Chapter 5: Chemical Reactions

Home , Aqueous solution

11/3/09

Aqueous Solubility of Compounds • Not all compounds dissolve in water. • Solubility varies from compound to compound.

Chapter 5: Chemical Reactions

Soluble ionic compounds Most molecular dissociate. Ions are compounds stay solvated associated in water.

Aqueous Solubility of Ionic Compounds Aqueous Solubility of Ionic Compounds

Aqueous Solubility of Ionic Compounds Exchange Reactions: Precipitation

(a) Nitrates (b) Hydroxides (soluble) (insoluble)

AgNO Cu(NO ) 3 3 2 Cu(OH)2 AgOH

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Precipitation Reaction Precipitation Reactions

When ionic solutions mix, a precipitate may form: Na2SO4(aq) and Ba(NO3)2(aq) are mixed. Will they react?

+ 2- Products? Na SO4 Not all ions react 2+ - Ba NO3

KNO3(aq) + NaCl(aq) No reaction NaNO3 and BaSO4 . Insoluble? A reaction occurs if a product is insoluble.

The solubility rules help predict reactions.

Net Ionic Equations Net Ionic Equations Soluble ionic compounds fully dissociate: + - AgNO3(aq) Ag (aq) + NO3 (aq) + - KCl(aq) K (aq) + Cl (aq) On mixing: + - + - Ag (aq) + NO3 (aq) + K (aq) + Cl (aq) + - AgCl(s) + K (aq) + NO3 (aq)

+ - Net ionic equation: Ag (aq) + Cl (aq) → AgCl(s)

Net Ionic Equations Net Ionic Equations

(NH4)2S(aq) + Hg(NO3)2(aq) → HgS + 2 NH4NO3 Product Solubility HgS – insoluble + (All sulfides – except group 1A, 2A and NH4 salts)

NH4NO3 – soluble (All ammonium salts; all nitrates)

(NH4)2S(aq) + Hg(NO3)2(aq) → HgS + NH4NO3 not balanced ammonium sulfide mercury(II) sulfide mercury(II) nitrate ammonium nitrate

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Net Ionic Equations Acids Increase the concentration of H+ ions in water.

+ 2- 2+ - 2 NH4 (aq) + S (aq) + Hg (aq) + 2 NO3 (aq) + - → HgS(s) + 2 NH4 (aq) + 2 NO3 (aq) • Change the color of pigments (indicators) . Litmus, phenolphthalein… S2-(aq) + Hg2+(aq) → HgS(s)

Acids Bases Increase the concentration of OH- (hydroxide ion) in water. Bases: • Counteract an acid (neutralize an acid). • Change an indicator’s color (phenolphthalein…). • Have a bitter taste. • Feel slippery.

H2O

Common Acids and Bases Neutralization Reactions acid + base → salt + water.

Neutralizations are exchange reactions.

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Net Ionic Equations for Acid-Base Reactions Net Ionic Equations for Acid-Base Reactions

Gas-Forming Exchange Reactions Gas-Forming Exchange Reactions

Carbonic acid (unstable)

Tums® H2CO3(aq) → H2O(l) + CO2(g) Alka-Seltzer ® often written:

CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)

Gas-Forming Exchange Reactions Gas-Forming Exchange Reactions

Sulfurous acid (unstable)

H2SO3(aq) → H2O(l) + SO2(g) or

Na2SO3(aq) + 2 HCl(aq) →

2 NaCl(aq) + H2O(l) +SO2(g)

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Oxidation-Reduction Reactions Oxidation-Reduction Reactions In all cases: • If something is oxidized, something must be reduced.

