UNIT OBJECTIVES: When you complete this unit you will be able to do the following……….. 1. 2. 3. 4. 5. 6. 7. 8. 9.
Determine the oxidation numbers of atoms and ion is a chemical reaction Determine if a reaction is a Redox reaction Determine which species is oxidized and which is reduced Compose redox half reactions Construct a balanced redox reaction Determine the flow of electrons in a battery (voltaic cell) Identify the anode and cathode in a voltaic cell Identify an electrolytic cell Differentiate between an electrolytic cell and a voltaic cell
CCHS Chemistry
UNIT 9: ELECTROCHEMISTRY LESSON 1: Oxidation Numbers Objective: • Determine the oxidation numbers of atoms and ion is a chemical reaction Problem: What is electricity and how is it formed? In electrochemical reactions, electrons are transferred from one species to another. In order to keep track of what loses electrons and what gains them, we assign ____________________________________________________.
Rules for Assigning Oxidation Numbers 1. Uncombined elements (not combined with any other element) have an oxidation # of ZERO (this includes diatomic elements) Example: Na = 0 Cl2 = 0 2. If an element has only one charge listed on the periodic table, then that is its oxidation number. Example: Group 1 are always +1 and group 2 are always +2 3. If a nonmetal atom is the negative ion in an ionic compound, then the top charge listed is its oxidation Examples: HF NaCl 4. H is +1 in the front and -‐1 in the back. Examples: H3P P is -‐3 H is +1 LiH Li must be +1 H is -‐1 5. The Sum of the oxidation #’s in a compound must = ZERO. Be sure to multiply the oxidation # by the # of atoms indicated by the subscript. Example: CaCl2
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UNIT 9: ELECTROCHEMISTRY LESSON 1: Oxidation Numbers 6. Sum of the oxidation # in an ion must equal the charge of the ion listed Example: Cr2O72-‐ Cr: 2(+6) = +12 O: 7(-‐2) = -‐14 -‐2 7. If an element has more than one charge listed, use the other charges to figure it out. n Assign oxidation number to elements with 1 oxidation # first. n Assign it an oxidation number which will make the compound = ZERO Example: FeCl3 PRACTICE: Determine the Oxidation Number of each atom Sn
LiF
N2
NaBrO3
Cl-‐
MgSO4
Ca+2
CaClO3
CHECK YOUR UNDERSTANDING: Determine the Oxidation Number of each atom K2Cr2O7 Exceptions to the rule: Oxygen is -‐2 except when bonded to fluorine
Example: OF2
F must be -‐1
O is +2
Oxygen is -‐1 in peroxides
Example: K2O2 each K is +1
O is -‐1
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UNIT 9: ELECTROCHEMISTRY LESSON 2: Identifying a REDOX Reaction Objective: • Determine if a reaction is a Redox reaction • Determine which species is oxidized and which is reduced REDOX Reactions = Reactions involving the transfer of electrons OXIDATION: ______________________________________________ by an atom or ion § §
OXIDATION NUMBER goes UP/INCREASES Becomes more (+) charged
REDUCTION: ______________________________________________ by an atom or ion § §
OXIDATION NUMBER goes DOWN/REDUCES Becomes MORE (-‐) charged
EXAMPLE:
§
Zinc loses two electrons (oxidized) to go from neutral zinc metal to the Zn2+ ion
§
Each of the H+ gains an electron (reduced) and they combine to form H2
Trick: LEO (the lion says…) GER LEO: GER:
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UNIT 9: ELECTROCHEMISTRY LESSON 2: Identifying a REDOX Reaction How to Identify Changes in Oxidation #’s § §
Assign oxidation numbers Determine which species is oxidized and which is reduced
EXAMPLE: Oxidation #’s can change as a result of a rxn. 2Na + Cl2 2NaCl
Na went from 0 to +1: Na is _______________________
Cl went from 0 to -‐1: Cl is _________________________
EXAMPLE: Oxidation #’s can change as a result of a rxn. H2 + O2 2H2O
H went from 0 to +1: H is _______________________
O went from 0 to -‐2: O is _________________________
***Tip: For Polyatomic Ions § §
Look up charge on TABLE E This is the oxidation # (easier than assigning oxidation numbers to each individual atom) Ex. NO3-‐1
§ SPECTATOR IONS: The ion that does have a change in oxidation # Example: §
Cu0 + 2Ag+1 NO3-‐1 Cu+2(NO3)2-‐1 + 2Ag0
§
NO3 is the spectator ion
PRACTICE: Determine which species is oxidized and reduced?
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UNIT 9: ELECTROCHEMISTRY LESSON 2: Identifying a REDOX Reaction 2Mg + O2 2MgO K2O + Li Li2O + K Zn(s) + HCl(aq)
H2(g) + ZnCl2(aq)
CHECK YOUR UNDERSTANDING: Determine which species is oxidized and reduced? Zn(s) + CuSO4(aq) à ZnSO4(aq) + Cu(s) TRICK: Single Replacement Rx’s are ALWAYS Redox Zn(s) + HCl(aq) à H2(g) + ZnCl2 (aq) Double Replacement Rx’s are NEVER redox NaOH(aq) + HCl(aq) à H2O(l) + NaCl(aq)
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UNIT 9: ELECTROCHEMISTRY LESSON 3: Half Reactions Objective: • Compose redox half reactions • Construct a balanced redox reaction HALF REACTIONS: Show either the oxidation or reduction portion of a redox reaction, including the electrons gained or lost.
