Structure and Bonding:  The Water Molecule

How do the melting points and boiling points of H2S, H2Se, and H2Te vary with molar mass?

Using the data in the table below, draw a graph of melting point against molar mass and boiling point against molar mass.  Develop a linear regression model using Excel or the graphing calculator for the behavior of both melting and boiling points and then predict the melting point and boiling point of water from these models.  Calculate the liquid range by subtracting: boiling point - melting point.

Compound Molar Mass Melting Point Boiling Point Liquid Range
H2O 18 g/mole      
H2S 34 g/mole -85.5oC -59.55oC  
H2Se 81 g/mole -65.73oC -41.25oC  
H2Te 130 g/mole -49oC -2oC  

The actual melting and boiling points for water are 0o and 100oC; hence, giving a liquid range of 100oC.  How do your predicted melting and boiling points compare to the actual measurements?

Why are the actual melting point and boiling point of water so different?

The structure of the water molecule, H2O, is given below.  If you place the cursor on the molecule and click you can move it around.    Describe the shape of the molecule.

Right click, select Display from the menu, and change it to space fill and van der Waals radius.  This will give you a more realistic picture of the water molecule.

The next box shows a cluster of water molecules in the liquid state.  View the cluster and which type of nearest neighbors do you find- H to H, H to O or O to O?  Why?

Water is a polar molecule, with the oxygen (red) being the negative area and the hydrogen (white) being the more positive area.   In ice, solid water, the arrangement is very highly organized (see below).

The molecules of water themselves are held together by covalent bonds between the hydrogen and oxygen.  While in ice or even liquid water, the molecules are bound to one another by hydrogen bonding, a strong intermolecular force between hydrogen and highly electronegative elements such as fluorine, oxygen, or nitrogen.

Looking at the ice structure, measure the distance in a O-H covalent bond and in the O.....H of the hydrogen bond (see the Chime Guide for help).  The shorter a bond, the stronger the force of attraction.  How would you characterize a H-bond compared to a covalent bond?  

Hydrogen bonding is very important as an attractive force especially in proteins and nucleotides.  The hydrogen bond, a strong intermolecular force, is about 10% the strength of a covalent bond.  Water also hydrates ions and other molecules due to its polar nature.

Here is the chloride ion, Cl-, a common ion in blood and other cellular fluids.

How would you describe the orientation of the water molecules surrounding, hydrating, the chloride ion?

Would the arrangement of the water molecules be the same around a potassium ion, K+?  Explain why or why not.

The water molecules orient their positive ends, the hydrogens, toward the negative chloride ion, while they would orient their negative ends, the oxygen, toward the positive potassium ion.  The ions are hydrated or surrounded by an organized layer of water molecules.  The polar nature gives water the amazing power to dissolve most ionic salts, such as sodium chloride or potassium iodide.  The polar nature of the water molecule can be seen in the electrostatic potential image below.  Red is negative, while blue is positive.

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