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Unit 5 – Aqueous Reactions & Solution Stoichiometry

Unit 5 – Aqueous Reactions & Solution Stoichiometry. Properties of Aqueous Solutions Predicting Products of Chemical Reactions Concentrations of Solutions Solution Stoichiometry and Chemical Analysis. Aqueous Solutions. Many chemical reactions take place in an aqueous solution:

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Unit 5 – Aqueous Reactions & Solution Stoichiometry

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  1. Unit 5 – Aqueous Reactions & Solution Stoichiometry Properties of Aqueous Solutions Predicting Products of Chemical Reactions Concentrations of Solutions Solution Stoichiometry and Chemical Analysis

  2. Aqueous Solutions • Many chemical reactions take place in an aqueous solution: • a solution in which water is the dissolving medium • (aq) • Solution:a homogeneous mixture of two or more substances

  3. Aqueous Solutions • Examples of Aqueous Reactions or Processes • Biological or physiological processes • photosynthesis • metabolism of food • transportation of oxygen or nutrients in blood • Rust formation • Acid rain

  4. Aqueous Solutions • Components of Solutions • Solvent: • the dissolver • usually present in the greatest quantity • Solute: • the substance that dissolves

  5. Aqueous Solutions • Sodium Chloride/Water • Solute: sodium chloride • Solvent: water • Sugar/water • Solute: sugar • Solvent: water • Denatured alcohol • Solutes: methyl alcohol, water • Solvent: ethyl alcohol

  6. NaCl Aqueous Solutions sucrose H2O NaCl in H2O sucrose in H2O

  7. Aqueous Solutions • Compare and contrast salt (NaCl) water solution and sucrose (C12H11O22) dissolved in water • Similarities: Differences:

  8. Aqueous Solutions • Salt water solutions are good electrical conductors. • Sucrose (C12H11O22)/water solutions are not good electrical conductors. • Salt (NaCl) forms ions when dissolved in water • ions carry electrical charge • Sugar is a molecular compound. • Molecules stay intact when the compound dissolves so no ions are formed.

  9. Aqueous Solutions • Electrolyte: • a substance that forms ions when dissolved in water • resulting solutions conduct electricity • all ionic compounds • MgCl2 • NaHCO3 • Ca(OH)2 • some molecular compounds • HCl • Acetic acid

  10. Aqueous Solutions • Nonelectrolyte: • a substance that does not form ions when dissolved in water • solutions do not conduct electricity • most molecular compounds • sugar • methyl alcohol • acetone

  11. Aqueous Solutions • When an ionic compound like NaCl dissolves in water, the ions are separated from their orderly crystalline structure in a process called dissociation. • The water molecules form a “shell” around the ions which helps stabilize the ions and keeps them from recombining.

  12. Aqueous Solutions • Ions and their “shell” of water molecules are free to move around in solution. • The ions become uniformly dispersed throughout the solution. • a homogeneous solution forms

  13. d+ H d- O d+ H Aqueous Solutions • How does water stabilize ions in solution? • Water is an electrically neutral, molecular compound but the electrons are not shared equally between the H and O atoms. • Oxygen attracts the electrons in the molecule more strongly than the hydrogen atoms. • The O atom in water has a partial negative charge (high electron density). • The H atoms in water have a partial positive charge (low electron density).

  14. + - Aqueous Solutions • Water solvates (surrounds and stabilizes) the ions. • the H’s (d+) interacts with the negatively charged anions • the O (d-) interacts with the positively charged cations Solvation of anion Solvation of cation

  15. Aqueous Solutions Example: What ions (and how many of each) are formed when KBr dissolves in water? Example: What ions (and how many of each are formed when Na2SO4 dissolves in water?

  16. Aqueous Solutions Example: What ions (and how many of each form when Ba(NO3)2 dissolves in water? Example: What ions (and how many of each form when Al2(SO4)3 dissolves in water?

