OXIDATION NUMBERS 3. In a compound: O has ox. no. = -2 (except in peroxides: in H 2 O 2, 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

20.2 Oxidation Numbers > 19 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. K 2 Cr 2 O –2 CrK(SO 4 ) –2 What is the oxidation number of chromium in each compound? Chromium potassium dichromate (K 2 Cr 2 O 7 ) is Orange chromium(III) potassium sulfate (CrK(SO 4 ) 2 ) is purple Cr

20 OXIDATION NUMBERS NH 3 N = ClO - Cl = H 3 PO 4 P = MnO 4 - Mn = Cr 2 O 7 2- Cr = C 3 H 8 C = Oxidation number of F in HF?

20.2 Oxidation Numbers > 21 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Oxidation-Number Changes in Chemical Reactions How are oxidation and reduction defined in terms of a change in oxidation number?

22 1.increase in oxidation number of same element = OXIDATION 2. decrease in oxidation number of same element = REDUCTION REDOX REACTIONS

20.2 Oxidation Numbers > 23 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. This figure illustrates a redox reaction that shows what occurs when a shiny iron nail is dipped into a solution of copper(II) sulfate. The iron reduces Cu 2+ ions in solution and is simultaneously oxidized to Fe 2+. The iron becomes coated with metallic copper. Cu 2+ SO 4 + Fe → Fe 2+ SO 4 + Cu

20.2 Oxidation Numbers > 24 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Oxidation-Number Changes in Chemical Reactions 2AgNO 3 (aq) + Cu(s) → Cu(NO 3 ) 2 (aq) + 2Ag(s) – –2 0 What happens when copper wire is placed in a solution of silver nitrate?

20.2 Oxidation Numbers > 25 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Use changes in oxidation number to identify which atoms are oxidized and which are reduced in the following reaction. 2HNO 3 (aq) + 3H 2 S(g) → 2NO(g) + 4H 2 O(l) + 3S(s) –2 +1 –2 +2 –2 +1 –2 0 Sulfur is oxidized because its oxidation number increases (–2 to 0). Nitrogen is reduced because its oxidation number decreases (+5 to +2). 2HNO 3 (aq) + 3H 2 S(g) → 2NO(g) + 4H 2 O(l) + 3S(s)

20.2 Oxidation Numbers > 26 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Use changes in oxidation number to identify which atoms are oxidized and which are reduced in the following reactions. a.Cl 2 (g) + 2HBr(aq) → 2HCl(aq) + Br 2 (l) b.C(s) + O 2 (g) → CO 2 (g)

20.3 Describing Redox Equations > 27 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Which of the following are redox reactions? A.NH 3 + HCl → NH 4 Cl B.SO 3 + H 2 O → H 2 SO 4 C.NaOH + HCl → NaCl + H 2 O D.H 2 S + NHO 3 → H 2 SO 4 + NO 2 + H 2 O

20.3 Describing Redox Equations > 28 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Which of the following are redox reactions? A.NH 3 + HCl → NH 4 Cl B.SO 3 + H 2 O → H 2 SO 4 C.NaOH + HCl → NaCl + H 2 O D.H 2 S + NHO 3 → H 2 SO 4 + NO 2 + H 2 O

20.2 Oxidation Numbers > 29 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 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) + 2MnO 2 (s) + 2NH 4 Cl(aq) → ZnCl 2 (aq) + Mn 2 O 3 (s) + 2NH 3 (g) + H 2 O(l)

20.3 Describing Redox Equations > 30 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Identifying Redox Reactions What are the two classes of chemical reactions?

20.3 Describing Redox Equations > 31 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. All chemical reactions can 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.

20.3 Describing Redox Equations > 32 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. During an electrical storm, oxygen molecules and nitrogen molecules in air react to form nitrogen monoxide. N 2 (g) + O 2 (g) → 2NO(g) How can you tell if this is a redox reaction?

20.3 Describing Redox Equations > 33 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 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.

20.3 Describing Redox Equations > 34 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Identifying Redox Reactions Examples of reactions that are not redox reactions include double-replacement reactions and acid-base reactions.

35 Review of Terminology for Redox Reactions OXIDATION—loss of electron(s) by a species; increase in oxidation number.OXIDATION—loss of electron(s) by a species; increase in oxidation number. REDUCTION—gain of electron(s); decrease in oxidation number.REDUCTION—gain of electron(s); decrease in oxidation number. OXIDIZING AGENT—electron acceptor; species is reduced.OXIDIZING AGENT—electron acceptor; species is reduced. REDUCING AGENT—electron donor; species is oxidized.REDUCING AGENT—electron donor; species is oxidized. OXIDATION—loss of electron(s) by a species; increase in oxidation number.OXIDATION—loss of electron(s) by a species; increase in oxidation number. REDUCTION—gain of electron(s); decrease in oxidation number.REDUCTION—gain of electron(s); decrease in oxidation number. OXIDIZING AGENT—electron acceptor; species is reduced.OXIDIZING AGENT—electron acceptor; species is reduced. REDUCING AGENT—electron donor; species is oxidized.REDUCING AGENT—electron donor; species is oxidized.

20.2 Oxidation Numbers > 36 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Redox reactions are identified by changes in oxidation number. BIG IDEA Reactions

20.2 Oxidation Numbers > 37 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Oxidation-Number Changes in Chemical Reactions You can define oxidation and reduction 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.