What is Stoichiometry?
In simple words, Stoichiometry is defined as the calculation of products and reactants in a given chemical reaction. It is principally concerned with numbers.
The word “ stoichiometry” originated from the Greek word “stoikhein” which means element and the word “metron” means to measure.
This term Stoichiometry was first proposed by a well-known German chemist Jeremias Richter.
Stoichiometry is a significant concept in chemistry that helps us balanced calculate the quantities of reactants and products in a given chemical equation.
For instance, oxygen and hydrogen react to form a substance called water in which one mole of oxygen reacts with two moles of hydrogen and hence forms two moles of water.
In addition to this, stoichiometry can also be used to find quantities such as the amount of products that can be formed with a given quantity of reactants and percent yield.
Importance of Stoichiometry
Stoichiometry helps us govern how much substance is needed or is present in the given compound.
Things that can be measured using stoichiometry are;
1. Reactants’ mass and Products mass
2. The molecular weight of a given compound
3. Chemical equations
4. Formulas of different compounds or elements
Stoichiometric Coefficient or Stoichiometric Number
Stoichiometric coefficient or stoichiometric number is defined as the number of molecules that participate in the given chemical reaction. A balanced equation has an equal number of elements on both sides. The stoichiometric coefficient is mostly the number that is present before atoms, molecules, or ions.
Stoichiometric coefficients can be in the form of fractions as well as whole numbers. In addition to that, the stoichiometric coefficients help to establish the mole ratio between reactants and the products of a given chemical equation.
Stoichiometry in Chemical Analysis
Stoichiometric calculations follow a numerical analysis methodology that is frequently used by pharmacists to determine the concentrations of materials present in a given sample.
There are chiefly two main types of analysis given below;
1. Gravimetric Analysis
Gravimetric analysis describes the numerical determination of the analyte based on the mass of the given solid compound.
The gravimetric analysis gives the most precise results as compared to other analytical analyses Gravimetric analysis can be categorized into the following types mentioned below;
1. Precipitation gravimetry involves isolation of ions in a given solution by a precipitation method then filtering, washing the precipitate, and lastly weighing the precipitate and determining its mass by difference.
2. Volatilization gravimetry involves separating components of a mixture either by heating or chemically decomposing the given sample.
3. Electrogravimetry involves the electrochemical reduction of metal ions at the cathode and the immediate deposition of ions on the cathode.
This cathode is weighed before and after the electrolysis is done and the weight, difference thus parallels to the mass of analyte primarily present in the given sample.
2. Volumetric Analysis
The volumetric analysis involves the numerical measurement of a substance in terms of its volume.
In volumetric analysis, a known volume (V1) of the given substance whose concentration (N1) is known is made to react with the unknown volume (V2) of the given solution of the substance whose concentration(N2) is to be calculated. The volume, V1 is defined as the endpoint of the reaction.
Thus, the concentration N2 is calculated with the help of following equation mentioned below;
N1x V1 = N2 x V2
The endpoint of such a reaction is directed by a change in a color with the help of indicators or precipitation etc.
Stoichiometric Values in a Chemical Reaction
In some cases, it might be necessary to calculate the number of moles of a reagent or product under certain given conditions. To do this properly, the reaction needs to be balanced.
The law of conservation of matter thus states that the quantity of each element does not change in a given chemical reaction. Consequently, a chemical equation is balanced to make the quantity of each element in the equation the same.
Chemical reactions are balanced by adding coefficients in front of the given reactants and products.
Example: Balance the given equation;
N2 + O2 → NO
The given equation is not considered balanced since there are more N and O atoms on the left side of the equation as compared to the right side or product of the reaction. Allocate a stoichiometric coefficient of 1 to the given compound, NO.
N2 + O2 → 1NO
Now, balance the remaining element. To do this, we used fractional coefficients.
1/2N2 + 1/2O2 → 1NO
To get rid of the fractional coefficients multiply the reactants by 2 to this equation balanced.
N2 + O2 → 2NO
Thus, the above equation is balanced.
Limiting Reagent
In a chemical reaction, if any one of the reactants is present in a surplus amount then some of these excess reactants will, consequently, be left over when the given reaction is completed.
The given reaction will stop instantly as soon as one of the reactants is fully consumed. The substance that is fully consumed in a given reaction is known as a limiting reagent.
Example;
N2 + 3H2 ➝ 2NH3
Assume we have one mole of N2 that reacts with one mole of H2. But from the above-mentioned balanced chemical equation, one mole of N2 requires only three moles of H2. Thus, the limiting reagent in the above reaction is H2.
Stoichiometry Citations
- “Why not stoichiometry” versus “Stoichiometry–why not?” Part II: GATES in context with redox systems. Crit Rev Anal Chem . 2015;45(3):241-69.
- Stoichiometry and physical chemistry of promiscuous aggregate-based inhibitors. J Am Chem Soc . 2008 Jul 23;130(29):9606-12.
- Determining the stoichiometry and interactions of macromolecular assemblies from mass spectrometry. Nat Protoc . 2007;2(3):715-26.
- Defining the stoichiometry and cargo load of viral and bacterial nanoparticles by Orbitrap mass spectrometry. J Am Chem Soc . 2014 May 21;136(20):7295-9.
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