11: Oxidation-Reduction reactions

Question 1

law of conservation of charge

Answer 1

states that electrical charge can be neither created nor destroyed… so oxidation (loss of electrons) must occur simultaneously with reduction (gain of electrons)

Question 2

redox reaction

Answer 2

electron transfer involving oxidation and reduction

Question 3

oxidizing agent

Answer 3

causes another atom to be oxidized and itself reduced

*the term ox/red agent is applied specifically to the atom that loses/gains electrons, but often compound as a whole is described as oxidizing or reducing agent (ie. CrO₃ is compound described as oxidizing agent but Cr⁶⁺ is the actual oxidizing agent part

Question 4

reducing agent

Answer 4

causes another atom to be reduced and itself oxidized

Question 5

common oxidizing agents

Answer 5

O₂, H₂O₂

halogens: F₂, Cl₂, Br₂, I₂

H₂SO₄, HNO₃, NaClO, KMnO₄

CrO₃, Na₂Cr₂O₇

PCC, NAD⁺, FADH

Question 6

common reducing agents

Answer 6

CO, C, B₂H₆

Sn²⁺ and other pure metals

Zn(Hg), hydrazine, Lindlar’s catalysts

NaBH₄, LiAlH₄, NADH, FADH₂

Question 7

biochemical redox agents

Answer 7

such as NAD⁺ and FADH act as both oxidizing and reducing agents at different times during metabolic pathways… thus they act as mediators of energy transfer during many metabolic processes

Question 8

oxidation numbers

Answer 8

are assigned to atoms in order to keep track of the redistributionof electrons during chemical reactions… think of ox numbers as the typical charge of an element

*assumes unequal division of electrons in bonds UNLIKE formal charge, which asumes equal division

*conventions put cation first and anion second so HCl implies H⁺ and NaH implies H^-

Question 9

assigning oxidtion numbers

Answer 9

1. ox number of a free element is 0
2. ox number for a monoatomic ion is equal to the charge of the ion
3. group 1A elements is +1
4. group 2A elements is +2
5. group VIIA element is -1 except when combined witha more electronegative element
6. hydroen is usually +1 except whenit is with less electronegative elements
7. oxygen is usually -2 except in peroxides and compounds with more electronegative elements
8. sum of ox numbers of all atoms in a neutral compound is 0… equals charge in polyatomic ion

Question 10

half-reaction method/ion-electron method

Answer 10

equation is separated into 2 half-reactions (oxidation part and the reduction part) and each half-reaction is balanced seperately and then added to give overall reaction

• add H₂O and then H⁺ to balance acidic solution
• add OH- and H20 to balance basic solution
• then add electrons to balance charges
• then cancel out lectrons and terms that appear on both sides

Question 11

complete ionic equations

Answer 11

split various species into all the ions present including spectator ions

Question 12

net ionic equations

Answer 12

does not involve spectator ions

• all aquoeous compounds should be split into their consituent ions
• solid salts should be kept together as a single entity

Question 13

comibination reactions

Answer 13

2+ species come together to form a product

ex. H₂ (g) + F₂ (g) → 2HF

net ionic is H₂ (g) + F₂ (g) → 2H⁺ + 2F^-

Question 14

decomposition reactions

Answer 14

one product breaks down into 2+ species

ex. (NH₄)₂Cr₂O₇ → N₂ + Cr₂O₃ + 4H₂O

net ionic: 2NH₄⁺ + Cr₂O₇^2- → N₂ + Cr₂O₃ + 4H₂O

Question 15

combustion reactioins

Answer 15

feul (hydrocarbon) is mixed with an oxidant (oxygen) forming CO2 and H2O

ex. CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l)

*net ionic equation is identical to the overall balanced equation because there are no spectator ions nd no aqueous species

Question 16

double-displacement/methathesis reactions

Answer 16

involve switching of counterionsa

ex. AgNO₃ (aq) + HCl (aq) → HNO₃ (aq) + AgCl (aq)

net ionic: Ag⁺ + Cl^- → AgCl

*not ox-red because all species retain same oxidation numbers

Question 17

disproportionation reactions

Answer 17

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

• example is catalysis of peroxides by catalase, which is a critical biological enzyme
  
  • 2H₂O₂ (aq) → 2H₂O (l) + O₂ (g)

Question 18

oxidation-reduction titrations

Answer 18

follow transfer of charge (electrons) to reach equivalence point rather than movement of protons in acid-base

• use indicators that change color at particular voltage (emf) value
• ex. iodimetric titration which relies on titration of free iodine radicals

Question 19

potentiometric titration

Answer 19

form of redox titration where no inidcator is used… instead a voltemeter is used to measure electrical potential difference (voltage)

*analagous to using pH meter for acid base titration