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Questions and Answers
In the balancing of the equation, how many hydroxide ions (OH-) were added to the reactions?
In the balancing of the equation, how many hydroxide ions (OH-) were added to the reactions?
The overall oxidation reaction results in the formation of two moles of water.
The overall oxidation reaction results in the formation of two moles of water.
True
What is the final product when CN- reacts with H2O in the balancing procedure?
What is the final product when CN- reacts with H2O in the balancing procedure?
CNO-
To balance the charge in the reaction, electrons are added to MnO_4 to produce __________.
To balance the charge in the reaction, electrons are added to MnO_4 to produce __________.
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Match the following steps with their corresponding actions in the balancing reactions.
Match the following steps with their corresponding actions in the balancing reactions.
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During the balancing process, 3 electrons are produced in the reduction half-reaction.
During the balancing process, 3 electrons are produced in the reduction half-reaction.
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What is the role of electrons in the redox reaction process?
What is the role of electrons in the redox reaction process?
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What role does the standard hydrogen electrode (SHE) play in electrochemical cells?
What role does the standard hydrogen electrode (SHE) play in electrochemical cells?
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Half-cell potentials can be measured directly without needing a full cell.
Half-cell potentials can be measured directly without needing a full cell.
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What does a more positive $E^ heta$ indicate in terms of half-reaction tendencies?
What does a more positive $E^ heta$ indicate in terms of half-reaction tendencies?
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If $E^ heta$ is greater than 0, then the change in Gibbs free energy, $ riangle G^ heta$, is __________.
If $E^ heta$ is greater than 0, then the change in Gibbs free energy, $ riangle G^ heta$, is __________.
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In the balanced redox reaction, what is the total number of manganese (Mn) atoms on the reactant side?
In the balanced redox reaction, what is the total number of manganese (Mn) atoms on the reactant side?
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Match the following terms with their definitions:
Match the following terms with their definitions:
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The total charge is balanced with 4+ on the reactant side and 4+ on the product side.
The total charge is balanced with 4+ on the reactant side and 4+ on the product side.
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How many electrons are gained in the half-reaction 8H + 5e → Mn?
How many electrons are gained in the half-reaction 8H + 5e → Mn?
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What is the physical meaning of cell potentials?
What is the physical meaning of cell potentials?
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Electrons have higher potential at the anode and flow towards the cathode.
Electrons have higher potential at the anode and flow towards the cathode.
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What is the relationship between electric potential and potential energy?
What is the relationship between electric potential and potential energy?
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In basic solutions, redox reactions require ______ to balance the reaction.
In basic solutions, redox reactions require ______ to balance the reaction.
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At 5 volts, each electron carries double the energy as at 2.5 volts.
At 5 volts, each electron carries double the energy as at 2.5 volts.
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What is the significance of $E^ heta$ being an intensive quantity?
What is the significance of $E^ heta$ being an intensive quantity?
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Match the following components of the reaction to their descriptions:
Match the following components of the reaction to their descriptions:
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What is the resulting product from the half-reaction 4H + 2CO2 → 2CO?
What is the resulting product from the half-reaction 4H + 2CO2 → 2CO?
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What is the value of the charge carried by each electron?
What is the value of the charge carried by each electron?
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The standard cell potential is calculated by using the formula: Cell potential = ________ - ________.
The standard cell potential is calculated by using the formula: Cell potential = ________ - ________.
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What is the balance of hydrogen atoms in the overall balanced equation?
What is the balance of hydrogen atoms in the overall balanced equation?
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The charge balance is achieved only with electrons and does not need to consider other ions.
The charge balance is achieved only with electrons and does not need to consider other ions.
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How much work can each electron do at 1 volt?
How much work can each electron do at 1 volt?
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The anode is the electrode where reduction occurs.
The anode is the electrode where reduction occurs.
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What must be considered to accurately calculate cell potential?
What must be considered to accurately calculate cell potential?
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Which metal is most easily oxidized and commonly used in batteries?
Which metal is most easily oxidized and commonly used in batteries?
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Electrochemistry explores the relationship between light and chemical reactions.
Electrochemistry explores the relationship between light and chemical reactions.
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What is the primary method used to balance redox reactions?
What is the primary method used to balance redox reactions?
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In the context of electrochemistry, a __________ cell converts chemical energy into electrical energy.
In the context of electrochemistry, a __________ cell converts chemical energy into electrical energy.
