Chemical Equivalents

Stoichiometric calculations are extremely important in chemistry because every chemical reaction requires stringent measures of reactants to form the desired number of products. Various calculations amount to these measurements, and each of these comes under the general topic of stoichiometry or stoichiometric coefficients. 

Chemical equivalent, in general, is defined as, “Weight in grams of a substance that displaces or combines with one gram of hydrogen.” It is important to understand the relationship between reactants and products in a reaction regarding their masses, molecules, moles, and volumes. In electrochemical terms, chemical equivalents are defined as the “Amount of substance (in grams) produced or utilised when one coulomb of electricity is passed in an electrochemical reaction.” 

Chemical equivalents are entities that make relevant for any chemical reaction, and therefore, need to be calculated at different instances and in different ways, depending upon the requirement. The blog will explore the different stoichiometric calculations involving chemical equivalents and their methods.

Stoichiometry of Chemical Reactions

Stoichiometry, or to measure an element, as it means in Greek, calculates the coefficients in a balanced chemical reaction to get the details about the relationship as mentioned. As an example, the stoichiometric calculations of a balanced chemical reaction can be expressed as:

\(\mathrm{CaCO}_{3}+2 \mathrm{HCl} \rightarrow \mathrm{CaCl}_{2}+\mathrm{H}_{2} \mathrm{O}+\mathrm{CO}_{2}\)

Here, \(1\) mole of calcium carbonate (\(40+12+3 \times 16=100\) grams) reacts with \(2\) moles of \(\mathrm{HCl}\) (\(1+35.5=73\) grams) to yield \(1\) mole of calcium chloride (\(40+2 \times 35.5=111\) grams) and \(1\) mole of water (\(2 \times 1+16=18\) grams) and one mole of carbon dioxide (\(12+2 \times 16=44\) grams) or \(22.4\) litres at STP.

This quantitative information available from the chemical reaction can be used in many calculations, such as:

1. Mass to mass relationships of the reactants and products- if information about any one of the reactants or products is given, the other can be calculated.
2. Mass to volume relationship of products and reactants- again here, if one part is given, the other part can be calculated using the information available about the stoichiometric calculations.
3. Volume to volume relationship between reactants and products can also be calculated using the information.

Calculation of Chemical Equivalents: Reactions in Solution

As we know, chemical equivalents of any entity are given by the weight in grams of a substance that combines with or displaces one gram of hydrogen. Since we are talking about the solution here, the calculations can be done with the help of the measure of normality or molarity of the solution.

For example, to calculate the amount in grams or moles present, the information required includes:

1. The volume of the solution in molarity or normality
2. Known: \(1000 \mathrm{~cm}^{3}\) or \(1 \,\mathrm{M}\) solution has \(1\) mole of the solute or
3. \(1000 \mathrm{~cm}^{3}\) of \(1 \mathrm{~N}\) solution contains \(1\) gram equivalent of the solute.

Also, when a balanced equation is given, the amount of required substance in moles can be calculated from:

1. Volume of the solution in which the substance is dissolved or alternatively, from
2. Molarity of the solution 

When molarity is given, volume can be calculated and vice versa using the information available on the balanced equation. 

Equivalent Weight: Definition and Calculation

Equivalent weight can be defined as the mass of one equivalent of a given substance that will combine with or displace a fixed quantity of the other substance. It is represented by ‘E’ and is equal to the ratio of molecular weight and the valency factor. 

Equivalent Weight \(=\frac{\text { Molecular Weight }}{\text { Valency Factor }}\)

Number of equivalents \((\text {n})=\frac{\text { mass of the sample }}{\text { Equavalent weight of that species }}\)

• Equivalent weight is a pure number. When expressed in grams, it is called gram equivalent mass.
• The equivalent mass of the substance can have different values under different conditions.

Valency Factor Calculation

The valency factor is calculated differently for both acids and bases. 

For Acids:

Valency factor for acids \(=\) Number of replaceable \({{\rm{H}}^ + }\) ions in it.

In \(\mathrm{H}_{3} \mathrm{PO}_{3}\), although it has \(3\) hydrogen atoms in the molecule, only \(2\) are replaceable, since the other hydrogen goes on to form a bond with Phosphorous (since \(\text {P}\) forms five bonds with the elements- two single bonds with two oxygen, one single bond with hydrogen and one double bond with another oxygen).

For Base:

Valency factor for the base = Number of replaceable \({\rm{O}}{{\rm{H}}^ – }\) ions.

