It is one of the constituents of gastric juice which aids in the digestion process. HCl can be synthesized according to the following reaction by warming aqueous solution crystals of sodium chloride with sulphuric acid. NaCl aq.   +    H2SO4 (aq.)   ——->    2 HCl (aq)   +    Na2SO4aq. It is soluble in water and combines with alkali to form salt and water. HCl is very useful. HCl can be distinguished from a base by using indicators like turmeric, pH paper, litmus paper, etc. Dry HCl gas does not change the color of indicators because of the absence of H3O+ ions. In this article, we will discuss the acidic behavior of hydrochloric acid and compare the same with other acids. So, what is the ph of HCl? The pH of HCl generally lies around 3 classifying it as a strong acid. It is a strong acid because it dissociates completely in an aqueous solution to release H+ ions and Cl- ions. Acid is considered a strong acid if it completely dissociates to form H+ ions. We will study more about its fundamentals and concepts in detail. Stay connected.  

Why is HCl a Strong Acid?

Acids release H+ or H3O+ ions on dissolving in water. Some acids dissociate completely while some acids dissociate partially, and an equilibrium is established. HCl behaves like a strong electrolyte. HCl dissociates completely according to the following equation. HCl (aq.)   +   H2O(l)   —–>   H3O+(aq.)   +    Cl-(aq.) On the reaction of HCl with water, HCl completely breaks down to release hydronium and chloride ions. Cl- is the conjugate base. A strong acid leads to the formation of a weak conjugate base. It is believed that the weaker the conjugate base is, the stronger is the acid. For strong acids, the value of pKa is less than -1.74. For HCl, pKa is -6.3. pKa is inversely proportional to acidity.  

What are Acids?

Various scientists have given different definitions of acid. Some of them are-

  1. Arrhenius theory – It is also known as water theory. Acids dissociate in water to release H+ ions, while bases dissociate to release OH- ions. HA aq.+H2O (l) H3O+ (aq.)+ A- (aq.) Here, HA acts as an acid by dissociating to release H3O+ ions.
  2. The Brönsted-Lowry Concept – It is also known as the proton theory. A substance is acidic if it can donate H+ ion and basic if it can accept H+ ion in an aqueous medium. NH3 aq.   +   H2O(l)   ⇌    NH4+ aq.   +   OH-aq. Here, water acts as an acid by donating H+ ions, and ammonia acts as a base by accepting H+ ions.
  3. Lewis Acid-Base Concept – It is also known as electronic theory. Acids are those species that accept an electron pair while the base donates an electron pair. The Lewis Acid-base concept can explain acidic substances which do not have a proton. BF3  +  :NH3   —->   BF3:  +  NH3 Here BF3 acts as an acid while ammonia acts as the base. Only Lewis Acid-base concept is independent of solvent, while other theories assume water as solvent.

 

Type of Acids (explained on the basis of Arrhenius Theory)

  1. On the basis of strength, acids can be classified as • Weak acid Acids that do not dissociate completely (<5%) to release H+ ions in water. They are considered to be weak electrolytes. For example- acetic acid. • Strong acid  Acids that dissociate completely to release H+ ions in water. They are considered to be strong electrolytes. For example- HCl
  2. On the basis of the number of ionizable hydrogen, acids can be classified as • Monoprotic/Monobasic acid  Acids that have only one ionizable hydrogen per molecule. For example- HNO3 • Polyprotic/Polybasic acid  Acids that have two or more ionizable hydrogen per molecule. For example- H2SO4, H3PO4
  3. On the basis of source, acids can be classified as • Inorganic Mineral acid Acids that do not have carbon. They are obtained from minerals. They are relatively stronger than organic acids. For example- HCl • Organic Acid Acids that have carbon as a constituent. They are widely used in manufacturing processes. For example- acetic acid.
  4. On the basis of concentration, acids can be classified as • Dilute acid When an aqueous solution has a relatively less percentage of dissolved acid. For example- 0.001N HCl • Concentrated acid When an aqueous solution has a relatively more percentage of dissolved acid. For example- 10N HCl
  5. On the basis of the presence or absence of oxygen, acids can be classified as • Oxy-acid– acids that contain oxygen as a constituent. For example- HNO3 • Hydracid– Acids that do not have oxygen as a constituent. For example- HCl  

Which is More Acidic? HCl, HBr, or HI

According to Arrhenius’s theory, acids dissociate to release H+ ions in water. The acid which releases H+ ion more easily is considered to be a stronger acid. A weak bond will break easily to release the constituent ions. Longer bonds are weaker. Large-sized anions form longer and weak bonds and relatively small-sized anions form short and stable bonds. The decreasing order of size of anions is I->Br->Cl-

Thus, the strength of the bond follows the order HI<HBrHBr>HCl  

Which is More Acidic? HCl or H2SO4

Both are considered to be very strong acids and electrolytes. It is a common misconception that sulphuric acid is stronger because it is diprotic and HCl is monoprotic. The compound which releases proton more easily is considered to be a better acid. HCl (aq)   —–>     H+ (aq)   +   Cl- (aq) H2SO4(aq)    —–>    H+ (aq)   +   HSO4- (aq) HCl is more acidic because the Cl- ion so formed on dissociation is a weaker conjugate base than HSO4-. HSO4- is stabilized by resonance. For HCl, the pKa value is -6.3, while for H2SO4, it is ~-3. Thus, HCl releases the H+ ion more easily than H2SO4. HCl is a stronger acid than H2SO4.  

