c101 1 lecturenotes chapter5 Foster · A 4.554 g mixture of oxalic acid, H 2C 2O 4 and NaCl was...

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© 2008 Brooks/Cole 1

Chapter 5: Chemical Reactions


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

Soluble ionic compounds dissociate. Most molecular

compounds stay associated in water.

Ions are solvated


Aqueous Solubility of Ionic Compounds




(a) Nitrates (soluble)

AgNO3 Cu(NO3)2

CdS Sb2S3 PbS (NH4)2S soluble

insoluble

(c) Sulfides

(b) Hydroxides (insoluble)

Cu(OH)2 AgOH



Exchange Reactions: Precipitation

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When ionic solutions mix, a precipitate may form:

Not all ions react

The solubility rules help predict reactions.

A reaction occurs if a product is insoluble.

KNO3(aq) + NaCl(aq) No reaction

Precipitation Reaction


Products? Na+ SO42-

Ba2+ NO3-

Precipitation Reactions Na2SO4(aq) and Ba(NO3)2(aq) are mixed. Will they react?

NaNO3 and BaSO4 . Insoluble?


Soluble ionic compounds fully dissociate: AgNO3(aq) Ag+(aq) + NO3

-(aq) KCl(aq) K+(aq)

+ Cl-(aq) On mixing:

Net Ionic Equations

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 + NH4NO3 not balanced


Product Solubility

(NH4)2S(aq) + Hg(NO3)2(aq) → HgS + 2 NH4NO3

ammonium sulfide mercury(II) sulfide mercury(II) nitrate ammonium nitrate

HgS – insoluble (All sulfides – except group 1A, 2A and NH4

+ salts)

NH4NO3 – soluble (All ammonium salts; all nitrates)

Net Ionic Equations

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2 NH4+(aq) + S2-(aq) + Hg2+(aq) + 2 NO3

-(aq) → HgS(s) + 2 NH4

+(aq) + 2 NO3-(aq)

S2-(aq) + Hg2+(aq) → HgS(s)

Net Ionic Equations


Acids Increase the concentration of H+ ions in water.

•  Change the color of pigments (indicators)  Litmus, phenolphthalein…


Acids


Bases Increase the concentration of OH- (hydroxide ion) in water. Bases:

H2O

•  Counteract an acid (neutralize an acid). •  Change an indicator’s color (phenolphthalein…). •  Have a bitter taste. •  Feel slippery.


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


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

Carbonic acid (unstable)

H2CO3(aq) → H2O(l) + CO2(g) Tums®

Gas-Forming Exchange Reactions


Alka-Seltzer ®



or Na2SO3(aq) + 2 HCl(aq) →

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

Sulfurous acid (unstable)

H2SO3(aq) → H2O(l) + SO2(g)




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Oxidation-Reduction Reactions

(note: H2 is oxidized, so CuO is an oxidizing agent) © 2008 Brooks/Cole 26

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

•  Redox reactions move e-.

2 Ag+(aq) + Cu(s) 2 Ag(s) + Cu2+(aq)

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)

+2 e-

-2 e-


Redox Reactions and Electron Transfer


M is a reducing agent X is an oxidizing agent

M X

e-

M loses electron(s) X gains electron(s)

M is oxidized X is reduced

Redox Reactions and Electron Transfer


Oxidizing Agent Reaction Product O2 (oxygen) O2

- (oxide ion) H2O2 (hydrogen peroxide) H2O(l) F2, Cl2, Br2, I2 (halogen) F-, Cl-, Br-, I- (halide ion) HNO3 (nitric acid) nitrogen oxides (NO, NO2..) Cr2O7

- (dichromate ion) Cr3+ (chromium(III) ion) MnO4

- (permanganate ion) Mn2+ (manganese(II) ion)

Reducing Agent Reaction Product H2 (hydrogen) H+ or H2O C CO and CO2

M (metal: Na, K, Fe…..) Mn+ (Na+, K+, Fe3+…..)

Common Oxidizing and Reducing Agents


+ + A XZ AZ X

Redox: Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s) 0 +2 +2 0 Cu(s) + 2 AgNO3(aq) Cu(NO3)2(aq) + 2 Ag(s) 0 +1 +2 0

Displacement Reactions

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lithium potassium

barium strontium calcium sodium

magnesium aluminum

manganese zinc

chromium iron

nickel tin

lead

antimony copper mercury

silver palladium platinum

gold

Displace H2 from steam or acid

Displace H2 from H2O (l), steam or acid

Displace H2 from acid

No reaction with H2O, steam or acid

hydrogen

Ease of oxidation increases

Activity Series of Metals


Powerful reducing agents at the top.

Higher elements displace lower ones:

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)

Sb Cu Hg Ag Pd Pt Au

Li K

Ba Sr Ca Na

Mg Al Mn Zn Cr

Fe Ni Sn Pb

H





Displacement Reactions Potassium + water

2 K(s) + 2 H2O(l) → 2 KOH(aq) + H2(g)


Relative amounts of solute and solvent.

