Chapter 4: Reactions in Aqueous Solution - PDF

MCHM011: Chapter 4 Reactions in Aqueous Solution Adapted by Mr. MT Olivier Aqueous Reactions...

MCHM011: Chapter 4 Reactions in Aqueous Solution Adapted by Mr. MT Olivier Aqueous Reactions Sefako Makgatho Health Sciences © 2015 Pearson Education University  Solutions Solutions are defined as homogeneous mixtures of two or more pure substances. The solvent is present in greatest abundance. All other substances are solutes. When water is the solvent, the solution is called an aqueous solution. Aqueous Reactions © 2015 Pearson Education  Aqueous Solutions Substances can dissolve in water by different ways: Ionic Compounds dissolve by dissociation, where water surrounds the separated ions. Molecular compounds interact with water, but most do NOT dissociate. Some molecular substances react with water when they dissolve. Aqueous Reactions © 2015 Pearson Education  Electrolytes and Nonelectrolytes An electrolyte is a substance that dissociates into ions when dissolved in water. A nonelectrolyte may dissolve in water, but it does not dissociate into ions when it does so. Aqueous Reactions © 2015 Pearson Education  Electrolytes A strong electrolyte dissociates completely when dissolved in water. A weak electrolyte only dissociates partially when dissolved in water. Aqueous A nonelectrolyte does NOT dissociate in water. Reactions © 2015 Pearson Education  Solubility of Ionic Compounds Not all ionic compounds dissolve in water. A list of solubility rules is used to decide what combination of ions will dissolve. Aqueous Reactions © 2015 Pearson Education  Precipitation Reactions When two solutions containing soluble salts are mixed, sometimes an insoluble salt will be produced. A salt “falls” out of solution, like snow out of the sky. This solid is called a precipitate. Aqueous Reactions © 2015 Pearson Education  Metathesis (Exchange) Reactions Metathesis comes from a Greek word that means “to transpose.” It appears as though the ions in the reactant compounds exchange, or transpose, ions, as seen in the equation below. AgNO3(aq) + KCl(aq) ⎯→ AgCl(s) + KNO3(aq) Aqueous Reactions © 2015 Pearson Education  Completing and Balancing Metathesis Equations Steps to follow 1) Use the chemical formulas of the reactants to determine which ions are present. 2) Write formulas for the products: cation from one reactant, anion from the other. Use charges to write proper subscripts. 3) Check your solubility rules. If either product is insoluble, a precipitate forms. 4) Balance the equation. Aqueous Reactions © 2015 Pearson Education  Ways to Write Metathesis Reactions 1) Molecular equation 2) Complete ionic equation 3) Net ionic equation (a) Predict the identity of the precipitate that forms when aqueous solutions of BaCl2 and K2SO4 are mixed. (b) Write the balanced chemical equation for the reaction. Aqueous Reactions © 2015 Pearson Education  Molecular Equation The molecular equation lists the reactants and products without indicating the ionic nature of the compounds. Pb(NO3)2(aq) + 2KI(aq) ⎯→ PbI2(s) + 2KNO3(aq) Write the net ionic equation for the precipitation reaction that occurs when aqueous solutions of calcium chloride and sodium carbonate are mixed. AgNO3(aq) + KCl(aq) ⎯→ AgCl(s) + KNO3(aq) Aqueous Reactions © 2015 Pearson Education  Complete Ionic Equation In the complete ionic equation all strong electrolytes (strong acids, strong bases, and soluble ionic salts) are dissociated into their ions. This more accurately reflects the species that are found in the reaction mixture. Ag+(aq) + NO3−(aq) + K+(aq) + Cl−(aq) ⎯→ AgCl(s) + K+(aq) + NO3−(aq) Aqueous Reactions © 2015 Pearson Education  Net Ionic Equation To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. The ions crossed out are called spectator ions, K+ and NO3−, in this example. The remaining ions are the reactants that form the product—an insoluble salt in a precipitation reaction, as in this example. Ag+(aq) + NO3−(aq) + K+(aq) + Cl−(aq) ⎯→ AgCl(s) + K+(aq) + NO3−(aq) Aqueous Reactions © 2015 Pearson Education  Writing Net Ionic Equations 1. Write a balanced molecular equation. 