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Introduction to Oxidation-Reduction Reactions

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1 Introduction to Oxidation-Reduction Reactions
Electron Transfer Reactions

2 Types of Chemical Reactions
There are four types of chemical reactions: Acid/Base Precipitation/Solubility Complex Formation/Complex Dissociation Oxidation/Reduction Any chemical reaction consists of one (or more) of these basic categories.

3 Oxidation/Reduction Reactions
Acid/Base reactions involve a donation /acceptance of protons Precipitation/ Solubility reactions involve a donation/ acceptance of negative charge what is being donated and accepted in a redox reaction?

4 Oxidation/Reduction Reactions
Electrons! Consider the reaction taking place in a disposable battery: Zn + 3MnO2  Mn3O4 + 2ZnO How can you tell that electrons are being donated and accepted? Which species is donating electron( s) and which is accepting electron (s)?

5 REDOX REACTIONS Redox reactions are characterized by ELECTRON TRANSFER between an electron donor and electron acceptor.

6 REDOX REACTIONS Transfer leads to— increase in oxidation number of some element = OXIDATION 2. decrease in oxidation number of some element = REDUCTION

7 Electron Transfer in Redox Reactions
Oxidation Loss of electrons Gain in oxygen Reduction Gain of electrons Loss of oxygen “LEO the lion goes Ger”

8 Mg S Mg 2+ S2- Example The reaction of a metal and non-metal
All the electrons must be accounted for! Mg S Mg 2+ S2- + +

9 Oxidation-Reduction Oxidation means an increase in oxidation state - lose electrons. Reduction means a decrease in oxidation state - gain electrons. The substance that is oxidized is called the reducing agent. The substance that is reduced is called the oxidizing agent.

10 Assigning Oxidation States
An Oxidation-reduction reaction involves the transfer of electrons. You should memorize these rules

11 Rules for Oxidation States
The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred. The oxidation state of elements in their standard states is zero. Example: Na, Be, K, Pb, H2, O2, P4 = 0

12 Assigning Oxidation States
Oxidation state for monatomic ions are the same as their charge. Example: Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2 Oxygen is assigned an oxidation state of -2 in its covalent compounds except as a peroxide.

13 Rules for Oxidation States
The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds. In these cases, its oxidation number is –1. Group IA metals are +1, IIA metals are +2 and fluorine is always –1. The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion.

14 Practice in Oxidation States
Assign the oxidation states to each element in the following. K2SO4 NO3- H2SO4 Fe2O3 Fe3O4

15 Identify the Oxidizing agent Reducing agent Substance oxidized
Substance reduced On the worksheet

16 Types of Oxidation-Reduction Reactions
Combination Reaction A + B C +4 -2 S + O SO2 Decomposition Reaction C A + B +1 +5 -2 +1 -1 2KClO KCl + 3O2

17 Types of Oxidation-Reduction Reactions
Displacement Reaction a.k.a Single Replacement A + BC AC + B +1 +2 Sr + 2H2O Sr(OH)2 + H2 Hydrogen Displacement +4 +2 TiCl4 + 2Mg Ti + 2MgCl2 Metal Displacement -1 -1 Cl2 + 2KBr KCl + Br2 Halogen Displacement

18 The Activity Series for Metals
Hydrogen Displacement Reaction M + BC AC + B M is metal BC is acid or H2O B is H2 Ca + 2H2O Ca(OH)2 + H2

19 Copper Demonstration Copper Pennies reacting with nitric acid.
Can you figure out the equation?

20 Types of Oxidation-Reduction Reactions
Disproportionation Reaction Element is simultaneously oxidized and reduced. +1 -1 Cl2 + 2OH ClO- + Cl- + H2O Chlorine Chemistry

21 Classify the following reactions.
Ca2+ + CO CaCO3 NH3 + H NH4+ Zn + 2HCl ZnCl2 + H2 Ca + F CaF2

22 Half-Reactions All redox reactions can be thought of as happening in two halves. One produces electrons - Oxidation half. The other requires electrons - Reduction half.

23 Half-Reactions Write the half reactions for the following.
Na + Cl2 → Na+ + Cl- SO3- + H+ + MnO4- → SO4- + H2O + Mn+2

24 Balancing Redox Equations
In aqueous solutions the key is the number of electrons produced must be the same as those required. For reactions in acidic solution an 8 step procedure.

25 Balancing Redox Equations
Write separate half reactions For each half reaction balance all reactants except H and O Balance O using H2O

26 Acidic Solution Balance H using H+ Balance charge using e-

27 Acidic Solution Multiply equations to make electrons equal
Add equations and cancel identical species Check that charges and elements are balanced.

28 Practice Balance the following reactions:
Sn 2+ (aq) + 2Fe 3+ → Sn 4+ (aq) + 2Fe 2+ MnO4- (aq) + C2O4-2 (aq) → Mn2+ (aq) + CO2 (g)

29 Practice The following reactions occur in aqueous solution. Balance them Cr(OH)3 + OCl- + OH- ® CrO4-2 + Cl- + H2O MnO4- + Fe+2 ® Mn+2 + Fe+3

30 Now for a tough one Fe(CN)6-4 + MnO4- ® Mn+2 + Fe+3 + CO2 + NO3-

31 Basic Solution Do everything you would with acid, but add one more step. Add enough OH- to both sides to neutralize the H+ CrI3 + Cl2 ® CrO4- + IO4- + Cl- CN MnO4- → CNO- + MnO2

32 Redox Titrations Same as any other titration.
the permanganate ion is used often because it is its own indicator. MnO4- is purple, Mn+2 is colorless. When reaction solution remains clear, MnO4- is gone. Chromate ion is also useful, but color change, orangish yellow to green, is harder to detect.

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