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 Incoming links: Kinetics of the Autoreduction of Hexavalent Americium in Aqueous Nitric Acid by Travis S. Grimes and others

Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 4 / b / Solution 2026-10-20

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Radiation changes the solution's chemistry by radiolysis; it does not turn Am(VI) nuclei into Am(V) nuclei. An emitted alpha particle deposits energy into many water and solute molecules, generating ionized/excited species, hydrated electrons, hydrogen atoms and molecular products including hydrogen peroxide. Reducing products can supply the electron needed for

Am O_{2}^{2 +} + e^{-} ⟶ Am O_{2}^{+} .

(1)

For example, hydrogen peroxide can be oxidized while reducing two americyl(VI) ions:

2Am O_{2}^{2 +} + H_{2} O_{2} ⟶ 2Am O_{2}^{+} + O_{2} + 2 H^{+} .

(2)

The

0.3

mM loss of Am(VI) equals the

0.3

mM appearance of Am(V), preserving the measured dissolved americium total while transferring

0.3 mmol L^{- 1}

of electron equivalents. Oxidizing radiolysis products are also present, and the net reduction depends on acid, dissolved gases and scavengers; not every radiolytic event is reducing. In nitric acid, primary kinetic work identifies hydrogen peroxide and radiolytically produced nitrous acid as important reductants, with a rate controlled by total americium activity rather than necessarily by the instantaneous Am(VI) concentration.^[ref]

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