Reactions in Aqueous Solution Chapter 4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4.1 A solution is a homogenous mixture of 2 or more substances = solute dissovled in solvent The solute is(are) the substance(s) present in the smaller amount(s) The solvent is the substance present in the larger amount SolutionSolventSolute Soft drink (l) Air (g) H2OH2O N2N2 Sugar, CO 2 O 2, Ar, CH 4 Reactions Between Ions in Aqueous Solutions General properties of aqueous solution

An electrolyte is a substance that, when dissolved in water, results in a solution that can conduct electricity. A nonelectrolyte is a substance that, when dissolved, results in a solution that does not conduct electricity. nonelectrolyte weak electrolyte strong electrolyte 4.1 Solute

Strong Electrolyte – 100% dissociation NaCl (s) Na + (aq) + Cl - (aq) H2OH2O Weak Electrolyte – not completely dissociated CH 3 COOH CH 3 COO - (aq) + H + (aq) Conduct electricity in solution? Cations (+) and Anions (-) 4.1 Strong and Weak Electrolytes A reversible reaction. The reaction can occur in both directions. Acetic acid is a weak electrolyte because its ionization in water is incomplete.

Nonelectrolyte does not conduct electricity? No cations (+) and anions (-) in solution C 6 H 12 O 6 (s) C 6 H 12 O 6 (aq) H2OH2O

4.1 How to Predict Electrolytes Water soluble and ionic = strong electrolyte (probably) Water soluble and molecular, and a weak acid or weak base = weak electrolyte Otherwise, the compound is probably a nonelectrolyte.

Copyright © Cengage Learning. All rights reserved 7 Why does a chemical reaction occur? What causes reactants “want” to form products? Chemical Reactions

Four Driving Forces Favor Chemical Change 1.Formation of a solid 2.Formation of water 3.Transfer of electrons 4.Formation of a gas Copyright © Cengage Learning. All rights reserved 8 Type of Chemical Reactions Or combination Double displacement AB+CD AD+CB A+B AB AB A+B Double displacement AB+CD AD+CB

Metathesis Reactions Double Displacement Reactions cations(A and C) & anions (B and D) change partners in the reaction AB + CD –> AD + CB Example: Pb(NO 3 ) 2 (aq) + 2KI(aq) –> 2KNO 3 (aq) + PbI 2 (s) Precipitate

Precipitation of Lead Iodide PbI Precipitation Reactions Precipitate – insoluble solid that separates from solution

4.2 Solubility is the maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature.

Molecular equation, Ionic equation and net Ionic equation molecular equation ionic equation net ionic equation Pb NO Na + + 2I - PbI 2 (s) + 2Na + + 2NO 3 - Na + and NO 3 - are spectator ions: ions that are not involved in the overall reaction Pb(NO 3 ) 2 (aq) + 2NaI (aq) PbI 2 (s) + 2NaNO 3 (aq) precipitate Pb I - PbI 2 (s) 4.2

Writing Net Ionic Equations 1.Write the balanced molecular equation. 2.Write the ionic equation showing the strong electrolytes completely dissociated into cations and anions. Weak and non electrolytes are written as molecules 3.Cancel the spectator ions on both sides of the ionic equation 4.Check that charges and number of atoms are balanced in the net ionic equation AgNO 3 (aq) + NaCl (aq) AgCl (s) + NaNO 3 (aq) Ag + + NO Na + + Cl - AgCl (s) + Na + + NO 3 - Ag + + Cl - AgCl (s) 4.2 Write the net ionic equation for the reaction of silver nitrate with sodium chloride.

The net ionic equation tells us: What actually changed during a reaction Example: Cd 2+ (aq) + S 2- (aq) –> CdS (s) Writing ionic equations, ask: 1. is substance soluble ? 2. is substance a strong electrolyte? **If yes to both questions, write substance as ions. 3. Weak and non electrolytes are written as molecules. Conditions that favor product formation: formation of a precipitate formation of a soluble weak electrolyte formation of a nonelectrolyte formation of a gas oxidation-reduction reactions (involves electron transfer!)

