Reactions in Aqueous Solutions Course Instructor: HbR GENERAL CHEMISTRY CHE 101 Reactions in Aqueous Solutions Course Instructor: HbR

Lecture Plan General properties Precipitation Reaction Acid-Base Reaction Oxidation-Reduction Reaction Electronegativity Concentration of Solutions Dilutions of Solutions

Solution A solution is a homogenous mixture of 2 or more substances. The solute is (are) the substance(s) present in smaller amount(s) in a solution. The solvent is the substance present in larger amount in solution Solution Solvent Solute Soft drink (l) H2O Sugar, CO2

Properties of Aqueous Solution Electrolyte : Is a substance that, when dissolved in water, results in a solution that can conduct electricity. Nonelectrolyte: is a substance, that, when dissolved, results in a solution that does not conduct electricity. Nonelectrolyte weak electrolyte strong electrolyte

Electrolytic Solutions Cations (+) and Anions (-) Strong electrolyte – 100% dissociation NaCl (s) Na+ (aq) + Cl- (aq) Weak electrolyte – not completely dissociated CH3COOH CH3COO-(aq) + H+(aq) H2O H2O

Electrolytic Solutions Ionization of acetic acid CH3COOH CH3COO-(aq) + H+(aq) A reversible reaction. The reaction can occur in both directions. Acetic acid is a weak electrolyte because its ionization in water is incomplete.

Electrolytic Solutions

Electrolytic Solutions Water is a very effective solvent for ionic compounds. Although water is an electrically neutral molecule, it has a positive region (the H atoms) and a negative region (the O atom), or positive and negative “poles”; for this reason it is a polar solvent.

Electrolytic Solutions When an ionic compound such as sodium chloride dissolves in water, the three-dimensional network of ions in the solid is destroyed. The Na+ and Cl- ions are separated from each other. NaCl (s) Na+ (aq) + Cl- (aq) H2O

Electrolytic Solutions

Reactions in aqueous solution Precipitation Reaction Acid-Base Reaction Oxidation-Reduction Reaction

Ca(OH)2 (s) Ca2+(aq) + OH-(aq) H2O Ca(OH)2 (s) Ca2+(aq) + OH-(aq) Dissolving in water breaks the ionic bonds. s aq Precipitation Dissolution Dissolution is the process by which a solute (solid) forms a solution in a solvent (water). Precipitation is the formation of a solid in a solution during a chemical reaction

Precipitation reactions Precipitate : insoluble solid that separates from solution. Solubility: Maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature. Solubility is a qualitative measure (e. g., soluble, slightly soluble, insoluble) Ionic compounds are strong electrolytes /solubility varies Insoluble compounds can also be soluble to a certain extent (depending on Temp)

Precipitation reactions

Examples of Precipitation Reaction CdS, PbS, Ni(OH)2, AL(OH)3

Classify the solubility

Precipitation reactions Molecular equation Pb(NO3)2(aq)+2KI(aq) PbI2(s)+2KNO3(aq) Ionic equation Pb2+(aq)+2NO3-(aq)+2K+(aq)+2I-(aq) PbI2(s)+2K+(aq)+2NO3-(aq) Net Ionic equation Pb2++ 2I- PbI2 (s) K+ and NO3- are spectator ions. PbI2

Precipitation reactions Pb2+ + 2I- PbI2 (s)

silver nitrate with sodium chloride. Net Ionic Equation Write the balanced molecular equation. Write the ionic equation showing the strong electrolytes completely dissociated into cations (+) and anions (-). Cancel the spectator ions on both sides of the ionic equation. Check that charges and numbers of atoms are balanced in the net ionic equation. Write the net ionic equation for the reaction of silver nitrate with sodium chloride. AgNO3(aq) + NaCl (aq) AgCl(s) + NaNO3(aq) Ag+(aq) +NO3-(aq) + Na+(aq) + Cl- (aq) AgCl (s) + Na+(aq) +NO3-(aq)

