What is Stoichiometry?

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Stoichiometry is a branch of chemistry that deals with the quantitative relationships that exist between the reactants and the products in chemical reactions. Reactants are the substances which participate in the chemical reaction, and products are the substances that are obtained as a result of the chemical reaction. Stoichiometry relies on the fact that elements behave in predictable ways, and that matter cannot be created or destroyed. Therefore, when elements are combined resulting in a chemical reaction, something known and specific will happen and the outcome of the reaction can be predicted on the basis of the elements and quantities involved. Stoichiometry is the mathematics behind the science of chemistry. Stoichiometry calculations can find how elements and components which are diluted in a solution of known concentration, react in experimental conditions. The word “Stoichiometry” is derived from the Greek word stoicheion, meaning “element” and metron meaning “measure”.


What is the law governing Stoichiometerty?

Stoichiometry rests upon the laws such as law of definite proportion and law of multiple proportions and law of conservation of mass.

  • Law of conservation of mass: Using the physical laws such as law of conservation of mass, which states that the mass of the reactants equals the mass of the products, Stoichiometry is used to gather information about the amounts of various elements used in a chemical reaction, and whether they took the form of gases, solids or liquids.
  • Law of definite proportion: It states that a chemical compound (substance consisting of 2 or more elements) always contains exactly the same proportion of elements (substance with one type of atom) by mass.
  • Law of multiple proportions: It is one of the basic laws of stoichiometry, along side the law of definite proportions. It is sometimes called as Dalton’s law. It says that, if 2 elements form more than 1 compound between them, then the ratio of the masses of the second element which combine with the fixed mass of the first element will both ratios of small whole number.


How can we explain Stoichiometry?

Based on the above laws, chemical reactions combine in definite ratios of chemicals. The amount of each element must be same throughout the reaction. In a balanced chemical reaction, the relations among the quantities of reactants and products typically form a ratio of whole numbers. For instance, in a reaction that forms ammonia (NH3), exactly 1 molecule of nitrogen (N2) reacts with 3 molecules of hydrogen (H2) to produce 2 molecules of NH3. This can be illustrated as below

  • N2 + 3H2   ------>  2NH3

So, Stoichiometry can be used to calculate quantities such as the amount of products that can be produced with given reactants and percentage of the given reactant that is made into the product.

What are the types of Stoichiometry?

  • Reaction stoichiometry: Stoichiometry is often used to balance chemical equations which can be found in reaction stoichiometry. It describes the quantitative relationships among the substances as they participate in chemical reactions. In the above example, of nitrogen and hydrogen reacting to form ammonia, reaction stoichiometry describes the 1:3:2 ratios of molecules of nitrogen, hydrogen and ammonia.
  • Composition stoichiometry: This describes the quantitative (mass) relationships among elements in compounds. For instance, composition stoichiometry describes the nitrogen to hydrogen (mass) relationship in the compound ammonia. i.e. 1 mole of nitrogen and 3 moles of hydrogen are in every mole of ammonia. Mole is the unit used in chemistry for the amount of a substance.
  • Gas Stoichiometry: This type of stoichiometry deals with the reactions involving gases, where the gases are at a known temperature, pressure and volume and can be assumed to be ideal gases. For gases, the volume ratio is ideally same by the ideal gas law, but the mass ratio of a single reaction has to be calculated from the molecular mass of the reactants and products, where molecular mass is the mass of 1 molecule of a substance. An ideal gas is a theoretical gas composed of a set of randomly-moving, non-interacting particles which obeys the ideal gas law. Ideal gas law is the equation of state of ideal gas. The equation of ideal gas law is PV = nRT, where P is the pressure, V is the volume and T is the absolute temperature, n is the moles of gas and R is the universal gas constant.


What is a stoichiometric ratio?

A stoichiometric amount or ratio of a reagent (substance added to a system in order to create a chemical reaction) is the amount or ratio where, assuming that the reaction proceeds to completion in the following basis:

  • All reagents are consumed
  • There is no deficit of reagent
  • No residue remains

Reactions will only occur in stoichiometric ratios


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