Acids and Bases.docx

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 gi...

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.** 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- ions Not limited to bases that produce OH- ions Does not take the existence of hydronium ions into account Takes the existence of hydronium ions into account Cannot explain substances that act as both an acid and a base Can explain substances that act as both an acid and a base 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 **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 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+ 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-