MCAT General Chemistry - Oxidation–Reduction Reactions

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Leigh disease

extremely rare mitochondrial disorder; number of key mitochondrial enzymes are disrupted and the process of oxidative phosphorylation is never achieved; when pyruvate cannot be oxidized to acetyl-CoA, it is instead fermented to lactic acid

Oxidation–Reduction (redox) reactions

Reactions that involve the transfer of electrons from one chemical species to another; loss and gain must happen simultaneously

oxidation

loss of electrons

reduction

gain of electrons

oxidizing agent

causes another atom in a redox reaction to undergo oxidation and is itself reduced; applied specifically to the atom that gains electrons

reducing agent

causes the other atom to be reduced and is itself oxidized; applied specifically to the atom that loses electrons

Oxidation numbers

assigned to atoms in order to keep track of the redistribution of electrons during chemical reactions; typical charge of an element based on its group number, metallicity, and general location in the periodic table; assumes unequal division of electrons in bonds, “awarding” the electrons to the more electronegative element

Oxidation Rules

1. Free element = 0

   1. N2 = 0

   2. He = 0

2. Monatomic ion = charge

   1. Cu²⁺ = +2

   2. Fe³⁺ = +3

   3. N^3- = -3

3. Group IA = +1

   1. Na⁺ = +1

4. Group IIA = +2

   1. Ca²⁺ = +2

5. Group VIIA = -1 except when combined with an element of higher electronegativity

   1. HCl - Cl^- = -1

   2. HOCl - Cl⁺ = +1

6. Hydrogen = +1, except with less electronegative elements

   1. HCl - H⁺ = +1

   2. NaH - H^- = -1

7. Oxygen = -2, except peroxides (-1 each) and more electronegative atoms

   1. OF₂ - O²⁺ = +2

8. Sum of oxidation numbers = 0 or charge of ion

formal charge

not the same as oxidation number; assumes equal division of electrons in bonds, “awarding” one electron to each atom in the bond

half-reaction / ion–electron method

equation is separated into two half-reactions—the oxidation part and the reduction part; Each half-reaction is balanced separately, and then added to give a balanced overall reaction

complete ionic equation

split the various species into all of the ions present

spectator ions

chemically inert ion during a reaction; not taking part in the overall reaction but simply remaining in the solution unchanged

net ionic equation

showing only the species that actually participate in the reaction

combination reactions

two or more species come together to form a product

decomposition reactions

one product breaks down into two or more species

combustion reactions

fuel (usually a hydrocarbon) is mixed with an oxidant (usually oxygen), forming carbon dioxide and water

Double-displacement / metathesis reactions

involve the switching of counterions; ions generally retain their oxidation state, so not usually Oxidation–Reduction reactions

Disproportionation (dismutation)

specific type of redox reaction in which an element undergoes both oxidation and reduction in producing its products

ex. many biological enzymes (catalase, superoxide dismutase)

Redox titrations

follow the transfer of charge (as electrons) to reach the equivalence point; utilize indicators that change color at a particular voltage (emf) value

iodimetric titration

use of starch indicators to identify iodine complexes; use of starch indicators to identify iodine complexes; dark solution in the presence of starch, and at the endpoint of the titration, a colorless solution develops

Potentiometric titration

redox titration where no indicator is used; electrical potential difference (voltage) is measured using a voltmeter