Solution Concentration


Molarity = moles solute liters of solution

= mol L

The brackets [ ] represent “molarity of ”

•  Shorthand: [NaOH] =1.00 M

Molarity

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Molarity Calculate the molarity of sodium sulfate in a solution that contains 36.0 g of Na2SO4 in 750.0 mL of solution.

nNa2SO4 = 36.0 g 142.0 g/mol

= 0.2534 mol

[Na2SO4 ] = 0.2534 mol 0.7500 L

[Na2SO4 ] = 0.338 mol/L = 0.338 M

Unit change! (mL to L)


Molarity

(a)  Al(NO3)3 FM = 26.98 + 3(14.00) + 9(16.00)

6.37 g of Al(NO3)3 are dissolved to make a 250. mL aqueous solution. Calculate (a) [Al(NO3)3] (b) [Al3+] and [NO3

-].

6.37 g 213.0 g/mol

g mol

= 213.0


Molarity 6.37 g of Al(NO3)3 in a 250. mL aqueous solution. Calculate (a) the molarity of the Al(NO3)3 , (b) the molar concentration of Al3+ and NO3

- ions in solution.

1 Al(NO3)3 ≡ 1 Al3+ 1 Al(NO3)3 ≡ 3 NO3-

[Al3+] = 0.120 M Al(NO3)3 = 0.120 M Al3+ 1 Al3+ 1 Al(NO3)3

[NO3-] = 0.120 M Al(NO3)3 = 0.360 M NO3

-


Solution Preparation

Solutions are prepared either by:

1.  Diluting a more concentrated solution.

or…

2.  Dissolving a measured amount of solute and diluting to a fixed volume.


Solution Preparation by Dilution

MconcVconc = MdilVdil

Mdil = = MconcVconc Vdil

17.8 M x 75.0 mL 1000. mL

= 1.34 M


Prepare a 0.5000 M solution of potassium permanganate in a 250.0 mL volumetric flask.

Mass of KMnO4 = 0.1250 mol x 158.03 g/mol = 19.75 g

Solution Preparation from Pure Solute

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•  Rinse all the solid from the weighing dish into the flask.

•  Fill the flask ≈ ⅓ full. •  Swirl to dissolve the solid.

•  Fill the flask to the mark on the neck. •  Shake to thoroughly mix.

Solution Preparation from Pure Solute


nA = [ A ] x V

[product] = nproduct / (total volume).

Molarity and Reactions in Aqueous Solution


H2SO4(aq) + 2 NaOH(aq) → Na2SO4(aq) + 2 H2O(l)

nNaOH = 0.0250 L 0.234 mol L = 5.850 x 10-3 mol



? mol 5.850 x 10-3 mol

2 NaOH ≡ 1 H2SO4

Vacid needed:

1 L 0.0875 mol

2.925 x 10-3 mol V = = mol H2SO4 [H2SO4]

= 0.0334 L = 33.4 mL

H2SO4(aq) + 2 NaOH(aq) → Na2SO4(aq) + 2 H2O(l)

H2SO4 required = 5.850 x 10-3 mol NaOH 1 H2SO4

2 NaOH = 2.925 x 10-3 mol



A 4.554 g mixture of oxalic acid, H2C2O4 and NaCl was neutralized by 29.58 mL of 0.550M NaOH. What was the weight % of oxalic acid in the mixture?

nNaOH = 0.02958 L x 0.550 mol/L = 0.01627 mol

1 H2C2O4 ≡ 2 NaOH Oxalic acid reacted =

0.01627 mol NaOH = 8.135 x10-3 mol 1 H2C2O4 2 NaOH



Mass of acid consumed = 8.135 x10-3 mol x (90.04 g/mol acid) = 0.7324 g of oxalic acid

Weight percent = x 100% weight of acid sample weight

Weight percent = x 100% 0.7324 g 4.554 g = 16.08%

A 4.554 g H2C2O4 / NaCl mixture was neutralized by 29.58 mL of 0.550M NaOH. What was the weight % of oxalic acid in the mixture?


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FeCl3(aq) + 3 NaOH(aq) → 3 NaCl (aq) + Fe(OH)3(s)

nNaOH = 0.0500 L x 0.453 mol/L = 0.02265 mol

25.0 mL of 0.234 M FeCl3 and 50.0 mL of 0.453 M NaOH are mixed. Which reactant is limiting? How many moles of Fe(OH)3 will form?

nFeCl3 = 0.0250 L x 0.234 mol/L = 0.005850 mol



FeCl3(aq) + 3 NaOH(aq) → 3 NaCl (aq) + Fe(OH)3(s) 0.005850 mol 0.02265 mol

0.00585 mol FeCl3 = 0.00585 mol Fe(OH)3 1 Fe(OH)3

1 FeCl3

25.0 mL of 0.234 M FeCl3 and 50.0 mL of 0.453 M NaOH are mixed. Which reactant is limiting? How many moles of Fe(OH)3 will form?


0.02265 mol NaOH = 0.00755 mol Fe(OH)3 1 Fe(OH)3 3 NaOH

FeCl3 is limiting; 0.00585 mol produced.


Often used to determine acid or base concentrations.

Aqueous Solution Titrations


Slowly add standard solution.

End point: indicator changes color.

Determine Vtitrant added.

Unknown acid + phenolphthalein (colorless in acid)… …turns pink

in base

Titrant: Base of known

concentration

Aqueous Solution Titrations

c101 1 lecturenotes chapter5 Foster · A 4.554 g mixture of oxalic acid, H 2C 2O 4 and NaCl was...

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