Acids-Base Reactions Acids *Have a sour taste. Vinegar owes its taste to acetic acid. *React with certain metals to produce hydrogen gas. *React with carbonates and bicarbonates to produce carbon dioxide gas 4.3 *Cause color changes in plant dyes. 2HCl (aq) + Mg (s) MgCl 2 (aq) + H 2 (g) 2HCl (aq) + CaCO 3 (s) CaCl 2 (aq) + CO 2 (g) + H 2 O (l) *Aqueous acid solutions conduct electricity.

Have a bitter taste. Feel slippery. Many soaps contain bases. Bases 4.3 Cause color changes in plant dyes. Aqueous base solutions conduct electricity.

Arrhenius acid is a substance that produces H + (H 3 O + ) in water Arrhenius base is a substance that produces OH - in water 4.3 Arrhennius’s definition can be applied only to aqueous solution.

A Brønsted acid is a proton donor A Brønsted base is a proton acceptor Brønsted acid Brønsted base Conjugate acid Conjugate base 4.3 A Brønsted acid must contain at least one ionizable proton!

Identify each of the following species as a Brønsted acid, base, or both. (a) HI, (b) CH 3 COO -, (c) H 2 PO 4 - HI (aq) H + (aq) + I - (aq)Brønsted acid CH 3 COO - (aq) + H + (aq) CH 3 COOH (aq)Brønsted base H 2 PO 4 - (aq) H + (aq) + HPO 4 2- (aq) H 2 PO 4 - (aq) + H + (aq) H 3 PO 4 (aq) Brønsted acid Brønsted base 4.3 Amphoteric species

Neutralization Reaction acid + base salt + water HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O H + + Cl - + Na + + OH - Na + + Cl - + H 2 O H + + OH - H 2 O 4.3 Neutralization occurs when a solution of an acid reacts with a base. The products are salt and water. Salt = ionic compound cation from a base + anion from an acid However, a weak base and an acid gives only a salt HCl +NH 3  NH 4 Cl

Oxidation-Reduction Reactions Oxidation number The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred. 1.Free elements (uncombined state) have an oxidation number of zero. Na, Be, K, Pb, H 2, O 2, P 4 = 0 2.In monatomic ions, the oxidation number is equal to the charge on the ion. Li +, Li = +1; Fe 3+, Fe = +3; O 2-, O = -2 3.The oxidation number of oxygen is usually –2. 4.4

4.The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds (e.g. LiH, CaH 2 ). In these cases, its oxidation number is –1. 6.The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion. For example,NH 4 +, the sum of oxidation numbers is -3+4(+1)=+1 5.Group IA metals are +1, IIA metals are +2 and fluorine is always –1. HCO 3 - O = -2H = +1 3x(-2) ? = -1 C = +4 Oxidation numbers of all the elements in HCO 3 - ? 4.4

The oxidation numbers of elements in their compounds 4.4 *Metallic elements: only positive oxidation number; nonmetallic elements: positive or negative oxidation number; *The highest oxidation number of an element in group1A-7A is its group number.

NaIO 3 Na = +1 O = -2 3x(-2) ? = 0 I = +5 IF 7 F = -1 7x(-1) + ? = 0 I = +7 K 2 Cr 2 O 7 O = -2K = +1 7x(-2) + 2x(+1) + 2x(?) = 0 Cr = +6 Oxidation numbers of all the elements in the following ? 4.4

Zn (s) + CuSO 4 (aq) ZnSO 4 (aq) + Cu (s) Zn is oxidizedZn Zn e - Cu 2+ is reducedCu e - Cu Zn is the reducing agent CuSO 4 is the oxidizing agent 4.4 Copper wire reacts with silver nitrate to form silver metal. What is the oxidizing agent in the reaction? Cu (s) + 2AgNO 3 (aq) Cu(NO 3 ) 2 (aq) + 2Ag (s) Cu Cu e - Ag + + 1e - AgAg + is reducedAgNO3 is the oxidizing agent Oxidation and Reduction always occur together

