*STUDENT* * STUDENT * Mr. Dolgos Regents Chemistry NOTE PACKET. Unit 10: Electrochemistry (Redox) REDOX NOTEPACKET 1

Transcription

1 *STUDENT* * STUDENT * Mr. Dolgos Regents Chemistry NOTE PACKET Unit 10: Electrochemistry (Redox) REDOX NOTEPACKET 1

2 *STUDENT* * STUDENT * UNIT 10: Electrochemistry (Redox) Redox Reduction Oxidation Reducing Agent Oxidizing Agent Oxidation Number Half reaction Vocabulary: Electrode Galvanic or Voltaic Cell Salt bridge Electrochemical Cell Electrolytic Cell Anode Cathode Unit Objectives: Define and identify oxidation reactions Define and identify reduction reactions Assign oxidation numbers to elements in a compound Write and balance half reactions Identify oxidizing agents and reducing agents Distinguish between voltaic and electrolytic cells Identify the components of an electrochemical cell Indicate the direction of electrons and ions through an electrochemical cell Determine, using Table J, whether a reaction if spontaneous or not REDOX NOTEPACKET 2

3 REDuction OXidation Reactions (AKA Redox): rxns that involve the ; both reduction and oxidation must happen! Reduction = by an atom or ion; goes / Oxidation = by an atom or ion; goes / A way to remember L E O the lion goes G E R *Oxidation and reduction happen because of the for electrons in a chemical reaction. Species prefer to either or electrons in a chemical reaction. AND **Oxidation and reduction are or reactions and one cannot happen without the other. If one atom electrons, there must be another atom that will electrons. Green Chunk Experiment: Example: Al + CuCl 2 + Aluminum is above Cu on Table J so it will replace it! Notice how Al is all by itself (on left of arrow) with a zero charge and then bonded (on right of arrow) where it takes on a charge REDOX NOTEPACKET 3

4 IDENTIFYING REDOX REACTIONS One way that we can begin to identify a redox reaction is to inspect the from reactant to product side (for every element involved in the reaction). Oxidation numbers are used to track the (electron transfer) from reactant to product side of rxn Oxidation Number (State) =,, OR ( ) values that can be assigned to atoms; identify how many electrons are being lost or gained by an atom/ion when they *top listed # to the upper right is the most common oxidation number for that element Trick 1: reactions are always REDOX! Ex: Zn + HCl + * / are by themselves on one side and bonded on the opposite side Trick 2: DOUBLE REPLACEMENT REACTIONS are NOT REDOX! Ex: NaOH + HCl + *charges stay the same for all elements in the rxn REDOX NOTEPACKET 4

5 Rules for assigning OXIDATION STATES (numbers): 1) (elements not bonded to another element) have an oxidation number of. This includes any formula that has only one chemical symbol in it (single elements & diatomic elements). Examples: 2) In, the sum of the for all elements must. Ex: NaCl Ex: Mg 3 N 2 Na: 1(+1) = +1 Mg: Cl: 1(-1) = -1 N: 0! 0! Ex: HNO 3 H: N: O: 0! * The is the number the. It is the charge on atom of that element! ** Trick: You can keep polyatomic ions together and use the charge from Table E to determine the oxidation numbers for those elements. *** Remember that we almost always write the and the in a compound formula. EXAMPLE: EXCEPTION to this rule: REDOX NOTEPACKET 5

6 3) always have a oxidation number when in a compound (bonded to another species). always have a oxidation number when located within a compound. (Assign) Ox #: Ex: LiCl MgCl 2 4) is always a in compounds. The other (ex: Cl, Br, I) are also as long as they are the most electronegative element in the compound. (Assign) Ox #: Ex: HF CaCl 2 NaBr 5) is a in compounds unless it is combined with or, in which case it is. (Assign) Ox #: Ex: HCl LiH 6) is in compounds. (Assign) Ox #: Ex: H 2 O When combined with ( ), which is more electronegative,. (Assign) Ox #: Ex: OF 2 When in a. A peroxide is a compound that has a formula of. (Assign) Ox #: Ex: Na 2 O 2 H 2 O 2 7) The sum of the oxidation numbers in polyatomic ions must equal the ( ). Ex: Cr 2 O 7 2- Cr: 2( ) = O: 7( ) = = (charge on ion) REDOX NOTEPACKET 6