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Match the following concepts with their descriptions:
Match the following concepts with their descriptions:
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What is a significant concern regarding engineers and chemistry knowledge?
What is a significant concern regarding engineers and chemistry knowledge?
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The relationship between ΔG° and standard cell potential is an important aspect of electrochemistry.
The relationship between ΔG° and standard cell potential is an important aspect of electrochemistry.
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Name one application of chemistry that is critical for the future, as stated in the content.
Name one application of chemistry that is critical for the future, as stated in the content.
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What is the purpose of adding OH− to each side of a half-reaction in acidic solution?
What is the purpose of adding OH− to each side of a half-reaction in acidic solution?
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It is acceptable to cancel out H2O molecules from both sides of a half-reaction.
It is acceptable to cancel out H2O molecules from both sides of a half-reaction.
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What must be equalized before adding the two half-reactions together?
What must be equalized before adding the two half-reactions together?
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The process of balancing redox reactions typically involves balancing the half-reactions for _____ and _____ first.
The process of balancing redox reactions typically involves balancing the half-reactions for _____ and _____ first.
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Match the following concepts with their corresponding descriptions:
Match the following concepts with their corresponding descriptions:
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Which of the following is a key step in balancing redox reactions?
Which of the following is a key step in balancing redox reactions?
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Balancing the number of hydroxide ions (OH−) is not required when balancing half-reactions in acidic solutions.
Balancing the number of hydroxide ions (OH−) is not required when balancing half-reactions in acidic solutions.
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In the half-reaction CN− → CNO, what is the oxidation state change for carbon?
In the half-reaction CN− → CNO, what is the oxidation state change for carbon?
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Study Notes
Lecture Announcements
- Lecture 14 covers topics in Brown Chapter 20 including: Electrochemistry, Oxidation States and Oxidation-Reduction Reactions, Balancing Redox Equations, Voltaic Cells, Cell Potentials under Standard Conditions, and Free Energy and Redox Reactions.
- Problem Set 12 is not graded.
- Problem Set 13 is due tomorrow by 14:00 and posted on Moodle.
- Problem Set 14 is posted with solutions on Moodle and does not need to be handed in.
- Professor Norris and Brisby have office hours today.
- The exam covers Lectures 1-14, Problem Sets 1-14, Exercises 1-13.
- Allowed one A4 "Cheat Sheet," German-English dictionary, and standard scientific calculator.
- A list of standard scientific calculators is available on page 3
Standard Scientific Calculators
- A list of standard scientific calculators is provided.
Credits
- A list of teaching assistants (TAs) with their photos is listed.
Review of Lecture 13
- Lecture 13 covered acid-base equilibria topics, including autoionization of water, ion-product constant, Kw, proton-transfer reactions, Brønsted-Lowry Acids and Bases, acid-base pairs, conjugate acids and conjugate bases, acid-dissociation constant, Ka, base-dissociation constant, Kb, pH, pOH, pKa, pKb, pKw, common-ion effect, and buffers, Henderson-Hasselbalch Equation.
Review of Chemical Concepts
- The autoionization of water is represented as: H₂O(l) + H₂O(l) = H₃O⁺(aq) + OH⁻(aq).
- The acid dissociation constant, Ka, is represented as: Ka = [A⁻][H₃O⁺]/[HA].
- The base dissociation constant, Kb, is represented as: Kb = [HB⁺][OH⁻]/[B].
- The ion-product constant, Kw, is represented as: Kw = [H₃O⁺][OH⁻].
- The pH is represented as: pH = - log [H₃O⁺].
Electrochemistry
- Oxidation-Reduction reactions (redox) is the transfer of electron(s) between reactants.
- Involved in batteries, fuel cells, electroplating, corrosion, etc.
- Oxidation is substance losing electrons, and reduction is substance gaining electrons (LEO the lion says GER).
- Redox reactions are a bookkeeping system for electrons.
Oxidation Numbers
- Atoms in elemental form have an oxidation number of 0.
- Monoatomic ions have an oxidation number equal to their charge.
- In ionic/molecular compounds, nonmetals usually have negative oxidation numbers.
- Oxygen usually has an oxidation number of -2.
- Hydrogen usually has an oxidation number of +1.
- Fluorine always has an oxidation number of -1.
- Chlorine, bromine, and iodine typically have an oxidation number of -1 (except when bonded to oxygen, they may have a positive number), these exceptions are noted.
