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Introduction to Oxidation- Reduction Reactions Electron Transfer Reactions.

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Presentation on theme: "Introduction to Oxidation- Reduction Reactions Electron Transfer Reactions."— Presentation transcript:

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: 2Zn + 3MnO 2 Mn 3 O 4 + 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 are characterized by ELECTRON TRANSFER between an electron donor and electron acceptor. REDOX REACTIONS

6 Transfer leads to 1. increase in oxidation number of some element = OXIDATION 2.decrease in oxidation number of some element = REDUCTION REDOX REACTIONS

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

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 4.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. 5.Group IA metals are +1, IIA metals are +2 and fluorine is always –1. 6.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. K 2 SO 4 NO 3 - H 2 SO 4 Fe 2 O 3 Fe 3 O 4

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 S + O 2 SO 2 Decomposition Reaction 2KClO 3 2KCl + 3O 2 C A + B

17 Displacement Reaction a.k.a Single Replacement A + BC AC + B Sr + 2H 2 O Sr(OH) 2 + H 2 TiCl 4 + 2Mg Ti + 2MgCl 2 Cl 2 + 2KBr 2KCl + Br 2 Hydrogen Displacement Metal Displacement Halogen Displacement Types of Oxidation-Reduction Reactions

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

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

20 Disproportionation Reaction Cl 2 + 2OH - ClO - + Cl - + H 2 O Element is simultaneously oxidized and reduced. Types of Oxidation-Reduction Reactions Chlorine Chemistry 0 +1

21 Ca 2+ + CO 3 2- CaCO 3 NH 3 + H + NH 4 + Zn + 2HCl ZnCl 2 + H 2 Ca + F 2 CaF 2 Classify the following reactions.

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 + Cl 2 Na+ + Cl - SO H + + MnO 4 - 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 - CrO Cl - + H 2 O MnO Fe +2 Mn +2 + Fe +3

30 Now for a tough one Fe(CN) MnO 4 - Mn +2 + Fe +3 + CO 2 + NO 3 -

31 Basic Solution Do everything you would with acid, but add one more step. Add enough OH - to both sides to neutralize the H + CrI 3 + Cl 2 CrO IO Cl - CN - + MnO 4 - CNO - + MnO 2

32 Redox Titrations Same as any other titration. the permanganate ion is used often because it is its own indicator. MnO 4 - is purple, Mn +2 is colorless. When reaction solution remains clear, MnO 4 - is gone. Chromate ion is also useful, but color change, orangish yellow to green, is harder to detect.

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