<p> 1.1.3 Acids and bases</p><p>What are acids and bases? Acids can be described as proton donors; this is known as the Lowry- Bronsted definition of an acid. Any substance which has the ability to produce an H+ ion is capable of acting as an acid. This means that compounds will be capable of acting as acids if they: a) are ionic, and contain hydrogen with a negative counter ion eg HNO3; b) are covalent and contain H bonded to an electronegative atom eg HOCH3. Compounds can exist with hydrogen as a negative ion (hydride); when the counter ion is an electropositive metal eg in NaH. These compounds are not acidic: not all compounds with hydrogen present, act as acids.</p><p>The release of a proton in aqueous solution can be shown by the reaction below, for hydrochloric acid.</p><p>+ - HCl(aq) + H2O(l) → H3O (l) + Cl (aq)</p><p>The proton is actually taken up by a water molecule.</p><p>In order to classify substances as bases or alkalis, we need to test their solubility and whether or not they react with an acid</p><p>We can summarise the results in the table below.</p><p>Substance Solubility in Effect on pH Did it Alkali or water of the HCl neutralise the base (or hydrochloric neither) acid? MgO (s) Insoluble Increases it √ Base NaOH (s) Soluble Increases it √ Alkali NaHCO (s) 3 Soluble Increases it √ Alkali sulphamic acid (s) Lowers (or Soluble doesn’t X Acid change it)</p><p>Technical: Requires 8 test tubes, spatula, substances, UI, rack, bung</p><p>1 A base can then be described as a proton acceptor; this is known as the Lowry-Bronsted definition of a base. If a substance is capable of accepting a proton, then it is capable of neutralising an acid; or raising the pH of a solution. Common bases are metal oxides, metal hydroxides and ammonia. The release of hydroxide in aqueous solution can be shown by the equation below.</p><p>+ - NaOH(s) + aq → Na (aq) + OH (aq)</p><p>Activity 1. Match the acids with their formulae</p><p>Name of acid Formula hydrochloric HCOOH</p><p> sulphuric HNO3</p><p> nitric H3CCOOH ethanoic HCl</p><p> methanoic H2SO4</p><p> phosphoric H3PO4</p><p>Activity 2. Match up the formulae with the bases</p><p>Name of base Formula sodium hydroxide Ca(OH)2</p><p> ammonia H3CNH2 calcium hydroxide NaOH methyl amine MgO</p><p> magnesium oxide NH3</p><p>You may also see aqueous ammonia described as ammonium hydroxide. This is because the ammonia reacts with water to produce ammonium hydroxide.</p><p>Dot and cross diagram for ammonium hydroxide</p><p>2 The reactions of acids</p><p>Acids commonly react with alkalis, bases and carbonates. An alkali is a soluble base. A base is a compound which can neutralise an acid.</p><p>Acids reacting with alkalis</p><p>Write balanced equations, with state symbols, to show the following reactions (in pencil). a) hydrochloric acid with sodium hydroxide HCl + NaOH → NaCl + H O ______(aq) (aq) (aq) 2 (l) b) nitric acid with calcium hydroxide</p><p>2HNO3(aq) + Ca(OH)2(aq) → CaCl2(aq) + 2H2O(l) ______c) sulphuric acid with ammonia H SO + NH → (NH ) SO ______2 4(aq) 3(aq) 4 2 4(aq)</p><p>Acids reacting with bases</p><p>Write balanced equations, with state symbols, to show the following reactions. a) hydrochloric acid with magnesium oxide</p><p>2HCl(aq) + MgO(s) → MgCl2(aq) + H2O(l) ______b) sulphuric acid with calcium oxide H SO + CaO → CaSO + H O ______2 4(aq) (s) 4(aq) 2 (l) c) phosphoric acid with copper (ii) oxide 2H PO + 3CuO → Cu (PO ) + 3H O ______3 4(aq) (s) 3 4 2(aq) 2 (l)</p><p>Acids reacting with carbonates</p><p>Write balanced equations, with state symbols, to show the following reactions. a) hydrochloric acid with calcium carbonate</p><p>2HCl(aq) + CaCO3(aq) → CaCl2(aq) + H2O(l) + CO2(g) ______</p><p>3 b) sulphuric acid with lithium carbonate</p><p>H2SO4(aq) + Li2CO3(aq) → Li2SO4(aq) + H2O(l) + CO2(g) ______c) ethanoic acid with sodium carbonate</p><p>2H3CCOOH(aq) + Na2CO3(aq) → 2H3CCOONa(aq) + H2O(l) + CO2(g) ______</p><p>Ionic equations We can also represent the reaction of an acid with an alkali by using an ionic equation. So the reaction between hydrochloric acid and sodium hydroxide becomes:</p><p>+ - H (aq) + OH (aq) → H2O(l)</p><p>The spectator ions, Na+ and Cl- have been removed from the original equation; they don’t really do anything or change their oxidation state.