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RedOx Reactions & Electrochemistry Outline and Section Review 8.3 Usefulness of RedOx • Making Batteries • Corrosion Control • Isolating Elements from Compounds • Used for dyes and pigments Reduction + Oxidation = RedOx RedOx Reduction – Gaining of electrons Oxidation – Loss of electrons OIL RIG Oxidation is Losing Electrons Reduction is Gaining Electrons Reduction and oxidation always happens in pairs thereby Reduction / Oxidation 2Al + 3CuCl2 2AlCl3 + 3Cu 0 2Al 2+ 3Cu + 3+ 2Al 6e + 6e 0 3Cu Lab – Single Replacement Section Review 19.1 Questions 2, 3 (b-d), 5 Electrochemistry OPTIONAL READING: Section 20.1, pg 658 and pg 662-665 WS – Voltaic Cell We will be discussing how to produce electrical energy from a RedOx Reaction just as is done in a common battery. Batteries are a bank of voltaic cells. A battery’s usefulness is measured thru Voltage (or Electric Potential) & Wattage / Current. We will discuss how to calculate the expected voltage and how to improve on the voltage Electric Potential (or Voltage) is the driving force behind a transfer of electrons. Voltage can be thought of as Electrical Pressure. Voltage is created by a RedOx Reaction RedOx of Two Metals • Redox Reactions half reactions – Oxidation: Zn0 – Reduction: Cu2+ + Zn2+ + 2e 2e- Cu0 The repulsion of the electrons at the site of oxidation creates a flow of electrons towards the site of reduction. If we separate the metals and channel the electrons through a circuit we can use the flow of electrons (electricity) Electrodes • The Zinc and Copper Strips are our Electrodes. • Electrodes are conductors that make a connection with the nonmetallic parts of the solution. • Electrodes are also the location of oxidation and reduction Electrodes • ANODE is the site of OXIDATION – In a battery the anode is negative because it is the site where electrons are released. • CATHODE is the site of REDUCTION – In a battery the cathode is positive because it is the site where electrons are absorbed. Simple Battery Activity Cu and Zn Do Not Dip the Alligator Clips and Rinse them off We have a table that is similar to the activity series, but gives data that can be used to calculate the possible voltages. It is called a Standard Reduction Potential Table. All the reactions on a Reduction Potential Table are shown as Reductions and Compared to Hydrogen’s ability to be reduced. How to calculate voltage • We know that in a redox reaction one substance / element has to gives up electrons and one absorb electrons. • The one that gives up electrons (oxidation) will be lower on the reduction potential table. • We reverse the reduction half-reaction for the element that has a lower reduction potential because it is actually oxidized instead of reduced. Find the Reduction Half-Rxns and Reverse the One That’s Oxidized Cu2+ + 2e- Cu0 Zn2+ + 2e- Zn0 +0.34 -0.76 Volts Volts Zn is lower so………… 0 Zn +2 Zn + 2e +0.76 Volts Calculating Cell Potential Zn0 Zn+2 + 2eCu+2 + 2e- Cu0 +0.76 V +0.34 V +1.10 V Combined force with which the reaction takes place 1.10V E cell = E cathode + E anode E cell = E reduction + E oxidation This equation assumes that the sign of the voltage for the oxidized substance has already been changed! Substances will have different electron releasing or absorbing abilities as evidenced by the activity series which is really an oxidation table and the reduction potential table. Larger distances between metals on reduction potential table the larger the voltage. Better means more voltage (electrical potential). Lab – Making a Battery Lab – Building a Reduction Potential Table Lab – Powering an LED Corrosion of Metals 25% of metal production is to replace corroded metals RUSTING 4Fe(s) +3O2 0 4Fe 6 (g) 3+ 4Fe 0 O + 12e 2Fe2O3 (s) + 12e 26O Issues Corrosion Creates •Safety •$$$$$$ •Aesthetics / Cleanliness Videos of Corrosion Impacts • Aloha Airlines Flight 243 • I-35 Bridge Galvanic Corrosion GALVANIC CORROSION GALVANIC Corrosion • Insulating material between Copper Statue and Iron Scaffolding broke down and needed to be replaced at the cost of $90,000,000 What is a half reaction & net equation? • Redox Reactions half reactions – Oxidation: – Reduction: Fe0 Fe2+ + 2eCu2+ + 2e- Cu0 • Net Ionic Reaction Fe0 + Cu+2 Fe+2 + Cu0 • Corrosion will occur between any two substances (in the case of galvanic corrosion – two metals) which have a positive voltage • Anytime two dissimilar metals are in contact there will be corrosion. How do you prevent corrosion ? • Avoid contact – separation • Paint or Plastic Insulation • Coat with metal that makes a good oxide layer like Cr (Stainless) or Zn (Galvanized) • Keep materials dry – Salt water exacerbates the issue Sacrificial Anode – Prevents Corrosion • Connect structurally important metal to a more easily oxidized metal. • Metal is not important to structure or function • Must be replaced • Ex: Zinc coated nails, zinc piece on boat motor, bridges have zinc slabs. Lab – Corrosion of a Nail What can speed or impede corrosion of an Fe nail? ?????????? Cu Wrap Zn Wrap Electroplating What is electroplating? • Electroplating is coating one metal onto another through precipitation of metal