Which is More Acidic? HCl or CH3COOH

HCl is a strong electrolyte, while acetic acid is a weak electrolyte. For the same concentration of HCl and CH3COOH, the dissociation of HCL is 100%, while it is less than 5% for acetic acid. HCl (aq)   ——>    H+(aq)   +    Cl- (aq) CH3COOH(aq)   ⇌    CH3COO-(aq)   +   H   +   (aq) Thus, the amount of H+ ions released is relatively less for acetic acid. Most of the acetic acid is in undissociated form. Since fewer protons are dissociated from acetic acid, it is a weaker acid. HCl is a stronger acid than acetic acid.

 

How to calculate the pH of HCl?

pH is used for measuring the concentration of H+ ions in a substance. Concentration is measured in terms of molarity, i.e., the number of moles of H+ ions per liter of solution. pH is calculated by taking the negative logarithmic value of H+ ions concentration. pH= – log10[H+]

A standard pH scale ranges from 0 to 14. For a neutral solution, the pH is 7. It indicates that the concentration of H+ in pure water is 10-7M. In an aqueous solution, both water and acid contribute to increasing H+ ion concentration. The dissociation of water follows the given equation. Kw is the dissociation constant of water. 2H2O(l)    ⇋    H3O+(aq)   +   OH-(aq) Keq = [OH-][H3O+]/[H2O] Kw = [OH-][H3O+] = 10^-14 For water, [OH-] = [H3O+] = 10^-7 The dissociation of acid follows the given equation HCl aq   +   H2O(l)    ⇋    H3O+(aq)    +    Cl-(aq) Here, the concentration of H3O+ is equal to the concentration of HCl. pH  +   pOH  =  14 For measuring pH, we adopt different methods depending on the concentration of HCl.

Case 1: When the concentration of HCl is greater than ~10-5 M

Here, the concentration of HCl is considered to be high, and we assume that acid is the only source of all the H+. Example 1: 10-3M solution of HCl we assume, [H+ ]=10-3M. In the logarithmic bracket, we put a unitless quantity. Hence, we divide the given concentration by a unit concentration to obtain a unitless quantity. pH = -log10(10-3 M/1M) = -log10(10-3) = 3log10(10) =3 Example 2: 2* 10-2M solution of HCl pH= -log10(210-2 M/1M) = -log10(210-2) = -log10(2) – log10(10-2) = -log10(2)+2 = -0.301+2 = 1.699  

Case 2: Concentration of HCl is less than 10-5 M

In this case, the concentration of HCl is not considered to be high, and we consider the H+ ions coming from water. Example For 10-8M HCl In water [OH-] = [H3O+] = x Kw = [OH-][H3O+] = x(10-8 +x) = 10-14 We solve the equation x^2 + 10^-8x – 10^-14 = 0 to get x which gives [OH-] x = 9.5* 10-8 pOH = -log109.5 * 10-8M1M = -log10(9.5*10-8) = -log10(9.5) – log10(10-8) = -log10(9.5) + 8 = -0.98 + 8 pOH =7.02 pH+pOH =14 pH = 6.98  

HCl Properties

  1. The molar mass of HCl is 36.458 g/mole.
  2. Density of 37% HCl is 1.2 g/mL.
  3. HCl reacts with metals like Zn, Al, K, etc., to release H2 gas.
  4. The aqueous solution is a colorless liquid with a pungent smell.
  5. The boiling point, melting point, and pH depend on the concentration of the solution of HCl.
  6. HCl is a polar covalent compound as Cl is more electronegative than H.    

HCl Uses

  1. H2 production
  2. ClO2 synthesis
  3. Hydrometallurgical synthesis
  4. Cleaning and etching
  5. Petroleum well activation
  6. Leather processing
  7. Gelatin formation
  8. Preparation of chloride-containing salts
  9. Used in titration to determine an unknown concentration
  10. Pickling of steels
  11. Regulates pH in the treatment/manufacturing process.  

Conclusion

Hydrochloric acid is a strong acid. It dissociates completely in water to give H+ ions. Three theories have been successful in explaining the meaning of acid. The three theories are- Arrhenius Theory, Brönsted-Lowry Concept, and Lewis Acid-base concept. On the basis of strength, acids can be classified as strong and weak. On the basis of the number of ionizable hydrogen, acids can be classified as monoprotic and polyprotic. On the basis of concentration, acids can be classified as dilute and concentrated. On the basis of source, acids can be classified as organic and inorganic. On the basis of the presence of O, acids can be classified as oxyacid and hydracid. Considering the various factors affecting acidity, we can conclude that the acidity order is HI>HBr>HCl HCl>H2SO4 HCl>CH3COOH To measure the pH of HCl, we ignore the contribution of water for highly concentrated solutions, while for very dilute solutions, we consider the contribution from water. Happy Learning

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