2. Dissociate all strong electrolytes. 3. Cross out anything that remains unchanged from the left side to the right side of the equation. 4. Write the net ionic equation with the species that remain. Aqueous Reactions © 2015 Pearson Education  Acids The Swedish physicist and chemist S. A. Arrhenius defined acids as substances that increase the concentration of H+ when dissolved in water. Both the Danish chemist J. N. Brønsted and the British chemist T. M. Lowry defined them as proton donors. Aqueous Reactions © 2015 Pearson Education  Bases Arrhenius defined bases as substances that increase the concentration of OH− when dissolved in water. Brønsted and Lowry defined them as proton acceptors. Aqueous Reactions © 2015 Pearson Education  Strong or Weak? Strong acids completely dissociate in water; weak acids only partially dissociate. Strong bases dissociate to metal cations and hydroxide anions in water; weak bases only partially react to produce hydroxide anions. Aqueous Reactions © 2015 Pearson Education  Acid-Base Reactions ❑In an acid–base reaction, the acid (H2O above) donates a proton (H+) to the base (NH3 above). ❑Reactions between an acid and a base are called neutralization reactions. ❑When the base is a metal hydroxide, water and a salt (an ionic compound) are produced. Aqueous Reactions © 2015 Pearson Education  Neutralization Reactions When a strong acid (like HCl) reacts with a strong base (like NaOH), the net ionic equation is circled below: HCl(aq) + NaOH(aq) ⎯→ NaCl(aq) + H2O(l) H+(aq) + Cl−(aq) + Na+(aq) + OH−(aq) ⎯→ Na+(aq) + Cl−(aq) + H2O(l) H+(aq) + OH−(aq) ⎯→ H2O(l) Aqueous Reactions © 2015 Pearson Education  Gas-Forming Reactions Some metathesis reactions do not give the product expected. When a carbonate or bicarbonate reacts with an acid, the products are a salt, carbon dioxide, and water. CaCO3(s) + 2 HCl(aq) ⎯→CaCl2(aq) + CO2(g) + H2O(l) NaHCO3(aq) + HBr(aq) ⎯→NaBr(aq) + CO2(g) + H2O(l) Aqueous Reactions © 2015 Pearson Education  Gas-Forming Reactions This reaction gives the predicted product, but you had better carry it out in the hood—the gas produced has a foul odor! Na2S(aq) + H2SO4(aq) ⎯→ Na2SO4(aq) + H2S(g) Aqueous Reactions © 2015 Pearson Education  Oxidation-Reduction Reactions Loss of electrons is oxidation. Gain of electrons is reduction. One cannot occur without the other. The reactions are often called redox reactions. Aqueous Reactions © 2015 Pearson Education  Oxidation Numbers To determine if an oxidation–reduction reaction has occurred, we assign an oxidation number to each element in a neutral compound or charged entity. Aqueous Reactions © 2015 Pearson Education  Oxidation Numbers Aqueous Reactions © 2015 Pearson Education Rules to Assign Oxidation Numbers Elements in their elemental form have an oxidation number of zero. The oxidation number of a monatomic ion is the same as its charge. Aqueous Reactions © 2015 Pearson Education  Rules to Assign Oxidation Numbers Nonmetals tend to have negative oxidation numbers, although some are positive in certain compounds or ions. – Oxygen has an oxidation number of −2, except in the peroxide ion, in which it has an oxidation number of −1. – Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal. Aqueous Reactions © 2015 Pearson Education Rules to Assign Oxidation Numbers – Fluorine always has an oxidation number of −1. – The other halogens have an oxidation number of −1 when they are negative; they can have positive oxidation numbers, most notably in oxyanions. Aqueous Reactions © 2015 Pearson Education Rules to Assign Oxidation Numbers The sum of the oxidation numbers in a neutral compound is zero. The sum of the oxidation numbers in a polyatomic ion is the charge on the ion. Aqueous Reactions © 2015 Pearson Education  Displacement Reactions In displacement reactions, ions oxidize an element. In this reaction, silver ions oxidize copper metal: Cu(s) + 2 Ag+(aq) ⎯→ Cu2+(aq) + 2 Ag(s) Aqueous The reverse reaction does NOT occur. Why not? Reactions © 2015 Pearson Education  Activity Series Elements higher on the activity series are more reactive. They are more likely to exist as