Rules for Setting Up Half Reactions: 1. Assign oxidation numbers to all elements in reaction and determine if it is a redox reaction (look for the change in oxidation # of 2 elements 2. Determine which species is oxidized (loses electrons) and which is reduced (gains electrons) [use brackets]. 3. Then break the overall reaction into oxidation and reduction reactions called HALF REACTIONS by pulling out brackets 4. Fill in electrons for each half reaction: oxidation: electrons (lost) on right side reduction: electrons (gained) on left side 5. Check to see if charges are equal on both sides of each half reaction Example:
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UNIT 9: ELECTROCHEMISTRY LESSON 3: Half Reactions PRACTICE: Set up both half reactions for the following reaction: Ca(s) + Cu2+(aq) à Ca2+(aq) + Cu(s) Balancing Redox Reactions In All redox reactions there is a CONSERVATION of: §
____________________________
§
____________________________
Steps to balance redox reactions: 1. Write out the 2 half reactions (including electrons) 2. Multiply the half-‐reactions by the number of electrons in the other half-‐reaction 3. Multiply through and put the resulting coefficients into the original equation EXAMPLE: Balance this Reaction Ca(s) + Cu2+(aq) à Ca2+(aq) + Cu(s) PRACTICE: Balance this Reaction K + ZnCl2 à KCl + Zn
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UNIT 9: ELECTROCHEMISTRY LESSON 3: Half Reactions CHECK YOUR UNDERSTANDING: Balance this Reaction 2HCl + Zn à H2 + ZnCl2 Table J and Spontaneous Reactions (TELLS OF IF A REDOX REACTION WILL TAKE PLACE) SPONTANEOUS REACTION: occurs w/out adding energy to system • If the “single” element is MORE active than the “combined” element, the reaction will be spontaneous. Ex: Zn + PbCl2 à ZnCl2 + Pb NONSPONTANEOUS REACTON: Reaction WILL NOT occur unless energy is added to system • If the “single” element is LESS active than the “combined” element, the reaction will be non-‐spontaneous Ex: Zn + AlCl3 à No rxn PRACTICE: Comparing nickel and aluminum. Which would be oxidized and which would be reduced?
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UNIT 9: ELECTROCHEMISTRY LESSON 4a: Electrochemical Cells (Voltaic Cells) Objective: • Determine the flow of electrons in a battery (voltaic cell) • Identify the anode and cathode in a voltaic cell Problem: What is electricity and how is it formed? § §
In a spontaneous redox rx, electrons are transferred and ENERGY is released. This energy can be used to do work is a voltaic cell Voltaic Cell (battery)
Converts ____________________________ into __________________________________
§
The flow of electrons produces electricity
How does a voltaic cell work? §
Electrons flow SPONTANEOUSLY from the __________________________ (more active metal) to the ________________________________ (less active metal)
§
Anode: _______________________________ electrode where oxidation (loss of e-‐) occurs
§
Cathode: _____________________________ electrode where reduction (gain of e-‐) occurs
Remember: EXAMPLE: §
Zn is higher on table J so electrons
§
flow from Zn to Cu.
§
Zn is anode and Cu is cathode
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UNIT 9: ELECTROCHEMISTRY LESSON 4a: Electrochemical Cells (Voltaic Cells) Parts of a Voltaic Cell § § § § §
2 half cells (1 for oxidation and 1 for reduction half reactions) Electrodes (site of ox and red) Wire (connects to electrodes-‐ allows electrons to flow) Salt Bridge (allows ions to flow and prevents polarization of cells Voltmeter (measures electric current) How to Label a Voltaic Cell
§ § § §
Determine which electrode is the anode and which is the cathode Identify where oxidation and reduction take place Determine the direction of electron flow Remember: o Red Cat o An Ox
Using table J to determine flow of electrons § More active metal is oxidized § ELECTRONS FLOW FROM __________________________________________________
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UNIT 9: ELECTROCHEMISTRY LESSON 4a: Electrochemical Cells (Voltaic Cells) What happens to the electrodes as electrons flow? §
Cathode (Cu) ___________________________________
§
Anode (Zn) _____________________________________
Function of the Salt Bridge n The salt bridge allows for the flow of ions to prevent a buildup of charge at each electrode n Positive ions flow to _____________________________ n Negative ions flow to ____________________________ Removing the salt bridge would result in: EXAMPLE: Label the following: anode, cathode, where ox and red take place and direction of e-‐ flow
CCHS Chemistry
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UNIT 9: ELECTROCHEMISTRY LESSON 4a: Electrochemical Cells (Voltaic Cells) PRACTICE: Label the following: anode, cathode, where ox and red take place and direction of e-‐ flow
CHECK YOUR UNDERSTANDING: Label the following: anode, cathode, where ox and red take place and direction of e-‐ flow
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UNIT 9: ELECTROCHEMISTRY LESSON 4b: Electrolytic Cells Objective: • Identify an electrolytic cell • Differentiate between an electrolytic cell and a voltaic cell ELECTROLYTIC CELL: § Non-‐Spontaneous reaction § No flow of e-‐ to produce a current § Outside power source must be supplied (Battery) § Converts ___________________________________ into ______________________________________. ELECTROPLATING: • The process of adding a layer (plate) of metal on the surface of another object. Ex. Gold plated jewelry & Chrome Bumpers ELECTROPLATING PROCESS: § § § §
The battery forces electrons to travel to the spoon. The spoon is negative and will attract Silver (+) ions. The silver ions will reduce (stick) onto the spoon, plating it. The Ag bar anode loses e-‐ and will eventually disappear.
At the anode:
At the cathode: CCHS Chemistry
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UNIT 9: ELECTROCHEMISTRY LESSON 4b: Electrolytic Cells COMPARISON BETWEEN ELECTROLYTIC AND VOLTAIC CELLS Voltaic Cells Same Different
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