Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2013

Lecture Notes 12: 18 February

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Balancing Redox Equations, cont.

Basic Solution:

Example: Balance the equation above in basic solution (I don't believe this reaction actually occurs in basic solution, but due to our time constraints we'll pretend it does.)

MnO4- + Cl- right arrow Mn2+ + Cl2

First we balance as above to give:

16 H+ + 2 MnO4- + 10 Cl- right arrow 2 Mn2+ + 8 H2O + 5 Cl2

Next add 16 OH- to each sides to cancel H+

16 OH- + 16 H+ + 2 MnO4- + 10 Cl- right arrow 2 Mn2+ + 8 H2O + 5 Cl2 + 16 OH-

Combine OH- and H+ to give 16 H2O

16 H2O + 2 MnO4- + 10 Cl- right arrow 2 Mn2+ + 8 H2O + 5 Cl2 + 16 OH-

canceling waters then gives the final equation:

8 H2O + 2 MnO4- + 10 Cl- right arrow 2 Mn2+ + 5 Cl2 + 16 OH-

(Additional examples for balancing Redox equations in acidic solution can be found in the Discussion Module.)

Acid-Base Reactions

Neutralization

When we combine equal numbers of moles of hydrogen ion and hydroxide ion a neutralization occurs. That is, there is no reactive component left, all of the acid has been consumed by all of the base, and water has been synthesized.

Consider the reaction of 50.0 mL of 0.25 M hydrochloric acid with 25.0 mL of 0.50 M sodium hydroxide.

H+ + Cl- + Na+ + OH- right arrow H2O + Cl- + Na+

Giving: H+ + OH- right arrow H2O

Oxidation Numbers

For simple elemental ions it is easy to determine the charge on an atom, but in many other circumstances this is not the case. In order to name compounds and understand reactions we frequently need this information which is obtained from oxidation numbers.

Oxidation numbers are in essence an electronic accounting method in which electrons are assigned to a particular atom in a bond or interaction. As such they give an approximate picture of where electrons actually reside in compounds. We will find this information very useful later when we look at particular types of chemical reactions. Oxidation numbers are essential for nomenclature.

For simple elemental ions it is easy to determine the charge on an atom, but in many other circumstances this is not the case. In order to name compounds and understand reactions we frequently need this information which is obtained from oxidation numbers.

Oxidation numbers are most readily assigned using a simple set of rules:

  1. In the formula for any substance the sum of the oxidation numbers of all the atoms in the formula is equal to the charge shown. Thus:
  2. In compounds fluorine is always assigned an oxidation number of -1.
  3. Alkali metals in compounds will always (for our class) be assigned an oxidation number of +1.
  4. Alkaline-earth metals in compounds will always (for our class) be assigned an oxidation number of +2
  5. In compounds oxygen is usually assigned an oxidation number of -2.
  6. In compounds hydrogen is usually assigned an oxidation number of +1
  7. Aluminum will always (for our class) be assigned an oxidation number of +3, other elements in this Group will usually be assigned an oxidation number of +3.

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© R A Paselk

Last modified 18 February 2013