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Chapter 4

Aqueous Reactions and Solution Stoichiometry. Chapter 4. Predicting whether or not a reaction will occur. There are two issues Whether the reaction wants to occur Ag + Mg(NO 3 ) 2 Whether there is enough energy to start the process Formation of a solid Formation of a liquid

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Chapter 4

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  1. Aqueous Reactions and Solution Stoichiometry Chapter4

  2. Predicting whether or not a reaction will occur • There are two issues • Whether the reaction wants to occur Ag + Mg(NO3)2 • Whether there is enough energy to start the process • Formation of a solid • Formation of a liquid • Formation of a gas • Transfer of an electron

  3. Reactions in Aqueous Solutions • The starting solids are ionic compounds • They are neutral • They dissociate in water • They conduct electricity in solution (electrolytes)

  4. Solutions: • Homogeneous mixtures of two or more pure substances. • The solvent is present in greatest abundance. • All other substances are solutes.

  5. Dissociation • When an ionic substance dissolves in water, the solvent pulls the individual ions from the crystal and solvates them.

  6. Dissociation This process is called dissociation.

  7. Water The Common Solvent A. Structure of water 1. Oxygen's electronegativity is high (3.5) and hydrogen's is low (2.1) 2. Water is a bent molecule 3. Water is a polar molecule

  8. Hydration of Ionic Solute Molecules 1. Positive ions attracted to the oxygen end of water 2. Negative ions attracted to the hydrogen end of water

  9. "Like Dissolves Like" 1. Polar and ionic compounds dissolve in polar solvents like water 2. Nonpolar compounds like fats dissolve in nonpolar solvents like ____?_____

  10. Electrolytes • Substances that dissociate into ions when dissolved in water. • will produce enough ions to support an electric current

  11. Nonelectrolytes • Anonelectrolyte may dissolve in water, but it does not dissociate into ions when it does so. No charge

  12. Electrolytes and Nonelectrolytes Soluble ionic compounds tend to be electrolytes.

  13. Electrolytes and Nonelectrolytes Molecular compounds tend to be nonelectrolytes, except for acids and bases.

  14. Electrolytes • A strong electrolyte dissociates completely when dissolved in water. • A weak electrolyte only dissociates partially when dissolved in water.

  15. Strong Electrolytes Are… • Strong acids

  16. Strong Electrolytes Are… • Strong acids • Strong bases

  17. Strong Electrolytes Are… • Strong acids • Strong bases • Soluble ionic salts

  18. Precipitation Reactions When one mixes ions that form compounds that are insoluble (as could be predicted by the solubility guidelines), a precipitate is formed.

  19. AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq) Metathesis (Exchange) Reactions • Metathesis comes from a Greek word that means “to transpose”

  20. AgNO3 (aq) + KCl (aq) AgCl(s) (s) +KNO3 (aq) Metathesis (Exchange) Reactions • Metathesis comes from a Greek word that means “to transpose” • It appears the ions in the reactant compounds exchange, or transpose, ions

  21. AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq) Metathesis (Exchange) Reactions • Metathesis comes from a Greek word that means “to transpose” • It appears the ions in the reactant compounds exchange, or transpose, ions

  22. 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). • If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction.

  23. AgNO3 (aq) + KCl(aq) AgCl(s) + KNO3 (aq) Molecular Equation The molecular equation lists the reactants and products in their molecular form.

  24. Ag+(aq) + NO3- (aq) + K+ (aq) + Cl- (aq)  AgCl(s) + K+(aq) + NO3- (aq) Ionic Equation • In the 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.

  25. Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) AgCl(s) + K+(aq) + NO3-(aq) 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.

  26. Ag+(aq) + Cl-(aq) AgCl(s) 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 only things left in the equation are those things that change (i.e., react) during the course of the reaction.

  27. Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) AgCl(s) + K+(aq) + NO3-(aq) 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 only things left in the equation are those things that change (i.e., react) during the course of the reaction. • Those things that didn’t change (and were deleted from the net ionic equation) are called spectator ions.