</p><p>The guidance from the examiners A spectator ion does not change. If the state changes, then the ion is not a spectator. For example, solid sodium carbonate with acid should be shown as an ionic equation as follows: </p><p>+ + Na2CO3(s) + 2H (aq) –> 2Na (aq) + CO2(g) + H2O(l)</p><p>The sodium ions have clearly changed as they have gone from part of an ionic lattice to hydrated ions.</p><p>But aqueous sodium carbonate and acid is:</p><p>2– + CO3 (aq) + 2H (aq) –> CO2(aq) + H2O(l)</p><p>The sodium ions can be omitted as they are (aq) throughout and do not change.</p><p>Write ionic equations, with state symbols, to show the following reactions (in pencil). i) hydrochloric acid with sodium hydroxide</p><p>+ + + - H (aq) + NaOH(s) → Na (aq) + H2O(l) or... H (aq) + OH (aq) → H2O(l) ______ii) hydrochloric acid with magnesium oxide</p><p>+ 2+ 2H (aq) + MgO(s) → Mg (aq) + H2O(l) ______iii) sulphuric acid with lithium carbonate</p><p>+ + 2H (aq) + Li2CO3(s) → 2Li (aq) + H2O(l) + CO2(g) ______</p><p>+ 2- or... 2H (aq) + CO3 (aq) → H2O(l) + CO2(g)</p><p>4 Strong and weak acids</p><p>When a strong acid ionises in water, its molecules fully dissociate fully to form hydrogen ions and chloride ions. The hydrogen ions are also known as + protons. The proton is donated to the water to form an oxonium ion (H3O ).</p><p>+ - HCl(aq) + H2O(l) → H3O (aq) + Cl (aq)</p><p>The position of equilibrium lies so far to the right hand side that all the reactants have been converted into product.</p><p>A weak acid only partially dissociates in water and an equilibrium is established which lies to the left hand side; most of the acid has not been dissociated. This is the case for most organic acids.</p><p>+ - CH3COOH(aq) + H2O(l) → H3O (aq) + CH3COO (aq)</p><p>Some of the protons are donated to water; there are four species in the reaction mixture.</p><p>Activity True or false (write a T or F next to each statement)</p><p>T a) Hydrochloric acid is classed as a strong acid because it fully dissociates in aqueous solutions. F b) A strong acid has a high number of moles per volume of solution. T c) A weak acid is capable of reacting with carbonates. T d) Methanoic, ethanoic and citric acids are all weak acids. F e) Aqueous sulphuric acid contains more sulphate ions than oxonium 2- + ions.(H2SO4(aq) + 2H2O(l) → SO4 (aq) + 2H3O (l))</p><p>Calculation to find formula of a hydrated salt</p><p>0.05 moles of the salt H2C2O4.nH2O weighs 6.3 g</p><p>Find the mass of 1 mole of hydrated salt Use this to find the value of n</p><p>Mass of 1 mole hydrated salt = (1/0.05) x 6.3 = 126 g Mr H2C2O4 = (2 x 1.0) + (2 x 12.0) + (4 x 16.0) = 90.0 Therefore total mass of H2O in 1 mole hydrated salt = 126 – 90 = 36 g So n = 36/18 = 2</p><p>Alternative ratio method (without calculating mass of 1 mole) Mr H2C2O4 = (2 x 1.0) + (2 x 12.0) + (4 x 16.0) = 90.0 Mass of 0.05 moles H2C2O4 = 90.0 x 0.05 = 4.5 g</p><p>Assume mass H2O = 6.3 – 4.5 = 1.8 g H2O Moles H2O = 1.8/18.0 = 0.1 moles</p><p>Molar ratio H2C2O4 : H2O is 0.05 : 0.1 ≡ 1 : 2 therefore n = 2</p><p>5 Salts</p><p>A salt is produced when the H+ ion of an acid is replaced by a metal ion or + NH4 . The reactions described above all produce salts; these are normally produced as aqueous solutions. </p><p>Activity How could you make the following salts?</p><p> a) Ammonium chloride; HCl + NH3 → NH4Cl ______</p><p> b) Copper (ii) nitrate; 2HNO3 + CuO → Cu(NO3)2 + H2O ______</p><p> c) Sodium iodide; HI + NaOH → NaI + H2O or 2HI + Na2CO3 → 2NaI + H2O + CO2 ______</p><p> d) Lithium phosphate;</p><p>H3PO4 + 3LiOH → Li3PO4 + 3H2O or 2H3PO4 + 3Li2CO3 → 2Li3PO4 + 3H2O + 3CO2 ______</p><p> e) Calcium ethanoate. 2H3CCOOH + Ca(OH)2 → Ca(H3CCOO)2 + 2H2O</p><p> or... 2H3CCOOH + CaO → Ca(H3CCOO)2 + H2O ______</p><p> or... 2H3CCOOH + CaCO3 → Ca(H3CCOO)2 + H2O + CO2</p><p>If the solutions are left to evaporate, at room temperature, then the hydrated salts are formed. A hydrated salt has the metal ion, the counter ion and water of crystallisation. </p><p>Example Name hydrated copper (ii) sulphate Metal ion Cu2+ 2- Counter ion SO4 Formulae CuSO4.5H2O</p><p>If the blue hydrated copper (ii) sulphate is heated strongly the water of crystallisation is removed to produce the white anhydrous salt, CuSO4.</p><p>6 7</p>