Explaining Acids and Bases Section 6.4 pg. 248-253 Do you remember? • Arrhenius Definition of Acids: ▫ A substance that forms an acidic solution by dissolving in water to + produce free hydrogen ions (H (aq)) in solution + ‐ ▫ Example: HCl (aq) H (aq) + Cl (aq) • Arrhenius Definition of Bases: ▫ A substance that forms a basic solution by dissolving in water to ­ produce free hydroxide ions (OH (aq)) in solution + – ▫ Example: NaOH(aq) Na (aq) + OH (aq) • What was the problem?? Arrhenius • Arrhenius did not know that a hydrogen ion is better described as a hydronium ion; a hydrogen ion bonded to a water molecule • We will be doing Investigation 6.2 (see pg. 260) which will show the limited ability of Arrhenius’ definition to predict acidic or basic properties of substances in aqueous solution + ‐ ▫ Easy to predict HCl(aq): HCl (aq) H (aq) + Cl (aq) ACID ▫ But what about: NaHCO3(aq) or CO2(g) ACID or BASE?? Do you remember? • Modified Definition of an Acid: ▫ A species that forms an acidic solution by reacting with water to + produce hydronium ions (H3O (aq)) + - ▫ Example: HCl (aq) + H2O(aq) H3O (aq) + Cl (aq) + - ▫ Example: NaHSO4(aq) Na (aq) + HSO4 (aq) Dissociation... - + 2- HSO4 (aq) + H2O(aq) H3O (aq) + SO4 (aq) Then reacts with water Do you remember? • Modified Definition of a Base: ▫ A species that forms an basic solution by reacting with water to produce ­ hydroxide ions (OH (aq)) 2+ ‐ ▫ Example: Ca(OH)2 (aq) Ca (aq) + 2 OH (aq) There is no need to show the reaction with water because hydroxide ions are already present in Ca(OH)2 so a simple dissociation produces the hydroxide ions + – ▫ Example: NH3 (aq) + H2O(aq) NH4 (aq) + OH (aq) The modiied deinition (reaction with water) is necessary here to explain how hydroxide ions form in solution. + 2‐ ▫ Example: Na2CO3 (aq) 2 Na (aq) + CO3 (aq) Dissociation... 2‐ ‐ ‐ CO3 (aq) + H2O(aq) HCO3 (aq) + OH (aq) then reacts with water Summary 1. Write the chemical formulas for the reactants: molecule or polyatomic ion water. • Ignore any Group 1 and 2 cations, and Group 7 anions in the compound formula. Evidence from many compounds shows that these ions do not produce acidic or basic solutions. • If the substance is a non-metal oxide (e.g. CO2(g), SO2(g)), use two moles of water for every mole of the substance in the reactants. 2. Note the evidence provided. If the final solution is acidic, write hydronium ions as the first product. If the final solution is basic, then hydroxide ions are the first product. 3. Complete the other product by determining the combination of atoms and charge required to balance the chemical equation. • The other product should be a recognizable chemical formula—usually a polyatomic ion on your polyatomic ion chart (see the inside back cover). Practice • In a test of the modiied Arrhenius theory, a student tested the pH of a solution made by dissolving solid sodium cyanide in water, and found it to have a pH greater than 7. • Can the modiied Arrhenius theory explain this evidence? Provide your reasoning. NaCN(s) soluble ionic compound – therefore dissociates: + ‐ ‐ NaCN (s) Na (aq) + CN (aq) then CN reacts with water ‐ ‐ CN (aq) + H2O(aq) HCN(aq) + OH (aq) • The modified Arrhenius theory can explain the basic character of a sodium cyanide solution because it is possible to write a balanced chemical equation with valid products, including the hydroxide ion. Practice • Carbon dioxide is a major air pollutant from the combustion of fossil fuels. Suggest a possible chemical reaction that explains the acidity of a carbon dioxide solution. ‐ + CO2(g) + 2 H2O(aq) HCO3 (aq) + H3O (aq) • The modified Arrhenius theory can explain the acidic character of a carbon dioxide solution because it is possible to write a balanced chemical equation with valid products, including the hydronium ion. More Practice.... • HI(aq) – explain acidic properties + ‐ ▫ HI(aq) + H2O(l) H3O (aq) + I (aq) • NaCH3COO(aq) – explain basic properties + ‐ ▫ NaCH3COO(aq) Na (aq) + CH3COO (aq) Simple Dissociation ‐ ‐ ▫ CH3COO (aq) + H2O(l) CH3COOH (aq) + OH (aq) • HOCl(aq) – explain acidic properties + ‐ ▫ HOCl + H2O(l) H3O (aq) + OCl (aq) Try Three More... • H3PO4(aq) – explain acidic properties + ‐ ▫ H3PO4(aq) + H2O(l) H3O (aq) + H2PO4 (aq) • Na2SO4 (aq) – explain basic properties + 2‐ ▫ Na2SO4 (aq) 2 Na (aq) + SO4 (aq) Simple Dissociation 2‐ ‐ ‐ ▫ SO4 (aq) + H2O(l) HSO4 (aq) + OH (aq) • Sr(OH)2(aq) – explain basic properties 2+ ‐ ▫ Sr(OH)2(aq) Sr (aq) + 2 OH (aq) Will I ask you to predict if something is an acid or base? • No, for example: + 2- Na2HPO4(s) → 2 Na (aq) HPO4 (aq) • Will the solution be acidic, basic, or neutral? We can write valid equations to predict that either hydronium ions or hydroxide ions will form when hydrogen phosphate ions react with water: 2- + 3- HPO4 (aq) + H2O(l) → H3O (aq) PO4 (aq) Or 2- - - HPO4 (aq) + H2O(l) → OH (aq) H2PO4 (aq) • Nothing that you have studied so far in this textbook enables you to predict which is correct. If you know that the solution turns blue litmus red, then you can select one of the equations to explain the evidence. Neutralization Reactions • Can now be defined as the reaction between hydronium and hydroxide ions to produce water. Example: HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) • If we mix an acid and a base, we must be mixing solutions of hydronium and hydroxide ions, so the modified Arrhenius theory predicts: + - H3O (aq) + OH (aq) H2O(l) Summary • Acids are substances that react with water to produce hydronium ions • Most bases are substances that react with water to produce hydroxide ions • Neutralization can be explained as the reaction between hydronium ions and hydroxide ions to produce water. • Try pg. 253 #4-5 .