January 16th - An Intro to Stoichiometry

Today, we started our new unit! Of course, that unit is stoichiometry!

What is stoichiometry, you ask? Well, stoichiometry is a branch of chemistry that deals with the quantitative analysis of chemical reactions. It is a generalization of mole conversions to chemical reactions. Simply put, this means that it is an analysis of the ratio of moles found in a chemical reaction.

Just as Mr. Doktor said, understanding the six types of chemical reactions is key to understanding stoichiometry.

The key to stoichiometry.

The six types of reactions are synthesis, decomposition, single replacement, double replacement, neutralization, and combustion.

First, if you need practice balancing equations, now is the time:

Moving on:

• In a synthesis reaction, A + B à AB. It is usually found in the shape of element + element à compound.

Ex. 1) Write the formula equation for the following reaction: Barium oxide (BaO) reacts with water to form barium hydroxide.

BaO + HOH à Ba(OH)₂

We know that barium has a change of 2+ and hydroxide has a charge of 1-. This forms Ba(OH)_2.

Oh look, it’s already balanced. How convenient!

• In a decomposition reaction, AB à A + B. It is the opposite of synthesis. In this course, we can always assume that compounds decompose into elements during decomposition.

Ex. 2) Manganese(II) iodide decomposes to form manganese and iodine.

MnI₂ à Mn + I₂

Manganese(II) has a change of 2+ (remember that it is multivalent), while iodine has a charge of 1-. Remember, though, it is diatomic.

Again, it seems that fate is on our side. It’s already balanced!

• In a single replacement reaction, A + BC à B + AC. The stand-alone element switches with the other cation/anion in the reaction.

Ex. 3) Write the formula equation for the following reaction: Zinc reacts with copper(II) nitrate (Cu(NO₃)₂) to form zinc nitrate and copper.

Zn + Cu(NO₃)₂ à Zn(NO₃)₂ + Cu

As you can see, we simply switch the copper out for the zinc. Remember to check the charge of zinc, as it could change the subscript after the nitrate ion.

Oh, and it’s already balanced. How about that? >_>

• In a double replacement reaction, AB + CD à AD + BC. Like single replacement reaction, the elements switch compounds. However, in this case, there are two switches. There are no stand-alone elements.

Ex. 4) Write a balanced equation for the reaction in which sulphuric acid (H₂SO₄) reacts with potassium hydroxide (KOH) to produce potassium sulphate (K₂SO₄) and water.  

H₂SO₄ + KOH à K₂SO₄ + HOH

This is a peculiar reaction, as one of the products is water. Nonetheless, we can still switch the potassium atom and the hydrogen atoms. We can balance the final equation like so:

H₂SO₄ + 2KOH à K₂SO₄ + 2HOH

• In a neutralization reaction, acid + a base à salt + water. It is a special case of double replacement. Remember, acids have hydrogen on the left side of the chemical equation (water is an exception). Bases have hydroxide on the right side of the chemical equation. Salts are simply ionic compounds.

Ex. 5) Write the formula for a reaction between hydrochloric acid and sodium hydroxide, which react to form sodium chloride and water.

HCL + NaOH à NaCl + HOH

Luckily, this equation is a pinch. There’s nothing to balance! Also, you may notice that through neutralization, we have just performed double replacement. Still, we produced a salt and water from an acid and a base. Superb!

Here's a picture of a salt for no reason!

• In a combustion reaction, a hydrocarbon + oxygen à carbon dioxide + water + energy. A hydrocarbon is a compound containing only hydrogen and carbon. Oxygen is found in its diatomic form in this reaction. Combustion reactions also release energy.

Ex. 6) Write the chemical equation for a reaction between methane (CH₄) and oxygen in a combustion reaction.

CH₄ + 2O₂ à CO₂ + HOH + energy

I’ve gone ahead and balanced everything for you. Why, you ask? Well, because you know the formula of the hydrocarbon, everything else is done for you!

Fun, eh? Need more practice? Here’s an introduction to stoichiometry. It even includes some stuff that will be covered in the next lesson:

Posted by Michael.