Structure and Bonding:  The Acetic Acid Molecule

Let's examine the structure and some bonds in acetic acid and the acid anion produced on dissociation, the acetate ion.  How do the geometries of the two carbon atoms in acetic acid differ?

CH3COOH  =  CH3COO-   +    H+

acetic acid                                        acetate ion

    

Right click on each image and go to select, then to mouse click action, and click on distance.  This will allow you to determine the bond distance or length by clicking on the two atoms that form the bond.  Watch the lower left corner of your browser screen for the results.  Record the bond lengths.  This distance x 100 will give you picometers, pm.

C=O    ________ pm                        C-O  _________ pm

C-O    ________ pm                        C-O  _________ pm

O-C-O bond angle _________        O-C-O bond angle _________

In acetic acid, do the two difference bond lengths make sense?  Explain why or why not.

In the acetate ion, what do the bond lengths tell you about the bonds?

Right click on each image and go to select, then to mouse click action, and click on angle. Measure the O-C-O bond angle in acetic acid and acetate ion.   What can you conclude from the measures?

Acetic acid is a weak acid in water; however, it is a non-electrolyte in non-polar solvents such as benzene.  Which form will be present in benzene?   acetic acid or acetate ion

In benzene, the acetic acid exists in the form shown in the image below.  This was discovered by freezing point measurements (i = 0.5) as non-electrolytes have an i = 1.  How is the acetic acid dimer held together?

How long are the hydrogen bonds in the acetic acid dimer?  Illustrate how you measured them.

For a review of hydrogen bonding in water- click here to go to the "Water Molecule" activity.

Look at the image of the acetate ion below where the electrostatic potential is displayed.  The electrostatic potential surface is the distribution of charge on the molecule, where red is negative and blue is positive.  See the Student Guide for Using Chime for more explanation of the electrostatic potential surface- click here.

How would you describe the distribution of charge on the ion?

On dissociation of the hydrogen ion from acetic acid, the acetate ion forms a resonance structure, where the two carbon-oxygen bonds are of equal length.  The negative charge is spread over the end of the molecule equally too.  The two bonds are shorter than a single bond but longer than double bond, plus the O-C-O angle is a little greater than 120o due to slightly greater repulsion.

For the dissociation of acetic acid, address the following questions:

CH3COOH  =  CH3COO-   +    H+

       Which form is present at very high pH?

        Which form is present at very low pH?

        At what pH does (CH3COOH) = (CH3COO-)?

        If the Ka of acetic acid was to increase, how would that influence the dissociation?  

Click here to get an interactive Excel spreadsheet to see how the Ka influences the distribution of species for a monoprotic acid, HA and A-.  Complete the worksheet using this spreadsheet - click here to get the pdf file.

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