Acids and Bases PDF

Acids and Bases Arrhenius Theory of Acids and Bases **Definition: Arrhenius definition – An acid is a substance that dissociates in water to produce H+ ions.** For example: When hydrogen chloride is added to water, it dissociates into H+ ions and Cl- ions. HCl → H+ + Cl- - Acids which dissociate to...

Acids and Bases Arrhenius Theory of Acids and Bases **Definition: Arrhenius definition – An acid is a substance that dissociates in water to produce H+ ions.** For example: When hydrogen chloride is added to water, it dissociates into H+ ions and Cl- ions. HCl → H+ + Cl- - Acids which dissociate to give one H+ ion in aqueous solution are called monobasic acids. - Acids which dissociate to give two H+ ions in aqueous solution are called dibasic acids. - Acids which dissociate to give three H+ ions in aqueous solutions are called tribasic acids. HCl → H+ + Cl- (monobasic as one H+ produced) H2SO4 → 2H+ + SO4 2- (dibasic as two H+ produced) H3PO4 → 3H+ + PO4 3- (tribasic as three H+ produced) Acids such as hydrochloric acid, sulfuric acid and, nitric acid are described as strong acids because they fully dissociate in water. **Definition: Arrhenius definition – A strong acid is a substance that almost completely dissociates in water to give hydrogen ions.** Acids such as ethanoic acid and methanoic acid are described as weak acids because they dissociate only to a slight extent in water **Definition: Arrhenius definition – A weak acid is a substance that only slightly dissociates in water to give hydrogen ions.** H+ ions cannot exist on their own in water. They bond with a water molecule to form a hydronium ion, H3O+. Bases **Definition: Arrhenius definition – A base is a substance that dissociates in water to produce OH- ions.** NaOH → Na+ + OH- Mg(OH)2 → Mg2+ + 2OH- Ca(OH)2 → Ca2+ + 2OH- The OH- ion is commonly called the hydroxide ion. Since Sodium Hydroxide dissociates almost completely in water to give OH- ions it is referred to as a strong base. **Definition: Arrhenius definition – A strong base is a substance that almost completely dissociates in water to give hydroxide ions.** 1 Calcium hydroxide and magnesium hydroxide dissociate only to a slight extent in water and hence are referred to as weak basis. **Definition: Arrhenius definition – A weak base is a substance that only slightly dissociates in water to give hydroxide ions.** Shortcomings on the Arrhenius Theory - Arrhenius’s theory of acids and bases is limited to solutions dissolved in water. - Not all acid-base reactions need water, or even involve OH- ions. Bronsted – Lowry Theory of Acids and Bases **Definitions – A Bronsted – Lowry Acid is a proton donor. A Bronsted – Lowry base is a proton acceptor.** HCl + H2O → H2O+ Cl- (HCl donates a proton, H2O accepts a proton) NH3 + H2O → NH4+ + OH- (NH3 accepts a proton, H2O donates a proton) Some substances can act as both an acid and a base in the Brønsted-Lowry theory, depending on what they react with. HCl + H2O →H3O+ + Cl- NH3 + H2O → NH4 + + OH- H2O accepts proton → base H2O donated proton → acid A substance which can act as both an acid and a base is called amphoteric. **Definitions- Bronston-Lowry A strong acid is a good proton donor. A weak acid is a poor proton donor. A strong base is a good proton acceptor. A weak base is a poor proton acceptor.** Comparing the Arrhenius Theory and the Bronsted-Lowry Theory Arrhenius Theory Bronsted-Lowry Theory Limited to reactions in water Not limited to reactions in water Limited to bases that produce OH- Not limited to bases that produce OH- ions ions Does not take the existence of Takes the existence of hydronium ions hydronium ions into account into account Cannot explain substances that act as Can explain substances that act as both an acid and a base both an acid and a base 2 Conjugate Acid-Base Pairs **Definition: An acid changes into its conjugate base when it donates a proton.