Acids, Bases and Salts (Food Science)-1.pptx

food science

1. Chalimbana University School of Mathematics & Science Education Department of Chemistry Course Name : Biochemistry Chemistry Course Code : BCH 1100 Lecturer : Mwale James Phone : [phone] Email : [email protected]

2. Acids, Bases and Salts

3. Acids If hydronium ions are found in a solution, the solution is acidic in nature. Hydronium ions are the only positively-charged ions (cations) formed when an acid dissolve in water. All the properties of an acid are due to the presence of these ions. The chemical formula of a hydronium ion is H3O+ simplified as H+. Acids can also be defined as compounds that contain Hydrogen as the only cation when in solution e.g. Hydrochloric, HCl ; Sulphuric , H2SO4; Nitric, HNO3.

4. Cont ; Sulphuric , Hydrochloric and Nitric acids are inorganic. There are also organic acids such as Acetic acid (found in vinegar) which has the formula CH3CO2H . Strength of acids An acid is said to be strong if it can ionize completely. For example, hydrochloric, nitric, and sulphuric acids when in dilute form are completely ionized therefore they are strong acids, see the examples above On the other hand, acids like dilute ethanoic acid out of 1000 molecules in a dilute solution only 4 molecules ionize. These are weak acids

5. Cont ; Some properties s of acids 1.Acids have a sour taste. Lemons, vinegar, and sour candies all contain acids. 2. Acids change the color of certain acid-base indicates. Two common indicators are litmus and phenolphthalein. Blue litmus turns red in the presence of an acid, while phenolphthalein turns colorless. 3. Some acids are corrosive 4. All acids only show acidic properties when water is present. 5. All acids are proton donors

6. Cont ; 6. Metals above hydrogen in the activity series react with hydrochloric and Sulphuric acids to produce hydrogen and a salt . 7. Almost all acids liberate carbon dioxide from a carbonate . 8. Acids react with bases to produce a salt and water only. This is what is known as neutralization.

7. Cont ; Basicity of an acid This is the number of hydrogen ions H + ( aq ) which can be produced by one molecule of an acid. E.g. Hydrochloric acid ( HCl ) has a basicity of 1. It is monobasic Sulphuric acid (H 2 SO 4 ) has a basicity of 2. It is dibasic Phosphoric acid (H 3 PO 4 ) has a basicity of 3. It is tribasic Ethanoic acid (C 2 H 4 CO 2 ). has a basicity of 1. It is monobasic Bases A base is a proton acceptor. It is a substance which produces hydroxide ion (OH - ) as the only negatively changed ion when in solution. The common bases are basic oxides and hydroxides.

8. Cont ; Basic oxides (or hydroxide) are oxides (or hydroxides) of metals which contains the ions, O 2- (or OH - ), and will react with an acid to form a salt and water only . It is vital to realize the importance of the word only in this definition. if it was omitted, certain compounds, which are quite different from metallic oxides and hydroxides, would be included under the definition of base, for example lead (iv)oxide reacts with hydrochloric acid to form lead (ii) chloride (a salt), water and chlorine. The word only excludes lead (iv) Oxide from the class of bases because chlorine instead of salt and water only is also produced.

9. Cont ; Properties of bases Bases have properties that mostly contrast with those of acids. Aqueous solutions of bases are also electrolytes. Bases can be either strong or weak, just as acids can. Bases often have a bitter taste and are found in foods less frequently than acids. Many bases, like soaps, are slippery to the touch. Bases also change the color of indicators. Litmus turns blue in the presence of a base while phenolphthalein turns pink. Bases do not react with metals in the way that acids do. Bases react with acids to produce a salt and water.

10. The P/H Scale What is it? The pH scale measures how acidic or basic a substance is and It ranges from 0 to 14. If a pH is lower than 7 it indicates the solution is an acid . If it is above 7 it is a base or Alkaline. If a pH is a 7 it is neutral. Strong acids have lower pHs than weak acids and strong bases have lower pHs than weak bases . A neutral solution is neither an acid or base . Scientists use a pH number to show the strength of an acid or base. A pH is measured by dipping litmus into solution such as water or other substances.

11. Salts A salt is defined as a compound formed by the complete or incomplete replacement of the hydrogen ion of an acid by a basic radical. It is formed by a metallic radicle combining with an acidic radicle Types of Salts There are three types of salts; normal, acidic, and basic salts . A normal salt is formed by the complete replacement of the hydrogen ion of an acid by a basic radical An acid salt is formed by the incomplete replacement of the hydrogen ion of an acid by a basic radical.

12. Cont ; Phosphoric acid, a tribasic acid, reacts with sodium hydroxide an alkali in three ways; One mole of the acid can react with three moles of the alkali to form sodium phosphate, a normal salt, and water . In another reaction one mole of the acid can react with two moles of the alkali to form Disodium hydrogen phosphate, an acid salt, and water.

13. Cont ; In yet another reaction one mole of the acid can react with one moles of the alkali to form sodium dihydrogen phosphate, an acid salt, and water . In 1 Above a normal salt is formed because all the ionisable hydrogens are replaced. In 2 and 3 acid salts are formed because incomplete replacement of hydrogen ions has taken place. Acid salts solutions have properties of an acid because of the presence of hydrogen ions.