Stoichiometry: Limiting Reagent & Balanced Reactions

Stoichiometry is termed to the quantitative study of reactants and products that are involved in a chemical reaction. The word “stoichiometry” has been derived from the Greek word “stoicheion” which means element, while “metron” means measure.

• Stoichiometry is the evaluation of products and reactants taking part in any chemical reaction.
• Stoichiometry deals with the calculation of millions (sometimes volumes also) of the reactants and the products involved in a chemical reaction.
• Stoichiometric calculations involve various processes that include balancing equations, evaluation of moles of substances produced in the reaction, conversion of units to intelligence, and vice versa with the help of various conversion factors. 
• This term was first coined by a German chemist, Jeremias Richter.

Read Also: Rate of a Chemical Reaction

Key Terms: Stoichiometry, Gravimetric Analysis, Volumetric Analysis, Moles, Mole Concept, Reactants, Avogadro’s Number, Molar Mass, Stoichiometric Coefficient

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Stoichiometry Meaning

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The word stoichiometry is derived from two Greek words, “stoicheion” (meaning element) and “metron” (meaning measure). Stoichiometry can be further divided into two categories:

• Gravimetric Analysis
• Volumetric Analysis 

Stoichiometry can be defined as:

Stoichiometry can be used for balanced chemical equations in order to evaluate the amounts of reactants and products. Herein, the ratios from the balanced equations are used. Typically, every reaction which takes place depends upon one factor, i.e. how much of a substance is present.

Stoichiometric Coefficient

The stoichiometric coefficient or stoichiometric number is the number of molecules which participate in a reaction. In a balanced reaction, there is an equal number of elements on each side of the equation.

• The stoichiometric coefficient can be defined as the number in front of atoms, molecules or ions.
• Stoichiometric coefficients can be fractions or whole numbers.
• The coefficients help to determine the mole ratio between reactants and products.

In case of stoichiometric ratio, it can be defined as the exact ratio that is between air and flammable gas or vapor at which complete combustion occurs.

Stoichiometry Calculations

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Solution Stoichiometry and Chemical Analysis

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Stoichiometry can be classified into two types:

• Gravimetric Analysis
• Volumetric Analysis

Gravimetric Analysis

In analytical chemistry, gravimetric analysis helps to demonstrate the quantitative determination of an analyte on basis of the mass of the solid. The gravimetric analysis offers most accurate results out of every analytical analysis. Gravimetric analysis can be further classified into the following types:

• Precipitation gravimetry: This method involves the isolation of ions in solution by means of a precipitation reaction, called filtering, which helps to wash the precipitate from contaminants. Thus, it helps to evaluate and weigh the precipitate to determine its mass by difference.
• Volatilization gravimetry: Volatilization gravimetry is a process by which we can separate mixture components by heating or chemically decomposing the sample.
• Electrogravimetry: This process involves the electrochemical reduction of metal ions at the cathode, along with the deposition of ions at the cathode. The cathode is then weighed before and after the process of electrolysis. The difference can be seen to correspond to the mass of the analyte present initially in the sample.

Volumetric analysis 

Volumetric analysis is a quantitative analytical process which is used to measure the volume of a solution whose concentration is known.

Principle: In volumetric analysis, a substance’s known volume (V₁) with concentration (N₁) reacts with the unknown volume (V₂) of the solution of a substance with concentration(N₂) is to be estimated. The volume, V₁ can be found at the end of the reaction. The concentration N₂ can be evaluated by using the following equation.

The end of the reaction is denoted by a change in colour, precipitation or more.

The terms that are involved in volumetric analysis include:

• Titration: It is a method of determining the volume of solution which is required to completely react with the volume of another solution.
• Titrant: The solution which has a known strength is known as titrant.
• Titrate: The solution which has a concentration that needs to be estimated.
• Indicator: Indicators can be defined as reagents that change colour when the reaction is done.

Stoichiometry Infograph

Read More: What is Partial Pressure?

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Stoichiometry Calculations

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The factors which impact Stoichiometry calculation is:

• Formula mass: It's defined as the sum of the atomic weights of each grain present in the patch of the substance. 
• Avogadro number: It's defined as several titles present in exactly 12g of C-12. Avogadro's number is valued as 6.023 x 10²³ 
• Molar Mass: It's defined as the sum of the total mass of all the titles that make up a patch per moles. 

Steps of Stoichiometry Calculations

The steps required for Stoichiometry Calculations are:

• Balancing the equation 
• Converting substance unit to moles
• Using moles rate to calculate moles of yielded substances in the reaction 
• Converting moles of demanded rudiments to the demanded units. 

