# Balancing Redox Equations, cont.

### Basic Solution:

• balance as in acid, then:
1. Add enough OH- (equal numbers to both sides) to cancel the H+. (This is necessary because there will not be protons present in a basic solution!). (There are a couple of other conventions for balancing in basic solutions. If you are familiar with another and prefer it, you may use it instead.)
2. Combine the H+ and OH- on the appropriate side of the equation to give waters.
3. Go back and cancel waters which appear on both sides to give the final equation.

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- Mn2+ + Cl2

First we balance as above to give:

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

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

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

Combine OH- and H+ to give 16 H2O

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

canceling waters then gives the final equation:

8 H2O + 2 MnO4- + 10 Cl- 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.

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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.

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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.).

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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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