- Sum of oxidation numbers of all atoms in a compound equals the compound's overall charge
Redox Reaction Procedures
- For balancing redox reactions in acidic solutions:
- Divide the reaction into oxidation and reduction half-reactions.
- Balance each half-reaction:
- Balance elements other than H and O.
- Balance O atoms by adding H₂O.
- Balance H atoms by adding H⁺.
- Balance charge by adding electrons (e⁻).
- Multiply half-reactions to balance electrons lost and gained.
- Combine half-reactions and cancel common species.
- Check for balanced atoms and charges.
- For balancing redox reactions in basic solutions:
- Balance the reaction as if in an acidic solution.
- Add the same number of OH⁻ ions to both sides of the equation as there are H⁺ ions.
- Combine H⁺ and OH⁻ ions to form H₂O on the same side.
- Cancel any H₂O that appears on both sides.
- Proceed as in acidic solutions. Check for balanced atoms and charges.
Voltaic Cells
- Voltaic cells involve a spontaneous redox reaction converting chemical energy into electrical energy.
- A good example is a Nicad battery.
- Example reaction in a Cu-Zn battery: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq)+ Cu(s).
- Useful if the overall Ecell is positive (spontaneous).
- Oxidized substance loses electrons and is at the anode
- Reduced substance gains electrons and is at the cathode.
Battery Components
- Electrodes are solid metals that are connected to the external circuit. In a Cu-Zn reaction, zinc loses mass at the anode, and copper gains mass at the cathode.
- Electrolyte is a liquid containing ions that conduct electrical current which maintains charge neutrality in the electrolyte solution.
- Salt bridge allows ion migration between anode and cathode through the electrolyte components.
Energy and Batteries
- Electric potential is potential energy difference per unit charge (measured in volts).
- Each electron carries 1.6 × 10⁻¹⁹ J of energy per volt.
- Half-reactions are combined to form a complete redox reaction and determine useful voltage.
Calculating Cell Potentials
- Ecell is the cell voltage at standard conditions.
- Ecell = Ered(cathode) - Ered(anode)
- Standard Reduction Potentials (Ered) are the potential energy if a substance is reduced. This value is at standard conditions.
Standard Reduction Potentials
- Ered values are referenced to the standard H atom.
- Voltage values are always referenced to a standard electrode.
Oxidizing and Reducing Agents
- Oxidizing agents are substances that easily reduce/gain electrons, while reducing agents are substances that easily oxidize/lose electrons.
- Oxidizing agents have positive values of Ered.
- Reducing agents have negative values of Ered.
Determining Cell Potentials
- The cell potential (Ecell) is the difference between the standard reduction potentials of the cathode and anode.
- Ecell = Ered(cathode) – Ered(anode)
Standard Hydrogen Electrode (SHE)
- The SHE serves as a reference for standard reduction potential values.
- It operates as either an anode or a cathode and depends on the other half-cell.
Relationship between AG° and Ecell
- Gibbs Free Energy (ΔG°) is related to the cell potential (Ecell), and the amount of work (nFEcell), where "n" is the number of electrons in the reaction, and "F" is Faraday's constant .
- A positive Ecell corresponds to a negative ΔG° indicating a spontaneous reaction.
Big Picture - Batteries
- Batteries are designed combining half-cell redox reactions.
- Lithium, being easily oxidized, is used in batteries.
- Batteries can be recharged by reversing the redox reaction.
- Chemistry is vital for engineering in fuel cells, electroplating, corrosion, and other technologies.
Summary of Key Concepts
- Oxidation numbers are assigned to atoms in compounds based on rules.
- Balancing redox reactions involves balancing atoms and charges in half-reactions (acidic, basic environments.)
- Voltaic cells are where spontaneous redox reactions generate electrical energy (e.g., a Cu-Zn battery).
- Standard reduction potentials (Ered) determine the tendency of a species/element to gain electrons. The Ecell is the difference of two Ered values.
- The standard hydrogen electrode (SHE) is a reference for standard reduction potentials.
- A spontaneous reaction occurs when Ecell is positive, meaning ΔG is negative.
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Description
This quiz covers essential topics from Brown Chapter 20, focusing on electrochemistry, redox reactions, and voltaic cells. Test your understanding of balancing redox equations and cell potentials under standard conditions. Prepare to apply these concepts in your upcoming exam.
