Electrochemistry E-Notes - Flashcards

Question and Answer flashcards based on the provided electrochemistry notes, covering definitions, key concepts, examples, calculations, and common problems.

What is Electrochemistry?

Branch of chemistry that deals with the interconversion of electrical energy and chemical energy (electrical energy to chemical energy or vice versa).

What is a Redox reaction?

A reaction in which oxidation and reduction occur simultaneously.

How is oxidation defined in terms of electrons and oxidation state?

Oxidation involves loss of electrons and an increase in oxidation state.

How is reduction defined in terms of electrons and oxidation state?

Reduction involves gain of electrons and a decrease in oxidation state.

In Zn + Cu2+ → Zn2+ + Cu, which species is oxidized and which is reduced?

Zn is oxidized (Zn -> Zn2+); Cu2+ is reduced (Cu2+ -> Cu).

What is a disproportionation reaction?

A redox reaction where the same element is simultaneously oxidized and reduced in the same reaction.

How do you find the oxidation state of chromium in K2Cr2O7?

Let Cr have oxidation state x: 2(+1) + 2x + 7(-2) = 0, so x = +6.

What are galvanic (Daniell) cells?

Cells that convert chemical energy into electrical energy via spontaneous redox reactions.

What is an electrolytic cell?

A cell that converts electrical energy into chemical energy via non-spontaneous redox reactions, requiring an external power source.

In a Daniell cell, which electrode is the anode and what is the reaction?

Anode: Zn(s) → Zn2+ + 2e−; Cathode: Cu2+ + 2e− → Cu(s); Overall: Zn + Cu2+ → Zn2+ + Cu.

What are the standard reduction potentials for Zn2+/Zn and Cu2+/Cu?

Zn2+ + 2e− → Zn: E°red = −0.76 V; Cu2+ + 2e− → Cu: E°red = +0.34 V.

How is the standard cell potential E°cell calculated for a galvanic cell?

E°cell = E°cathode − E°anode (e.g., for Zn/Cu, 0.34 − (−0.76) = 1.10 V).

What is the standard electrode potential (SHE) and its value?

A universal reference electrode with E° = 0 V; used to measure standard potentials of other half-cells.

Why is H2 gas used in the Standard Hydrogen Electrode (SHE)?

To maintain a specific H+ ion activity in the solution and stabilize the reference potential.

What is the Faraday constant (rough value) and what does it represent?

Approximately 96485 C/mol; the charge carried by one mole of electrons.

State the basic relation between charge, current, and time in electrolysis.

Q = I × t, where Q is charge (C), I is current (A), and t is time (s).

How is Avogadros number related to electrolysis (F = NA × e−)?

F is the Faraday constant (≈96485 C/mol) = Avogadro's number × elementary charge. 1 Faraday corresponds to 1 mole of electrons.

What is the general idea of using electrolysis to estimate Avogadro’s number?

Deposit a known mass of metal under a known current for a known time, relate the charge to moles of electrons and to moles of metal deposited to estimate NA.

What is a salt bridge and what is its function?

A U-shaped connector filled with electrolyte that maintains electrical neutrality by allowing ion flow between half-cells.

How do you determine which species is anode and which is cathode in a galvanic cell based on reduction potentials?

The species with the lower reduction potential is oxidized (anode); the species with the higher reduction potential is reduced (cathode).

What happens if the salt bridge is removed in a galvanic cell?

Electricity production would stop due to charge build-up; ions cannot balance the solutions. Salt bridge maintains ongoing current.

What are the standard electrode potentials for the Daniell cell (Zn and Cu) and the cell potential?

Zn2+/Zn: −0.76 V; Cu2+/Cu: +0.34 V; E°cell = 0.34 − (−0.76) ≈ 1.10 V.

What is the role of the Standard Hydrogen Electrode in measuring half-cell potentials?

SHE serves as the zero reference (0 V) against which other half-cell potentials are measured.

What happens at the anode and cathode in an electrolytic cell?

Anode: oxidation occurs; Cathode: reduction occurs. External power drives electrons from anode to cathode.

What is a typical outcome in electrolysis of aqueous NaCl (brine) at concentrated conditions?

Anode: Cl− oxidized to Cl2; Cathode: H2 produced (Na+ remains in solution). Net: 2Cl− → Cl2 + 2e− and 2H2O + 2e− → H2 + 2OH−.

What is a typical outcome in electrolysis of concentrated aqueous NaCl (brine) at the cathode?

Hydrogen gas is produced from water reduction: 2H2O + 2e− → H2 + 2OH−.

What is a typical outcome in electrolysis of dilute aqueous NaCl at the anode and cathode?

Anode: OH− is oxidized to O2; Cathode: H2 is produced from water reduction. Net: 2H2O → 2H2 + O2.

What is meant by a reduction potential and how does it relate to feasibility of a reaction?

Reduction potential indicates tendency to gain electrons; if E°cell > 0, the redox reaction is feasible (spontaneous).

What is meant by a half-cell potential and its relation to SHE?

The potential of a single electrode relative to the SHE; the SHE has 0 V by definition.

What is the mass-to-charge relationship used in electrolysis calculations?

The amount of substance deposited or evolved is proportional to the total charge Q = I t and to F (and to the molar mass/valence in deposition calculations).

What is the general rule for which species deposits at the cathode during electrolysis of a solution containing multiple cations?

Species with higher reduction potential (or easier reduction) tends to plate first; otherwise water or other ions may discharge depending on concentrations and activities.

What is the oxidation state of oxygen in H2O2?

−1.

Which species acts as the reducing agent in the reaction Zn → Zn2+ + 2e−?

Zinc acts as the reducing agent because it is oxidized and donates electrons.

What is the general equation for oxidation state balancing using electrons?

Balance the electron transfer between oxidation and reduction halves, then combine and balance H and O to obtain the full balanced redox equation.

What is meant by the term 'standard electrode potential' (E°)?

The electrode potential of a half-cell under standard conditions (1 atm, 1 M, 25°C) relative to the SHE.

Why is the activity series of metals useful in electrochemistry?

It helps predict which metals can displace others from compounds and predicts feasibility of redox reactions and corrosion tendencies.

What is the net ionic equation for the electrolysis of aqueous NaCl (dilute) at the cathode and anode?

Cathode: 4H2O + 4e− → 2H2 + 4OH−; Anode: 4OH− → O2 + 2H2O + 4e−; Net: 2H2O → 2H2 + O2.

What is the relation for E°cell in terms of the cathode and anode potentials?

E°cell = E°cathode − E°anode.

What is the significance of a positive E°cell value?

It indicates that the redox reaction is spontaneous under standard conditions.

In a redox balancing problem, what does the term 'disproportionation' specifically require about the elements involved?

The same element must be both oxidized and reduced within the same reaction.