Reactions in aqueous solution Precipitation Reaction Acid-Base Reaction Oxidation-Reduction Reaction

Acid-Base Reaction Properties of acid: Have a sour taste. (Vinegar owes its taste to acetic acid. Citrus fruits contain citric acid.) Cause color changes in litmus test (blue- red) React with certain metals to produce hydrogen gas. 2HCl(aq) + Mg(s) MgCl2(aq) +H2(g) React with carbonates and bicarbonates to produce carbon dioxide gas. 2HCl(aq) + CaCO3(s) CaCl2(aq) + CO2(g) + H2O(l) Aqueous acid solutions conduct electricity.

Acid-Base Reaction Properties of Base: Have a bitter taste. Feel slippery. Many soaps which contain bases, show this property. Cause color changes in litmus test (red-blue) Aqueous base solutions conduct electricity. Example: NH3 OH-

Acid-Base Reaction Arhenius acid is a substance that produces H+ (H30+) in water. Arhenius base is a substance that produces OH- in water.

Acid-Base Reaction A Bronsted acid is a proton donor. A Bronsted base is a proton accepter. A Bronsted acid must contain at least one ionizable proton.

Acid-Base Reaction Monoprotic acids HCl H+ + Cl- Strong electrolyte, strong acid HNO3 H+ + NO3- Strong electrolyte, strong acid CH3COOH H+ + CH3COO- weak electrolyte, weak acid Diprotic acids H2SO4 H+ + HSO4- Strong electrolyte, strong acid HSO4- H+ + SO42- weak electrolyte, weak acid Triprotic acids H3PO4- H+ + H2PO4- weak electrolyte, weak acid H2PO42- H+ + HPO42- weak electrolyte, weak acid HPO42- H+ + PO4- weak electrolyte, weak acid

Neutralization Reaction A neutralization reaction is a reaction between an acid and a base. (if they are strong) Acid + Base  Salt + Water HCl(aq) + NaOH(aq) NaCl(aq) + H2O H+ + Cl- + Na+ + OH- Na+ + Cl- + H2O H+ + OH- H2O

Neutralization Reaction Weak acid + base salt + water HCN(aq) + NaOH (aq) NaCN (aq) + H2O HCN + Na+ + OH- Na+ + CN- + H2O HCN + OH- CN- + H20 Neutralization reaction producing a Gas Acid + base salt + water + gas HCl(aq) + Na2CO3(aq) 2NaCl(aq) + H2O + CO2 2H+ + 2Cl- + 2Na+ + CO32- 2Na+ + Cl- + H2O + CO2 2H+ + CO32- H2O + CO2

Reactions in aqueous solution Precipitation Reaction Acid-Base Reaction Oxidation-Reduction Reaction

Redox Reaction-Transfer of electrons ‘X’ id oxidized Loss of electron Oxidation number increase. Mg is reducing agent. ‘Y’ is reduced Gain of electron Oxidation number decreases O2 is oxidizing agent. Y 2Mg(s) + O2 (g)  2MgO (s) X

Oxidation-Reduction Reaction 2Mg 2Mg2+ + 4e- Oxidation half-reaction (lose e-) O2 + 4e- 2O2- Reduction half-reaction (gain e-) 2Mg + O2 + 4e- 2Mg2+ + 2O2- + 4e- 2Mg + O2 2MgO

Electronegativity Tendency to attract electron to it. What happens if two atoms of equal electronegativity bond together? example, H2 or Cl2molecules. What happens if B is slightly more electronegative than A? example; H2O What happens if B is a lot more electronegative than A? example; NaI

Electronegativity The most electronegative element is fluorine (F). Exception is Hydrogen (H).

