Free TCE Chemistry Flashcard Review
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Define oxidation.
Oxidation is loss of electrons or increase in oxidation state.
Define reduction.
Reduction is gain of electrons or decrease in oxidation state.
What is an oxidising agent?
An oxidising agent accepts electrons and is itself reduced.
What is a reducing agent?
A reducing agent donates electrons and is itself oxidised.
Explain oxidation state.
Oxidation state is a formal charge assigned to an atom using agreed electron-allocation rules; in monatomic ions it equals the ionic charge.
In a redox reaction, which element is oxidised and which is reduced?
The oxidised element's oxidation state increases; the reduced element's oxidation state decreases. Both must occur together.
How do you identify the oxidising agent in Zn + Cu²⁺ → Zn²⁺ + Cu?
Cu²⁺ is the oxidising agent because it accepts electrons (is reduced from +2 to 0).
What does oxidation state = ionic charge for?
In a monatomic ion; for molecules or polyatomic ions, oxidation states sum to zero or the ion charge respectively.
In permanganate MnO₄⁻, what is manganese's oxidation state?
Manganese is +7; oxygen contributes −2 each (−8 total across 4 oxygens), so Mn + (−8) = −1, giving Mn = +7.
Distinguish a spontaneous redox reaction from a galvanic cell.
A spontaneous reaction may have electrons transfer at the point of contact; a galvanic cell separates the half-reactions so electrons travel through an external circuit and can do electrical work.
What is a half-equation?
A half-equation shows one electron-transfer process (oxidation or reduction) and balances atoms, charge and electrons.
In acidic solution, which species balance oxygen in a half-equation?
Water H₂O(l) is added to the side deficient in oxygen to balance oxygen atoms.
Which species balance hydrogen in acidic half-equations?
H⁺(aq) is added to balance hydrogen atoms; never use H⁻ (hydride). Current course version 4e specifies H⁺.
In the half-equation MnO₄⁻ → Mn²⁺, how many electrons are transferred?
Five electrons; manganese changes from +7 to +2, a decrease of 5 oxidation states, so MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.
Balance the reduction half-equation for permanganate in acid.
MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ → Mn²⁺(aq) + 4H₂O(l).
What charge balance check should you perform after adding electrons?
Sum the charges on each side; they must be equal as signed integers. Electrons correct charge, not atom count.
How do you combine two half-equations?
Make the number of electrons equal by multiplying each half by an integer, then add and cancel identical species on opposite sides.
In the dichromate reduction Cr₂O₇²⁻ → Cr³⁺ in acid, how many electrons are gained?
Six electrons; chromium changes from +6 to +3 in each Cr atom (−3 each), and there are two chromium atoms, so 2 × 3 = 6e⁻.
Balance Cr₂O₇²⁻(aq) + 14H⁺(aq) → 2Cr³⁺(aq) + ?
Cr₂O₇²⁻(aq) + 14H⁺(aq) + 6e⁻ → 2Cr³⁺(aq) + 7H₂O(l).
After combining and cancelling half-equations, what must be verified?
Check atom balance, charge balance, and state symbols; verify all atoms and total charge are conserved.