Chromic Acid: Formula, Structure & Use

Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

There are many acids, and they all contain hydrogen. In this lesson, we will work with chromic acid, which is hydrogen bonded with chromate. We will learn chromic acid's structure and uses for it.

Art and Science

Music includes three arts meshed into one. The most obvious artistic aspect of music is the different sounds and melodies. The second is the performance facet of music. People attend live performances to watch the musicians perform. The third is the visual art of the instruments. Guitars and violins are made of various woods, pianos have the black and white keys, and the brass instruments have a brilliant shine or artistic patina. This is where chromic acid comes in. It used to be used to give brass instruments a patina, which is a decorative tarnish. Let's leave the arts and focus on the science of chromic acid.

Brass knocker with natural patina. Chromic acid can help make brass have a nice patina, or shine.
knocker

Formula

Whenever you meet someone for the first time, you want to know their name. Today, you are meeting chromic acid, and since you already know its name, let's learn its formula. The ''chromic'' in chromic acid comes from the polyatomic ion chromate. A polyatomic ion is a molecule with an electric charge. Chromate has a charge of negative two with the formula CrO4 -2. A hydrogen atom loses its one electron to become a hydrogen ion H+1.

Since chromate has a negative two charge, and hydrogen has a positive one charge, we need two hydrogen ions to bond with chromate. This results in an electrically neutral compound with the formula H2 CrO4 (aq). The (aq) indicates that it is an acid in aqueous form, which means it is dissolved in water. Now we can work on determining its structure.

Structure

We start the structure with a chromium atom in the center, and attach four oxygen atoms to it with single bonds. A single bond is two shared electrons. Diagram 1 shows the first step in determining the structure of chromic acid.


Diagram 1
cro4


We have two hydrogen atoms to add to the structure. We will bond them to two oxygen atoms that are opposite each other. Diagram 2 shows this stage of the structure.


Diagram 2
chromic_acid


Now we have to determine how many valence (outer shell) electrons there are in the molecule. Hydrogen has one valence electron, each oxygen has six valence electrons, and chromium has six as well. Let's calculate the total number of valence electrons there are in this molecule.

Element Quantity of atoms Quantity of Valence Electrons Total Valence Electrons
H 2 1 2
O 4 6 24
Cr 1 6 6

Total valence electrons = 2 + 24 + 6 = 32 valence electrons

Each line represented as a bond in Diagram 2 represents two electrons. This gives us a total of 12, and we need 32. The oxygen atoms want eight valence electrons so we add the appropriate number of dots (electrons) to all of the oxygen atoms so they all have eight electrons. Hydrogen atoms only want two electrons. Diagram 3 shows the molecule with 32 valence electrons.


Diagram 3
s3


The final step is to check that the molecule is electrically neutral and that each charged ion has the lowest charge possible. Diagram 4 shows that each oxygen atom without the bonded hydrogen atom has a -1 charge, and the chromium has a +2 charge.


Diagram 4
D4


Those two oxygen atoms with the -1 charge can change their charge to zero by sharing one of their lone pairs of electrons with the chromium atom. This will reduce chromium's charge to zero as well. The final structure is shown in Diagram 5 with two double bonds (four shared electrons).


Diagram 5, the structure of chromic acid
final


Uses

As an aqueous solution, a hydrogen ion attached to the chromate ion gets pulled off by a water molecule. The water molecule bonds with the free hydrogen ion forming a hydronium ion, H3 O+1. This is what makes chromic acid a strong acid. There are a few other uses for chromic acid. Let's see what they are.

Wood is a common building material. The problem using wood for this purpose is that it rots over time. Chromic acid is used to treat wood so that it can withstand exposure to the elements slowing its deterioration.

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