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Biochemistry > Redox chemistry! > Flashcards

Redox chemistry! Flashcards

(61 cards)

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1
Q

What is a redox reaction?

A

A chemical reaction involving transfer of electrons; oxidation and reduction occur simultaneously.

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2
Q

What is oxidation?

A

Loss of electrons.

(OILRIG: Oxidation Is Loss, Reduction Is Gain)

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3
Q

What is reduction?

A

Gain of electrons.

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4
Q

What is an oxidising agent?

A

A substance that gains electrons and gets reduced.

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5
Q

What is a reducing agent?

A

A substance that loses electrons to reduce something else and gets oxidised.

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6
Q

What are half-reactions?

A

Equations showing either oxidation or reduction separately in a redox reaction.

They show the movement of e-

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7
Q

Split Cu²⁺ + Zn → Cu + Zn²⁺ into half-reactions.

A

Oxidation: Zn → Zn²⁺ + 2e⁻; Reduction: Cu²⁺ + 2e⁻ → Cu.

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8
Q

What is the oxidation number of a free element (e.g. Fe, O₂)?

A

0

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9
Q

What is the oxidation number of H in most compounds?

A

+1

(except in metal hydrides like LiAlH₄, where it is -1)

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10
Q

What is the oxidation number of O in most compounds?

A

-2

(except in compounds like F₂O, where O is +2)

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11
Q

How do you find the oxidation number in polyatomic ions?

A

The sum of oxidation numbers equals the charge on the ion.

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12
Q

What happens at the anode?

A

Oxidation occurs; electrons flow out.

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13
Q

What charge is the anode?

A

Negative

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14
Q

What happens at the cathode?

A

Reduction occurs; electrons flow in.

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15
Q

What charge is the anode in a galvanic cell?

A

Negative (-)

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16
Q

What charge is the cathode in a galvanic cell?

A

Positive (+)

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17
Q

What direction do e- flow in an electrochemical cell?

A

From the anode to the cathode

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18
Q

What does a more positive standard electrode potential (E°) mean?

A

The species is more likely to be reduced.

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19
Q

What does the degree symbol (as in ΔG⁰ and E⁰) mean?

A

The value under standard conditions

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20
Q

How is the cell potential (ΔE°) calculated?

A

ΔE° = E°(cathode) – E°(anode)

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21
Q

What are the half-reactions for the lead accumulator?

A

Oxidation: Pb + SO₄²⁻ → PbSO₄ + 2e⁻;
Reduction: PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O.

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22
Q

What is the overall reaction in a lead battery?

A

Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O.

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23
Q

What is the ΔE° for the lead accumulator?

A

2.035 V

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24
Q

What is the formula linking ΔG° and ΔE°?

A

ΔG° = -nFΔE°
* n = moles of e-
* F = Faraday constant
* ΔE° = standard cell potential in volts

This is not the standard Gibbs’ free energy equation!!

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25
What does a negative ΔG° imply?
The reaction is spontaneous.
26
What does the Nernst equation allow you to calculate?
It allows you to find ΔE°(cell) under non-standard conditions
27
How would you calculate ΔE°(cell) under standard conditions?
ΔE°(cell) = E°(cathode) - E°(anode)
28
What is the Nernst Equation?
E = E°(cell) - RT/nF ln [products]/[reactants] * E = actual cell potential (in volts) * E⁰₍cell₎ = standard cell potential (from tables) * n = number of moles of electrons transferred * R = gas constant * T = temperature in kelvin (use 298 K if at room temp) * F = Faraday constant
29
How does pH affect E° for the hydrogen electrode?
At pH = 0, E = 0.000 V; at pH = 7, E ≈ -0.420 V.
30
What is a first-kind electrode?
An electrode made of the same metal as the ion being measured (e.g., Cu/Cu²⁺).
31
What is a second-kind electrode?
An electrode based on an insoluble salt (e.g., calomel electrode Hg/Hg₂Cl₂).
32
What is a redox electrode?
An inert metal (Pt, Au) immersed in a solution of a redox couple.
33
What is a concentration cell?
An electrochemical cell where both electrodes are the same material but solutions have different concentrations.
34
How does a pH electrode work?
Measures voltage that changes with [H⁺]; calibrated because activity and concentration differ.
35
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)
36
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.
37
How can you calculate cell potential?
You can calculate the overall battery power (voltage) by: ΔE° = E°(cathode) – E°(anode)
38
What does it mean when ΔE° is positive?
Reaction runs spontaneously - the battery works - it really wants to gain electrons
39
What is Gibbs Free Energy (ΔG)?
A measure of the usable energy available to do work in a chemical reaction.
40
What does a negative ΔG mean?
The reaction is spontaneous (happens by itself).
41
What does a positive ΔG mean?
The reaction is non-spontaneous (needs energy to happen).
42
What is the formula linking Gibbs Free Energy and cell potential?
ΔG° = -zFΔE°
43
What does z stand for in the formula ΔG° = -zFΔE°?
The number of electrons transferred in the redox reaction.
44
What does F stand for in the formula ΔG° = -zFΔE°?
Faraday's constant (approximately 96,485 Coulombs per mole of electrons).
45
What does ΔE° stand for in the formula ΔG° = -zFΔE°?
The standard cell potential (voltage) under standard conditions.
46
If ΔE° is positive, what can you say about ΔG°?
ΔG° will be negative, meaning the reaction is spontaneous.
47
If ΔE° is negative, what can you say about ΔG°?
ΔG° will be positive, meaning the reaction is non-spontaneous.
48
What are the units of Gibbs Free Energy (ΔG)?
Joules (J) or kilojoules (kJ).
49
What is the value of Faraday’s constant (F)?
Approximately 96,485 Coulombs per mole (C/mol).
50
What two things determine whether a battery reaction is spontaneous?
The sign of ΔG° (negative = spontaneous) and the sign of ΔE° (positive = spontaneous).
51
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
52
What is an oxidation number?
An oxidation number is the hypothetical charge an atom would have if: All bonds were considered completely ionic, even if they’re covalent
53
What oxidation number do you give elements in their standard state? e.g. O2, N2
0
54
What oxidation number would you give monoatomic ions?
Give them the charge the have on them e.g. Cl- is -1
55
What oxidation number would you give group 1 and group 2 metals?
+1 for group 1 +2 for group 2 Group 1 metals (alkali metals) have 1 outer electron → easily lost → +1 Group 2 metals (alkaline earth metals) have 2 outer electrons → lost → +2 These elements always form the same oxidation states in compounds
56
What oxidation number would you give hydrogen?
+1 usually -1 in metal hybrids e.g. NaH
57
What oxidation number would you give oxygen?
Usually –2 –1 in peroxides (e.g., H₂O₂) +2 in OF₂
58
What oxidation number would you give fluorine?
-1 Always the most electronegative
59
How would you find the oxidation number in polyatomic compound?
Do the oxidation numbers you know Then if it's neutral make them add to 0 If it's not neutral, make it add to its overall ion charge number
60
Describe what electrochemical series values mean
Higher E° (more positive) → more likely to gain electrons = reduction = strong oxidising agent Lower E° (more negative) → more likely to lose electrons = oxidation = strong reducing agent
61
Does positive G or positive E mean the reaction is spontaneous?
Positive E These are opposites, negative G means a spontaneous reaction

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