  17. Aqueous Solutions • Most molecular compounds dissolve in water without forming ions. • Water molecules solvate the molecular compound, but the structural integrity of the molecule is maintained. The dissolution (dissolving) of methanol.

  18. Aqueous Solutions • Some molecular compounds interact strongly with water and form ions. • Acids • HCl (g) + H2O  H+ (aq) + Cl- (aq) • Ammonia • NH3 (g) + H2O  NH4+ (aq) + OH- (aq) • Molecular compounds that ionize are electrolytes.

  19. Aqueous Solutions • There are two kinds of electrolytes: • Strong electrolyte: • a substance that ionizes completely when dissolved in water • ionic compounds • strong acids • strong bases

  20. Aqueous Solutions • Weak Electrolyte: • a substance that partially ionizes when dissolved in water • part of the substance exists as ions and part exists as molecules HC2H3O2 (aq) H+ (aq) + C2H3O2- (aq)

  21. Aqueous Solution • The double arrow indicates that the reaction goes in both directions. • HC2H3O2 is ionizing to form H+ and C2H3O2- • H+ and C2H3O2- are recombining to form HC2H3O2 • A single arrow indicates the reaction goes only in one direction. and (at the same time)

  22. Predicting Chemical Reactions • Precipitation Reactions • Acid/Base Reactions • Oxidation/Reduction (Redox) Reactions

  23. Precipitation Reactions • One visible sign of a chemical reaction is the formation of a solid: AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq) • One driving force for a chemical reaction (i.e. thing that pushes a reaction to occur) is the formation of a solid product (precipitate).

  24. Precipitation Reactions • A reaction that forms an insoluble product is called aprecipitation reaction. AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq) • Precipitate:the insoluble product formed by a reaction in solution • In the reaction above, AgCl is the precipitate.

  25. Precipitation Reactions • AgCl forms a precipitate because it has very low solubility in water. • Solubility: • the amount of a substance that can be dissolved in a given quantity of solvent • AgCl is insoluble in water while NaNO3 is soluble. • Soluble: “dissolves” • Insoluble: “does not dissolve”

  26. Precipitation Reactions • In order to predict whether a precipitation reaction will occur, you must know if an ionic compound is soluble in water or not. • Experimental data have led to a series of guidelines used to predict solubility. • On your exam, you must be able to use the guidelines in Table 4.1 to predict the solubility of a compound. • I will give you a copy of these guidelines on the exam.

  27. Precipitation ReactionsSolubility Guidelines Guidelines Based on Cation • All ionic compounds of the 1A metals are soluble. • NaCl, NaHCO3, K2CO3, LiOH • All ammonium salts (ionic compounds) are soluble. • NH4OH, NH4Cl, (NH4)3PO4

  28. Precipitation ReactionsSolubility Guidelines SOLUBLE Compounds AnionImportant Exceptions NO3 - None C2H3O2 - None ClO4- Few common exceptions SO42- Cpds. With Ca2+, Sr2+, Ba2+,Pb2+, Ag+

  29. Precipitation ReactionsSolubility Guidelines SOLUBLE Compounds AnionImportant Exceptions Cl - Cpds. with Ag+, Pb2+, Cu+, Hg22+ Br - Cpds. with Ag+, Pb2+, Cu+, Hg22+ I - Cpds. with Ag+, Pb2+, Cu+, Hg22+ F- Cpds. With Pb2+, Group 2A

  30. Precipitation ReactionsSolubility Guidelines INSOLUBLE Compounds AnionImportant Exceptions S2- Cpds. with NH4+, Group 1A, Ca2+, Sr2+, or Ba2+ CO3 2- Cpds. with NH4+ or alkali metal cations PO43- Cpds. with NH4+, alkali metal cations OH - Cpds. with Group 1A, NH4+, Ca2+, Sr2+, or Ba2+

  31. Precipitation Reactions Example: Use the solubility guidelines in Table 4.1 to predict the solubility of the following compounds. Ba(NO3)2 CaCl2 Ca3(PO4)2 NH4Cl PbSO4 Ba(OH)2

  32. Precipitation ReactionsPredicting Products Consider the reaction between AgNO3 and NaCl in aqueous solution: AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq) Notice that the anions and cations appear to have exchanged partners.