• Redox reactions move e-. +2 e- 2 Ag+(aq) + Cu(s) 2 Ag(s) + Cu2+(aq) -2 e- Here: • Cu changes to Cu2+. • Cu loses 2 e-; each Ag+ gains one e- + • Ag is reduced (ore turned to metal) • Gain of e- = reduction (so, loss of e- = oxidation)

(note: H2 is oxidized, so CuO is an oxidizing agent)

Redox Reactions and Electron Transfer Redox Reactions and Electron Transfer

M X M loses electron(s) X gains electron(s) M is oxidized X is reduced M is a reducing agent X is an oxidizing agent

Common Oxidizing and Reducing Agents Displacement Reactions

Oxidizing Agent Reaction Product + + O (oxygen) O - (oxide ion) 2 2 A XZ AZ X H2O2 (hydrogen peroxide) H2O(l) - - - - Redox: F2, Cl2, Br2, I2 (halogen) F , Cl , Br , I (halide ion)

HNO3 (nitric acid) nitrogen oxides (NO, NO2..) Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s) - 3+ Cr2O7 (dichromate ion) Cr (chromium(III) ion) 0 +2 +2 0 - 2+ MnO4 (permanganate ion) Mn (manganese(II) ion) Cu(s) + 2 AgNO3(aq) Cu(NO3)2(aq) + 2 Ag(s) 0 +1 +2 0 Reducing Agent Reaction Product + H2 (hydrogen) H or H2O

C CO and CO2 M (metal: Na, K, Fe…..) Mn+ (Na+, K+, Fe3+…..)

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lithium Activity Series of Metals potassium Displacement Reactions Li barium K strontium Ba Powerful reducing agents at the top. Sr Displace H from H O (l), calcium 2 2 Ca steam or acid sodium Na magnesium Mg aluminum Higher elements displace lower ones: Al manganese Mn Displace H from steam or 2 zinc Zn(s) + CuSO (aq) ZnSO (aq) + Cu(s) Zn acid chromium 4 → 4 Cr Ease of iron oxidation Mg(s) + 2 HCl(aq) → MgCl (aq) + H (g) Fe nickel 2 2 increases Ni tin Sn Displace H2 from acid lead Pb hydrogen H antimony Sb copper Cu No reaction with H2O, mercury Hg steam or acid silver Ag palladium Pd platinum Pt Au gold © 2008 Brooks/Cole 31 © 2008 Brooks/Cole 32

Displacement Reactions Displacement Reactions

Potassium + water

2 K(s) + 2 H2O(l) →

2 KOH(aq) + H2(g)

Solution Concentration Molarity

moles solute mol Relative amounts of solute and solvent. Molarity = = liters of solution L

• Shorthand: [NaOH] =1.00 M

The brackets [ ] represent “molarity of ”

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Molarity Molarity

Calculate the molarity of sodium sulfate in a solution 6.37 g of Al(NO3)3 are dissolved to make a 250. mL 3+ that contains 36.0 g of Na2SO4 in 750.0 mL of aqueous solution. Calculate (a) [Al(NO3)3] (b) [Al ] - solution. and [NO3 ]. 36.0 g nNa2SO4 = = 0.2534 mol 142.0 g/mol (a) Al(NO3)3 FM = 26.98 + 3(14.00) + 9(16.00) = 213.0 g mol 0.2534 mol [Na2SO4 ] = Unit change! 0.7500 L (mL to L) 6.37 g 213.0 g/mol

[Na2SO4 ] = 0.338 mol/L = 0.338 M

Molarity Solution Preparation

6.37 g of Al(NO3)3 in a 250. mL aqueous solution. Calculate (a) the molarity of the 3+ - Al(NO3)3 , (b) the molar concentration of Al and NO3 ions in solution. Solutions are prepared either by:

1. Diluting a more concentrated solution.

3+ - 1 Al(NO3)3 ≡ 1 Al 1 Al(NO3)3 ≡ 3 NO3 or…

3+ [Al3+] = 0.120 M Al(NO ) 1 Al = 0.120 M Al3+ 3 3 2. Dissolving a measured amount of solute and 1 Al(NO3)3 diluting to a fixed volume. - - [NO3 ] = 0.120 M Al(NO3)3 = 0.360 M NO3

Solution Preparation by Dilution Solution Preparation from Pure Solute Prepare a 0.5000 M solution of potassium MconcVconc = MdilVdil permanganate in a 250.0 mL volumetric flask.

