Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2015

Lecture Notes 12: 21 February

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Balancing Redox Equations, cont.

Finish Acid example of last time.

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 basic solution can be found in the Discussion Module.)

Water

Water: water is so ubiquitous, and has so many important and even special properties, that we will talk a bit more about it.

Water is a very unusual, even incredible substance whose amazing properties are often unappreciated because of its ubiquitousness. Water's special properties include extremely high mp and bp (0 °C & 100 °C, compare to methane, -183 °C & -161 °C, with a MW of 16 vs. water's 18); a high heat capacity (18 cal/°C mol vs. 8 cal/°C mol for methane); it has a high viscosity; its solid form is less dense than the liquid form at the same temperature (ice floats on water - very rare), it has a high dielectric constant (78.5 vs. 1.9 for hexane; 'blocks electric fields') and it has a high surface tension.

The high mp, bp, heat capacity and surface tension all predict relatively strong bonding between water molecules, H-bonding. Note environmental consequences - Earth's weather is much more pleasant because it is moderated by water, especially along coasts. Ice floating prevents "solid" seas, definitely a downer in environmental terms, many animals use surface tension to "float."

Water of course is a covalent structure: H-O-H. But what gives it its special properties is the polarity of its O-H bonds and the resultant dipole moments of the bonds and the molecule itself.

The water molecule itself is bent, with an angle of 104.5° between the hydrogens (compare to 109.5° for sp3 tetrahedron) as seen in Figure 4.1 on p 140 of your text.

Structural diagram of water molecule showing bond angles and lengthsSpace-filling image of water molecule

public domain images via Wikipedia Creative Commons

Because of the very strong dipole moments of these bonds and the very small size of the hydrogen substituents on water, a slight degree of orbital overlap occurs between adjacent water oxygens and hydrogens to give partial covalent bonds known as H-bonds (effectively, can only form with O, N, & F). Note that the partial covalent character means that they are directional!

Within solid bulk water (ice) every water molecule is bonded to 4 others.

3-D image of H-bonding between space-filling images of water molecules

public domain image via Wikipedia Creative Commons

In liquid water the molecules are still bonded to a large degree (the heat of fusion for ice is only 13% of the heat of vaporization for ice, thus most of the H-bonds must survive melting). Of course in liquid water the bonds are very unstable (average lifetime about 10 psec = 10-11 sec), exchanging constantly to give a "flickering cluster" structure. The various properties of water arise from this structure. (Note hi bp & mp, heat cap., viscosity, and, less obviously, that ice floats. This is because the molecules are in an open lattice rather than close-packed. G&G note that close-packed molecules would only occupy about 57% of volume. This would lead one to expect that ice would float "high." It doesn't because most of the structure remains in the liquid phase at 0° C.)

Water is also an excellent solvent for polar substances since its dipolar structure enables it to insulate them from each other and it can make good dipole-dipole and dipole-charge bonds. Figure 4.2 on pg. 140 shows the hexavalent liganding of water to sodium and chloride ions to form hydration shells (For sodium ions, the waters in the inner hydration-shell exchange every 2-4 nsec.).

Flattened image of the hydration of sodium ion

public domain image via Wikipedia Creative Commons

We say that substances dissolved in water are hydrated. (More generally, solvents solvate, with hydration being the special case for aqueous solvation.) Anything which can H-bond (such as alcohol or acetic acid) will also of course be quite soluble.

As a general rule for solubility we can say that "Like dissolves like.

Electrolyte Solutions

A major family of solutes are the electrolytes. An electrolyte is a substance whose solution conducts electricity. Electrolytes are ionic substances which dissociate in solution to conduct charge. In contrast, a nonelectrolyte gives a solution which does not conduct electricity. Nonelectrolytes are generally non-ionic substances.

Electrolytes are in turn classified in two broad categories:

Strong electrolytes: solutions are generally good conductors; substances are completely ionized in solution.

Weak electrolytes: solutions are not very good conductors; substances only partially ionized in solution.

Notice that electrolytes can be ionic compounds, like sodium chloride, or they can be covalent, molecular compounds like hydrogen chloride or hydrogen fluoride, both of which exist as perfectly good polar covalent molecules in the gas phase, but which react in water to dissociate into ions.

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© R A Paselk

Last modified 17 February 2015