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Sample Problem 4.1 Determining Moles of Ions in Aqueous Ionic Solutions PROBLEM: How many moles of each ion are in the following solutions? (a) 5.0 mol of ammonium sulfate dissolved in water (b) 78.5 g of cesium bromide dissolved in water (c) 7.42x1022 formula units of copper(II) nitrate dissolved in water (d) 35 mL of 0.84 M zinc chloride PLAN: We have to relate the information given and the number of moles of ions present when the substance dissolves in water. SOLUTION: (a) (NH4)2SO4(s) 5.0 mol (NH4)2SO4
H2O
2NH4+(aq) + SO42-(aq) = 10. mol NH4+ 5.0 mol SO42-
2 mol NH4+ 1 mol (NH4)2SO4
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Sample Problem 4.1 continued
H2O
Determining Moles of Ions in Aqueous Ionic Solutions
(b) CsBr(s) 78.5 g CsBr
Cs+(aq) + Br-(aq) mol CsBr 212.8 g CsBr
H2O
= 0.369 mol Cs+ = 0.369 mol Br= 0.123 mol Cu2+
-
= 0.369 mol CsBr
(c) Cu(NO3)2(s)
Cu2+(aq) + 2NO3-(aq)
mol Cu(NO3)2 7.42x1022 formula = 0.123 mol Cu(NO3)2 units Cu(NO3)2 6.022x1023 formula units = 0.246 mol NO (d) ZnCl2(aq) 35 mL ZnCl2 1L 103mL
H2O
3
Zn2+(aq) + 2Cl-(aq) L = 2.9x110-2 mol ZnCl2 = 5.8x10-2 mol Cl-
0.84 mol ZnCl2
= 2.9x110-2 mol Zn2+
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Figure 4.4
The hydrated proton.
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Sample Problem 4.2
Determining the Molarity of H+ Ions in Aqueous Solutions of Acids
PROBLEM: Nitric acid is a major chemical in the fertilizer and explosives industries. In aqueous solution, each molecule dissociates and the H becomes a solvated H+ ion. What is the molarity of H+(aq) in 1.4M nitric acid? PLAN: Use the formula to find the molarity of H+.
SOLUTION: One mole of H+(aq) is released per mole of nitric acid (HNO3) HNO3(l)
H2O
H+(aq) + NO3-(aq)
1.4M HNO3(aq) should have 1.4M H+(aq).
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Writing Equations for Aqueous Ionic Reactions
The molecular equation
shows all of the reactants and products as intact, undissociated compounds.
The total ionic equation
shows all of the soluble ionic substances dissociated into ions.
The net ionic equation
eliminates the spectator ions and shows the actual chemical change taking place.
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Figure 4.5
A precipitation reaction and its equation.
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Precipitation reactions occur when two soluble ionic compounds (salts) react to form an insoluble salt.
In this experiment, a solution of silver nitrate is being added to a solution of sodium chloride. AgNO3 and NaCl are soluble salts, but AgCl is insoluble. When the solutions are mixed, Ag+ ions react with Cl- ions to form solid AgCl. The solid that is formed is called a precipitate. This is called a double displacement (metathesis) reaction.
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The molecular equation for this reaction is written as follows: AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq) The Ag+ and Na+ ions displace each other. The total ionic equation for this reaction is written as follows: Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) AgCl(s) + Na+(aq) + NO3- (aq) The net ionic equation for this reaction is written as follows: Ag+(aq) + Cl-(aq) AgCl(s) Only the ions that participate in the reaction are shown. Na+ and NO3- are spectator ions.
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Figure 4.7
The reaction of Pb(NO3)2 and NaI.
NaI(aq) + Pb(NO3)2 (aq) PbI2(s) + NaNO3(aq) 2NaI(aq) + Pb(NO3)2 (aq) PbI2(s) + 2NaNO3(aq) 2Na+(aq) + 2I-(aq) + Pb2+(aq) + 2NO3-(aq) PbI2(s) + 2Na+(aq) + 2NO3-(aq) 2NaI(aq) + Pb(NO3)2(aq) PbI2(s) + 2NaNO3(aq) double displacement reaction (metathesis)
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Predicting Whether a Precipitate Will Form
1. Note the ions present in the reactants. 2. Consider the possible cation-anion combinations. 3. Decide whether any of the ion combinations is insoluble. See Table 4.1 (next slide) for solubility rules.
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Table 4.1 Solubility Rules For Ionic Compounds in Water
Soluble Ionic Compounds 1. All common compounds of Group 1A(1) ions (Li+, Na+, K+, etc.) and ammonium ion (NH4+) are soluble. 2. All common nitrates (NO3-), acetates (CH3COO- or C2H3O2-) and most perchlorates (ClO4-) are soluble. 3. All common chlorides (Cl-), bromides (Br-) and iodides (I-) are soluble, except those of Ag+, Pb2+, Cu+, and Hg22+. All common fluorides (F-) except those of Pb2+ and group 2A(2). 4. All common sulfates (SO42-) are soluble, except those of Ca2+, Sr2+, Ba2+, Ag+, and Pb2+.
Insoluble Ionic Compounds 1. All common metal hydroxides are insoluble, except those of Group 1A(1) and the larger members of Group 2A(2)(beginning with Ca2+). 2. All common carbonates (CO32-) and phosphates (PO43-) are insoluble, except those of Group 1A(1) and NH4+. 3. All common sulfides are insoluble except those of Group 1A(1), Group 2A(2) and NH4+.
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Predicting Whether a Precipitation Reaction Occurs; Writing Ionic Equations PROBLEM: Predict whether a reaction occurs when each of the following pairs of solutions are mixed. If a reaction does occur, write balanced molecular, total ionic, and net ionic equations, and identify the spectator ions. (a) sodium sulfate(aq) + strontium nitrate(aq) (b) ammonium perchlorate(aq) + sodium bromide(aq) SOLUTION: PLAN: write ions (a) Na2SO4(aq) + Sr(NO3)2 (aq) 2NaNO3(aq) + SrSO4(s) 2Na+(aq) +SO42-(aq)+ Sr2+(aq)+2NO3-(aq) 2Na+(aq) +2NO3-(aq)+ SrSO4(s) combine anions & cations check for insolubility Table 4.1 eliminate spectator ions for net ionic equation SO42-(aq)+ Sr2+(aq) (b) NH4ClO4(aq) + NaBr (aq) SrSO4(s) NH4Br (aq) + NaClO4(aq)
Sample Problem 4.3
All reactants and products are soluble so no reaction occurs.
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