STOICHIOMETRY

Understanding Stoichiometry

Stoichiometry comes from two languages ​​ie the language meaning "element" and Metron which means "measurement".

Stoichiometry is a section of chemistry that involves using relationships between reactants and/or products in a chemical reaction to determine desired quantitative data. In Greek, stoikhein means element and metron means measure, so stoichiometry literally translated means the measure of elements. In order to use stoichiometry to run calculations about chemical reactions, it is important to first understand the relationships that exist between products and reactants and why they exist, which require understanding how to balanced reactions.

a.    Reactants to Products

A chemical equation is like a recipe for a reaction so it displays all the ingredients or terms of a chemical reaction. It includes the elements, molecules, or ions in the reactants and in the products as well as their states, and the proportion for how much of each particle is create relative to one another, through the stoichiometric coefficient. The following equation demonstrates the typical format of a chemical equation:

2Na(s)+2HCl(aq)→2NaCl(aq)+H2(g)(1)(1)2Na(s)+2HCl(aq)→2NaCl(aq)+H2(g)

In the above equation, the elements present in the reaction are represented by their chemical symbols. Based on the Law of Conservation of Mass, which states that matter is neither created nor destroyed in a chemical reaction, every chemical reaction has the same elements in its reactants and products, though the elements they are paired up with often change in a reaction. In this reaction, sodium (NaNa), hydrogen (HH), and chloride (ClCl) are the elements present in both reactants, so based on the law of conservation of mass, they are also present on the product side of the equations. Displaying each element is important when using the chemical equation to convert between elements.

b.    Stoichiometric Coefficients

In a balanced reaction, both sides of the equation have the same number of elements. The stoichiometric coefficient is the number written in front of atoms, ion and molecules in a chemical reaction to balance the number of each element on both the reactant and product sides of the equation. Though the stoichiometric coefficients can be fractions, whole numbers are frequently used and often preferred. This stoichiometric coefficients are useful since they establish the mole ratio between reactants and products. In the balanced equation:

2Na(s)+2HCl(aq)→2NaCl(aq)+H2(g)(2)(2)2Na(s)+2HCl(aq)→2NaCl(aq)+H2(g)

Balancing reactions involves finding least common multiples between numbers of elementspresent on both sides of the equation. In general, when applying coefficients, add coefficients to the molecules or unpaired elements last. 

A balanced equation ultimately has to satisfy two conditions.

1. The numbers of each element on the left and right side of the equation must be equal.

1. The charge on both sides of the equation must be equal. It is especially important to pay attention to charge when balancing redox reactions.

c.     Stoichiometry and Balanced Equations

In stoichiometry, balanced equations make it possible to compare different elements through the stoichiometric factor discussed earlier. This is the mole ratio between two factors in a chemical reaction found through the ratio of stoichiometric coefficients. Here is a real world example to show how stoichiometric factors are useful.

Types of Reactions

There are 6 basic types of reactions.

• Combustion: Combustion is the formation of CO₂ and H₂O from the reaction of a chemical and O₂

• Combination (synthesis): Combination is the addition of 2 or more simple reactants to form a complex product.

• Decomposition: Decomposition is when complex reactants are broken down into simpler products.

• Single Displacement: Single displacement is when an element from on reactant switches with an element of the other to form two new reactants.

• Double Displacement: Double displacement is when two elements from on reactants switched with two elements of the other to form two new reactants.

• Acid-Base: Acid- base reactions are when two reactants form salts and water

The Concept of Mole

a.      Molar Mass

Before applying stoichiometric factors to chemical equations, you need to understand molar mass. Molar mass is a useful chemical ratio between mass and moles. The atomic mass of each individual element as listed in the periodic table established this relationship for atoms or ions. For compounds or molecules, you have to take the sum of the atomic mass times the number of each atom in order to determine the molar mass.

Molar mass = mass: mol

Mass = mol x Mr / Ar (molar mass)

b.      Density

Density (ρρ) is calculated as mass/volume. This ratio can be useful in determining the volume of a solution, given the mass or useful in finding the mass given the volume. In the latter case, the inverse relationship would be used.

Volume x (Mass/Volume) = Mass

Mass x (Volume/Mass) = Volume

c.       Percent Mass

Percents establish a relationship as well. A percent mass states how many grams of a mixture are of a certain element or molecule. The percent X% states that of every 100 grams of a mixture, X grams are of the stated element or compound. This is useful in determining mass of a desired substance in a molecule.

d.      Molarity

Molarity (moles/L) establishes a relationship between moles and liters. Given volume and molarity, it is possible to calculate mole or use moles and molarity to calculate volume. This is useful in chemical equations and dilutions.

Empirical Formulas and Molecular Formulas

1.      Determining Empirical Formulas

An empirical formula can be determined through chemical stoichiometry by determining which elements are present in the molecule and in what ratio. The ratio of elements is determined by comparing the number of moles of each element present.

2.      Determining Molecular Formulas

To determine a molecular formula, first determine the empirical formula for the compound as shown in the section above and then determine the molecular mass experimentally. Next, divide the molecular mass by the molar mass of the empirical formula (calculated by finding the sum the total atomic masses of all the elements in the empirical formula). Multiply the subscripts of the molecular formula by this answer to get the molecular formula.

Molecular formula ≡ (empirical formula) n

Molecular formula = n × empirical formula, n ={ 1, 2, 3, ... }