A Note on phase symbols, solubility and soluble compounds. Ch 4. Aqueous Reactions and Solution Stoichiometry. Dissociation.

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1 Ch 4. Aqueous Reactions and Solution Stoichiometry Understand the nature of ions in solutions Recognize and write common chemical reactions Use the activity series to predict products of reactions Write net-ionic equations Understand solution stoichiometry using molarity, solution preparation and dilutions of solutions A Note on phase symbols, solubility and soluble compounds In a balanced equation we will now begin to use phase symbols to indicate the physical state of the reactants/products. There are 4 phase symbols: 1. (s): solid 2. (l): liquid 3. (g): gas 4. (aq): aqueous. This means dissolved in water to form a homogeneous solution. By definition any soluble compound will have the (aq) phase symbol in a reaction where water is present. Soluble = (aq), Insoluble = (s). 1 2 Solutions: Homogeneous mixtures of two or more pure substances. The solvent is present in greatest abundance. All other substances are solutes. Dissociation When an ionic substance dissolves in water, the solvent pulls the individual ions from the crystal and solvates them. This process is called dissociation

2 Electrolytes Electrolytes and Nonelectrolytes Substances that dissociate into ions when dissolved in water. A nonelectrolyte may dissolve in water, but it does not dissociate into ions when it does so. Soluble ionic compounds tend to be electrolytes. 5 6 All Soluble Compounds have Electrolytic Properties! Electrolytes and Nonelectrolytes Molecular compounds tend to be nonelectrolytes, except for acids and bases. Non electrolyte Weak electrolyte Strong electrolyte 7 8 2

3 Strong Electrolytes Are Strong acids, Strong bases, Soluble ionic salts 9 10 Reaction Types in this Chapter 1.Precipitation 2.Acid-Base Ionization/Dissociation Reactions 3.Acid-Base Chemical Reactions 4.Gas Formation 5.Oxidation-Reduction You must be familiar with these and be able to predict the products (if possible) for each of these types of reactions! When one mixes ions that form compounds that are insoluble (as could be predicted by the solubility guidelines), a precipitate is formed. These reactions are often metathesis reactions. Precipitation Reactions PbCrO4 ppt rxn.mov

4 1. Precipitation or Exchange (Metathesis) Reactions Predict the products of the following exchange reactions: a) BaCl 2 (aq) + K 2 SO 4 (aq) > b)fe(no 3 ) 3 (aq) + NaOH(aq) > c) (NH 4 ) 2 S(aq) + Pb(NO 3 ) 2 (aq) > Solution Chemistry It is helpful to pay attention to exactly what species are present in a reaction mixture (i.e., solid, liquid, gas, aqueous solution)i.e look at the phase labels. If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction Acids: Substances that increase the concentration of H + when dissolved in water (Arrhenius). Proton donors (Brønsted Lowry). 15 Acids There are only seven strong acids: Hydrochloric (HCl) Hydrobromic (HBr) Hydroiodic (HI) Nitric (HNO 3 ) Sulfuric (H 2 SO 4 ) Chloric (HClO 3 ) Perchloric (HClO 4 ) 16 4

5 Bases: Bases Substances that increase the concentration of OH when dissolved in water (Arrhenius). Proton acceptors (Brønsted Lowry). The strong bases are the soluble salts of hydroxide ion: Alkali metals Calcium Strontium Barium Acid-Base Reactions In an acid-base reaction, the acid donates a proton (H + ) to the base. 2. Acid-Base Ionization Reactions Acids: Proton (H + ) Donors Acids: compounds that form hydrogen ions in water. Examples of ACID IONIZATION EQUATIONS: a) HCl(aq) > H + (aq) + Cl (aq) 100% ionized since HCl is a strong acid/electrolyte b) CH 3 COOH(aq) << > H + (aq) + CH 3 COO (aq) HC 2 H 3 O 2 (aq) << > H + (aq) + C 2 H 3 O 2 (aq) 1% ionized since CH 3 COOH is a weak acid/electrolyte c) H 2 SO 4 (aq) > H + (aq) + HSO 4 (aq) 100% ionized since H 2 SO 4 is a strong acid/electrolyte 19 d) HSO 4 (aq) << > H + (aq) + SO 4 2 (aq) 1% ionized since HSO 4 is a weak acid/electrolyte 20 5

6 2. Strong Acid Ionization - 100% ions formed 2. Weak acid ionization, only a few % of the acid molecules ionize QuickTime and a Animation decompressor are needed to see this picture. Strong acid.mov We write H + (aq) to show the proton is ionized from the acid. HCl(aq) H + (aq) + Cl (aq) 100% ionization for strong acids! In reality that proton is always attached to water forming the hydronium ion, H 3 O + (aq). Weak acid ionization is written with a double arrow to indicate less than 100% ionization. Examples: Oxalic acid: H 2 C 2 O 4 (aq) H + (aq) + HC 2 O 4 (aq) Acetic acid: CH 3 CO 2 H(aq) H + (aq) + CH 3 CO 2 (aq) Strong Base Dissociation 100% ions formed QuickTime and a Graphics decompressor are needed to see this picture. Strong base.mov Weak Base Hydrolysis 1% ions formed Types of Reactions: 3. Acid-Base Often called Neutralization Reactions Acids: compounds that can donate a proton, H +, to a base. Bases: compounds that can accept a proton, H +, from an acid. Reaction: Transfer the proton(s) to the base to make the products. Example reaction: Ba(OH) 2 (aq) + 2 HNO 3 (aq) > Ba(NO 3 ) 2 (aq) + 2 H 2 O(l) The insoluble hydroxide salts can also act as bases when in the presence of an acid: Al(OH) 3, Fe(OH) 2, Mg(OH) 2, etc. Example reaction: Al(OH) 3 (s) + 3 HCl(aq) > AlCl 3 (aq) + 3 H 2 O(l) 23 The weak base NH 3 also accepts a proton from an acid: NH 3 (aq) + HNO 3 (aq) > NH 4+ (aq) + NO 3 (aq) 24 6

7 3. Acid-Base (Neutralization) Predict the products of the following reactions. React all acidic protons on the acid, complete neutralization. a) HI(aq) + LiOH(aq) > Types of Reactions: 4. Gas-Forming There are several reactions that produce a gas as the product. We call these Acid-base Reactions with Gas Formation. b) H 2 SO 4 (aq) + Fe(OH) 3 (s) > c) CH 3 COOH(aq) + NH 3 (aq) > QuickTime and a Cinepak decompressor are needed to see this picture. Gummy Bear rxn.mov Gas-Forming Reactions Carbonates + acid > CO 2 (g) + H 2 O(l) + salt Sulfides + acid > H 2 S(g) + salt Predict the products of the following reactions: a) HI(aq) + NaHCO 3 (aq) > b) H 2 SO 4 (aq) + CaS(s) > c) CH 3 COOH(aq) +Al 2 (CO 3 ) 3 (s) > Writing Net-ionic Equations (for all reaction types) Net-ionic equations are used to describe the actual chemical species involved in an aqueous reaction. To write net-ionic equations you must identify ALL the STRONG ELECTROLYTES! Steps: 1. Write the correct, balanced molecular equation 2. Add phase symbols: (s), (l), (g), (aq). 3. Identify all the STRONG ELECTROLYTES. 1. Search only the (aq) species for the strong electrolytes! 4. Rewrite the strong electrolytes as ions wherever they occur. All other species are rewritten without modification. 5. Cancel any ions that appear on both sides of the reaction. 6. Balance the final net-ionic equation

8 Writing Net-Ionic Equations Na 2 CO 3 (aq) + HCl(aq) > NaCl + H 2 O + CO 2 Reaction Types: 5. Oxidation and Reduction (redox) Oxidation and reduction involve the transfer of electrons from one species to another. Oxidation: The lose of electrons Reduction: The gain of electrons One cannot occur without the other! AgNO 3 + KCl AgCl + KNO First step: Assigning Oxidation Numbers to Atoms I. For an atom in its elemental form the oxidation number is zero. II. For any monatomic ion the oxidation number equals the charge of the ion. III. For nonmetals the oxidation number is usually negative. a) Oxygen is usually -2 in all compounds. b) Fluorine is -1 in all compounds. c) Hydrogen is +1 when bonded to nonmetals and -1 when bonded to metals (metal hydrides). IV. The sum of the oxidation numbers for all atoms is zero for neutral compounds or equals the charge for polyatomic ions. Assign oxidation numbers to each atom in the following: 5. Oxidation of Metals by Acids and Salts Elemental metals can be oxidized in the presence of a salt or acid. The general form is: M + AX > MX + A where M is a metal and AX is a salt or acid. Examples: Cu(s) + 2 AgNO 3 (aq) > Cu(NO 3 ) 2 (aq) + 2 Ag(s) Net Ionic: Cu(s) + 2 Ag + (aq) > Cu 2+ (aq) + 2 Ag(s) Mg(s) + 2 HCl(aq) > MgCl 2 (aq) + H 2 (g) Net ionic: Mg(s) + 2 H + (aq) > Mg 2+ (aq) + H 2 (g) (a) NaCl (b) H 2 O (c) S 8 (d) SO 4 2- (e) Na 2 SO 4 (f) Mn(NO 3 )