7 A reaction is REDOX if Reduction (GER) = by an atom or ion; goes / Oxidation (LEO) = by an atom or ion; goes / Assign oxidation numbers for all elements and complete the tables: Example 1: C + H 2 O CO + H 2 C 0 H +1 Charge: Increases/Decreases e - : Lost/Gained Oxidized/Reduced Example 2: MnO 2 + 4HCl MnCl 2 + Cl 2 + 2H 2 O Cl -1 Mn +4 Charge: Increases/Decreases e - : Lost/Gained Oxidized/Reduced Oxidizing Agent (OA) = that is ; species that Reducing Agent (RA) = that is ; species that *NOTE: OA & RA are ALWAYS located on the! Assign oxidation numbers for all elements and identify the OA and RA: Example: C + H 2 O CO + H 2 OA = RA = REDOX NOTEPACKET 7

8 HALF REACTIONS Half reactions allow us to show the in a redox rxn. For each redox reaction, we can illustrate two. One half-reaction shows and other shows. Example of a Reduction Half Reaction: Fe e- Fe *Electrons on left hand side, in the rxn ( ). Notice also how the charge for Fe goes down from left to right, ( ). Charge goes down because Fe e -. Example of an Oxidation Half Reaction: Fe Fe e- *Electrons on left hand side, in the rxn ( ). Notice also how the charge for Fe goes up from left to right, ( ). Charge goes up because Fe e -. NOTICE: Always to the side of rxn that has a (remember: electrons are!) FOLLOWING THE LAW OF CONSERVATION: Half reactions follow the. This means that there must be the on both sides of the reaction arrow. There must also be a. In half reactions, the of the equation, although it doesn t necessarily need to equal zero. REDOX NOTEPACKET 8

9 RULES FOR SETTING UP HALF REACTIONS 1) Assign oxidation numbers to all elements in reaction 2) Draw brackets and identify oxidation & reduction 3) Begin to set up half reactions. Pull out brackets bringing element symbol and assigned charge with you. Set up as a reaction with arrow connecting two sides that have different oxidation numbers assigned. Only trick: diatomics must be pulled out as a pair. This is the only time you ever bring subscripts with you in creating half reactions! 4) FOR REACTIONS INVOLVING DIATOMIC ELEMENTS ONLY: Balance mass 1 st (make sure there are the same number of elements on each side of each half reaction) 5) Lastly, balance charge in each half reaction by inserting appropriate amount of electrons into each half reaction to attain conservation of charge. Always add electrons to side that has a more positive charge. REMEMBER, electrons are negative in nature! Net charges on each side of rxn should be equal after adding electrons. Assign oxidation numbers to all elements or polyatomic ions. Label the brackets for reduction (red) or oxidation (ox). Ex. 1: Mg + ZnCl 2 MgCl 2 + Zn OXIDATION Half Reaction: Mg 0 + REDUCTION Half Reaction: Zn +2 + REDOX NOTEPACKET 9

10 Ex. 2 (balance masses): Hg + I 2 HgI OXIDATION Half Reaction (make sure to balance the masses): + REDUCTION Half Reaction: + Ex. 3 (balance charges): Cu + AgNO 3 Cu(NO 3 ) 2 + Ag OXIDATION Half Reaction: + REDUCTION Half Reaction: + Now, we need to balance the charges: x (Ag e - Ag 0 ) = Ag +1 + e - Ag 0 REDOX NOTEPACKET 10

