Reactions in aqueous solutions Precipitation Reactions

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Ethelbert Hart
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1 Reactions in aqueous solutions Precipitation Reactions

2 Aqueous solutions Chemical reactions that occur in water are responsible for creation of cenotes. When carbon dioxide, CO2, dissolves in water, the resulting solution is slightly acidic and reacts with CaCO3 in the limestone: CaCO3(s) + H2O(l) + CO2 (aq) Ca(HCO3)2 (aq) A solution in which water is the dissolving medium is called an aqueous solution CO 2 H 2 O 1 Atmospheric CO 2 dissolves in groundwater, forming H + and HCO 3 H HCO 3 2 Acidic water reacts with CaCO 3, dissolving limestone and forming a cenote CaCO 3 2 H Ca 2 HCO 3 cenotes in Yucatan - Mexico

3 Aqueous solutions A solution is a homogeneous mixture of two or more substances. The substance present in the greatest quantity is usually called the solvent, and the other substances are called solutes, they are said to be dissolved in the solvent. Pure water is a very poor conductor of electricity. The conductivity of bathwater originates from the substances dissolved in the water, not from the water itself. Some substances (e.g NaCl) that dissolve in water make the resulting solution conducting.

4 Electrolytic Properties Two aqueous solutions, 1. NaCl in water 2. Table sugar (sucrose) in water. Both solutions are colorless, but they have different conductivities: the salt solution is a good conductor of electricity, whereas the sugar solution is not. Current is a flow of electrically charged particles. In a solution the ions carry electrical charges. Thus, the conductivity of NaCl solutions indicates the presence of ions. The lack of conductivity of sucrose solutions indicates the absence of ions. When NaCl dissolves in water, the solution contains Na + and Cl - ions, each surrounded by water molecules. When sucrose (C12H22O11) dissolves in water, the solution contains only neutral sucrose molecules surrounded by water molecules. A substance whose aqueous solutions contain ions is called an electrolyte. A substance that does not form ions in solution is called a non electrolyte. Not lit Not lit Lit Pure water, H 2 O(l) does not conduct electricity Sucrose solution, C 12 H 22 O 11 (aq) Nonelectrolyte does not conduct electricity Sodium chloride solution, NaCl(aq) Electrolyte conducts electricity

Ionic Compounds in Water When NaCl dissolves in water, each ion separates from the solid structure and disperses in the solution. The ionic solid dissociates into its component ions.

ionic compounds. The O atom is rich in electrons and has a partial negative charge, denoted by δ -. Each H atom has a partial positive charge (δ + ).

5 Ionic Compounds in Water When NaCl dissolves in water, each ion separates from the solid structure and disperses in the solution. The ionic solid dissociates into its component ions. 1 H 2 O molecules separate Na + and Cl ions from solid NaCl Ionic compound dissolves in water d d d 3 Na + and Cl ions disperse throughout the solution Water is polar and is an effective solvent for ionic compounds. The O atom is rich in electrons and has a partial negative charge, denoted by δ -. Each H atom has a partial positive charge (δ + ). Cl Na Cations are attracted by the negative end of H2O, anions are attracted by the positive end. 2 H 2 O molecules surround Na + and Cl ions As an ionic compound dissolves, the ions become surrounded by H2O molecules (solvation). In chemical equations, solvated ions are noted as Na + (aq) and Cl - (aq), whereas aq is an abbreviation for aqueous. Solvation stabilizes the ions in solution and prevents cations and anions from recombining.

Molecular Compounds in Water When a molecular compound dissolves in water, the molecules remain intact and dispersed throughout the solution.

6 Molecular Compounds in Water When a molecular compound dissolves in water, the molecules remain intact and dispersed throughout the solution. Consequently, most molecular compounds are non-electrolytes. Molecular substances like methanol, CH3OH, dissolve without forming ions. Methanol A few molecular substances have aqueous solutions that contain ions. Acids are the most important of these solutions. For example, when HCl(g) dissolves in water to form hydrochloric acid, HCl(aq), it ionizes; that is, it dissociates into H + (aq) and Cl - (aq) ions.

