15 Facts on HCl + CuS: What, How To Balance & FAQs

The reaction between hydrochloric acid and copper monosulfide is a common inorganic reaction. Let us see the mechanism behind the reaction between HCl and CuS.

Copper sulfide is a borderline element where Cu is borderline acid and makes a bond with soft acid with sulfur. Due to the electronegativity of sulfur, the bond between Cu-S weakened and easily cleaved by the reaction with HCl. This reaction has not required any type of catalyst or external parameters.

To estimate the amount of the Cu and S in the CuS we can react between CuS and HCl.  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 CuS?

In the reaction between HCl and CuS Cupric chloride is formed as major product and some amount of hydrogen sulfide gas also had been released.

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Product of Reaction between HCl + CuS

2. What type of reaction is HCl + CuS?

HCl + CuS reaction is an example of a double 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 + CuS?

HCl + CuS = CuCl2 + H2S 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 CuS = C CuCl2 + 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, Cu = 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 + CuS = CuCl2 + H2S  

4. HCl + CuS titration

To estimate the quantity of sulfur or copper we can perform a titration between CuS and HCl

Apparatus used

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

Titre and titrant

HCl versus CuS, HCl acts as titrant which is taken in the burette and the molecule to be analyzed is CuS 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 CuS 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 copper as well as sulfur quantity by the formula V1S= V2S2.

5. HCl+ CuS net ionic equation

The net ionic equation between HCl + CuS is as follows,

H+ + Cl + Cu2+ + S2- = Cu2+ + 2Cl + H2S

  • To derive the net ionic equation the following steps are required,
  • First HCl will be ionized as it is a strong electrolyte
  • After that CuS also ionized to form strong electro-charged particle
  • After that, the corresponding ions interact to form CuCl2 and formed ionized.

6. HCl+ CuS conjugate pairs

HCl + CuS conjugate pairs will be the corresponding de-protonated and protonated form of that particular species which are listed below-

  • Conjugate pair of HCl = Cl
  • Conjugate pair of S2- = H2S

7. HCl and CuS intermolecular forces

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

van der waal’s
Intermoleuclar Forces

8.HCl + CuS reaction enthalpy

HCl + H2reaction enthalpy for the is, 432.6 KJ/mol which can be obtained by the formula enthalpy of products – enthalpy of reactants, and here the change in enthalpy is positive.

Enthalpy of Reactants
and Products

9. Is HCl + CuS a buffer solution?

The reaction between HCl + CuS gives a buffer solution of CuCl2 and H2S and they can control the pH of the reaction.

10. Is HCl + CuS a complete reaction?

The reaction HCl + CuS is a complete reaction because it gives two complete products CuCl2 and H2S.

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

The reaction HCl + CuS is an endothermic reaction in terms of thermodynamics first law.

12. Is HCl + CuS a redox reaction?

HCl + CuS reaction is a redox reaction because in this reaction many elements get reduced and oxidized like chloride gets oxidized from the reactant to the product where Cu gets reduced.

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Redox Schematic of the
HCl and CuS Reaction

13. Is HCl + CuS a precipitation reaction

HCl + CuS is a precipitation reaction because CuCl2 and H2S get precipitated in the solution and they are not soluble in the reaction mixture.

14. Is HCl + CuS reversible or irreversible reaction?

HCl + CuS is irreversible because we get H2S gas as a product. When gas is formed during the reaction the entropy of the reaction increases and the equilibrium of the reaction shifts towards the right-hand side only.

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Irreversible Reaction Direction

15. Is HCl + CuS displacement reaction?

HCl + CuS is an example double displacement reaction because in this reaction Cu2+ displaced H+ in HCl. Whereas H+ also displaced Cu2+ in CuS and formed CuCl2 and H2S as products. It is also called Salt metathesis reaction.

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Double Displacement Reaction


HCl and CuS reaction mainly gives us hydrogen sulfide along with Cupric chloride so it is a commercially important reaction for the production of hydrogen sulfide gas. Also, the quantratric analysis of the HCl. CuS gives the amount of copper as well as sulfur in the respective sample.