Redox chemistry Flashcards
What is a redox reaction?
A chemical reaction involving transfer of electrons; oxidation and reduction occur simultaneously.
What is oxidation?
Loss of electrons.
(OILRIG: Oxidation Is Loss, Reduction Is Gain)
What is reduction?
Gain of electrons.
What is an oxidising agent?
A substance that gains electrons and gets reduced.
What is a reducing agent?
A substance that loses electrons to reduce something else and gets oxidised.
What are half-reactions?
Equations showing either oxidation or reduction separately in a redox reaction.
Split Cu²⁺ + Zn → Cu + Zn²⁺ into half-reactions.
Oxidation: Zn → Zn²⁺ + 2e⁻; Reduction: Cu²⁺ + 2e⁻ → Cu.
What is the oxidation number of a free element (e.g. Fe, O₂)?
0
What is the oxidation number of H in most compounds?
+1
(except in metal hydrides like LiAlH₄, where it is -1)
What is the oxidation number of O in most compounds?
-2
(except in compounds like F₂O, where O is +2)
How do you find the oxidation number in polyatomic ions?
The sum of oxidation numbers equals the charge on the ion.
What happens at the anode?
Oxidation occurs; electrons flow out.
What happens at the cathode?
Reduction occurs; electrons flow in.
What charge is the anode in a galvanic cell?
Negative (-)
What charge is the cathode in a galvanic cell?
Positive (+)
What does a more positive standard electrode potential (E°) mean?
The species is more likely to be reduced.
How is the cell potential (ΔE°) calculated?
ΔE° = E°(cathode) – E°(anode)
What are the half-reactions for the lead accumulator?
Oxidation: Pb + SO₄²⁻ → PbSO₄ + 2e⁻; Reduction: PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O.
What is the overall reaction in a lead battery?
Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O.
What is the ΔE° for the lead accumulator?
2.035 V
What is the formula linking ΔG° and ΔE°?
ΔG° = -zFΔE° - Gibb’s free energy equation
What does a negative ΔG° imply?
The reaction is spontaneous.
What is the Nernst Equation for dilute solutions?
E = E° - (0.059/n) × log([oxidised]/[reduced])
How does pH affect E° for the hydrogen electrode?
At pH = 0, E = 0.000 V; at pH = 7, E ≈ -0.420 V.
What is a first-kind electrode?
An electrode made of the same metal as the ion being measured (e.g., Cu/Cu²⁺).
What is a second-kind electrode?
An electrode based on an insoluble salt (e.g., calomel electrode Hg/Hg₂Cl₂).
What is a redox electrode?
An inert metal (Pt, Au) immersed in a solution of a redox couple.
What is a concentration cell?
An electrochemical cell where both electrodes are the same material but solutions have different concentrations.
How does a pH electrode work?
Measures voltage that changes with [H⁺]; calibrated because activity and concentration differ.
Give 3 rules for oxidation numbers
Pure elements (Fe, O₂) = 0
H is usually +1 (unless in special cases like hydrides where it is -1).
O is usually -2 (except with fluorine, crazy exception)
Explain a standard electrode potential
Each half-reaction has a number called E° (standard electrode potential).
If E° is positive, it wants to gain electrons (good reducer).
If E° is negative, it wants to lose electrons.
When you put two half-cells together (like Zn and Cu), the one with a bigger E° will win electrons.
How can you calculate cell potential?
You can calculate the overall battery power (voltage) by:
ΔE° = E°(cathode) – E°(anode)
What does it mean when ΔE° is positive?
Reaction runs spontaneously - the battery works - it really wants to gain electrons
What is Gibbs Free Energy (ΔG)?
A measure of the usable energy available to do work in a chemical reaction.
What does a negative ΔG mean?
The reaction is spontaneous (happens by itself).
What does a positive ΔG mean?
The reaction is non-spontaneous (needs energy to happen).
What is the formula linking Gibbs Free Energy and cell potential?
ΔG° = -zFΔE°
What does z stand for in the formula ΔG° = -zFΔE°?
The number of electrons transferred in the redox reaction.
What does F stand for in the formula ΔG° = -zFΔE°?
Faraday’s constant (approximately 96,485 Coulombs per mole of electrons).
What does ΔE° stand for in the formula ΔG° = -zFΔE°?
The standard cell potential (voltage) under standard conditions.
If ΔE° is positive, what can you say about ΔG°?
ΔG° will be negative, meaning the reaction is spontaneous.
If ΔE° is negative, what can you say about ΔG°?
ΔG° will be positive, meaning the reaction is non-spontaneous.
What are the units of Gibbs Free Energy (ΔG)?
Joules (J) or kilojoules (kJ).
What is the value of Faraday’s constant (F)?
Approximately 96,485 Coulombs per mole (C/mol).
What two things determine whether a battery reaction is spontaneous?
The sign of ΔG° (negative = spontaneous) and the sign of ΔE° (positive = spontaneous).
How do you apply the concept of standard electrode potential to batteries?
In a battery, you combine two materials:
The one with a higher E° becomes the cathode (where reduction happens).
The one with a lower E° becomes the anode (where oxidation happens).
Electrons always flow from anode to cathode
