NAME: DATE: AP Chemistry POGIL: Oxidation and Reduction Why? Many of the items you use on a daily basis rely on batteries for power. Most batteries supply electricity through oxidation-reduction reactions that exchange electrons from one atom to another. Current materials do not always allow for an efficient transfer of these electrons, so overheating may occur PHENOMENA - Cool Light Demonstration Observations: Possible Explanation: Success Criteria Determine the oxidation number for an element in a compound. Separate a reaction into oxidation and reduction into balanced half-reactions. Calculate the electron transfer during a redox reaction. MODEL 1: Oxidation Numbers Redox (reduction-oxidation) reactions involve the transfer of one or more electrons from a reductant (reducing agent) to an oxidant (oxidizing agent). This transfer is complete, and does not need to involve pair of electrons. The best way to keep track of this transfer is to use a bookkeeping trick called oxidation numbers. This assigns each atom of a compound virtual charges, or electron numbers.
P O G I L : O x i d a t i o n a n d R e d u c t i o n Page 2 Key Questions 1. For neutral compounds, the sum of all oxidation numbers must equal. For all polyatomic ions, the sum of all oxidation numbers must equal. a. The sum of all the oxidation numbers for H 2 O must equal. b. The sum of all the oxidation numbers for SO 4 must equal. 2. Show how all the oxidation numbers in KMnO 4 obey the rule for neutral compounds. 3. Why does C have an oxidation number of zero by itself, but 2 in the compound CO? Exercises 4. Assign oxidation numbers to every element in the following: Li 2 O P 4 CaH 2 Fe 2 O 3 MnCl 3 O 2 O 3 H 2 O H 2 O 2 HO 2 OF 2 S 8 H 2 Cs 2 O 2 CH 2 Cl 2
P O G I L : O x i d a t i o n a n d R e d u c t i o n Page 3 5. Assign oxidation numbers to every element in the following polyatomic ions: ClO 4 SO 4 PO 3 3 MnO 4 MnO 4 6. Assign oxidation numbers to every element in the following ternary compounds: K 2 TiO 3 (NH 4 ) 3 PO 4 Ti(SeO 3 ) 2 Na 2 SO 4 Pb(IO 4 ) 2 7. Assign oxidation numbers to the atoms in these covalent compounds. You must take into account the atoms bonded to each carbon. Ethanol Acetic Acid Model 2: Exchange of Electrons
Redox Reactions P O G I L : O x i d a t i o n a n d R e d u c t i o n Page 4 2 A. Zn (s) Cu (aq) Zn 2 (aq) Cu (s) B. 2I (aq) S 2 O 8(aq) I 2(aq) 2SO 4(aq) C. 4Fe (s) 3O 2(g) 2Fe 2 O 3(s) D. 4H (aq) MnO 4(aq) 3Fe 2 (aq) 3Fe 3 (aq) MnO 2(aq) 2H 2 O (l) Non-Redox Reactions E. HCl (g) H 2 O (l) H 3 O (aq) Cl (aq) F. 2NaOH (aq) H 2 SO 4(aq) Na 2 SO 4(aq) 2H 2 O (l) G. Ba 2 (aq) 2OH (aq) Ba(OH) 2(s) H. 2AgNO 3(aq) CaCl 2(aq) Ca(NO 3 ) 2(aq) 2AgCl (s) Key Questions 8. What two types of reactions are shown in the model? 9. Examine the redox and non-redox reactions. Is/are there any feature(s) in the redox reactions that would allow you to identify them as redox reactions? If yes, use specific examples from the model to support your answer. 10. In the space under each reaction in the model, write the oxidation number for every atom. An example is shown: 4Fe (s) 3O 2(g) 2Fe 2 O 3 0 0 3-2 0 3-2 -2
P O G I L : O x i d a t i o n a n d R e d u c t i o n Page 5 11. Identify any elements that changed oxidation number in the reactions in the model. Connect the starting and ending oxidation numbers with a line. An example is shown. 4Fe (s) 3O 2(g) 2Fe 2 O 3 0 0 3-2 0 3-2 -2 12. Based on the oxidation number analysis you just performed, are there any features of the redox reactions that would allow you to identify them? Use specific examples to support your answer. 13. Identify the following reactions as either redox or non-redox using oxidation numbers as evidence. a. Pb(NO 3 ) 2(aq) 2NaI (aq) PbI 2(s) 2NaNO 3(aq) b. 2H 2 O (g) 2H 2(g) O 2(g) c. CH 4(g) 2O 2(g) 2H 2 O (g) CO 2(g) d. HCl (aq) NaOH (aq) H 2 O (l) NaCl (aq) MODEL 3: Single Replacement Reactions Back in first year chemistry, you did a lab where you placed metal pieces in solutions with other metal cations. When you place the copper metal in the silver solution, the metal turned gray and the solution went from clear and colorless to a light blue. When you place the copper metal in a solution with zinc ions, nothing happened. From this data, you were able to determine the relative reactivity of the metals.