  28. Writing Net Ionic Equations • Write a balanced molecular equation. • Dissociate all strong electrolytes. • Cross out anything that remains unchanged from the left side to the right side of the equation. • Write the net ionic equation with the species that remain. End 10/1

  29. Acids: • Substances that increase the concentration of H+ when dissolved in water (Arrhenius). • Proton donors (Brønsted–Lowry).

  30. Acids There are only seven strong acids: • Hydrochloric (HCl) • Hydrobromic (HBr) • Hydroiodic (HI) • Nitric (HNO3) • Sulfuric (H2SO4) • Chloric (HClO3) • Perchloric (HClO4)

  31. Bases: • Substances that increase the concentration of OH− when dissolved in water (Arrhenius). • Proton acceptors (Brønsted–Lowry).

  32. Bases The strong bases are the soluble salts of hydroxide ion: • Alkali metals • Calcium • Strontium • Barium

  33. Acid-Base Reactions In an acid-base reaction, the acid donates a proton (H+) to the base.

  34. HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l) Neutralization Reactions Generally, when solutions of an acid and a base are combined, the products are a salt and water.

  35. Salts • Salts are ionic compounds composed of cations and anions so that the product is electrically neutral. These component ions can be inorganic such as chloride (Cl−), as well as organic such as acetate (CH3COO−) and monoatomic ions such as fluoride (F−), as well as polyatomic ions such as sulfate (SO42−).

  36. HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l) H+ (aq)+ Cl- (aq)+ Na+ (aq) + OH-(aq) Na+ (aq)+ Cl- (aq)+ H2O (l) Neutralization Reactions When a strong acid reacts with a strong base, the net ionic equation is…

  37. HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l) H+ (aq)+ Cl- (aq)+ Na+ (aq) + OH-(aq) Na+ (aq)+ Cl- (aq)+ H2O (l) H+ (aq)+ Cl- (aq)+ Na+ (aq) + OH- (aq) Na+ (aq) + Cl- (aq) + H2O (l) Neutralization Reactions When a strong acid reacts with a strong base, the net ionic equation is…

  38. Neutralization Reactions Observe the reaction between Milk of Magnesia, Mg(OH)2, and HCl.

  39. CaCO3 (s) + HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l) NaHCO3 (aq) + HBr (aq) NaBr (aq)+ CO2 (g) + H2O (l) SrSO3 (s) + 2 HI(aq)SrI2 (aq) + SO2 (g) + H2O (l) Gas-Forming Reactions • These metathesis reactions do not give the product expected. • The expected product decomposes to give a gaseous product (CO2 or SO2).

  40. Na2S (aq) + H2SO4 (aq) Na2SO4 (aq) + H2S (g) Gas-Forming Reactions • This reaction gives the predicted product, but you had better carry it out in the hood, or you will be very unpopular! • Just as in the previous examples, a gas is formed as a product of this reaction: End 10/4

  41. Oxidation-Reduction Reactions • An oxidation occurs when an atom or ion loses electrons. OIL • A reduction occurs when an atom or ion gains electrons. RIG

  42. Oxidation-Reduction Reactions One cannot occur without the other.

  43. 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.

  44. Oxidation Numbers An increase in oxidation number is called oxidation 0 +1 K(s) → K+ (aq) A decrease in oxidation number is called reduction 0 -1 Br2(g) → 2Br– (aq)

  45. Oxidation Numbers Elemental form: 0 (O2, F2, H2, Ag…) 1. The sum of oxidation #s in a species equals its charge (like formal charges)

  46. Oxidation Numbers 2. Alkali metals ions (Group I): +1 Alkaline earth ions (Group II): +2 Group III: usually +3

  47. Oxidation Numbers • Nonmetals tend to have negative oxidation numbers, although some are positive in certain compounds or ions.

  48. Oxidation Numbers • O usually –2, Occasionally -1, peroxides • except with F: OF2 -1 -1 • compounds with O–O bonds: HOOH • (peroxide, superoxide) KO2 Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal.

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