7 Strong and Weak Electrolytes Strong electrolytes are those solutes that exist in solution completely or nearly completely as ions. Essentially all water-soluble ionic compounds (such as NaCl) and a few molecular compounds (such as HCl) are strong electrolytes. Weak electrolytes are those solutes that exist in solution mostly in the form of neutral molecules. For example, in a solution of acetic acid (CH3COOH) most of the solute is present as CH3COOH (aq) molecules. Only a small fraction of the CH3COOH is dissociated into H + ions (aq) and CH3COO - (aq) ions. When a weak electrolyte, such as acetic acid, ionizes in solution, we write the reaction as CH3COOH (aq) CH3COO - (aq) + H + (aq) The arrows pointing in opposite directions mean that the reaction is at the chemical equilibrium. At any given moment some CH3COOH molecules are ionizing to form H + and CH3COO - ions but those are recombining to form CH3COOH.

8 PRECIPITATION REACTIONS Reactions that result in the formation of an insoluble solid are called precipitation reactions. A precipitate is formed by a reaction in solution. Pb 2 NO 3 I K Reactants 2 KI(aq) Pb(NO 3 ) 2 (aq) Pb 2 (aq) and I (aq) combine to form precipitate Products PbI 2 (s) 2 KI(aq) The figure shows two clear solutions being mixed. One solution contains potassium iodide, KI, dissolved in water and the other contains lead nitrate, Pb(NO3)2, dissolved in water. The reaction between these two solutes produces a water-insoluble yellow solid. The precipitate is lead iodide (PbI2), a compound that has a very low solubility. The other product of this reaction, potassium nitrate (KNO3), remains in solution. Pb(NO3)2 (aq) + KI (aq) PbI2 (s) + 2 KNO3 (aq)

9 PRECIPITATION REACTIONS Solubility Guidelines for Ionic Compounds Precipitation reactions occur when pairs of oppositely charged ions attract each other so strongly that they form an insoluble ionic solid. The solubility of a substance is the amount of the substance that can be dissolved in a given quantity of solvent at the given temperature. In the case of an insoluble substance the attraction between the ions in the ionic solid is too great for the water molecules to separate the ions. All common ionic compounds of the alkali metal ions (group 1A of the PTE) and of the ammonium ion (NH4 + ) are soluble in water.

10 PRECIPITATION REACTIONS Solubility Guidelines for Ionic Compounds Experimental observations, however, have led to guidelines for predicting solubility for ionic compounds. E.g. all common ionic compounds that contain the nitrate anion, NO3 -, are soluble in water. Solubility guidelines for common ionic compounds Soluble Ionic Compounds Important Exceptions Compounds containing NO - 3 None CH3COO - None Cl - Compounds of Ag +, Hg2 2+, and Pb2 + Br - Compounds of Ag +, Hg2 2+, and Pb 2+ I - Compounds of Ag +, Hg2 2+, and Pb 2+ SO4 2- Compounds of Sr 2+, Ba 2+, Hg2 2+, and Pb 2+ Insoluble Ionic Compounds Important Exceptions Compounds containing S 2- Compounds of NH4 +, the alkali metal cations, Ca 2+, Sr 2+, CO3 2- and Compounds Ba 2+ of NH4 + and the alkali metal cations PO4 3- Compounds of NH4 + and the alkali metal cations OH - Compounds of NH4 +, the alkali metal cations, Ca 2+, Sr 2+, and Ba 2+

11 PRECIPITATION REACTIONS Solubility Guidelines for Ionic Compounds To predict whether a precipitate, we must 1.note the ions present in the reactants 2.consider the possible cation-anion combinations, 3.use the Solubility guidelines for common ionic compounds table to determine if any of these combinations is insoluble. E.g. Will a precipitate form when solutions of Mg(NO3)2 and NaOH are mixed? Both substances are soluble ionic compounds and strong electrolytes (see the Solubility guidelines for common ionic compounds table). Mixing the solutions first produces a solution containing Mg2 +, NO3 -, Na +, and OH - ions. Will either cation interact with either anion to form an insoluble compound? Knowing that Mg(NO3)2 and NaOH are both soluble in water, our only possibilities are Mg2 + with OH- and Na + with NO3 -. From the table we see that hydroxides are generally insoluble. Because Mg2 + is not an exception, Mg(OH)2 is insoluble and thus forms a precipitate. NaNO3, however, is soluble, so Na + and NO3 - remain in solution. The balanced equation for the precipitation reaction is Mg(NO3)2 (aq) + NaOH (aq) Mg(OH)2 (s) + 2 NaNO3 (aq)