Thus, the same can be shown by stoichiometry examples:

Step 1

Working the unstable equation to balance the equation:

The substances of a chemical equation are not destroyed or lost; the yield of a reaction must exactly correspond to the original reagents. It is not only for the reactants achieved, but also the number. Given our unstable equation:

C₃H₈ (g) O₂ (g) → CO₂ (g) H₂O (l) (unstable equation) 

Since the equation is unstable, first we have to balance it. The number of carbons on the left side is 3 so, CO₂ should also be 3 to balance the equation. It is also to balance the oxygen and hydrogen in the chemical equation. 

After balancing, 

C₃H₈ (g) 5O₂(g) → 3CO₂ (g) 4H₂O (l) 

Step 2

Converting substance unit to moles:

Below are the conversion factors that are used between moles and volumes of substances.

C₃H₈ (g) 5O₂ (g) → 3CO₂ (g) 4H₂O (l) 

One mole of CH₄ (g) reacts with two moles of O₂ (g) to give one mole of CO₂ (g) and two moles of H₂O (g) 

16 g of CH₄ (g) reacts with 2 × 32 g of O₂ (g) to give 44 g of CO₂ (g) and 2 × 18 g of H₂O (g). 

Step 3

Using moles rate to calculate moles of yielded substances in the reaction:

 C₃H₈ (g) 5O₂ (g) → 3CO₂ (g) 4H₂O (l) 

 Also, 1 mole of propane reacts with 5 molecules of oxygen to give 3 moles of CO₂ and 4 moles of 4 H₂O. So we can say that the reactants react at the rate of 1/5. 

Step 4

Converting moles of demanded rudiments to the demanded units. The last step involves the conversion of moles to original units asked in a particular problem. 

Steps of Stoichiometry Calculation

Read More: Relation Between Normality and Molarity

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Limiting Reagent

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For a chemical reaction, it can be a possibility that one of the reactants is in excess amount. Some of these excess reactants can be further left over following the completion of the reaction. It is because the reaction immediately stops as soon as one of the reactants has been fully consumed.

The substance which is totally consumed in a reaction can be termed a limiting reagent.

The same can be shown via an example of a chemical reaction to demonstrate limiting reagent concept:

Assume that we have one mole of N₂ which is reacted with one mole of H₂. From a balanced chemical equation, one mole of N₂ needs three moles of H₂.

Thus, it can be said that the limiting reagent for this specific reaction is H₂.

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Balanced Reactions

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Atoms and molecules are negligible and very small in size. The numbers in a significantly small amount of a substance are very large. Hence, in order to show the atoms and molecules in bulk, a process of mole concept was brought into effect. One mole of a substance has 6.022 x 10²³ numbers of that specific substance. This is also termed Avogadro’s number.

Thus,

The molar mass of one mole of a given substance is seen to be numerically equivalent to the atomic/molecular formula mass.

For the same, a balanced chemical equation can be shown:

3Fe(s) + 4H₂O(l) ⇾ Fe₃O₄ (s)+ 4H₂ (g)

The quantitative information that can be represented from this balanced chemical equation is:

• 3 mole of Fe is seen to react with 4 moles of H₂O to give one mole of Fe₃O₄ and 4 moles of H₂.
• 168g (56 × 3) of Fe is seen to react with 72g (18 × 4) of H₂O to give 231g of Fe₃O₄ and 8g of H₂ gas.

If the reactants and products are in gaseous form, the molar volume can be considered.

Thus, one mole of any gas typically occupies 22.4 L.

Hence,

CH₄(g) + 2O₂(g)⇾ CO₂(g)+ 2H₂0 (g)

Here, 22.4 Litres of CH₄ is seen to react with 44.8 (2 x 22.4) litres of O₂ to give 22.4 Litres of CO₂ along with 44.8 litres of H₂O.

Read More: Difference between Elements and Compounds

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Things to Remember

• Stoichiometry can be defined as the quantitative study of reactants and products that are involved in a chemical reaction.
• Stoichiometry is of two types: Gravimetric Analysis and Volumetric Analysis
• The substance fully consumed in a reaction is known as a limiting reagent.
• The molar mass of one mole of a specific substance is typically numerically equal to the atomic/molecular formula mass.

Also Read: 

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Previous Year Questions

• An element, X has the following isotopic composition….[NEET 2007]
• An organic compound contains carbon….[NEET 2008]
• The number of oxygen atoms in 4.4g of CO_2​ is….[NEET 1989]
• Assuming fully decomposed, the volume of CO₂​ released….[NEET 2000]
• At STP, the density of CCl₄​ vapour….[NEET 1988]
• Boron has two stable isotopes….[NEET 1990]
• Haemoglobin contains 0.33% of iron by weight.….[NEET 1998]
• How many moles of lead (II) chloride will be….[NEET 2008]
• If avogadro number N_A​, is changed from…..[NEET 2015]