Types of Solution Reactions. Precipitation Reactions (also called double-displacement reaction) Acid-Base Reactions Oxidation-Reduction Reactions System most commonly used by practicing chemists. Precipitation Reactions.
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Types of Solution Reactions • Precipitation Reactions (also called double-displacement reaction) • Acid-Base Reactions • Oxidation-Reduction Reactions • System most commonly used by practicing chemists.
Precipitation Reactions • When two solutions are mixed, an insoluble substance (precipitate) sometimes forms. • In other words, a solid forms and separates from the solution. • Example: When a yellow aqueous solution of K2CrO4 (aq) is added to a colorless aqueous solution of Ba(NO3)2, a yellow solid forms. • What is the equation that describes this reaction? • The reactants have been described: K2CrO4 (aq) and Ba(NO3)2 (aq). • Can we predict the products? • If so, what is the yellow solid?
In the previous example Ba(NO3)2 (aq) and K2CrO4 (aq) were mixed. • Remember (aq) means dissolved in water and that when dissolved in water an ionic substance breaks apart into its constituent ions.
Ba(NO3)2 (aq) = dissolved in H2O = independent Ba2+ and NO3-ions in solution K2CrO4 (aq) = dissolved in H2O = independent K+ and CrO42-ions Reaction: 2K+ (aq) + CrO42- (aq) + Ba2+ (aq) + 2NO3- (aq) → products
The mixed solution contains the ions: • K+ CrO2- Ba2+ NO3- • We know our reactants but what is the product or products? • Predicting the products: • When ions form a solid compound, the compound must have a zero net charge. Thus the products of this reaction must contain both anions and cations. • Most ionic materials contain only two types of ions: one type of cation and one type of anion (for example, NaCl, KOH, Na2SO4, Co(NO3)2).
In our example, the possible combinations from the list of ions K+, CrO42-, Ba2+, and NO3- are • K2CrO4 KNO3 BaCrO4 Ba(NO3)2 Reactant Reactant • Since K2CrO4 and Ba(NO3)2 are reactants our products must be KNO3 and BaCrO4. • But which one is the yellow precipitate?
Products = KNO3 and BaCrO4 • Simple Rules for the Solubility of Salts in Water (p. 144) • Most nitrate (NO3-) salts are soluble. • Most chromate (CrO42-) are only slightlysoluble. • Thus BaCrO4 would be the precipitate. • What happens to the K+ and NO3- ions? • These ions are left in solution; they do not form a precipitate in this amount of water.
The overall equation for this precipitation reaction using the formulas of the reactants and products is: • K2CrO4 (aq) + Ba(NO3)2 (aq) → BaCrO4 (s) + 2KNO3 (aq) • As long as water is present, the KNO3 remains dissolved as separated ions.
Memorize the solubility rules in Table 4.1 on page 144. • The phrase slightlysolubleused in the solubility rules means the tiny amount of solid that dissolves is notnoticeable. • The solid appears to be insoluble to the naked eye. • Thus the terms insoluble and slightly soluble are often used interchangeably.
Describing reactions in solution • Overall or formula equation: • K2CrO4 (aq) + Ba(NO3)2 (aq) → BaCrO4 (aq) + 2KNO3 (aq) • Complete ionic equation: • 2K+ (aq) + CrO42- (aq) + Ba2+ (aq) + 2NO3– (aq) → • BaCrO4 (s) + 2K+ (aq) + 2NO3– (aq) • In a complete ionic equation, all substances that are strongelectrolytes are represented as ions.
2K+ (aq) + CrO42- (aq) + Ba2+ (aq) + 2NO3– (aq) → • BaCrO4 (s) + 2K+ (aq) + 2NO3– (aq) • The complete ionic equation shows that only some of the ions participate in the reaction. • Notice the K+ and NO3– ions are the same before and after the reaction. • They are called spectator ions. • Net ionic equation: • Ba2+ (aq) + CrO4 (aq) → BaCrO4 (s) • A net ionic equation shows only those solution components that are directlyinvolved in the reaction.
Three Types of Equations are used to Describe Reactions in Solutions The formula equation gives the overall reaction stoichiometry but not necessarily the actual forms of the reactants and products in solution. The complete ionic equation represents as ions all reactants and products that are strong electrolytes. The net ionic equation includes only those solution components undergoing a change. Spectator ions are not included.
Acid-Base Reactions • Arrhenius Definition of Acid and Base • Acid – produces H+ ions when dissolved in water • Base – produces OH- ions when dissolved in water • More general definition of acids and bases came from Johannes N. BrØnsted (1879-1947) and Thomas M Lowry (1874 – 1936). • An acid is a proton (H+) donor. • A base is a proton (H+) acceptor. • Includes bases that do not contain OH-ions.
What happens in an acid-base reaction? • Suppose we mix an aqueous solution of HCl with an aqueous solution of NaOH. • The combined solution contains the ions H+, Cl-, Na+, and OH-. • We have H+ and OH- ions which do not exist in large quantities in solution so they combine to give H2O which is a nonelectrolyte.
Net Ionic Equation when we mix HCl (aq) and NaOH (aq): • H+ (aq) + OH- (aq) → H2O (l)
What if we mix solutions of an acid that is a weakelectrolyte with a strong base? • Remember in solution weak electrolytes do not dissociate to any great extent. • Let’s mix together acetic acid (HC2H3O2), a weak electrolyte, and potassium hydroxide (KOH), a strong base. • In solution we would have HC2H3O2, K+ ions, and OH- ions. • We know that KOH is soluble so no precipitation occurs. • What about a reaction between OH- which is a protonacceptor and a proton donor, HC2H3O2? • The OH- ion has a strong affinity for protons so it can strip them from the HC2H2O3 molecules. • The net ionic equation is: • OH- (aq) + HC2H3O2 (aq) → H2O (l) + C2H3O2– (aq)
Acid-Base Titrations • Volumetric analysis: technique for determining the amount of a certain substance by titration. • Titration: involves delivery (from a buret) of a measured volume of a solution of known concentration (titrant) into a solution containing the substance being analyzed (the analyte). • The titrant contains a substance that reacts in a known manner with the analyte.
The point in the titration where enough titrant has been added to fully react with the analyte is called the equivalence point. • This point is marked by the use of an indicator. • An indicator is a substance that changes color based on the pH of its surroundings. • The point where the indicator changes color is called the endpoint of the titration. • The goal is to choose an indicator that allows the endpoint and the equivalence point to be very close together.
Acid-Base Titration • Commonly used indicator is phenolphathalein. • Phenolphathalein is colorless in acidic solution and pink in basic solution. Analyte before Titration Too Far!!! Too Much Titrant Added Proper Endpoint