MCAT Oxidation-Reduction Reactions

Reaction that involve the transfer of electrons from one chemical species to another can be classified as what type of reactions?

Oxidation-reduction (redox) reactions

What is the mnemonic for redox reactions?

OIL RIG

Oxidation Is Losing, Reduction is Gaining

The loss of electrons is called what?

oxidation

Gain of electrons is called what

reduction

An oxidizing agents causes another atom in a redox reaction to undergo oxidation itself and itself is ________________

reduced

A reducing agent causes the other atom to be reduced and itself is ______________

oxidized

What are some common oxidizing agents?

O2, H2O2, F2, Cl2, Br2, I2

H2SO4

HNO3

NaCLO

KMnO4

CrO3

Na2Cr2O7

PCC

NAD+, FADH

What are some common reducing agents?

CO, C, B2H6,

Sn2+ and other pure metals

Hydrazine

Zn(Hg)

Lindlar's catalyst

NaBH4

LiAlH4

NADH

FADH2

What are oxidation numbers?

Assigned to atoms in order to keep track of the relationship of electrons during chemical reactions

What is the oxidation number of a free element?

0

What is the oxidation number of a monoatomic ion?

its charge;

Na+ = 1

Cu2+ = 2

What is the oxidation number for each group IA element in a compound?

+1

What is the oxidation number of each group IIA element in a compound?

+2

The oxidation number of each group VIIA element in a compound is what? What is the exception?

-1, except when combined with an element of higher electronegativity.

What is the oxidation number of HCl?

-1

What is the oxidation number of HOCL?

+1

What is the oxidation number of hydrogen?

+1

What is the oxidation number or hydrogen in compounds with more electronegative elements?

-1

What is the oxidation number for HCl?

+1

What is the oxidation number in NaH?

-1

In most compounds, the oxidation number of oxygen is what?

-2

What is the oxidation number of peroxides?

-1

What is the oxidation number of OF2?

+2

The sum of the oxidation numbers of all the atoms present in a neutral compound is

0

The sum of the oxidation numbers of the atoms present in a polyatomic ion is equal to the ________ of the ion

charge

How is oxidation number different than formal charge?

Both account for the perceived charge on an element, but oxidation numbers assumes unequal division of electrons in bonds. Formal charge assumes equal division of electrons in bonds, awarding one electron to each atom in the bond.

To balance a redox reaction, both the ____________ charge and the number of ______________ must be equal on both sides of the equation

net; atoms

The most common method for balancing redox reactions is the _______________-________________ method, also known as the ion-electron method.

Half-reaction

What is the complete ionic equation?

an equation that shows dissolved ionic compounds as dissociated free ions

What is a spectator ion?

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

What is the net ionic equation?

Shows only the species that actually participate in the reaction

What are combination reactions?

Two or more species come together to form a product

For this combination reactions: H2 + F2 --> 2HF.

What is the relevant half reaction?

What is the net ionic equation?

Half: H2 --> 2 H+ + 2e-

F2 + 2 e- --> 2 F-

Net: H2 + F2 --> 2 H+ + 2F-

What happens in decomposition reactions?

One product breaks down into two or more species?

What is a combustion reaction?

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

What are double displacement or metathesis reactions?

These types of reactions involve the switching of counterions.

What is a disproportionation reaction?

~ Where a single element is simultaneously oxidized and reduced at the same time

What is the goal of an oxidation-reduction titration?

Redox titrations follow the transfer of charge (as electrons) to reach the equivalent point

Bipyrimidine metal complexes. What is the voltage of color change? What is its oxidized form? Reduced form?

About +1V

Oxidized: Colorless (Ru), Cyan (Fe)

Reduced: Yellow (Ru), Red (Fe)

Diphenylamine. What is the voltage of color change? What is its oxidized form? Reduced form?

+.76 V

Oxidized: Violet

Reduced: Colorless

Safranin. What is the voltage of color change? What is its oxidized form? Reduced form?

+.24 / -.29 V

Oxidized: Red-Violet

Reduced: Colorless

What is a potentiometric titration?

Form of a redox titration where no indicator is used. Instead, the electrical potential difference is measured using a voltmeter

Potentiometry is an oxidation-reduction titration. This is analogous to performing an acid-base titration with a(n)

pH meter

Consider the following equation:

6 Na (s) + 2 NH3 (aq) --> 2 Na3N (s) + 3 H2 (g)

Which speciews acts as an oxidizing agent?

A. Na

B. N in NH3

C. H in NH3

D. H2

C; The oxidizing agent is the species that is reduced in any given equation. In this problem, six hydrogen atoms with +1 oxidation states in NH3 are reduced to three neutral H2 molecules.

How many electrons are involved in the following half reaction after it is balanced?

Cr2O7 2- + H+ + e- --> Cr2+ + H2O

A. 2

B. 8

C. 12

D. 16

B

Lithium Aluminum Hydride is a strong _____________ agent

reducing

A strong oxidizing agent will be easily reduced, meaning that it will have the tendency to gain electrons. Atoms usually gain electrons if they are one or two electrons away from filling up their ______________ shell .

Valence

During the assigning of oxidation numbers, which of the following elements would most likely be determined last?

Ar

F

Sr

Ir

Ir

As methanol is converted to methanal, and then methanoic acid, the oxidation number of the carbon

increases

decreases

increases, then decreases

decreases, then increases

Increases

methanal (formaldehyde)

HCHO

In general, it is often easier to think of oxidation as a gain of bonds to ___________ or loss of bonds to _________

oxygen; hydrogen

In the compound KH2PO4, which element has the highest oxidation number?

K is a group I metal, so it has +1

Hydrogen is almost always +1, but there is 2, so +2

Oxygen has -2, but there are 4, so -8.

By default, P has +5, so it has the largest oxidation number.

After balancing the following oxidation-reduction reaction, what is the sum of the stoichiometric coefficients of all of the reactants and products?

S8 (s) + NO3^- (aq) -> SO3^2- (aq) + NO (g)

4

50

91

115

115

Utilize the method described earlier to balance this redox reaction. The balanced half reactions are:

S8 + H2O --> 8 SO3 2- + 48 H+ + 32 e-

NO3- + 4H+ + 3 e- --> NO + 2 H2O

To get equal numbers of electrons in each half reaction, the oxidation half reduction will have to be multiplied by 3, and the reduction half-reaction will have to be multiplied by 32

3 S8 + 72 H2O --> 24 SO3 2- + 144 H+ + 96 e-

32 NO3- + 128 H+ + 96 e- --> 32 NO + 64 H2O

This makes the overall reaction:

3 S8 + 32 NO3- + 8H2O --> 24 SO3 2- + 32 NO + 16 H+

3 + 32 + 8 + 24 + 32 + 16 = 115