Acid and base theories

In chemistry, acids and bases have been defined differently by three sets of theories. One is the Arrhenius definition, which revolves around the idea that acids are substances that ionize (break off) in an aqueous solution to produce hydrogen (H+) ions while bases produce hydroxide (OH-) ions in solution.

1. ACIDS AND BASES Presented By: Ms. Kad Dhanashree R. Assistant Prof. (Pharmaceutical Chemistry) PES Modern College of Pharmacy (For Ladies) Moshi,Pune-412105

2. OBJECTIVE: To Understand the Definations of Acid and Base To learn theories of acid and base CONTENTS: Examples of common acid base household and lab. Arrhenius Theory Neutralization

3. Common household acids Citric acid Ethanoic acid Lactic acid Stearic acid Acetylsailicylic Acid

4. Hydrochloric acid Nitric acid Sulfuric acid Phosphoric acid HCl HNO 3 H 2 SO 4 H 3 PO 4

5. Arrhenius was a Sweedish chemist Put forward a theory of acids in the 1880’s Stated that: An acid is a substance that dissociates in water to form H + ions.

6. For example: when HCl is added to water: H + HC l + C l - In general: HA H + + A -

7. Acids HCl and HNO 3 are monobasic acids as they donate H + one H + ion. HN O 3 + N O 3 - H 2 SO 4 is a dibasic acid as it donates two H + ions. H 2 S O 4 2 H + + S O 4 2 - H 3 PO 4 is a tribasic acid as it donates three H + ions. H 3 P O 4 3H + + P O 4 3-

8. A strong acid is one which dissociates fully in water Example: HCl, H 2 SO 4 , HNO 3 HCl + H 2 O H 3 O + + Cl - A weak acid is one which does not fully dissociate in water Example: CH 3 COOH (ethanoic acid) C H 3 COOH + H 2 O H 3 O + + CH 3 COO -

9. M a gnesium hydroxide Ammonia Sodium hydrogen carbonate Common household bases Sodium hydroxide Calcium hydroxide

10. Sodium hydroxide Calcium hydroxide Ammonia Sodium carbonate NaOH Ca ( OH ) 2 NH 3 Na 2 CO 3

11. Arrhenius theory of bases Arrhenius defined a base as: A substance that dissociates in water to produce OH - ions. For example: when NaOH is added to water: NaOH Na + + OH - In general: XOH X + + O H -

12. A strong base is one which dissociates fully in water Example: NaOH A weak base is one which does not fully dissociate in water Example: Mg(OH) 2

13. Arrhenius theory Co m bining: HA XOH H + X + + A - + O H - we get: HA + XOH acid + base AX + H 2 O salt + water

14. Limitations of Arrhenius theory The acids and bases must be in aqueous solutions (i.e. water). This prevents the use of other solvents benzene. Not all acid – base reactions are in solution, e.g. ammonia gas and hydrogen chloride gas produce ammonium chloride. According to Arrhenius, the salt produced should not be acidic or basic. This is not always the case, for example in the above reaction ammonium chloride is slightly acidic

15. Arrhenius thought that an acid gives off H + ions in solution. H + ions are protons and can not exist independently. When the acid dissociates, the H + ions react with water molecules: Hydronium Ion H + + H 2 O H 3 O + The H 3 O + ion is called the hydronium ion. This is another limitation of the Arrhenius theory.

16. Brønsted-Lowry Theory In 1923, Johannes Brønsted (a Danish chemist) and Thomas Lowry (an English chemist) proposed new definitions of acids and bases. Brønsted Lo w ry

17. Brønsted-Lowry Theory Brønsted and Lowry had worked independently of each other but they both arrived at the same definitions: An acid is a substance that donates protons (hydrogen ions). A base is a substance that accepts protons.

18. Acid = Proton Donor + Cl - The HCl donates a proton and so is an acid The H 2 O, in this case, accepts a proton and so is a base Remember: Proton = H + Donates a Proton HC l + H 2 O H 3 O + Accepts a Proton

19. Likewise: HNO 3 + H 2 O H 3 O + + NO 3 - and H 2 SO 4 HSO 4 - + H 2 O + H 2 O H 3 O + + HSO 4 - H 3 O + + SO 4 - 2

20. Base = Proton Acceptor NH 3 + H 2 O The NH 3 accepts a proton and so is a base. The H 2 O, in this case, donates a proton and so is an acid. Accepts a proton NH 4 + OH + - Donates a proton

21. Amphoteric As can be seen from the previous two examples, water is capable of acting as both and acid and a base. Any substance that can act as both an acid and a base is said to be amphoteric .

22. Acid – Base Reaction Cl - + NH 4 + Acid – Donates Protons HCl + NH 3 Base – Accepts Protons

23. Neutralisation The reaction between an acid and a base to produce a salt and water A salt is formed when the hydrogen of an acid is replaced by a metal (or ammonium ion)

24. Neutralisation A c id + B a s e Sa l t + Wat e r HCl + NaOH NaCl + H 2 O but since the acid and base dissociate in water we can write: H + + Cl - + Na + + OH - Na + + Cl - + H 2 O we can cancel the Na + and Cl - on both sides leaving: H + + OH - H 2 O

25. Everyday Examples of Neutralisation Indigestion remedies are bases that neutralise excess stomach acid Lime is a base that neutralises acid in soil Toothpaste is a base that neutralises acid in the mouth

26. Wasp stings are basic They can be neutralised with vinegar or lemon j u ice Nettle, bee and ant stings are acidic They can be neutralised with baking soda

27. Conjugate Acid-Base Pairs Acids and bases exist in pairs called conjugate acid-base pairs. Every time an acid donates/loses a proton, it becomes its conjugate base . C H 3 COO - + H 3 O + Example: C H 3 COOH + H 2 O Acid Conjugate Base

28. Likewise: When a base accepts a proton, it becomes its conjugate acid . 4 N H + + O H - Example: N H 3 + H 2 O Base C o n juga te Acid

29. Examples: H 2 S O 4 H S O 4 - N H 4 + Ac i d C o njug a te Base + H 2 O Base + H 3 O + C o njug a te Acid Ac i d N H 3 + H 2 S Base Conjugate Base + HS - Conjugate Acid