12 PRECIPITATION REACTIONS PRACTICE EXERCISE Classify these ionic compounds as soluble or insoluble in water: (a) sodium carbonate, Na2CO3 (b) lead sulfate, PbSO4 SOLUTION We are given the names and formulas of two ionic compounds and asked to predict whether they are soluble or insoluble in water. We can use the Solubility guidelines for common ionic compounds table to answer the question. Thus, we need to focus on the anion in each compound because the table is organized by anions. (a) According to the table, most carbonates are insoluble. But carbonates of the alkali metal cations (such as sodium ion) are an exception to this rule and are soluble. Thus, Na2CO3 is soluble in water. (b) Although most sulfates are water soluble, the sulfate of Pb2 + is an exception. Thus, PbSO4 is insoluble in water. Classify the following compounds as soluble or insoluble in water: (a) cobalt(ii) hydroxide (b) barium nitrate (c) ammonium phosphate.

13 PRECIPITATION REACTIONS (Exchange Reactions) In the previous reaction Mg(NO3)2 (aq) + NaOH (aq) Mg(OH)2 (s) + 2 NaNO3 (aq) the reactant cations exchange anions (Mg2 + ends up with OH -, and Na + ends up with NO3 - ). Reactions in which cations and anions appear to exchange partners conform to the general equation AX + BY AY + BX Mg(NO3)2 (aq) + NaOH (aq) Mg(OH)2 (s) + 2 NaNO3 (aq) AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq) Such reactions are called either exchange reactions and precipitation reactions conform to this pattern.

14 PRECIPITATION REACTIONS PRACTICE EXERCISE (a) Predict the identity of the precipitate that forms when aqueous solutions of BaCl2 and K2SO4 are mixed. (b) Write the balanced chemical equation for the reaction. SOLUTION We are given two ionic reactants and asked to predict the insoluble product that they form. We need to write the ions present in the reactants and exchange the anions between the two cations. Once we have written the chemical formulas for these products, we can use the Solubility guidelines for common ionic compounds table to determine which is insoluble in water. Knowing the products also allows us to write the equation for the reaction. (a) The reactants contain Ba 2+, Cl -, K +, and SO4 2- ions. Exchanging the anions gives us BaSO4 and KCl. According to the Solubility guidelines for common ionic compounds table, most compounds of SO4 2- are soluble but those of Ba 2+ are not. Thus, BaSO4 is insoluble and will precipitate from solution. KCl is soluble. BaCl (aq) + K2SO4 (aq) BaSO4 (s) + KCl (aq) (b) From part (a) we know the chemical formulas of the products, BaSO4 and KCl. The balanced equation is: BaCl (aq) + K2SO4 (aq) BaSO4 (s) + 2 KCl (aq)

15 PRECIPITATION REACTIONS PRACTICE EXERCISE (a) What compound precipitates when aqueous solutions of Fe2(SO4)3 and LiOH are mixed? (b) Write a balanced equation for the reaction. (c) Will a precipitate form when solutions of Ba(NO3)2 and KOH are mixed? Answers: (a) Fe(OH)3, (b) Fe2(SO4)3(aq) + 6 LiOH (aq) 2 Fe(OH)3 (s) + 3 Li2SO4 (aq) (c) no (both possible products, Ba(OH)2 and KNO3, are water soluble)

16 PRECIPITATION REACTIONS PRACTICE EXERCISE Using the Solubility guidelines for common ionic compounds table, predict whether each of the following compounds is soluble or insoluble in water: A. MgBr2 B. PbI2 C. (NH4)2CO3 D. Sr(OH)2 E. ZnSO4 Write balanced equations for the reactions that occur in each of the following cases: Cr2(SO4)3(aq) + (NH4)CO3 (aq) Ba2(NO3)2(aq) +K2SO4 (aq) Fe(NO3)2(aq) + KOH (aq)

The solvent is the dissolving agent -- i.e., the most abundant component of the solution

The solvent is the dissolving agent -- i.e., the most abundant component of the solution SOLUTIONS Definitions A solution is a system in which one or more substances are homogeneously mixed or dissolved in another substance homogeneous mixture -- uniform appearance -- similar properties throughout