ACID BASE AND SALT : EASY WAY TO EXPLAIN THE CONCEPT

ACID BASE AND SALT

CONCEPT OF ACID AND BASE

According to Arrhenius theory of electrolytic dissociation,  an acid is a hydrogen containing compound which dissociates in aqueous solution. By this it produces H⁺ ion or    H₃ O⁺ (hydronium ion) as only cation.  On the other hand compound which produces OH^– ions as the only anion in their aqueous solution we term it as base. Some examples of acids are H₂ SO₄, HCl, HNO₃ etc.  Some examples bases are NaOH, KOH, NH₄ OH. Study of acid base and salt gives a clear idea about inorganic chemistry.

IDENTIFICATION OF ACID IN ACID BASE AND SALT AS ARRHENIUS THEORY

 We can identify Hydrochloric acid (HCl) by using aqueous solution of silver nitrate which form white ppt of silver chloride (AgCl).

Identification of Sulphuric acid (H₂SO₄) we do by using barium chloride to form white ppt of Barium Sulphate (BaSO₄).

We can identify Nitric acid (HNO₃ ) by ring test.

GENERAL PROPERTIES OF ACID IN ACID BASE AND SALT AS ARRHENIUS THEORY

• In aqueous solution acid forms H⁺ ion or hydronium ion , in this way it can conduct electricity.
• Sour in taste.
• Acid turns blue litmus paper into red.
• It produces hydrogen gas while reacting with metal.
• With reaction with carbonates and bicarbonates, acid produces carbon dioxide gas.
• Alkali reacts with acid to form salt and water in acid base and salt.

QUALITATIVE CONCEPT OF pH IN ACID BASE AND SALT AS ARRHENIUS THEORY

pH helps to determine the acidity and basicity of a compound

The H⁺ ion concentration of a dilute solution we term as pH.  A neutral solution has pH 7 at 25°C. Any acidic solution has pH less than 7. Acidity increases with decrease of pH value. As ph 3 or 4 indicates more acidic solution. Alkaline solution has pH greater than 7. We use pH paper as universal indicator to get an idea about the pH value of a solution. The paper shows different colors in solution of different pH.

DIFFERENT TYPES OF SALTS IN ACID BASE AND SALT AS ARRHENIUS THEORY

ACIDIC OXIDE:  Oxides which reacts with bases to produce salt and water we term it as acidic oxides. All non-metallic oxides are acidic oxide like Carbon dioxide , Nitrogen Oxide.

BASIC OXIDE:  Metallic oxides which react with acids to produce salt and water we call them basic oxide like Sodium oxide, manganese oxide et.

AMPHOTERIC OXIDE:  It reacts with both acids and bases to produce salt and water like oxides of aluminium and zinc.

NORMAL SALT:  The salt formed when all the replaceable hydrogen atoms of an acid are completely replaced by metal or some other basic radical that we term as normal salt like sodium chloride, ammonium sulphate etc.

QUESTIONS ANSWERS

Qs1: Why do free H⁺ ions not exist in water or aqueous solution?

Ans1: H⁺  ions combine with electronegative oxygen atoms in water to produce hydronium ions ( H₃O⁺) . So free H⁺ ions do not exist in water or aqueous solution.

Qs2: What are inorganic or mineral acids and give some example.

Ans2: Acids which we obtain from minerals or inorganic substances are term as mineral acids. Some examples are HCl, HNO₃, H₂SO₄.

Qs3: Mention the limitations of Arrhenius theory acid –base concept.

Ans3: The limitations of Arrhenius theory of acid-base concepts are-

• According to this concept [ARRHENIUS THEORY], we define an acid or base on the basis of their dissociation in aqueous solutions. Thus, the concept refers to aqueous solution only. It cannot explain the acidic and basic property of compounds in solvents other than water or non-aqueous solvent like liquid ammonia, benzene etc in ARRHENIUS THEORY.
• This ARRHENIUS THEORY concept demonstrate that a base must yield OH^– in aqueous medium. Thus, the theory fails to explain the basic nature of compounds like ammonia (NH₃), methyl amine ( CH₃NH₃), aniline  (C₆ H₅ NH₂) etc. similarly acidic nature of compounds like BF₃ or AlCl₃ web cannot explain by this ARRHENIUS THEORY. As these compounds do not produce H⁺ ions in aqueous solutions.

CONCEPTUAL QUESTIONS IN ARRHENIUS THEORY

Qs4: What is Passive iron in acid base salt?

Ans4: iron looses its chemical reactivity when it comes in contact with hot and concentrated nitric acid. This inactive iron we term as passive iron. In presence of concentrated HNO₃ a layer of iron oxide forms on the surface of metallic iron. This layer prevents any further reaction of the acid with the metal iron. In this way, it loses the reactivity as told in ARRHENIUS THEORY.

Qs5: We should  dilute acid by adding concentrated acid into water,  but we should not add water into acid in ARRHENIUS THEORY. Why?

Ans5: If we add water to acid then large amount of heat will be produced. It causes the acid to boil. As a result, the acid may spurt out from the beaker and cause serious accident. Apart from this, the glass beaker may break due to evolution of excess amount of heat. So, we must add acid gradually to large amount of water with continuous stirring to distribute the heat evenly.

HIGHER ORDER SKILL ON ACID BASE SALT /ARRHENIUS THEORY               

Qs6: What do you mean by acid burns and alkali burns.?

Ans6: When concentrated mineral acids and concentrated alkalis come in contact with our skin , it causes blisters on our skin and even burn. Burn caused by acid or alkali we term as acid burn or alkali burn respectively.

Qs7: What are the steps we should take immediately in case of acid burn or alkali burn?

Ans7: Concentrated acid or alkalis cause blisters on the skin. In such cases, we must wash the affected part with plenty of water repeatedly. In case of acid burn, we should treat  the affected area with 1% dilute sodium bicarbonate solution. In case of alkali burn, we should wash the affected area with 1% acetic acid solution. After first aid, patient must consult doctors.   

Qs8: We should not store acid in metal container. Why?

Ans8: Metals placed above hydrogen in the activity series react with acids to produce salts and hydrogen gas. So, acids  cannot be placed in contact with active metal container. For example iron reacts with dilute Sulphuric acid to produce Ferrous sulphate salt and hydrogen gas. So we should not keep acid in metal container due to corrosive nature.