** CH3COOH → CH3COO- + H+ Acid → Conjugate base + proton CH3COOH is said to be the conjugate acid of CH3COO-. **Definition: A base changes into its conjugate acid when it accepts a proton.** CH3COO- + H+ → CH3COOH Base + proton → conjugate acid **Definition: A conjugate acid-base pair (conjugate pair) is any pair consisting of an acid and a base that differ by one proton.** Since CH3COOH and CH3COO- only differ by one proton, we refer to this as a conjugate pair. CH3COOH + H20 ↔ CH3COO- + H3O+ acid base base acid - Every acid has a conjugate base. - Every base has a conjugate acid. What is (i) the conjugate acid, (ii) the conjugate base of HC2O4-? HC2O4- must accept a proton to change into its conjugate acid HC2O4- + H+ → H2C2O4 base conjugate acid Conjugate acid is H2C2O4 HC2O4- must donate a proton to change into its conjugate base HC2O4- → H+ + C2O42_ Conjugate base is C2O42- Neutralisation **Definition: Neutralisation is the reaction between an acid and a base to form a salt and water. acid + base → salt + water** HCl + NaOH → NaCl + H2O Acid + base → Salt + water 3 **Definition: A salt is the substance formed when the hydrogen ion from an acid is replaced by a metal or an ammonium ion.** Uses of Neutralisation Medicine - Excess hydrochloric acid in the stomach causes heartburn. - Gaviscon contains sodium hydrogen-carbonate (a base) to neutralise the acid. HCl + NaHCO3 → NaCl + H2O + CO2 Hydrochloric acid + sodium hydrogen-carbonate → Sodium chloride + Water + Carbon dioxide Agriculture - If soil is too acidic, the yield of crops is low - Farmers spread lime on the soil to neutralise the acidity in the soil CaO + H2O → Ca(OH)2 Calcium oxide + water → Calcium hydroxide Environmental Protection - In areas that suffer from acid rain, limestone is added to lakes to neutralise the acidity H2SO4 + CaCO3 → CaSO4 + H2O + CO2 Sulfuric acid + limestone → Calcium sulfate + water + carbon dioxide Miscellaneous - Toothpaste is slightly basic to neutralise acids in food - Hair conditioner is slightly acidic to neutralise the base in shampoo - Vinegar is used o neutralise the alkaline sting of wasps 4 Exam Questions 2014 – HL – Section B – Question 7 Nitrous acid (HNO2) is a weak acid that is readily oxidised to the strong acid, nitric acid (HNO3). (a) Distinguish between a strong acid and a weak acid according to the Brønsted-Lowry theory. Strong acid: good proton donor. Weak acid: poor proton donor (b) Nitrous acid dissociates in water as follows: HNO2 + H2O → H3O+ + NO2 – Identify the two substances acting as bases in this equilibrium. H2O NO2- 2011 – Hl – Section B – Question 7 7. Sulfuric acid is a strong dibasic acid. The formula HA represents a weak monobasic acid. (a) How do strong acids differ from weak acids in their behaviour in water according to (i) the Arrhenius theory, Strong: Almost completely dissociated to give hydrogen ions (H+) in solution. Weak: Only slightly dissociated to give hydrogen ions (H+) in solution. (ii) the Brønsted-Lowry theory? Strong: Good proton donor. Weak: Poor proton donor. (b) What is the conjugate base of (i) sulfuric acid, HSO4- (ii) the weak acid HA? A- Which of these conjugate bases is the stronger? Explain. A- conjugate base of weak acid (c) Explain, by giving a balanced equation for its dissociation in water, that the conjugate base of sulfuric acid is itself an acid. HSO4 – + H2O → SO4 2– + H3O+ 5 2010 – HL – Section B – Question 8 8. (a) Define (i) acid, Proton (H+) donor. (ii) conjugate acid, A base with a proton added according to the Brønsted-Lowry theory. In acting as an acid-base indicator methyl orange behaves like a weak acid. Letting HX represent methyl orange, it dissociates as follows: HX → H+ + X– In aqueous solution, the undissociated form (HX) is red and the dissociated form (X– ) is yellow. Distinguish between a strong acid and a weak acid. Strong acid: Good proton donor. Weak acid: Poor proton donor What is the conjugate base of HX? X- 6