Water Chemistry

Hydrogen bonding of Water molecules: Due to the polar covalent bonds that hold a water molecule together, Hydrogen bonds form where the negative Oxygens and the positive Hydrogens are located.

 

Drawings of Hydrogen bonded water molecules:

 

 

 

 

 

The results of these bonds are as follows:

 

1. Cohesion: is the sticking together of similar molecules. Water is very cohesive. This allows water to be pulled along a pathway with relative ease.

2. Surface Tension: cohesion allows water to pull together and form droplets or form an interface between it and other surfaces. The measure of how hard it is to break this interface is its surface tension.

Water allows materials to rest upon it if the surface tension is not broken. Pollen, dust, water insects, and other biological materials are able to remain on the surface of the water because of this tension.

3. Adhesion: The sticking of one substance to another. Water is a good adhesive. It will cling on to many objects and act as a glue. Capillary Action is an example of cohesion and adhesion working together to move water up a thin tube.

4. Imbibition: The process of soaking into a hydrophilic substance. Water being taken into a sponge, into a seed, into paper towels.

5. High Specific Heat: Specific heat of a substance is the heat needed (gained or lost) to change the temperature of 1g. of a substance 1degree Celsius. Heat is the total quantity of kinetic energy due to molecular motion. Temperature measures the intensity of the average kinetic energy of the molecules.Heat and temperature are not the same thing. A Kilocalorie or large C equals 1,000 small calories.It takes 1,000 calories to raise 1,000g. of water 1 degree C. Nutritional Packaging has the calorie measurements in Kilocalories. One gram of Protein = 3 calories. This means 3,000 small calories or 3 Kilocalories.

This high specific heat allows water to act as a heat sink. Water will retain its temperature after absorbing large amounts of heat, and retain its temperature after losing equally large amounts of heat. The reason for this is that Hydrogen bonds must absorb heat to break. They must release heat when they form.The Ocean acts as a tremendous heat sink to moderate the earth's temperature. 

6. High Heat of Vaporization: Water must absorb a certain amount of additional heat to change from a liquid into a gas. This extra heat is called heat of vaporization. In humans, this value is 576 cal/g. This results in evaporative cooling of the surface. Alcohol has a value of 237cal/g. and chloroform 59cal/g.

As one can see water removes much more heat from a surface upon evaporation than does either alcohol or chloroform.

7. Freezing and Expansion of Water: Water is most dense at 4 degrees C. At ) degrees C. it is 10% less dense. Ice floats because maximum Hydrogen bonding occurs at 0 degrees C.

 


 

8. Versatile Solvent: Water is a major solvent in nature. When water and another substance is mixed the resulting solution is called an aqueous solution. Any solution contains the following parts:

Solute (what's being dissolved) + Solvent (what is doing the dissolving) = Solution.

Solute Concentration: The concentration of the dissolved materials in relation to the solvent. This is always measured in moles. A mole is the amount of a substance whose mass is in grams is

numerically equivalent to the molecular weight in daltons. One must first find the atomic weights of the substance involved and add them together for the representative molecule and change the value to grams.

Molarity occurs when the mole (gram atomic weight of the substance) is placed in a container and dissolved in one Liter of water.

 

pH: Refers to the dissociation of water molecules.

 


The pH constant is Kw = 1.0 x 10-14 (mol/L)2

 

This constant shows that water dissociates at the rate of 1 molecule for every 554 million.

We have an even split of H+ and OH- ions.

If 1.0 x 10-14 = H+ and OH- Then the conc. of the H ion is 1 x 10-7 and the conc of the OH ion is also 1x10-7

The true definition of pH is the negative log of the hydrogen ion concentration.

 

pH Scale:

 


 

Problems Determining pH

pH is the negative log of the H+ concentration of a solution.
pH = - log [ H+].

pH constant Kw = 1.0 x 10-14 (mol/L)2

Water splits into equal H+ and OH- ions in solution.

pH = - log [ H+]
pH = - log [ 1x 10 -7]
pH = [ log 1 + log 10 -7 ]
pH = [ 0.0 + (-7) ]
pH = 7 Neutral

1. The H+ ion concentration of of a solution is 1x 10 -10 mol/L, what is the pH of the solution?

pH = - log [ H+]
pH = - log [ 1x 10 -10 ]
pH = [ log 1 + log 10 -10 ]
pH = [ 0.0 + (-10) ]
pH = 10 Basic

2. The OH- ion concentration is 1 x 10-2 mol/L, what is the pH of the solution? Since you know the OH- ion concentration , subtract it from 14 to get the H+ ion concentration.

pH = - log [ H+]
pH = - log [ 1x 10 -12]
pH = [ log 1 + log 10 -12 ]
pH = [ 0.0 + (-12) ]
pH = 12 Basic

3. The concentration of [OH-] = 4.0 x 10 -11 mol/L , what is the pH of the solution?

Calculate for H+ by using Kw = [OH-] x [H+]

[H+] = Kw / [OH-]

1x 10 -14 / 4.0 x 1x 10 -11 = 0.25 x 1x 10 -3

[H+] = 0.25 x 1x 10 -3 = 2.5 x 1x 10 -4

pH = - log [ H+]
pH = - log [2.5 x 1x 10 -4]
pH = [ log 2.5 + log 10 -4 ]
pH = - (0.4 ) + - (-4)
pH = 3.60 Acid

pH represents a 10 fold difference in the concentrations of each ion. A pH of 1 is 10x smaller than a pH of 2 and 100x smaller than a pH of 3, etc.

 

Buffers: Abrupt changes in pH is harmful to the cell and any living organism. In order to minimize this harm cells contain buffering systems. In order to change the pH of a solution H ions must be added or taken from it. Buffers do just that.