ions onto a negatively charged part (anode) • Electroplating helps with corrosion resistance and appearance. • Zn, Cr, Ni are coated onto Fe to prevent rust (redox rxns) because they form an impervious and durable oxide layer. Chrome Plating Chrome and Iron Electrodes in Nickel Ion +2 + 2eo = +0.26 V Ni Ni E solution Cu+2 + e- Cu Eo = +0.34 V Fe3+ + 3e- Fe0 Eo = -0.45 V Etotal = 0.60 V Cr0 Cr3+ + 3eEo = 0.75 V Etotal = 0.30 V Electroplating Cell + Power Supply - Oxidation Reduction Cr Cr3+ Electrodes Cleaned off Cr3+ Fe Plating Plating Nickel is a Nonspontaneous plating reaction Naturally Ni is reduced and Fe is oxidized Fe3+ + 3e- Fe0 Ni0 Ni+2 + 2e- Eo = 0.26 V Eo = -0.45 V Etotal = -0.19 V To reverse the process we would need to add 0.19 Volts to the process. Types of Redox Reactions • Decomposition • Synthesis • Single-Replacement – Follows activity series (NH4)2Cr2O7 (s) Cr2O3 (s) + N2 (g) + 4H2O (g) Decomp of Ammonium Dichromate (NH4)2Cr2O7(s)Cr2O3(s)+ N2(g) +4 H2O(g) Heat – Exothermic Reaction Decomposition Reaction Self Sustaining Electrolytic Cells • Because reactions are merely the exchange of electrons we can use electricity and LeChatelier’s Principle to force reactions to proceed in whatever direction we desire. • Demonstration of electrolysis of H2O – Indicators around both the cathode and anode to detect the presence of OH- and H+ • Aluminum metal and Sodium metal Bleaches • Color caused by the movement of electrons up and down energy levels • Removing electrons – OXIDATION – will prevent this movement The End Confusion of terms Reducing agents get oxidized and Oxidizing agents get reduced The exchange of electrons can be real (ionization and charges) or merely helpful (oxidation numbers for covalent compounds) Oxidation number Na Charge Na 1+ +1 Charge - 1+ Na Oxidation #s - 1Cl +1 (H )2 -2 O Pairing of oxidation and reduction -3 • 2N N2 + 6 +6 • 2Cr +6 e e +3 2Cr Example charge conservation Black and White Photography • Br Br + e- OXIDATION • Ag + e- Ag REDUCTION Reaction of Silver Halide in Black and White Photos Ag+ hv Br- Ag+ + Br + (s) e e Ag + Ag+ Ag + Ag Ag Ag Ag Ag Ag + Ag Ag Ag Ag Ag Use the newly developed negative to make a print through contact copying Common Dry Cell Batteries Zn (s) Zn+2 (s) + 2e- Oxidation 2MnO2 (s) +2NH4+(aq) +2e- Mn2O3 (s) +2NH3 (aq) +H2O(l) Reduction Alkaline batteries also use the reduction of manganese and oxidation of zinc to create an electrical current Dry Cell Alkaline NH3Cl reacts with KOH replaces Zn even when not NH3Cl and barriers in use installed Cheap More Expensive Inconsistent voltage Consistent voltage and short life and longer life Rechargeable Batteries Reversed poles extracts electrons from reduced and supplies electrons to oxidized How do Hybrid Cars work? Gelatin Battery Demo p 703 Each cell will become more and more positively charged moving from the right to left if the flow of electrons does not increase. This will build-up current to the point of internal resistance and then voltage is built up. When it is disconnected you are creating a deficit of electrons in the left most cell. e- e- H+ e- Fuel Cells eH2 Gas Catalyst converts H2 into H+ H2O Gas Battery and Engine 2H+ e Catalyst Converts O2 into O- and 2H+ + O- H2O O2 Gas Fuel Cell Chemistry Chemistry of a Fuel Cell • Anode side: 2H2 => 4H+ + 4e• Cathode side: O2 + 4H+ + 4e- => 2H2O • Net reaction: 2H2 + O2 => 2H2O Character of Fuel Cell • Low Temperature 176 degrees Celsius • Each cell is 0.7 volts so many are needed • Water is pure enough to drink • Good efficiency (26% compared to 20%) • Hydrogen is being taken from fossil fuels, alcohol, and methane Rules for finding oxidation numbers • Periodic Table Position Helps –1A +1 (except hydrogen w/ a metal) –2A +2 –3A +3 –6A -2 (most of the time) –7A -1 More Rules • Uncombined elements always have a charge of zero • Charge on any monoatomic ion = ionic charge • Oxidation numbers for elements in a compound are written on a per atom basis • The sum of all the individual atom’s oxidation numbers in a polyatomic ion equal to the charge on the polyatomic ion • The sum of all oxidation numbers in a compound must equal zero Practice Finding Oxidation Numbers for Elements within a Compound You can Mass Balance a complex redox reaction by Charge Balancing and Oxidation Numbers Charge is conserved S + HNO3 SO2 + NO + H2O Oxidation Numbers Change Left Right Side Side +4 S=0 S = +4 N = +5 N = +2 -3 3(+4) = +12 S + HNO3 SO2 + NO + H2O 4(-3) = -12 3(+4) = +12 3S + 4HNO3 SO2 + NO + H2O 4(-3) = -12 Finish Balancing 3S 3S ++ 4HNO 4HNO33 3SO 3SO22 ++ 4NO 4NO +2H +__H 2O 2O Steps used to Balance Equations Using Ox #s 1. Find oxidation numbers 2. Identify what is being reduced and what is being oxidized 3. Draw arrows overhead reduced and oxidized substances with the degree of oxidation over top of the arrow 4. Balance these oxidation numbers with coefficients 5. Use these coefficiencts for a first guess 6. Check the oxidized and reduced species to see if they are balanced in the entire equation, make adjustments as necessary 7. Finish balancing the equation by placing coefficients on reactants and products which were not involved in the oxidation or reduction.