Chemistry 30: Reduction-Oxidation Reactions. Single replacement Formation Decomposition Combustion. Double replacement

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Chemistry 30: Reduction-Oxidation Reactions BIG IDEA: Reduction-oxidation (redox) reactions occur by the transfer of one or more electrons from one atom to another.

1 Chemistry 30: Reduction-Oxidation Reactions BIG IDEA: Reduction-oxidation (redox) reactions occur by the transfer of one or more electrons from one atom to another. By assigning oxidation numbers, we can track the electron movement. Redox reactions Single replacement Formation Decomposition Combustion Non-redox Double replacement A reduction-oxidation (redox) reaction is a reaction in which electrons are transferred from one atom to another. The term oxidation refers to the loss of electrons by one atom (L.E.O.) The term reduction refers to the gain of electrons by another atom (G.E.R.) o L.E.O. the lion says G.E.R. O.I.L. R.I.G. The substance that supplies the electrons is called the reducing agent because it helps something else get reduced. The substance that accepts the electrons is called the oxidizing agent because it helps something else get oxidized. The reducing agent is the substance that is oxidized. The oxidizing agent is the substance that is reduced. How can we track the movement of electrons? Oxidation Numbers/States This is a process devised by chemists to keep track of electron transfers in redox reactions. It also helps chemists determine if a reaction is, or is not, a reduction-oxidation. Each atom in an entity is assigned a value called an oxidation number (oxidation state) which provide a measure of whether the atom is neutral, electron-rich (negative), or electron-poor (positive). Industry Connection Barium is a metal that is easily oxidized in air to form BaO, thus it is difficult to obtain this metal in its pure form. Since barium reacts so easily with oxygen, it s often used to create vacuum tubes by actually using it to remove the last of the oxygen. By comparing the oxidation number of an atom before and after reaction, we can tell whether the atom has gained or lost electrons. Note that oxidation numbers don t necessarily imply ionic charges. Chemistry 30 Lesson 2-01 Page 1

2 Rules for assigning oxidation numbers. 1. The oxidation number of any element in its natural free state (not in a compound) is zero, regardless of how complex its molecules might be. This is because it has neither being oxidized nor reduced yet. Examples: Na, H 2, S 8 2. The sum of the oxidation states of all the atoms or ions in a neutral compound is zero. 3. The oxidation number of a monatomic ion is the same as its charge. For example, the oxidation state of the Na + ion is +1, for S 2- it is 2. Essentially this rule applies to ions in groups 1, 2, The sum of all the oxidation numbers of atoms in a molecule or polyatomic ion must equal the net charge on the particle. Thus oxidation numbers of metals in groups 3 12 in ionic compounds are determined by the overall net charge of the polyatomic ion. 5. In its compounds, fluorine has an oxidation number of 1 (always). 6. In its compounds, oxygen has an oxidation number of 2. Except when combined with fluorine (which must always be 1). Except when in peroxides like H 2 O 2 and Na 2 O 2 where it has an oxidation number of In its compounds, hydrogen has an oxidation number of When trying to figure out the oxidation numbers of atoms in an entity, apply the above rules first, then assign an oxidation to the last atom so that the sum of all oxidation numbers equals the net charge on the entity. In complex ionic compounds, metals use their ion charge in the compound as the oxidation state. Examples: PbSO 4 Zn 3 (BO 3 ) 2 8. Atoms can have fraction or decimal oxidation numbers. Example: C 3 H 8 Oxidation number examples: HCl NaClO 3 KMnO 4 MnO 4 2- C 4 H 10 CH 4 Chemistry 30 Lesson 2-01 Page 2

3 Identifying a Redox Reaction. A redox reaction is a chemical reaction in which changes in oxidation numbers occurs due to a transfer of electrons Two processes occur simultaneously: - oxidation leads to an increase in oxidation number (becomes more positive) - reduction leads to a decrease in oxidation number (becomes more negative) - visualize a number line: The oxidizing agent (undergoing reduction) and the reducing agent (undergoing oxidation) may be identified in a redox reaction equation by examining changes in oxidation numbers. o Note: Individual atoms get oxidized or reduced, not the entity they are found in. The entity that contains an atom being reduced or oxidized is the agent of change. Only reactants are oxidized or reduced. Only reactants are oxidizing agents (O.A.) or reducing agents (R.A.) A disproportionation reaction is redox reaction where some atoms of an element undergo oxidation and other atoms of the same element undergo reduction. This also means that a single species (molecule, atom, ion) can be both the oxidizing agent and the reducing agent. Chemistry 30 Lesson 2-01 Page 3

4 Example Problems Identify the oxidizing agent, the reducing agent and the atoms being oxidized and reduced in the following examples. Example 1: ! 1. Identify all oxidation numbers Fe+H Cl H 2+FeCl ! 2. Identify oxidation numbers that change. Fe is being oxidized (0 "+3) thus Fe is the reducing agent H is being reduced (+1 " 0) thus HCl is the oxidizing agent Example 2: PbS + 4 H 2 O 2 PbSO H 2 O Example 3: (photosynthesis) 6 CO H 2 O C 6 H 12 O O 2 Example 4: (disproportionation) Cu 2 O + H 2 SO 4 Cu + CuSO 4 + H 2 O Chemistry 30 Lesson 2-01 Page 4

5 Chemistry 30 Oxidation Numbers Formative Problem Set Assigning oxidation numbers 1. Assign an oxidation number to carbon in the following entities: a) C 2 H 5 OH 2. Determine the oxidation number for the element noted in each of the following entities: a) Ga in Ga 2 O 3 b) C 6 H 6 c) CO b) Pb in PbO 2 c) Br in KBrO 4 d) CO 2 e) CH 3 COOH d) Mn in K 2 MnO 4 e) Cr in CrO 3 f) Na 2 CO 3 g) C h) CH 4 i) CH 3 OH 1- f) Al in Al(OH) 4 1- g) I in IO 3 2- h) Zn in Zn(OH) 4 1- i) As in H 2 AsO 3 Chemistry 30 Lesson 2-01 Page 5

6 Identifying Reducing and Oxidizing Agents Formative Problems Identify the entity oxidized and the entity reduced, as well as the oxidizing and reducing agents. (Hint: assign oxidation numbers to all entities first.) a) 2 Mg + O 2 2 MgO b) 2 HNO H 3 AsO 3 2 NO + 3 H 3 AsO 4 + H 2 O c) NaI + 3 HOCl NaIO HCl d) 2 KMnO H 2 C 2 O H 2 SO 4 10 CO 2 + K 2 SO MnSO H 2 O Chemistry 30 Lesson 2-01 Page 6

7 e) Cu + 2 H 2 SO 4 CuSO 4 + SO H 2 O f) I HNO 3 2 HIO NO H 2 O Chemistry 30 Lesson 2-01 Page 7

Oxidation-Reduction Reactions

Oxidation-Reduction Reactions Chapter 4 Section 9 & 10 Oxidation Reduction Reactions Reactions in which one or more electrons are transferred. That means elements change their charge from reactants to