  33. Precipitation Reactions • Metathesis Reactions • reactions in which the positive ions and negative ions present in the reactants appear to exchange partners • also called exchange reactions AX + BYAY + BX AgNO3+ NaClAgCl + NaNO3

  34. Precipitation Reactions • To write a chemical equation for a precipitation reaction: • Write the formula including charge for the ions formed by each reactant in aqueous solution. • Switch “partners” and write the correct formulas (electrically neutral) for the possible products. • Determine the solubility of each product. • Balance the equation. Include all physical states for reactants and products.

  35. Precipitation Reactions Example: Write a balanced equation for the reaction between aqueous solutions of NaF and Pb(NO3)2.

  36. Precipitation Reactions Example: Write the chemical equation for the reaction between aqueous solutions of Na2CO3 and Fe(C2H3O2)3.

  37. Precipitation Reactions Example: Write the chemical equation for the reaction between aqueous solutions of aluminum nitrate and sodium hydroxide.

  38. Precipitation Reactions Al(NO3)3 (aq) + 3 NaOH (aq)  Al(OH)3 (s) + 3 NaNO3 (aq) • At the end of this reaction, the beaker contains two components: • The solid aluminum hydroxide that precipitates out and settles to the bottom of the beaker. • The aqueous solution that contains Na+ (aq) and NO3- (aq)

  39. Precipitation Reactions Al(NO3)3 (aq) + 3 NaOH (aq)  Al(OH)3 (s) + 3 NaNO3 (aq) • This is called a molecular equation. • It shows the chemical formulas of reactants and products but does not indicate the ionic nature of the reactants or products. • In reality, the chemicals with (aq) beside them exist as ionsin solution

  40. Precipitation Reactions • In order to indicate the ionic nature of the chemicals, we can write acomplete ionic equation: • Explicitly indicates all of the ions in the reaction mixture • All soluble strong electrolytes are written as individual ions.

  41. Precipitation Reactions • To write a complete ionic equation: • Break all soluble strong electrolytes (ionic compounds or strong acids with (aq) beside them) into their ions. • indicate the correct charge of each ion • indicate the number of each ion • write (aq) after each ion • Bring down all weak electrolytes unchanged. • Bring down all compounds with (s), (g), or (l) unchanged.

  42. Precipitation Reactions Example: Write the complete ionic equation for the following reaction: Al(NO3)3 (aq) + 3 NaOH (aq) Al(OH)3 (s) + 3 NaNO3 (aq)

  43. Precipitation Reactions Al3+ (aq) + 3 NO3- (aq) + 3 Na+ (aq) + 3 OH- (aq)  Al(OH)3 (s) + 3 Na+ (aq) + 3 NO3- (aq) • Notice that the Na+ and NO3- ions appear unchanged on both sides of the reaction. • Spectator ions • ions that are present but play no direct role in a chemical reaction

  44. Precipitation Reactions • If we eliminate all spectator ions from the equation, we can write anet ionic equation • an equation that includes only the ions directly involved in the reaction and the insoluble products that are formed

  45. Precipitation Reactions • To write a net ionic equation: • Write the complete ionic equation. • Cross out the ions that appear in the same type, number, and physical state on both sides of the equation. • Write the “leftovers” as the net ionic equation. • Don’t forget to include the physical states of the reactants and products.

  46. Precipitation Reactions Net ionic equation: Al3+ (aq) + 3 NO3- (aq) + 3 Na+ (aq) + 3 OH- (aq)  Al(OH)3 (s) + 3 Na+ (aq) + 3 NO3- (aq)

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