Formal charge Formal charge: pretending all the electrons are shared/ perfectly covalent. H • • O • • H Formal charge (FC) is the charge assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative Electronegativity. Formal Charge = [# of valence electrons on atom] – [non-bonded electrons + number of bonds] Valence e- 1 6 1 Formal charge 0 0 0

Let’s try it! Calculate the formal charge on each atom in O3? 0 +1 -1 0 +1 -1 Can we infer it from Lewis structure ? Knowing the formal charge on a particular atom in a structure is an important part of keeping track of the electrons and is important for establishing and predicting the reactivity.

Rules to imply… For molecules, the sum of the charges must add up to zero because molecules are electrically neutral species For cations, the sum of formal charges must equal the positive charge For anions, the sum of formal charges must equal the negative charge

Oxidation number Oxidation number: assuming all the bonds are perfectly ionic bonds. H • • O • • H e- in ions 0 8 0 Oxidation num +1 -2 +1 (NH4)2MoO4   Calculate the oxidation number for each atom underneath… Oxidation number is assigned to an element in chemical combination that represents the number of electrons lost (positive) or gained (negative) (NH4)2MoO4 contains the NH4+ ion, in which H is (+1) and N is (-3). Because there are two NH4+ ions, the other half of the compound must be an MoO42- ion, in which Mo is (+6) and O is (-2)

Oxidation numbers

Oxidation-Reduction Reactions Combination Reaction A+B C 2Al + 3Br2 2AlBr3 Decomposition Reaction C A + B 2KClO3 2KCl + 3O2 Types of Redox reactions..

Oxidation-Reduction Reactions Combination Reaction A+B C 2Al + 3Br2 2AlBr3 0 0 +3 -1 Decomposition Reaction C A + B 2KClO3 2KCl + 3O2 +1/+5/-2 +1/-1 0

Oxidation-Reduction Reactions Combustion reaction A + O2 B S+ O2 SO2 2Mg +O2 2MgO

Oxidation-Reduction Reactions Displacement reaction: Hydrogen displacement Metal displacement Halogen displacement

Oxidation-Reduction Reactions Disproportion reaction: The same element is simultaneously oxidized and reduced. reduced oxidized Disproportion reactant always contains an element as reactant who have at least 3 oxidation numbers

Solution Stoichiometry The concentration of a solution is the amount of solute present in a given quantity of solvent or solution. Molarity(M) or Molar concentration = Dilution is the procedure for preparing a less concentrated solution from a more concentrated solution. X volume of solution(in liters) = moles of solute M V = n /// MiVi = MfVf Moles of solute Liters of solution Moles of solute Liters of solution

Solve: How many grams of potassium dichromate (K2Cr2O7 ) are required to prepare a 250-mL solution whose concentration is 2.16 M ? (K2Cr2O7 =294.2 g) Molarity(M) = 2.16 M Volume = 250 ml = 0.250 L We know, MV = n, moles of K2Cr2O7 = 2.16 M X 0.250 L = 0.540 mol of K2Cr2O7 the molar mass of K2Cr2O7 is 294.2 g So, 0.540 moles have (0.540 x 294.2) g = 159 grams

Solution Stoichiometry How would prepare 60.0 ml of 0.200 M HNO3 from a stock solution of 4.00 M HNO3? M1V1 = M2V2 M1 = 4.00 M M2= 0.200 M V1=0.0600 L V2= ? V2 = M1V1/M2 = 0.200 M x 0.0600 L/4.00 M = 0.00300 L = 3.00 ml Dilute 3.00 ml of acid with water to a total volume of 60.0 ml

Titration In a titration, a solution of accurately known concentration is added gradually to another solution of unknown concentration until the chemical reaction between the two solutions is complete. Equivalence point: the point at which the reaction is complete. Indicator: substance that changes color at (or near) the equivalence point. Slowly add base to unknown acid until the indicator changes color

Try It! Titration reveals that 11.6 mL of 3.0 M sulfuric acid are required to neutralize 25.00 mL of NaOH solution. What is the molarity of the NaOH solution? 2.8 M NaOH

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