9 5. Activity Series of Metals/Ions (to predict if a redox reaction will take place) 5. Oxidation Reduction Reactions Reactivity of Metal Increases Reactivity of Metal Ion Increases Any metal on the left will be oxidized (will react) with any ion on the right that is below the metal in the table. Predict the products (if any) of the following reactions: a) Mg(s) + FeCl 2 (aq) > b) HNO 3 (aq) + Ag(s) > Noble Metals (don t react with acids! 33 c) Zn(s) + CuSO 4 (aq) > 34 Definition of Molarity Molarity is the most common unit of concentration in chemistry. Molarity is defined as: (moles solute)/(1 L of solution) Solutions of precise molarity are made using volumetric flasks. Calculation of Molarity 1. Calculate the molarity of the of a CaCl 2 solution prepared by dissolving g of CaCl 2 into a 250-mL volumetric flask. a) What are the concentrations of each ion in solution? b) How many grams of CaCl 2 are contained in 75.0 ml of a M CaCl 2 solution? QuickTime and a Graphics decompressor are needed to see this picture

10 Calculation of Molarity 1. How many grams of Sr(OH) 2 are needed to prepare a solution that is 0.12 M in OH ion? a) What volume of this solution (ml) is needed to obtain 1.0 g of Sr(OH) 2? b) How many moles of Sr(OH) 2 are contained in 25 ml of a 0.12 M solution? Dilution of Known Concentrations Often solutions must be diluted before they can be used. The formula for dilution is: C 1 V 1 = C 2 V 2 = moles solute Where: C 1 is the concentration of the stock solution (before dilution) V 1 is the volume of the stock solution to be diluted C 2 is the concentration after dilution V 2 is the volume of the diluted solution QuickTime and a Graphics decompressor are needed to see this picture Dilution of Known Concentrations Using Molarity with Stoichiometry Often solutions must be diluted before they can be used. In lab you are asked to prepare a M NaOH solution from a 6.2 M stock solution. You need 300 ml of the diluted NaOH. How would you prepare this solution?

11 Stoichiometry of Aqueous Solutions Since M x V = moles solute, we can use M and volume of a reactant to find moles of reactant. With moles we can do reaction stoichiometry! Problem: Help! An acid spill! A student spills 500-mL of 3.4 M sulfuric acid on the floor. Your instructor quickly dumps 250 g of sodium bicarbonate on the acid to neutralize it. Was all the acid neutralized? This is a limiting reactant problem Titrations (Covered in lab as well) In a titration you are trying to determine the concentration of a solute in a solution of unknown concentration. A standard solution is used to react with the solute of interest. The equivalence point is when stoichiometrically equivalent quantities are brought together (balanced eqn. needed) The reaction can be of any form: acid-base, gas-formation or precipitation. HCl(standard) + NaOH(unknown conc.) > HNO 3 (standard) + 2 Na 2 CO 3 (unknown conc.) > 2 Al 2 (SO 4 ) 3 (standard) + 3 BaCl 2 (unknown conc.) > Acid-Base Titration Example Precipitation Examples What is the molarity of a NaOH solution that requires 37.5 ml of M HCl to titrate 50.0 ml of the NaOH? What is the molarity of a BaCl 2 solution that requires 14.5 ml of M Al 2 (SO 4 ) 3 to titrate 20.0 ml of the BaCl 2? What is the concentration of chloride ions (g/l) in solution if a 2.75 L sample of water requires 35.7 ml of a M AgNO 3 to precipitate all the chloride?

12 Ionic Salts - Solubility Rules HCl in Solution, 100% Ionized H 3 O + (aq) Cl (aq) Acetic Acid, a Weak Acid and Electrolyte Driving Forces for These Rxn s Why do these reactions take place? These reactions are said to be product-favored. Precipitation: the formation of the precipitate drives the reaction forward. Acid-base: the transfer of a proton drives the reaction forward. Gas-formation: the formation of a gaseous product drives the reaction forward. Reactions that do not take place or take place very little are said to be reactant-favored NaCl(aq) + HF(aq) < NaF(aq) + HCl(aq)

13 ph of Solutions To show the acid content of solutions we use the ph scale. ph = -log[h + ] Where [H + ] is the molarity of hydrogen ions in solution. (From now on [] represent concentration in molarity.) ph Scale Acidic solutions have ph values below 7 Basic solutions have ph values above 7 Neutral solutions have a ph = 7 What is the ph of a 1.0 M HCl solution? [H + ] in 1.0 M HCl is 1.0 M since HCl is a strong electrolyte. ph = -log(1.0) = -(0.0) = 0.0 What is the ph of a 6.0 M HCl solution? a M solution? Answers: and Acidic Neutral Basic Determining [H + ] from ph Common Strong Acids and Bases If the ph is known the [H + ] can be determined. [H + ] = 10 ph A 200-mL solution of acetic acid has a ph of What is the [H + ]? How many moles of H + are in solution? [H + ] = = M Moles H + = ( M H + )(0.200 L solution) = 3.6x10-4 mol H

14 More on Titrations in Lab