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Oxidation-Reduction

Oxidation-Reduction. aka…”Redox”. C 3 H 8 O + CrO 3 + H 2 SO 4  Cr 2 (SO 4 ) 3 + C 3 H 6 O + H 2 O. Why electrochemistry?. REDOX reactions are important in …. Purifying metals (e.g. Al, Na, Li). Electrical production ( batteries, fuel cells).

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Oxidation-Reduction

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  1. Oxidation-Reduction aka…”Redox”

  2. C3H8O + CrO3 + H2SO4 Cr2(SO4)3 + C3H6O + H2O Why electrochemistry? REDOX reactions are important in … • Purifying metals (e.g. Al, Na, Li) • Electrical production (batteries, fuel cells) • Producing gases (e.g. Cl2, O2, H2) • Electroplating metals • Protecting metals from corrosion • Balancing complex chemical equations • Sensors and machines (e.g. pH meter)

  3. Electrons • Remember…electrons are negatively charged. • If electrons are gained, the charge of the ion will decrease. • If electrons are lost, the charge of the ion will increase. Na+1 Cl-1

  4. What is Redox? • REDOX stands for REDuction/OXidation • Oxidation is often thought of as a combination of a substance with oxygen (rusting, burning) • Oxidation refers to a loss of e- • Reduction refers to a gain of e- Remember: LEO the lions says GERRRRRR! LossElectrons= Oxidation Gain Electrons = Reduction

  5. Reactions Fe --> Fe+3 +3e- What is happening to the Fe atom? Fe is going from 0 to +3 oxidation # It must be losing electrons Loss of Electrons = Oxidation In conclusion, the iron atom is being oxidized

  6. Reactions S +2e- --> S-2 What is happening to the Sulfur atom? S is going from 0 to -2 oxidation # It must be gaining electrons Gain of Electrons = Reduction In conclusion, the sulfur atom is being reduced

  7. Partners in crime • A reduction reaction cannot occur without the oxidation of another element. It is the electron(s) that is lost during the oxidation that causes another atom to be reduced.

  8.   Na0 H20 F20 Assigning OxidationNumbers 1. FREE ELEMENTS (Atoms) ATOMS ARE ELECTRICALLY NEUTRAL (# OF PROTONS = THE # OF ELECTRONS) AND Always have an oxidation number of zero Na H2 F2

  9.   +1 -1 -2 Assigning OxidationNumbers 2. IONS (CHARGED PARTICLES) AN ION HAS AN OXIDATION NUMBER EQUAL TO ITS CHARGE Na+1 H-1 S-2

  10. Assigning Oxidation Numbers 3. FLUORINE • F is always -1 WHY? IT IS THE MOST ELECTRONEGATIVE ATOM ON THE PERIODIC TABLE

  11. Assigning Oxidation Numbers 4. Group 1 metals (Alkali metals) • Are always +1

  12. Assigning Oxidation Numbers 5. Group 2 metals (Alkaline Earth) • Are always +2

  13. Assigning Oxidation Numbers 6. OXYGEN • O is almost always -2 EXCEPTION: HYDROGEN PEROXIDE (H2O2) WHERE OXYGEN IS -1

  14. Assigning Oxidation Numbers 7. HYDROGEN • H is almost always • +1 EXCEPTION: IN METAL HYDRIDES, WHERE THE H IS LISTED 2ND, ITS OXIDATION # IS -1

  15. Assigning Oxidation Numbers 8. If there is no rule for either element in the compound… • Always err with the most electronegative element

  16.  Na+1Cl-1 As+5I5-1  Cu+2(N+5O3-2)2  H2+1Cr2+6O7-2 Assigning OxidationNumbers 9. COMPOUNDS The sum of all oxidation numbers will equal ZERO NaCl AsI5 Cu(NO3)2 H2Cr2O7

  17.  (S+6O4-2)-2 (N+5O3-2)-  (Cr2+3O4)-2 Assigning Oxidation Numbers 10. POLYATOMIC IONS The sum of oxidation numbers will equal the charge of that ion (SO4)-2 (NO3)- (Cr2O4)-2

  18. Oxidation • Oxidation represents a loss (or an apparent loss) of electrons. • “leo” lose electrons oxidation! • If an atom loses electrons, then its oxidation number will increase • Ex: Mn0(s) Mn+7 + 7e-

  19. A llama at the library???

  20. Reduction • Reduction represents a gain (or an apparent gain) of electrons. • “ger” gain electrons reduction! • If an atom gains electrons, then its oxidation number will decrease. • Ex: Mg+2 + 2e- Mg0

  21. Oxidizing and Reducing Agents Oxidizing Agent  - causes the oxidation of another atom - it is actually the atom that is REDUCED - oxidation number decreases - causes the reduction of another atom Reducing Agent  - it is actually the atom that is OXIDIZED - oxidation number increases Mg + Cl2 MgCl2 What is the O.A.?  Cl What is the R.A.?  Mg

  22. HALF REACTIONS Shows only one half of the reaction, including the e- lost or gained Reduction: Sn+4+ 2e-  Sn+2 If electrons are gained (taken in) then we place them on the reactants side of the half reaction. Oxidation: Ni  Ni2++ 2e- If electrons are lost (given off) then we place them on the products side of the half reaction. -notice that there is a conservation of charge and mass

  23. BALANCING REACTIONS • In a chemical change, there must be a conservation of mass, energy and charge • We must make sure that the e- that one atom loses must equal the e- that another atom gains Try to balance this one: Cu (s) + AgNO3 Cu(NO3)2+ Ag(s)

  24. BALANCING REACTIONS Cu (s) + AgNO3 Cu(NO3)2 + Ag(s) 1. Assign ox #’s and write ½ reactions Ag+1+ 1e- Ag0 Cu0 Cu+2 + 2e- 2. Balance each ½ reaction with respect to atoms and then e- • 2(Ag+1+ 1e- Ag0) • Cu0 Cu+2 + 2e- 3. Distribute to all parts of the ½ reaction • 2Ag+1 + 2e- 2Ag0 • Cu0 Cu+2 + 2e-

  25. BALANCING REACTIONS Cu (s) + AgNO3 Cu(NO3)2 + Ag(s) 4. Insert new coefficients into original equation and balance the rest, if necessary. • Cu (s) + 2AgNO3 Cu(NO3)2+ 2Ag(s)

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