Methods of Determining Equivalent Mass

The below methods can be used to determine the equivalent mass of substances:

1. Hydrogen displacement method – The method calculates the mass of the metal which displaces about \(1.008\) parts by mass of hydrogen.
2. Chloride formation method: The method calculates the mass of an element that displaces or combines with \(35.5\) parts by mass of chlorine.
3. Metal displacement method: The method is used when metal with more electropositive character tries to displace a metal with a less electropositive character from its solution. The calculation is done as follows:
   \(\frac{\text { Mass of Metal added }}{\text { Mass of metal displaced }}=\frac{\text { Equivalent mass of metal added }}{\text { Equivalent mass of metal displaced }}\)
   Each of these methods can help in determining the equivalent masses of any chemical entity in a solution or in a reaction.
4. Oxide formation or oxide reduction method: The method calculates the mass of the metal which combines with 8 parts by mass of oxygen.

Examples of Calculations of Chemical Equivalents 

Below is an example of how the chemical equivalents can be derived using the information available. 

Calculate the volume of hydrogen liberated at STP when \(500 \mathrm{~cm}^{3}\) of \(0.5 \mathrm{~N}\) \(\mathrm{H}_{2} \mathrm{SO}_{4}\) reacts with an excess of \(\text {Zn}\). Given: \(\mathrm{H}=1, \mathrm{~S}=32\) and \(\mathrm{O}=16\).

The chemical equation for the reaction can be written as:

\(\mathrm{H}_{2} \mathrm{SO}_{4}+\mathrm{Zn} \rightarrow \mathrm{ZnSO}_{4}+\mathrm{H}_{2}\)

Molecular mass of sulphuric acid \(=(2 \times 1+32+4 \times 16)=98.0\) grams.

Hydrogen: \(22400 \mathrm{~cm}^{3}\) at STP

The first step is to calculate the amount of sulphuric acid in \(500 \mathrm{~cm}^{3}\) of sulphuric acid with normality \(0.5 \mathrm{~N}\).

\(\frac{\text { Strength of the acid }}{\text { Volume of solution in litres }}= \text {Nomalirt} \,\text {x}\, \text {eq}. \,\text {mass} \,=0.5 \times 49=24.5 \mathrm{~g} / \mathrm{lit}\)

So, \(1000 \mathrm{~cm}^{3}\) of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) contains \(24.5\) grams of pure sulphuric acid.

Hence, \(500 \mathrm{~cm}^{3}\) of sulphuric acid will have: \(24.5 \times 500 / 1000=12.25\) grams of pure sulphuric acid.

Next step is to calculate the volume of hydrogen released at STP:

\(98\) grams of sulphuric acid reacts with \(\text {Zn}\) to liberate \(22400 \mathrm{~cm}^{3}\) of hydrogen at STP.

So, \(12.25 \mathrm{~g}\) of pure sulphuric acid would liberate: \(\frac{22400}{98} \times 12.25 \mathrm{~cm}^{3}=2800 \mathrm{~cm}^{3}\) Volume of hydrogen liberated at STP \(2800\,{\rm{c}}{{\rm{m}}^3}\)

Summary

The chemical equivalent is defined as the weight in grams of a substance that displaces or combines with one gram of hydrogen. In terms of electrochemistry, chemical equivalents are defined as the amount is the substance (in grams) produced or utilised when one coulomb of electricity is passed in an electrochemical reaction. Chemical equivalents are essential to understand the relationship between products and reactants and the way they react with each other. It can be calculated in multiple ways depending upon where and what type of calculations are required. Equivalent weight is the ratio of molecular mass and valence factor. It can be calculated by knowing the valence factor for acids and bases, which are the number of replaceable hydrogen and hydroxyl groups, respectively. It can also be calculated using the hydrogen-displacement method, chloride formation method and metal displacement method.

FAQs

Q.1. What is chemical equivalent?
Ans: Chemical equivalent is defined as the weight in grams of a substance that displaces or combines with one gram of hydrogen.

Q.2. What is chemical equivalent in electrochemical terms?
Ans: In electrochemical terms, chemical equivalents are defined as the amount is a substance (in grams) produced or utilised, when one coulomb of electricity is passed in an electrochemical reaction.

Q.3. What is chemical equivalent weight?
Ans: Equivalent mass can be defined as the mass of one equivalent of a given substance that will combine with or displace a fixed quantity of the other substance.

Q.4. What is stoichiometric coefficient?
Ans: The coefficients in a balanced chemical reaction are called stoichiometric coefficients. For example, the moles or the number of various reactants and products represented in a balanced chemical reaction is their stoichiometric coefficient.

Q.5. What are the methods used to calculate the equivalent weight of a substance?
Ans: Equivalent mass can be calculated by: chloride formation method, metal displacement method, hydrogen displacement method, and oxygen formation method.

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