P O G I L : O x i d a t i o n a n d R e d u c t i o n Page 6 Suppose you have the following metals and corresponding solutions to perform similar reactions: Al (s) and Al(NO 3 ) 3(aq) ; Pb (s) and Pb(NO 3 ) 2(aq) ; Ni (s) and Ni(NO 3 ) 2(aq) ; and Ag (s) and AgNO 3(aq). The results are as follows. 2Al (s) 3Pb (aq) 2Al (aq) 3Pb (s) 2Al (s) 3Ni (aq) 2Al (aq) 3Ni (s) Al (s) 3Ag (aq) Al (aq) 3Ag (s) Ni (s) Al 3 (aq) NR Ni (s) Pb (aq) Ni (aq) Pb (s) Ni (s) 2Ag (aq) Ni (aq) 2Ag (s) 3 Pb (s) Al (aq) NR Pb (s) Ni (aq) NR Pb (s) 2Ag (aq) Pb (aq) 2Ag (s) 3 Ag (s) Al (aq) NR Ag (s) Pb (aq) NR Ag (s) Ni (aq) NR 14. Rank Al, Pb, Ni, and Ag from most reactive to least reactive. > > > 15. Refer to your copy of the Activity Series for Metals. Find Al, Pb, Ni, and Ag. How does the positon of the metal relate to its chemical reactivity? 16. If a strip of zinc metal is added to an aqueous solution of copper(ii) sulfate, over time one will notice that a brown precipitate accumulates on the strip of metal and that the solution becomes progressively less blue in color. Using the Activity Series, explain what chemical reaction is occurring and write a balanced net ionic equation for the 2. reaction between Zn (s) and Cu (aq) 17. Hydrogen is often written on the Periodic Table in Group IA (Group 1), because the chemical reactivity of hydrogen cations often resembles that of the cations of the alkali metals. In elemental form, hydrogen exists as a diatomic gas (H 2(g) ). Use the Activity Series to explain the following. a. Explain why the following occurs when calcium is added to water: Ca (s) 2H 2 O (l) Ca(OH) 2(aq) H 2(g) b. Explain why zinc metal dissolves in hydrochloric acid: Zn (s) 2HCl (aq) ZnCl 2(aq) H 2(g) c. Explain why copper metal does not dissolve in hydrochloric acid.
P O G I L : O x i d a t i o n a n d R e d u c t i o n Page 7 MODEL 4: Half reactions The process of oxidation and reduction can be thought of as a transfer of electrons from one atom to another. As a result of this process, the oxidation numbers of both atoms change. All redox reactions can be divided up into two reactions: an oxidation half-reaction and a reduction half-reaction. This allows for better understanding of the electron transfer process. A OVERALL B OVERALL C OVERALL D OVERALL 4H (aq) 2 Zn (s) Cu (aq) Zn 2 (aq) Cu (s) Zn Zn 2 2e Cu 2 2e Cu 2I (aq) S 2 O 8(aq) I 2(aq) 2SO 4(aq) 2I I 2 2e S 8 O 8 2e 2SO 4 4Fe (s) 3O 2(g) 2Fe 2 O 3(s) Fe Fe 3 3e O 2 4e 2O 3Fe 2 (aq) 3Fe 3 (aq) MnO 2(aq) 2H 2 O (l) Fe 2 Fe 3 e 4H MnO 4 3e MnO 2 2H 2 O MnO 4(aq) Key Questions 18. What does the e - symbol represent in the half-reactions? 19. Looking at the oxidation half-reactions: a. Which of the following types of particles may undergo oxidation? (Circle all that apply.) Neutral Atoms/Molecules Cations Anions b. Are electrons lost or gained by the atom during the process of oxidation? c. The oxidation number of the atom being oxidized ( Increases / Decreases ). 20. Looking at the reduction half-reactions: a. Which of the following types of particles may undergo oxidation? (Circle all that apply.) Neutral Atoms/Molecules Cations Anions b. Are electrons lost or gained by the atom during the process of reduction? c. The oxidation number of the atom being reduced ( Increases / Decreases ). 21. In a redox reaction, the oxidizer takes the electrons, so it is is the atom being ( oxidized / reduced ). The reducer is the atom losing the electrons, so it is the atom being ( oxidized / reduced ).
P O G I L : O x i d a t i o n a n d R e d u c t i o n Page 8 Read This! To remember which process is which, remember the phrase LEO the lion goes GER. LEO = Loss of Electrons is Oxidation GER = Gain of Electrons is Reduction 22. Consider the incomplete half-reactions: a. Use oxidation numbers to identify the reactions below as oxidation or reduction. b. Place the correct number of electrons on the appropriate side of the reaction to balance the equation charges. I 2 2I Cr 2 Cr 3 Sr Sr 2 ClO 2 H 2 O ClO 3 2H 23. Show that the two half reactions for Reaction A can be added together to give the overall redox reaction. A OVERALL 2 Zn (s) Cu (aq) Zn 2 (aq) Cu (s) Zn Zn 2 2e Cu 2 2e Cu 24. What happens when you try to add together the half reactions for C? C OVERALL 4Fe (s) 3O 2(g) 2Fe 2 O 3(s) Fe Fe 3 3e O 2 4e 2O 25. How would you remedy the issue found in #24?