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Electron Transfer Reactions

B.8- B. 9 In which you will learn about: redox definitions assigning oxidation numbers half reactions balancing redox equations. Electron Transfer Reactions. Electron transfer reactions are oxidation-reduction or redox reactions.

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Electron Transfer Reactions

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  1. B.8- B. 9 In which you will learn about:redox definitionsassigning oxidation numbershalf reactionsbalancing redox equations

  2. Electron Transfer Reactions • Electron transfer reactions are oxidation-reduction or redox reactions. • Results in the generation of an electric current (electricity) or be caused by imposing an electric current. • Therefore, this field of chemistry is often called ELECTROCHEMISTRY.

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

  4. You can’t have one… without the other! • Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons. • You can’t have 2 oxidations or 2 reductions in the same equation. Reduction has to occur at the cost of oxidation LEO the lion says GER! GER!

  5. Another way to remember • OIL RIG

  6. OXIDATION-REDUCTION REACTIONS Direct Redox Reaction Oxidizing and reducing agents in direct contact. Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)

  7. OXIDATION-REDUCTION REACTIONS Indirect Redox Reaction A battery functions by transferring electrons through an external wire from the reducing agent to the oxidizing agent.

  8. Practice Exercises Section 1 • 1) Why is it that when something gains electrons, it is said to be reduced? What is being reduced? • 2) Make a list of important vocabulary words from slides 1-7 and define each.

  9. Assigning Oxidation #s 1) Free elements are 0 2) Monatomic ions are their charge 3) Hydrogen is +1 4) Oxygen is -2 5) Group 1 elements are +1, Group 2 are +2, and Group 17 are -1 6) The sum of all oxidation numbers must equal 0 7) The sum of all oxidation numbers in a polyatomic ion must equal the charge of the ion

  10. Oxidation #s Examples • Assign oxidation numbers for each element in the following compound: • Al2O3 • Al = +3 according to rule 2 • O = -2 according to rule 4 • 2(+3) + 3(-2) = 0 according to rule 6 • XeF4 • F = -1 according to rule 5 • x + 4(-1) = 0 according to rule 6 • Solving for x, tells us that Xe = +4

  11. Practice Exercises Section 2 Assign oxidation states for all atoms in each of the following compounds: • KMnO4 • NiO2 • K4Fe(CN)6 [Fe only] • (NH4)2HPO4 • P4O6

  12. Balancing Equations for Redox Reactions Some redox reactions have equations that must be balanced by special techniques. MnO4- + 5 Fe2+ + 8 H+ ---> Mn2+ + 5 Fe3+ + 4 H2O Mn = +7 Fe = +2 Mn = +2 Fe = +3

  13. Balancing Equations Consider the reduction of Ag+ ions with copper metal. Cu + Ag+ --give--> Cu2+ + Ag

  14. Balancing Equations Step 1: Divide the reaction into half-reactions, one for oxidation and the other for reduction. Ox Cu ---> Cu2+ Red Ag+ ---> Ag Step 2: Balance each element for mass. Already done in this case. Step 3: Balance each half-reaction for charge by adding electrons. Ox Cu ---> Cu2+ + 2e- Red Ag+ + e- ---> Ag

  15. Balancing Equations Step 4: Multiply each half-reaction by a factor so that the reducing agent supplies as many electrons as the oxidizing agent requires. Reducing agent Cu ---> Cu2+ + 2e- Oxidizing agent 2 Ag+ + 2 e- ---> 2 Ag Step 5: Add half-reactions to give the overall equation. Cu + 2 Ag+ ---> Cu2+ + 2Ag The equation is now balanced for both charge and mass.

  16. Practice Exercises Section 3 Specify which of the following are redox reactions. Identify the oxidizing agent, reducing agent, substance being oxidized, and the substance being reduced. Write half reactions to find out how many electrons are being transferred. • CH4 (g) + 2 O2 (g)  CO2 (g) + H2O (g) • Zn (s) + 2 HCl (aq)  ZnCl2 (aq) + H2 (g) • Cr2O72- (aq) + 2 OH- (aq)  2 CrO42- (aq) + H2O (l)

  17. Tips on Balancing Equations • Never add O2, O atoms, or O2- to balance oxygen. • Never add H2 or H atoms to balance hydrogen. • Be sure to write the correct charges on all the ions. • Check your work at the end to make sure mass and charge are balanced. • PRACTICE!

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