Oxidation-Reduction Reactions

Question 1

Oxidation-reduction (redox) reactions involve the ___.

Answer 1

Transfer of electrons from one chemical species to another

Question 2

The law of conservation of charge

Answer 2

Electrical charge can be neither created nor destroyed

Question 3

OIL RIG

An oxidizing reagent causes another atom to undergo ___ and is itself ___.

Answer 3

Oxidation is loss
Reduction is gain

Oxidation
Reduced

Question 4

Common oxidizing agents

Answer 4

Almost always contain oxygen or a similarly electronegative element

Examples in biochem:
NAD+
FADH

Commonly seen in orgo rxns:
KMnO4
CrO3
Na2Cr2O7
PCC

Other: O2, H2O2, halogens, H2SO4, HNO3, NaClO

Question 5

Common reducing agents

Answer 5

Often contain metal ions and hydride ions (H-)

Examples in biochem:
NADH
FADH2

Commonly seen in orgo rxns:
Hydrazine
Zn(Hg)
Lindlar’s catalyst
NaBH4
LiAlH4

Other: CO, C, B2H6, Sn2+ and other pure metals

Question 6

The oxidation of a free element is ___.

Answer 6

Zero (e.g. N2, P4, S8)

Question 7

Can think of oxidation number as the ___.

Answer 7

Typical charge of an element based on its group number, metallicity, and general location on the periodic table

Question 8

The oxidation number for a monoatomic ion is equal to ___.

Answer 8

The charge of the ion (e.g. for Na+ the oxidation number is +1)

Question 9

The oxidation number for each Group IA element is ___.
For each Group IIA element is ___.

Answer 9

+1

+2

Question 10

The oxidation number of each Group VIIA element in a compound is ___, except when ___.

Answer 10

-1; combined with an element of higher e-neg.

In HCl, oxidation number of Cl- is -1

In HOCl, oxidation number of Cl- is +1

Question 11

The oxidation number of hydrogen is usually ___, however its oxid. number is ___ in compounds with ___.

Answer 11

+1

-1

Less electronegative elements

E.g. +1 in HCl but -1 in NaH

Question 12

In most compounds, the oxidation number of oxygen is ___.

Answer 12

-2

2 exceptions:

Peroxides for which the charge on each oxygen is -1

Compounds with more electroneg. elements like OF2, in which oxygen has a +2 charge

Question 13

The sum of the oxidation numbers of all the atoms present in a neutral compound is ___. The sum of the oxid. numbers of the atoms present in a polyatomic ion is ___.

Answer 13

Zero

The charge of the ion

Question 14

Oxidation number vs. formal charge?

Answer 14

Oxidation number: assumes unequal division of e- in bonds, awarding electrons to the more e-neg. element

Formal charge: assumes equal division of e- in bonds, awarding one electron to each atom in the bond

Question 15

SnCl2 + PbCl4 –> SnCl4 + PbCl2

Which is the reducing agent? Oxid. reagent?

Answer 15

Reactants:
Sn is +2
Pb is +4

Products
Sn is +4
Pb is +2

Sn loses e-, OIL so is oxidized –> reducing agent

Pb gains e-, RIG so is reduced –> oxidizing agent

Question 16

Steps to balancing a redox reaction?

Answer 16

1. Separate the two half-reactions (e.g. I- –> I2 and MnO4- –> Mn2+)
2. Balance the atoms of each half-reaction

- Balance all the atoms except H and O
- In an acidic solution, add H2O to balance the O atoms and then add H+ to balance the H atoms
- In a basic solution, use OH- and H2O to balance the O and H atoms

3. Balance the charges of each half-reaction; add electrons as necessary to one side of the reaction so that the charges are equal on both sides
4. Both half-reactions must have the same # of electrons so that they cancel each other out in the next step
5. Add the half-reactions, canceling out terms that appear on both sides of the reaction arrow
6. Confirm that mass and charge are balanced

Question 17

What is a spectator ion?

Answer 17

An ion that appears on both sides of an equation and is not directly involved in the reaction

E.g. Zn (s) + CuSO4 (aq) -> Cu (s) + ZnSO4 (aq)

SO4(2-) is not participating in the oxidation-reduction reaction

Question 18

Net ionic equation

Answer 18

An ionic equation that includes only the particles that participate in the reaction (e.g. leaves out SO4(2-) in the previous reaction)

Question 19

When writing net ionic equations, all aqueous solutions should be ___ while solid salts should be ___.

Answer 19

Split into their constituent ions

Kept together as a single entity

Question 20

What happens in a combustion reaction?

Answer 20

A fuel (usually hydrocarbon) is mixed with an oxidant (usually oxygen), forming CO2 and H2O

Question 21

Combustion half reaction for:

CH4 + 2O2 -> CO2 + 2H2O

Answer 21

CH4 + 2H2O -> CO2 + 8H+ + 8e-

2O2 + 8H+ + 8e- -> 4H2O

Question 22

Double-displacement or metathesis reactions - are they usually oxidation-reduction reactions?

Answer 22

No

Question 23

AgNO3 (aq) + HCl (aq) — AgCl (s) + HNO3 (aq)

What is the net ionic eqn?

Answer 23

Ag+ + Cl- -> AgCl

Question 24

Net ionic reaction in double-displacement reactions where the reactants and products are aqueous?

Answer 24

There is none

(Reactants and products are the same in the complete ionic equation - AKA they’re all just ions - so there is not NET ionic eqn)

Question 25

Disproportionation or dismutation reaction is a reaction in which ___.

Answer 25

(Redox reaction)

An element undergoes both oxidation and reduction in producing its products

(E.g. many biological enzymes)

Question 26

Example of an enzyme that uses disprop./dismutation?

Answer 26

Catalase

2H2O2 (aq) –(catalase)–> 2H2O (l) + O2 (g)

Oxygen is both REDUCED (from -1 to -2) and OXIDIZED (-1 to 0)

Superoxide dismutase

2O2(-0.5) + 2H+ –> H2O2 + O2

Oxygen in the free radical has oxid. state of -0.5
It is both reduced (from -0.5 to -1) and oxidized (from -0.5 to 0)

Question 27

Biochemical disprop. reactions are usually accomplished by enzymes, which often have ___ in their active sites that act as ___ agents.

Answer 27

Metals (Cu and Zn)

Reducing agents

Question 28

Redox titrations follow ___ to reach the equivalence point.

Answer 28

The transfer of charge (e-)

Question 29

How to do redox titration?

Answer 29

1. Balance the equations of the reactions WRT not only stoich, but also charge
2. Use stoich to determine the molarity of the substance in question (b/c we are given the molarity and volume of the standard solution, and the volume of the thing in question)

Question 30

Potentiometric titration

Answer 30

• A form of redox titration in which a voltmeter or external cell measures the electromotive force (emf) of a solution
• No indicator is used
• The equivalence point is determined by a sharp change in voltage

Analogous to acid-base titration where pH meter used and not color indicator