ACIDS, BASES & SALTS DR. RUCHIKA YADU

Transcription

1 ACIDS, BASES & SALTS DR. RUCHIKA YADU

2 Properties of Acids Acid is a compound which yields hydrogen ion (H+), when dissolved in water. Acid is sour to the taste and corrosive in nature. The ph value of acids is less than 7. Acids can be classified into organic and inorganic acids. Acetic acid (CH3COOH) is the best example of organic acid, and acids produced from minerals are termed as inorganic acids like sulfuric acid (H2SO4), hydrochloric acid (HCl), etc. Acid converts blue litmus paper to red in color. Acids have a tendency to corrode metal surfaces quickly.

3 On the basis of number of hydrogen ion, acids can be classified as Monoprotic acid Such type of acids produce one mole of H+ ions per mole of acid, e.g., HCl, HNO3, etc. Diprotic acid They can produce two moles of H+ ions per mole of acid, e.g., H2SO4. Triprotic acid They produce three moles of H+ ions per mole of acid, e.g., H3PO4. Polyprotic They can produce more than three H+ ion per mole of acid

4 On the basis of strengths or capacity to donate hydrogen ions, acids can be described as: Strong acids: Which are completely (100%) ionized in aqueous solutions. Hence at equilibrium, the concentration of acid molecules is very less and concentration of hydrogen ion reaches to maximum, e.g., HCl, HNO 3, HClO 4. Weak acids: They are only partially ionized in solution at equilibrium state. At equilibrium state, acid molecules are present and the concentration of hydrogen ion is less, e.g., HF, CH 3 COOH.

5 Properties of bases Bases are compounds which yield hydroxide ion (OH-), when dissolved in water. Bases are bitter to taste and corrosive in nature. They feel slippery and soapy. Bases show a ph value of more than 7. Bases convert red litmus paper to blue in color.

6 Properties of bases Bases also have the tendency to corrode metal surfaces. A reaction between a base and a metal is similar as for acid to form salt and release hydrogen gas. But this reaction can only occur when a metal is strong enough to displace another metal from its parent constituent. 2NaOH + Zn Na 2 ZnO 2 + H 2 Phenolphthalein solution turns pink in color in a basic solution. Bases turn methyl orange to yellow.

7 On the basis of strengths or capacity to donate hydroxyl ions, bases can be described as: Strong bases: They are completely ionized in water to produce hydroxide ions, e.g, sodium hydroxide: NaOH(s) Na + (aq) + OH - (aq) Weak bases: Partially ionize and equilibrium lies mostly towards reactants side, e.g., ammonia in water: NH 3 (aq) + H 2 O(l) NH 4+ (aq) + OH - (aq)

8 On the basis of the number of hydroxide groups present in a base, they can be classified as monobasic (one OH-), dibasic (two OH-), tribasic (three OH-) bases and so on

9 Arrhenius Concept of Acid and bases

10 Way back in the late 1800s, Svante Arrhenius came up with definitions of acids and bases while working on kinetics problems. Arrhenius concept is the oldest concept to explain acids and bases. According to this concept. An acid is a hydrogen-containing compound which can give hydrogen ion (H + ) in aqueous solution. For example, Hydrochloric acid (HCl) gives H + ion in its aqueous solution. HCl H+ + Cl- Such types of acids are called Arrhenius acids, like nitric acid (HNO 3 ), acetic acid (CH 3 COOH), etc.

11 Base is a compound, which can give hydroxide ion (OH - ) in an aqueous solution. For example, Sodium hydroxide in water form sodium ion (Na + ) and hydroxide ion (OH - ). NaOH Na + + OH - Such type of bases are known as Arrhenius bases. Other example of Arrhenius base are ammonium hydroxide (NH 4 OH), aluminium hydroxide (Al(OH) 3 ) and magnesium hydroxide(mg(oh) 2 ). H 2 O NaOH(s) Na + (aq) + Cl -1 (aq)

12 Brønsted-Lowry Acids and Bases

13 In the early 1900s, an alternate definition for acids and bases was proposed by Johannes Brønsted and Thomas Lowry A Brønsted-Lowry acid is defined as a compound that gives hydronium ions to another compound for example, hydrochloric acid gives H+ ions to compounds it reacts with. Brønsted-Lowry bases are compounds that can accept hydronium ions when ammonia gets a hydronium ion from HCl, it forms the ammonium ion. The following equation represents the reaction of a Brønsted-Lowry acid with a Brønsted-Lowry base: HNO 3 + NH 3 NO 3- + NH 4 + In this reaction, nitric acid behaves as an acid because it gives a proton to ammonia. Ammonia behaves as a base because it accepts the proton from nitric acid.

14 Conjugate Acid-Base pair The nitrate ion is based on the nitric acid molecule, so we say that it is the conjugate base of nitric acid. Likewise, the ammonium ion is the conjugate acid of ammonia. Together, an acid with its conjugate base (such as HNO 3 and NO 3- ) or a base with its conjugate acid (such as NH 3 and NH 4+ ) is referred to as a conjugate acid-base pair.

15 Lewis Acids and Bases

16 A Lewis base is a compound that donates an electron pair to another compound. A Lewis acid is a compound that accepts an electron pair. In this process, ammonia is the Lewis base and BH 3 is the Lewis acid. Generally, the Lewis definition of acids and bases is the most useful because it is the most inclusive of the three definitions

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18 SALTS

19 Properties of Salts Salts form by the combination of acid and base through neutralization reaction. The acidic and basic nature of salts depends on the acid and base combined in neutralization reaction. The most common salt is sodium chloride or table salt which forms by the combination of sodium hydroxide (base) and hydrochloric acid. Other examples include Epsom salts(mgso 4 ) used in bath salts, ammonium nitrate (NH 4 NO 3 ) used as fertilizer, and baking soda (NaHCO 3 ) used in cooking.

20 ACID BASE SALT EXAMPLE Strong Strong Neutral NaOH + HCl NaCl + H 2 O Strong Weak Acidic HCl + NH 4 OH NH 4 Cl + H 2 O Weak Strong Basic CH 3 COOH + NaOH CH 3 COONa + H 2 O Weak Weak Neutral CH 3 COOH + NH 4 OH CH3COONH 4 + H 2 O

21 The ph of a salt solution depends on the strength of acids and bases combined in the neutralization reaction. ACID BASE SALT ph strong strong ph=7 weak strong ph>7 strong weak ph<7 weak weak depends on which is stronger

22 Acids form hydrogen ions (H + ) in aqueous solution. In place of hydrogen ions, it s better to write hydronium ions (H 3 O + ) which form due to reaction of H + with water. For example, Hydrochloric acid (HCl) in water. HCl + H 2 O H 3 O + + Cl - At the same time bases are the substances that produce hydroxide ion OH - in water. Acids and bases react with one another to yield two products: water, and an ionic compound known as salt. The reaction is called as neutralization reaction. NaOH + HCl H 2 O + Na + Cl

23 Neutralization reactions are always exothermic in nature, which means they liberate the same amount of heat. This amount is fixed for any acid and base that is 57.7kJ per mole of H +. For any acid and base combination the net reaction is H + (aq) + OH - (aq) H 2 O Because salt produced in neutralization reaction is ionic in nature and exists in the form of ions just before the reaction. Hence salts are the main products of a neutralization reaction