mohd faisol mansor/chemistry form 4/chapter 7 CHAPTER 7 ACIDS AND BASES HCl (g) H 2 O H + (aq) + Cl - (aq) NaOH(s) H 2 O Na + (aq) + OH - (aq)

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1 CHAPTER 7 ACIDS AND BASES Arrhenius Theory An acid is a chemical compound that produces hydrogen ions, H + or hydroxonium ions H3O + when dissolve in water. A base defined as a chemical substance that can neutralise an acid to produce a salt and water. An alkali is defined as a chemical compound that dissolve in water to produce hydroxide ions, OH -. Example HCl (g) H 2 O H + (aq) + Cl - (aq) NaOH(s) H 2 O Na + (aq) + OH - (aq) The role of water In the presence of water an acid will ionise to form hydrogen ion and alkaline will dissociate into hydroxide ions, OH -. Therefore, water is essential for the formation of hydrogen ions, H + that cause acidity and hydroxide ions, OH - that cause alkalinity. 130

2 BASICITY OF AN ACID Is the number of ionisable hydrogen atoms per molecule of an acid. Monoprotic Acid Acid which produces 1 hydrogen ion when one molecule of an acid ionises in H2O. Diprotic Acid Triprotic Acid Example: a) CH3COOH? (ethanoic acid) = b) H2SO4? (sulphuric acid ) = c) HNO3? (Nitric acid) = d) H3PO4? (Phosphoric acid) = STRENGTH OF ACID & ALKALI 131

3 STRONG ACID WEAK ACID Strong acid will dissociate or ionize completely in water to produce hydrogen, H + ions. Degree of dissociation is higher. Thus, higher concentration of hydrogen ions in aqueous acid solution. Therefore, low ph value of the acid solution. STRONG ALKALI WEAK ALKALI 132

4 PHYSICAL PROPERTIES OF ACID & ALKALI ph Value Conduct electricity Litmus paper ALKALI ph Value Litmus paper ACID Corrosive Taste Taste Corrosive Conduct electricity 133

5 CHEMICAL PROPERTIES OF ACID & ALKALI Acids can react with, i) bases to produce salts and water eg : ii) metal to produce salts and hydrogen gas eg : iii) metal carbonates to produce salts, carbon dioxide and water eg : iv) alkali to produce salts and water (neutralization) eg : Alkali can react with, i) acid to produce salts and water (neutralization) eg : ii) ammonium salt to produce salts, water and ammonia gas eg : 134

6 The concentration of acid and alkali Concentration are measurement of the quantity of solutes dissolved in a quantity of solvent. Grams per dm 3 ( g dm -3 ) Moles per dm 3 ( mol dm -3 ) *known as Molarity Concentration Unit Conversion Grams per dm 3 ( g dm -3 ) molar mass molar mass Moles per dm 3 ( mol dm -3 ) Example 1. The molarity of a bottle of nitric acid, HNO3 solution is 2.0 mol dm -3. What is the concentration of the solution in g dm -3? [RAM: H, 1 ; N, 14 ; O, 16] 135

7 2. Calculate the molarity of a sodium sulphate, Na2SO4 solution with a concentration of 28.4 g dm -3. [RAM: O, 16 ; Na, 23 ; S, 32] Calculating Involving Concentration and Molarity No of mole = Molarity x Volume ( cm3 ) 1000 MV n = g of copper (II) sulphate is dissolved in water to form 500 cm 3 solution. Calculate the concentration of copper (II) sulphate in g dm A 250 cm 3 nitric acid solution contains 0.4 moles. Calculate the molarity of the nitric acid. 136

8 3. What is the mass of sodium carbonate required to dissolve in water to prepare 200 cm 3 solution contains 50 g dm Calculate the number of moles of ammonia in 150 cm 3 of 2 mol dm -3 aqueous ammonia. 5. Calculate the volume in dm 3 of a 0.8 mol dm -3 sulphuric acid that contains 0.2 mol g sodium carbonate powder, Na2CO3 is dissolved in water and made up to 250 cm 3. What is the molarity of the sodium carbonate solution. [RAM: C,12;O,16;Na,23] 137

9 7. Dilute hydrochloric acid used in the school laboratories usually has a concentration of 2.0mol dm -3. Calculate the mass of hydrogen chloride that found in 250 cm 3 of the hydrochloric acid? [RAM : H,1; Cl,35.5] 8. The concentration of a potassium hydroxide solution is 84.0 g dm -3. Calculate the number of moles of potassium hydroxide present in 300 cm 3 of the solution. [RAM: K,39,H,1 O,16] 138