ions. Aqueous Reactions © 2015 Pearson Education  Activity Series Zn(s)+ AgNO3(aq) Li(s)+KCl(aq) Ag(s) + CuSO4(s) Aqueous Reactions © 2015 Pearson Education Metal/Acid Displacement Reactions The elements above hydrogen will react with acids to produce hydrogen gas. The metal is oxidized to a cation. Aqueous Reactions © 2015 Pearson Education  Concentrations of the solutions Aqueous Reactions © 2015 Pearson Education  Molarity The quantity of solute in a solution can matter to a chemist. We call the amount dissolved its concentration. Molarity is one way to measure the concentration of a solution: moles of solute Molarity (M) = volume of solution in liters Aqueous Reactions © 2015 Pearson Education  Example 1 Calculate the molarity of a solution made by dissolving 23.4 g of sodium sulfate (Na2SO4) in enough water to form 125 mL of solution. The number of moles of Na2SO4 is obtained by using its molar mass: Converting the volume of the solution to liters: Thus, the molarity is: Aqueous Reactions © 2015 Pearson Education  Example 2 (a) How many grams of ethanol (CH3CH2OH), should you dissolve to make one liter of Vodka (which is an aqueous solution of 6.860 M ethanol). (b) Using the density of ethanol (0.789 g/ml), calculate the volume of ethanol you need to make 1.000 l of vodka). Example 2 In a sugar tolerance test, a patient needs to drink glucose (C6H12O6) solution containing 100 g of glucose. Given that one Aqueous cup = 350 ml, calculate the molarity in the glucose solution. Reactions © 2015 Pearson Education  Mixing a Solution To create a solution of a known molarity, weigh out a known mass (and, therefore, number of moles) of the solute. Then add solute to a volumetric flask, and add solvent to the line on the neck of the flask. Aqueous Reactions © 2015 Pearson Education  Dilution One can also dilute a more concentrated solution by – using a pipet to deliver a volume of the solution to a new volumetric flask, and – adding solvent to the line on the neck of the new flask. Aqueous Reactions © 2015 Pearson Education  Dilution The molarity of the new solution can be determined from the equation Mc  Vc = Md  Vd Where Mc and Md are the molarity of the concentrated and dilute solutions, respectively, and Vc and Vd are the volumes of the two solutions. Aqueous Reactions © 2015 Pearson Education  Example 1 How many milliliters of 3.0 M H2SO4 are needed to make 450 mL of 0.10 M H2SO4? Calculate the moles of H2SO4 in the dilute solution: Calculate the volume of the concentrated solution that contains 0.045 mol H2SO4: Converting liters to milliliters gives 15 mL. Aqueous Reactions © 2015 Pearson Education  Using Molarities in Stoichiometric Calculations Aqueous Reactions © 2015 Pearson Education  Titration A titration is an analytical technique in which one can calculate the concentration of a solute in a solution. Aqueous Reactions © 2015 Pearson Education  Titration A solution of known concentration, called a standard solution, is used to determine the unknown concentration of another solution. The reaction is complete at the equivalence point. Aqueous Reactions © 2015 Pearson Education  Assignment 1. A BSC I student from SMU was given 1.22 g sample of a pesticide, which he converted to AsO43- by suitable chemical treatment. It was then titrated using Ag+ to form Ag3AsO4 as a precipitate. (a) What is the oxidation state of As in AsO43-? (a) Name Ag3AsO4 by analogy to the corresponding compound containing phosphorus in place of arsenic. (a) If it took 25.0 mL of 0.102 M Ag+ to reach the equivalence Aqueous point in this titration, what is the mass percentage of arsenic Reactionsin ©the pesticide? 2015 Pearson Education  Assignment 2. A BSc I student from the department of chemistry performing titration as described as followed: (a) The titration, 15.0 mL of 0.1008 M sodium hydroxide is needed to neutralize a 0.2053 g sample of a weak acid. What is the molar mass of the acid if it is monoprotic? (b) An elemental analysis of the acid indicates that it is composed of 5.89% H, 70.6% C, and 23.5% O by mass. What is its molecular formula? Aqueous Reactions © 2015 Pearson Education