Electrochemistry and Voltaic Cells Comprehensive Study Guide

Reaction for Analysis:
- $2 F_2 + 2 H_2O \rightarrow 4 HF + O_2$
Oxidation States Breakdown:
- $F_2$ (Fluorine gas): Each element has an oxidation state of $0$ .
- $H_2O$ (Water):
- Hydrogen ( $H$ ): $+1$
- Oxygen ( $O$ ): $-2$
- $HF$ (Hydrogen fluoride):
- Hydrogen ( $H$ ): $+1$
- Fluorine ( $F$ ): $-1$
- $O_2$ (Oxygen gas): Each element has an oxidation state of $0$ .
Nature of the Reaction:
- This reaction is considered an unusual example because oxygen is being oxidized ( $O$ goes from $-2$ to $0$ ).
- Typically, oxygen acts as the oxidizer (oxidizing agent), which is how the process of oxidation derived its name.
- In this specific case, Fluorine ( $F_2$ ) is the oxidizing agent because it causes oxygen to be oxidized while it is reduced itself ( $F$ goes from $0$ to $-1$ ).

Rule 1: Hydrogen
- Hydrogen is usually assigned an oxidation state of $+1$ .
Rule 2: Oxygen
- Oxygen is usually assigned an oxidation state of $-2$ .
Rule 3: Monatomic Ions
- In ionic compounds, monatomic ions have oxidation states equal to their ionic charges.
- Examples: $Zn^{2+}$ has an oxidation state of $+2$ ; $Na^+$ has an oxidation state of $+1$ .
Rule 4: Free Elements
- Elements in their free (uncombined) state have an oxidation state of $0$ .
- Examples: $Zn$ , $O_2$ , $Cl_2$ .
Rule 5: Fluorine in Compounds
- Fluorine is always $-1$ when it is part of a compound.
- Example: In $OF_2$ , fluorine is $-1$ .

Texas Quest (TQ) Assignments:
- TQ #12: Oxidation States.
- TQ #13: Redox Reactions.
- Due Date: Sunday, 5/17 or Monday, 5/18, by 11:59 pm.
Standard Based Instructional Task (SBIT/SBT):
- Topic: Redox and Electrochemistry.
- Date: Monday/Tuesday (5/18 or 5/19).
- Exam Coverage:
- Assigning oxidation states.
- Determining if a reaction is a redox reaction.
- Determining what is being oxidized and what is being reduced.
- Identifying oxidizing and reducing agents.
- Labeling and interpreting voltaic cell diagrams.
- Study Resources:
- Kognity reading assignments.
- Redox Homework Video.
- Oxidation States and Redox Practice handout.
- Both Texas Quest Problem sets (#12 and #13).

Scenario: Chlorine gas ( $Cl_2$ ) is bubbled into a solution of sodium bromide ( $NaBr$ ), producing aqueous sodium chloride ( $NaCl$ ) and bromine ( $Br_2$ ).
Chemical Equation:
- $Cl_2(g) + 2 NaBr(aq) \rightarrow 2 NaCl(aq) + Br_2(l)$
Identification of Redox Species:
- Oxidized Species: Bromide ( $Br^-$ ). Its oxidation state increases from $-1$ in $NaBr$ to $0$ in $Br_2$ .
- Reduced Species: Chlorine ( $Cl_2$ ). Its oxidation state decreases from $0$ in $Cl_2$ to $-1$ in $NaCl$ .
- Oxidizing Agent: Chlorine ( $Cl_2$ ).
- Reducing Agent: Bromide ( $Br^-$ ).

Standard Components and Labels:
- Anode: The electrode where oxidation occurs.
- Cathode: The electrode where reduction occurs.
- Voltmeter: Measures the electrical potential difference between the two half-cells.
- Salt Bridge: Connects the half-cells to maintain electrical neutrality.
- Electrochemical Flow: Electrons ( $e^-$ ) flow from the anode to the cathode.
Al-Pb Voltaic Cell Example:
- Anode: Aluminum ( $Al$ ).
- Cathode: Lead ( $Pb$ ).
- Salt Bridge Contents: Often contains ions like $K^+$ and $NO_3^-$ .
Mass Changes in Electrodes (Zn-Al Example):
- If the Zinc ( $Zn$ ) electrode increases in mass and the Aluminum ( $Al$ ) electrode decreases in mass:
- The Aluminum electrode is the Anode. The loss in mass occurs because Aluminum metal is being oxidized and enters the solution as $Al^{3+}$ ions.
- The Zinc electrode is the Cathode. The increase in mass occurs because zinc ions in solution are being reduced and deposited as Zinc metal on the electrode.
- Electron Flow: From the $Al$ electrode to the $Zn$ electrode.

Determining Anode and Cathode:
- Pair 1: Al(s) in $Al(NO_3)_3$ vs. Ag(s) in $AgNO_3$
- Anode: $Al(s)$
- Cathode: $Ag(s)$
- Pair 2: Pb(s) in $Pb(SO_4)_3$ vs. Fe(s) in $Fe(SO_4)_3$
- Anode: $Fe(s)$
- Cathode: $Pb(s)$
Predicting Voltage:
- The farther apart two metals are on the Activity Series, the larger the voltage produced by the voltaic cell.
- This is because a greater distance signifies a larger difference in their tendency to lose or gain electrons (the difference in reduction potential).
- Comparison Example:
- Cell #1 ( $Al$ / $Ag$ ) will produce a larger voltage than Cell #2 ( $Pb$ / $Fe$ ) because Aluminum and Silver have a much greater difference in reactivity than Lead and Iron.
Multi-Metal Comparison (Pt, Ag, Ni):
- Out of Platinum ( $Pt$ ), Silver ( $Ag$ ), and Nickel ( $Ni$ ), the pair that produces the largest voltage is Platinum ( $Pt$ ) and Nickel ( $Ni$ ).
- Explanation: These two metals have the greatest difference in their tendency to gain or lose electrons, reflecting the largest distance between them on the activity series.

Purpose: The salt bridge allows ions to move between the two half-cells, which completes the electrical circuit and allows the voltaic cell to continue operating.
Consequences of Removing the Salt Bridge:
- Positive ions would build up rapidly in the anode compartment (as metal atoms oxidize into ions).
- Negative ions would build up in the cathode compartment (as positive ions are reduced and removed from the solution).
- This charge imbalance would create an electrical resistance that stops the flow of electrons almost immediately.
Direction of Ion Migration:
- Anions from the salt bridge always move toward the Anode.
- Cations from the salt bridge always move toward the Cathode.
- In a bridge containing $KNO_3$ :
- Nitrate ions ( $NO_3^-$ ) migrate to the anode compartment.
- Potassium ions ( $K^+$ ) migrate to the cathode compartment.

Cell Setup: A Magnesium ( $Mg$ ) electrode in Magnesium Nitrate solution and a Zinc ( $Zn$ ) electrode in Zinc Nitrate solution.
Labels:
- Anode: Magnesium ( $Mg$ ).
- Cathode: Zinc ( $Zn$ ).
- Reaction at Anode: $Mg \rightarrow Mg^{2+} + 2e^-$ . The electrode will physically degrade or shrink.
- Reaction at Cathode: $Zn^{2+} + 2e^- \rightarrow Zn$ . The electrode will physically grow or accumulate mass.
Particle Movement:
- Electrons ( $e^-$ ): Flow through the external wire from the $Mg$ electrode to the $Zn$ electrode.
- Salt Bridge ( $KNO_3$ ):
- $NO_3^-$ moves toward the $Mg^{2+}$ solution (Anode).
- $K^+$ moves toward the $Zn^{2+}$ solution (Cathode).