10 9. Calculate the number of moles of hydrogen ions present in 200 cm 3 of 0.5 mol dm -3 sulphuric acid. Preparation of Standard Solution A solution in which its concentration is accurately known is a standard solution. Preparation of a solution by dilution method Adding water to a concentrated solution changes the concentration of the solution but does not change the amount solutes ( number of moles)of solution present in solution. Add water Stock solution Dilute solution Moles of stock solution = moles of dilute solution n1 = n2 M1V1 = M2V2 139

11 Exercise 1. Find the volume of 2.0 mol dm -3 sulphuric acid, H2SO4 needed to prepare 100 cm 3 of 1.0 mol dm -3 sulphuric acid, H2SO4. 2. Calculate the volume of a concentrated solution needed to prepare each of the following dilute solution: a) 50 cm 3 of 0.1 mol dm -3 sodium hydroxide, NaOH solution from 2.0 mol dm -3 sodium hydroxide, NaOH solution. b) 100 cm 3 of 0.5 mol dm -3 potassium manganate(vii), KMnO4 solution from 1.0 mol dm -3 potassium manganate(vii), KMnO4 solution. 140

12 3. Calculate the volume of 2.0 mol dm -3 sulphuric acid, H2SO4 needed to prepare 2.5 dm 3 of 0.5 mol dm -3 of the same acid solution. 4. Calculate the molarity of potassium hydroxide, KOH, solution if 200 cm 3 of 2.0 mol dm -3 potassium hydroxide, KOH, solution is added to 200 cm 3 of water cm 3 of 0.5 mol dm -3 sodium hydroxide, NaOH, solution is diluted with 30 cm 3 of water. Calculate the molarity of the solution produced. 141

13 The ph Values and Molarity As the molarity of an acid increases, the ph value of the acid decreases, however the ph value of an alkali increases when the molarity of the alkali increases. Neutralisation Reaction of an acid and a base that produce salt and water. Acid-base titration Titration is a very useful laboratory technique in which one solution is used to analyse another solution. An acid of known concentration is carefully delivered from burette to completely neutralise a known volume of an alkali in a conical flask. Acid-base indicator is used to detect the end of titration. Eg: methyl orange, phenolphthalein and litmus. The point at which the colour of the solution change is called the end point. Titration using phenolphthalein 142

14 NEUTRALISATION ACID-BASE TITRATION 143

15 Example 1. Write a balanced equation for the neutralization of each of the following: a) Sulphuric acid, H2SO4 and barium hydroxide, Ba(OH)2 solution. b) Nitric acid, HNO3 and calcium hydroxide, Ca(OH)2 solution. c) Ethanoic acid, CH3COOH and potassium hydroxide, KOH solution. Acid-base Indicator Indicator Colour in alkalis Colour in neutral solution Colour in acids Methyl orange phenolphthalein litmus The end-point of neutralisation also can determined by another two method:- i) Measurement of ph values by computer ii) Measurement of electrical conductivity during titration 144

16 Numerical Problem involving Neutralisation Say the balance equation is aa + bb product which, A = acid a = no of mole of acid B = base b = no of mole of base MAVA = a and MBVB = b Therefore MAVA a = MBVB b Exercise 1. In an experiment, 25.0 cm 3 of a sodium hydroxide solution of unknown concentration required cm 3 of 1.0 mol dm -3 sulphuric acid to complete a reaction in titration. Calculate the molarity of sodium hydroxide. 145

17 2. What is the volume of 0.5 mol dm -3 sulphuric acid, H2SO4 needed to neutralize 25.0 cm 3 of 0.8 mol dm -3 ammonia, NH3 solution? 3. A sample of copper(ii) oxide, CuO was found to completely neutralize 100 cm 3 of 0.5 mol dm -3 hydrochloric acid, HCl. Calculate the mass of the sample. [RAM: O, 16 ; Cu, 64] 146

18 4. The volume of 0.15 mol dm -3 sulphuric acid, H2SO4 required to completely neutralize 25.0 cm 3 of potassium hydroxide, KOH solution is 30.5 cm 3. Calculate the molarity of the potassium hydroxide, KOH solution. 5. A student dissolved 3.65 g of hydrogen chloride gas, HCl in water to make 1.0 dm 3 of solution. Calculate the volume of a 0.1 mol dm -3 barium hydroxide, Ba(OH)2 solution required to completely neutralize 25.0 cm 3 of the acid solution. [RAM : H, 1 ; Cl, 35.5] 147