Types of Oxidation-Reduction Reactions Combination Reaction A + B C 2Al + 3Br 2 2AlBr 3 Decomposition Reaction 2KClO 3 2KCl + 3O 2 C A + B

Types of Oxidation-Reduction Reactions Combustion Reaction A + O 2 B S + O 2 SO Mg + O 2 2MgO

Single Displacement Reaction A + BC AC + B Sr + 2H 2 O Sr(OH) 2 + H 2 TiCl 4 + 2Mg Ti + 2MgCl 2 Cl 2 + 2KBr 2KCl + Br 2 Hydrogen Displacement Metal Displacement Halogen Displacement Types of Oxidation-Reduction Reactions

The Activity Series for Metals The Activity Series * Arranges metals according to their ease of oxidation *The higher the metal on the Activity Series, the more active that metal (the easier it is oxidized.)Any metal can be oxidized by the metal ions below it. *Any metal above hydrogen will displace it from water or from an acid.

The Activity Series arranges metals according to their ease of oxidation can be used to predict reactions The higher the metal on the Activity Series, Figure 4.16, the more active that metal (the easier it is oxidized.) Any metal can be oxidized by the ions of elements below it on Figure Cu + AgNO3 --> Cu(NO3)2 + Ag Cu is above Ag in the activity series; Cu is more active. Therefore Cu will displace Ag+ from a solution of AgNO3. Silver metal will come out of the solution (reduction.) The solution will begin to turn blue from the presence of Cu2+ as the copper metal reacts (oxidizes.)

3. Halogen Displacement F 2 > Cl 2 > Br 2 > I 2 F 2 is the greatest oxidizing halogen I 2 is the least oxidizing halogen Example: Br - + Cl 2 --> 2 Cl - + Br 2 Br 2 + Cl- --> no reaction (NR)

Disproportionation Reaction Cl 2 + 2OH - ClO - + Cl - + H 2 O Element is simultaneously oxidized and reduced. Types of Oxidation-Reduction Reactions Chlorine Chemistry

Solution Stoichiometry The concentration of a solution is the amount of solute present in a given quantity of solvent or solution. 4.5 M = molarity = moles of solute liters of solution = mole liters n V = What mass of KI is required to make 500 mL of a 2.80 M KI solution? M KI volume of KI solutionmoles KIgrams KI 500 mL= 232 g KI 166 g KI 1 mol KI x 2.80 mol KI 1 L soln x 1 L 1000 mL x

Dilution is the procedure for preparing a less concentrated solution from a more concentrated solution. Dilution Add Solvent Moles of solute before dilution (i) Moles of solute after dilution (f) = MiViMiVi MfVfMfVf = 4.5

How would you prepare 60.0 mL of M HNO 3 from a stock solution of 4.00 M HNO 3 ? M i V i = M f V f 4.5 M i = 4.00 M f = 0.200V f = 0.06 L V i = ? L V i = MfVfMfVf MiMi = x = L = 3 mL 3 mL of acid + 57 mL of water= 60 mL of solution

Titrations Slowly add base to unknown acid UNTIL the indicator changes color

What volume of a M NaOH solution is required to titrate mL of a 4.50 M H 2 SO 4 solution? WRITE THE CHEMICAL EQUATION! volume acidmoles acidmoles basevolume base H 2 SO 4 + 2NaOH 2H 2 O + Na 2 SO mol H 2 SO mL soln x 2 mol NaOH 1 mol H 2 SO 4 x 1000 ml soln mol NaOH x mL = 158 mL M acid rx coef. M base i a X M a X V a = i b X M b X V b Where a refers to acid and b refers to base. i a refers to number of H in the chemical formula of acid and i b refers to the number of OH in the chemical formula of base. Short-cut formula: