1. Predict the products of a Decomposition reaction.
2. Balance a chemical equation.
3. Identify the seven diatomic elements.
This type of chemical reaction was also discussed in the 01/14/2014 blog post about the "Decomposition of Sodium Bicarbonate".
Predicting Products of chemical reactions uses the skill of Formula Writing, knowledge of the Diatomic Elements, and the skill of Balancing Equations to produce a final reaction.
In a Decomposition Reaction a single compound is broken down to two or more chemicals.
For an introductory chemistry student, I am focusing on the reaction of one compound to produce two elements.
However, an easy reaction to perform in the classroom is the decomposition of hydrogen peroxide (H2O2) using yeast as a catalyst to produce oxygen gas.
2 H2O2 (aq) à 2 H2O (l) + O2 (g)
Other catalysts can be used to decompose the hydrogen peroxide, but yeast is easy to obtain at the local grocery store.
A wooden splint that has been lit then blown out produces a glowing splint – it will relight when place into a container with pure oxygen gas. A test tube held over the reaction container (another test tube concentrates the gas) can collect the gas easily.
This could be performed by students (wearing goggles of course!) and is a very exciting lab experience. A student will feel like a magician! And the glowing splint can be relit several times in the container of oxygen.
Here is a brief video showing the decomposition of hydrogen peroxide with potassium iodide – with soap to produce a foam:
Here is another video showing how to prepare this demonstration:
Here’s a video performing this reaction with yeast:
This reaction produces oxygen gas in an elemental state where the element is more stable bonded to itself, producing a diatomic molecule. There are seven elements found in a diatomic state – they are bromine, iodine, nitrogen, chlorine, hydrogen, oxygen, and fluorine. An easy pneumonic to remember this is “BrINClHOF” (pronounced “brin-cl-hoff).
Chemistry is the study of Matter and Energy and the changes they undergo. The Laws of Thermodynamics provide the parameters in which reactions will occur. The 1st Law of Thermodynamics states that matter and energy can neither be created nor destroyed but can only change forms.
To follow this 1st law of Thermodynamics, the number of atoms reacting (on the left side of the arrow of a reaction) must be the same as the number of atoms produced (on the right side of the arrow of a reaction).
In this reaction H2O2 (aq) à H2O (l) + O2 (g), there is 1 molecule of H2O2 composed of 2 atoms of hydrogen and 2 atoms of oxygen decomposing to produce 2 atoms of hydrogen and 1 atom of oxygen (in the H2O molecule) and 2 atoms of oxygen in a diatomic molecule. This reaction does not follow this 1st Law of Thermodynamics because it is not balanced.
To balance a chemical reaction, a chemist can only place numbers in front of a correctly written symbol or formula. Subscripts are not added to balance a reaction.
These numbers in front of a formula are called “coefficients” and The number “1” is never written out as a coefficient. If the formula is written without a coefficient there is one unit of that formula.
The final balanced reaction is:
2 H2O2 (aq) à 2 H2O (l) + O2 (g)
Here is a worksheet on Decomposition reactions w/ answers:
http://www.sciencegeek.net/Chemistry/chempdfs/EquationsWorksheet3.pdf You may have to aid your students with some chemical formulas.
I will post 3 more short posts through the end of the month of November to address the other types of reactions. I hope to have this done before Thanksgiving (11/27)!
Check out the Topic List to help you to find past Blog entries. I hope it helps!
The holidays approach, so buy my lab book "Chemistry on a Budget" (or ask for it as a gift) -- it is available for $20 at amazon.com or $23 at lulu.com. It will take a few weeks to get to you, so order now!
This book contains 13 labs using consumable materials purchased from local stores. There are two versions of each lab, one with a ten-question conclusion and one with directions for a full lab report. This way the teacher has the option! Each lab is two pages to allow for one two-sided handout.
Have a good week!