11 Table J and Spontaneous Reactions General Rule: elements on Table J are reactive than the elements below them Spontaneous rxn = rxn occurs w/out adding energy to system If the single element is more active than the combined element, the reaction will be spontaneous. Non-spontaneous rxn = rxn will not occur unless energy is added to system If the single element is less active than the combined element, the reaction will NOT be spontaneous. Complete the following equations by writing in the products formed or no rxn Ex 1: Ex 2: Ex 3: Ex 4: Ex 5: Ex 6: Zn + PbCl 2 Zn + BaO Ca + CrF 2 Mn + NiS Fe + MgI 2 Co + PbCl 2 REDOX NOTEPACKET 11

12 TWO TYPES of ELECTROCHEMICAL CELLS 1. Voltaic (similar to a battery) 2. Electrolytic (similar to alternator in cars) SIMILARITIES BETWEEN THE TWO: Both involve reactions; chemical reactions which involve the flow of Both involve the flow of, or, measured in Both have (conductive surfaces where oxidation or reduction occurs); called the and the or occurs in each half cell RED CAT ( ) AN OX ( ) Electrons flow through the from the to the. REDOX NOTEPACKET 12

13 Voltaic Cells Cells that convert energy into energy or electric. CATHODE The of the 2 metals (Table J) to it the electrode in a electrode where occurs ( ) ANODE The of the 2 metals (Table J) to cathode the electrode in a electrode where occurs ( ) Example 1: Wet Cell, are a form of battery Consists of LEAD ANODE and LEAD OXIDE CATHODE Both electrodes immersed in a solution Advantage: process is readily (by alternator) Disadvantage: very, somewhat Example 2: Dry Cell are the type of batteries in a portable radio, remote control, etc. CARBON (GRAPHITE) CATHODE surrounded by moist electrolyte paste Usually ZINC ANODE *SALT BRIDGE provides a path for the between the half-cells prevents the REDOX NOTEPACKET 13

14 Voltaic Cells (a.k.a Galvanic Cells) 1. Use Table J to predict the direction that electrons will spontaneously flow. Draw arrows to indicate the direction on the wire. 2. Based on your answer above, which would be the negative electrode and which would be the positive electrode? 3. Explain your answer to #2. 4. At which electrode or in which half-cell does reduction occur? 5. At which electrode or in which half-cell does oxidation occur? 6. Which electrode is the cathode? 7. Which electrode is the anode? *Electrons don t flow to the cathode, they flow through it to the ions in solution. That s why the cathode never becomes negative. REDOX NOTEPACKET 14

15 Electrolytic Cells Cells that use to force a to occur. This process is for and Example: (keeps the car battery replenished with energy) Electrolysis Experiment Animation (w/ tutorial): Standard Hydrogen Electrode (Zinc) Standard Hydrogen Electrode (Copper) Electrolysis of Water CATHODE electrode where are the electrode (opposite of voltaic cell) electrode where occurs ( ) ANODE electrode where are the electrode (opposite of voltaic cell) electrode where occurs ( ) REDOX NOTEPACKET 15

16 NOTICE: There is. This is a forced chemical reaction. You will always see a hooked up to an electrolytic cell which drives the 1. Predict the direction that electrons will flow. Draw arrows to indicate the direction on the wires. 2. Based on your answer above, which would be the negative electrode and which would be the positive electrode? 3. Explain your answer to #2. 4. At which electrode does reduction occur? 5. At which electrode does oxidation occur? 6. Which electrode is the cathode? 7. Which electrode is the anode? REDOX NOTEPACKET 16

17 Electrolysis of a Fused Salt = = is used to o Metals that are not found alone/uncombined in nature o and Metals REDOX NOTEPACKET 17

18 Compare and contrast the two types of electrochemical cells: GALVANIC/VOLTAIC ELECTROLYTIC Flow of e - (spontaneous or forced) (+) electrode (-) electrode *Direction of e - flow Reduction ½ cell Oxidation ½ cell *Direction of e - flow is either Anode Cathode or Cathode Anode *Fuel Cells: galvanic cells for which the reactants are continuously supplied. Hydrogen Fuel Cell Animation: REDOX NOTEPACKET 18