Mass of KMnO4 = 0.1250 mol x 158.03 g/mol M V 17.8 M x 75.0 mL Mdil = conc conc = = 1.34 M = 19.75 g Vdil 1000. mL

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Solution Preparation from Pure Solute Molarity and Reactions in Aqueous Solution

• Rinse all the solid from the weighing dish into the flask. • Fill the flask ≈ ⅓ full. • Swirl to dissolve the solid. nA = [ A ] x V

• Fill the flask to the mark on the neck. [product] = n / (total volume). • Shake to thoroughly mix. product

Molarity and Reactions in Aqueous Solution Molarity and Reactions in Aqueous Solution

H2SO4(aq) + 2 NaOH(aq) → Na2SO4(aq) + 2 H2O(l) ? mol 5.850 x 10-3 mol

2 NaOH ≡ 1 H2SO4

-3 1 H2SO4 H2SO4 required = 5.850 x 10 mol NaOH H2SO4(aq) + 2 NaOH(aq) → Na2SO4(aq) + 2 H2O(l) 2 NaOH = 2.925 x 10-3 mol V needed: 0.234 mol acid n = 0.0250 L = 5.850 x 10-3 mol NaOH L mol H SO 1 L V = 2 4 = 2.925 x 10-3 mol [H2SO4] 0.0875 mol = 0.0334 L = 33.4 mL

Molarity and Reactions in Aqueous Solution Molarity and Reactions in Aqueous Solution

A 4.554 g H C O / NaC mixture was neutralized by 29.58 mL of 0.550M A 4.554 g mixture of oxalic acid, H C O and NaCl 2 2 4 l 2 2 4 NaOH. What was the weight % of oxalic acid in the mixture? was neutralized by 29.58 mL of 0.550M NaOH. What was the weight % of oxalic acid in the mixture? Mass of acid consumed = 8.135 x10-3 mol x (90.04 g/mol acid) = 0.7324 g of oxalic acid

weight of acid nNaOH = 0.02958 L x 0.550 mol/L = 0.01627 mol Weight percent = sample weight x 100%

1 H2C2O4 ≡ 2 NaOH Oxalic acid reacted = 0.7324 g Weight percent = x 100% = 16.08% 1 H C O 4.554 g 0.01627 mol NaOH 2 2 4 = 8.135 x10-3 mol 2 NaOH

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Molarity and Reactions in Aqueous Solution Molarity and Reactions in Aqueous Solution

25.0 mL of 0.234 M FeCl and 50.0 mL of 0.453 M NaOH are mixed. Which 25.0 mL of 0.234 M FeCl3 and 50.0 mL of 0.453 M NaOH are 3 reactant is limiting? How many moles of Fe(OH)3 will form? mixed. Which reactant is limiting? How many moles of Fe(OH)3 will form? FeCl3(aq) + 3 NaOH(aq) → 3 NaCl (aq) + Fe(OH)3(s) 0.005850 mol 0.02265 mol FeCl3(aq) + 3 NaOH(aq) → 3 NaCl (aq) + Fe(OH)3(s) 1 Fe(OH)3 0.00585 mol FeCl3 = 0.00585 mol Fe(OH)3 1 FeCl3 nFeCl3 = 0.0250 L x 0.234 mol/L = 0.005850 mol 1 Fe(OH) 0.02265 mol NaOH 3 = 0.00755 mol Fe(OH) 3 NaOH 3 nNaOH = 0.0500 L x 0.453 mol/L = 0.02265 mol

FeCl3 is limiting; 0.00585 mol produced.

Aqueous Solution Titrations Aqueous Solution Titrations

Titrant: Base of known concentration Slowly add standard solution.

End point: indicator changes color.

Determine Vtitrant added.

Unknown acid + phenolphthalein Often used to determine acid or base concentrations. (colorless in acid)… …turns pink in base