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Demonstration Experiment on Video

Reaction of Carboxylic Acids with Magnesium

Objective: Relationship between Reactivity and Acid Strength

Peter Keusch






German version



Chemicals:
formic acid 98 - 100 %
acetic acid 100 %
propionic acid
magnesium ribbon

Glass wares:
3 petri dishes d = 7 cm
Pasteur pipette with rubber bulb


Hazards and safety precautions:

gassing Formic acid: Corrosive, causes severe burns. Harmful by inhalation, ingestion and through skin absorption. Readily absorbed through skin. Very destructive of mucous membranes and upper respiratory tract, eyes and skin. Inhalation may be fatal.
Acetic acid is strongly corrosive and causes serious burns. Lachrymator.

Safety glasses and nitrile gloves required. The experiment should be performed in a well ventilated room!


Experimental procedure:

Three petri dishes filled with 15 mL of a carboxylic acid are positioned on the glass surface of an overhead projector. The first dish contains formic acid, the second acetic acid and the third propionic acid. Using forceps a magnesium ribbon is placed in each of the three dishes. Afterwards the acids are diluted with distilled water using a Pasteur pipette.


Result:

In all three petri dishes a gassing occurs on the magnesium surface. The intensity of the gas evolution decreases in the order formic acid > acetic acid > propionic acid. Finally, a complete dissolution of magnesium occurs.
Already before the addition of water formic acid reacts with magnesium.




Video clip (Download RealPlayer .rm file)









Discussion:

pH Ka pKa Reaction with Mg
Formic acid   1.89     1.77 × 10 -4     3.75     excessive gas evolution  
Acetic acid 2.38 1.8 × 10 -5 4.74 moderate gassing
Propionic acid> 2.44 1.3 × 10 -5 4.87 slight bubbling


The three carboxylic acids react with magnesium to liberate hydrogen gas. The intensity of the reaction depends on the acid strength. The stronger the acid, the more hydrogen ions will be released into water. The hydrogen ions in aqueous solution attach to water molecules to form a hydroxonium ion (also known as a hydronium ion or an oxoniumion) H3O+. Two hydronium ions interact with two electrons at the metal-solution interface to produce a hydrogen molecule, which escapes the solution as hydrogen gas.



Magnitude of Ka and acid strength

In the general case

the equilibrium constant K can be written as follows


Because water is present in excess and its concentration remains relatively constant the [H2O] term is incorporated into the equilibrium constant, giving the acid dissociation constant Ka.


The larger the magnitude of Ka, the more the equilibrium is shifted to the product side, and the greater the acid strength of HA.

Ka is a better measure of the strength of an acid than pH because adding more water to the acid solution will not change the value of the equilibrium constant Ka, but it will change the H3O+ ion concentration on which pH depends.

Because Ka values range over many powers of 10, a logarithmic scale is used where

pKa  =  - log Ka

For acetic acid at 25°C:

pKa  =  - log Ka  =  - log ( 1.8 × 10 -5 )  =  4.74


The smaller the value of pKa, the stronger the acid.

Carboxylic acids are weak acids. But they are stronger than other protic substances like alcohols. The acidity of the carboxylic acids is attributed to

·   the electron-withdrawing inductive effect of the adjacent carbonyl group on the O - H bond. The presence of the polar carbonyl group in the carboxylic acid enhances the polarization of electrons leading to more electropositive hydrogen.


·   resonance stabilization of carboxylate ion. X-ray diffraction studies indicate the two carbon - oxygen bonds in the carboxylate ion to be equal in length (bond length between single and double bond) suggesting delocalisation of charge. The negative charge is shared between the two oxygen atoms. The stabilization is shown below and probably best represented by the 'composite' resonance structure.


The experiment shows that the acid strength decreases in the order  formic acid  <  acetic acid   <  propionic acid. The acid strength of a carboxylic acid depends on the nature of the moiety attached to the carboxyl. Strong electron-withdrawing groups increase the acid strength, whereas electron-donor moieties decrease it. Formic acid is ten times stronger an acid than acetic acid, confirming the electron donating character of an alkyl group relative to hydrogen. Due to the electron donating inductive ability of the moieties attached to the carboxyl group, the electron density on the oxygen of the hydroxyl group of carboxylic functional group increases in the series  formic acid  <  acetic acid   <  propionic acid.  The increase in the electron density on the oxygen of OH group leads to a decrease in the polarity of the O-H bond making its heterolytic cleavage difficult. Less hydrogen ions are released into the aqueous solution to produce hydronium ions.

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