15 Facts on HCl + FeS: With Several Elements Reaction

FeS is the non-stoichiometric iron complex that is water soluble and can react with several acids like hydrochloric. Let us see what happens when both are reacted.

In FeS, iron is present at a +2 oxidation state so it can be further oxidized by a suitable reagent, and when it reacted with a strong acid like hydrochloric it changes its oxidation state. FeS present most of the iron-sulfur protein like Rubredoxin protein cluster which is performed for one-electron transfer process.

Having more d electrons in iron sulfide or the presence of a vacant site of sulfur can participate in several chemical reactions. Let us discuss the mechanism of the reaction between hydrochloric acid and ferrous sulfide, the reaction enthalpy, the type of reaction, product formation, etc in the following part of the article.

1. What is the product of HCl and FeS?

Ferrous chloride is a major product is formed when HCl and FeS are reacted together and some amount of hydrogen sulfide gas also had been released.

2. What type of reaction is HCl + FeS?

HCl +Fes reaction is an example of a single displacement reaction along with redox and precipitation reactions also occur by a single mechanism. Here strong acid reacted with the nucleophile sulfur site so it is one kind of acid-base reaction too.

3. How to balance HCl + FeS?

HCl + FeS = FeCl2 + FeCl3 + H2O this reaction is not balanced yet because the atoms are not equal for the left side and right side, so we have to balance the equation in the following way,

  • Step 1 – Labelling all the reactants and products by the required number of alphabets
  • First, we labeled all the reactants and products by A, B, C, and D as there are only four molecules obtained for this reaction and the reaction looks like A HCl + B FeS = C FeCl2 + D H2S
  • Step 2 – Equating all the Coefficients for all the same type of elements by rearranging them
  • After the rearrangement of all the coefficients of the same elements by their stoichiometric proportion we get, H = A = 2D, Cl = A = 2C, Fe = B = C, S = B = D
  • Step 3 – Using Gaussian elimination to determine the coefficient values
  • Using the Gaussian elimination and equating all the equations we get, A = 2, B = 1, C = 1, D = 1
  • Step 4- now write the whole equation in the balanced form
  •  The overall balanced equation will be, 2HCl + FeS = FeCl2 + H2S

4. HCl + FeS titration

To estimate the quantity of sulfur or iron we can perform a titration between FeS and HCl

Apparatus used

We need a burette, conical flask, burette holder, volumetric flask, and beakers for this titration.

Titre and titrant

HCl versus FeS, HCl acts as titrant which is taken in the burette and the molecule to be analyzed is Fes which is taken in a conical flask.


The whole titration is done in an acidic medium or acidic pH so the best suitable indicator will be phenolphthalein which gives perfect results for this titration at given pH.


The burette was filled with standardized HCl and FeS was taken in a conical flask along with the respective indicator. HCl is added dropwise to the conical flask and the flask was shaking constantly. After a certain time when the endpoint arrived indicator changes its color and the reaction was done.

We repeat the titration several times for better results and then we estimate the iron as well as sulfur quantity by the formula V1S1 = V2S2.

5. HCl+ FeS net ionic equation

In the reaction between HCl +FeS different ions are formed like proton, chloride, and ferric ion because hydrogen sulfide does not ionize as it exists in gaseous form and also gets sulfide ion.

H+ + Cl + Fe2+ + S2- = Fe2+ + 2Cl + H2S

6. HCl+ FeS conjugate pairs

In the reaction between HCl + FeS Chloride is the conjugate base of strong acid hydrochloric and conjugate pair of FeS is the protonic formed of that molecule. The acidity and basicity are determined by the stability of their conjugate pair.

  • Conjugate pair of HCl = Cl
  • Conjugate pair of FeS = HFeS+

7. HCl and FeS intermolecular forces

The intermolecular force present in HCl is the strong electrostatic force between proton and chloride ion and due to polarity there some van der waal’s attraction force is present but in the case of FeS, covalent force as well as ionic interaction present, and for H2S only covalent force.

van der waal’s
Intermoleuclar Forces

8.HCl + FeS reaction enthalpy

For the reaction between HCl + FeS reaction enthalpy is, 77.3 KJ/mol which can be obtained by the formula enthalpy of products – enthalpy of reactants, and here the change in enthalpy is positive.

Enthaply of Reactants
and Products

9. Is HCl + FeS a buffer solution?

The reaction between HCl + FeS does not give any buffer because one is strong acid but the other is not a salt of a strong or weak base, so no chance of the formation of buffer in an aqueous solution.

10. Is HCl + FeS a complete reaction?

The reaction of HCl + FeS gives two complete products FeCl2 and H2S so it is a complete reaction. The reaction takes need some time to complete until all the reactants completely reacted the products are not formed.

11. Is HCl + FeS an exothermic or endothermic reaction?

The reaction HCl + FeS change in enthalpy is coming positive for which indicates it is an endothermic reaction in terms of thermodynamics first law. So the reaction takes temperature from the surroundings which helps to complete the reaction, Where δH is always positive.

Energy Profile Diagram of
Endothermic Reaction

12. Is HCl + FeS a redox reaction?

In the reaction HCl + FeS is one kind of redox reaction because in this reaction many elements gets reduced and oxidized like chloride gets oxidized from reactant to product where sulfur gets oxidized.

Redox Schematic of the
HCl and FeS Reaction

13. Is HCl + FeS a precipitation reaction

In the reaction between HCl + FeS, H2S is precipitated when the reaction is completed so it is a precipitation reaction. Because H2S is gaseous molecule so it cannot soluble in water but in high temperature it gets precipitated at the lower part of conical

H2S Precipitated

14. Is HCl + FeS reversible or irreversible reaction?

The reaction between HCl + FeS is also irreversible in nature because when acid reacts with other elements it is generally irreversible. Here also H2S gas is formed and it disappeared from the reaction vessel and cannot react with ferrous chloride in backword direction, it goes only forward direction.

Irreversible Reaction DIrection

15. Is HCl + FeS displacement reaction?

The above reaction between HCl + FeS is an example single displacement reaction because here sulfur just displaced from its initial position to another where Fe displaced sulfur and attached to chloride ion to get desired products.


HCl and FeS reaction mainly gives us the hydrogen sulfide along with ferrous chloride so it is a commercial important reaction for production of hydrogen sulfide gas. Also by the quantratric anylsis of the HCl and FeS give the amount of the iron as well as sulfur in the respective sample.

Biswarup Chandra Dey

Hi......I am Biswarup Chandra Dey, I have completed my Master's in Chemistry from the Central University of Punjab. My area of specialization is Inorganic Chemistry. Chemistry is not all about reading line by line and memorizing, it is a concept to understand in an easy way and here I am sharing with you the concept about chemistry which I learn because knowledge is worth to share it.

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