Chapter 4:

TYPES OF CHEMICAL REACTIONS AND

SOLUTION STOICHIOMETRY

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Aqueous Solutions

Water is the dissolving medium, or solvent.

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Some Properties of Water

1. Water is “bent” or V-shaped.

2. The O-H bonds are covalent.

3. Water is a polar molecule.

4. Hydration occurs when salts dissolve in water.

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Figure 4.1: (Left) The water molecule is polar. (Right) A space-filling model of the

water molecule.

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Figure 4.2: Polar water molecules interact with the positive and negative ions of a salt

assisting in the dissolving process.

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Figure 4.3: (a) The ethanol molecule contains a polar O—H bond similar to those in the water

molecule. (b) The polar water molecule interacts strongly with the polar O—H bond in ethanol. This

is a case of "like dissolving like."

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A Solute

dissolves in water (or other “solvent”);

changes phase (if different from the solvent);

is present in lesser amount (if the same phase as the solvent).

A Solvent

retains its phase (if different from the solute);

is present in greater amount (if the same phase as the solute).

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Figure 4.4: Electrical conductivity of aqueous solutions.

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Electrolytes

Strong Electrolytes - conduct current efficiently

NaCl, HNO3

Weak Electrolytes - conduct only a small current

vinegar, tap water

Nonelectrolytes - no current flows

pure water, sugar solution

※ Svante Arrhenius postulated: the extend to which a solution can conduct an electric current depends on the number of ions present.

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Strong electrolytes:

Substances that are completely ionized when they are dissolved in water .

(1)soluble salts,

(2) strong acids,

(3) strong bases.

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Figure 4.5: When solid NaCl dissolves, the Na+ and Cl- ions are randomly dispersed in

the water.

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Figure 4.6: HCl(aq) is completely ionized.

Arrhenius discoveries the nature of acids

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※ Arrhenius proposed that an acid is a substance that produces H+.

HCl H+(aq) + OH-(aq)

HNO3 H+(aq) + NO3-(aq)

H2SO4 H+(aq) + HSO4-(aq)

OH 2

OH 2

OH 2

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Figure 4.7: An aqueous solution of sodium hydroxide.

Strong bases - react completely with water to give OH- ions.

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Figure 4.8: Acetic acid (HC2H3O2) exists in water mostly as undissociated molecules. Only a small percentage of the molecules are ionized.

Weak electrolytes:

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Figure 4.9: The reaction of NH3 in water.

Weak bases - react only slightly with water to give OH- ions.

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Molarity

Molarity (M) = moles of solute per volume of solution in liters:

M

M

molaritymoles of soluteliters of solution

HClmoles of HCl

liters of solution3

62

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Common Terms of Solution Concentration

Standard solution- concentration is exactly known.

Stock solutions- routinely used solutions prepared in concentrated form.

Concentrated solution- relatively large ratio of solute to solvent. (5.0 M NaCl)

Diluted solution - relatively small ratio of solute to solvent. (0.01 M NaCl)

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Figure 4.10: Steps involved in the preparation of a standard aqueous solution.

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Figure 4.11: (a) A measuring pipet is graduated and can be used to measure various volumes of liquid accurately. (b) a volumetric (transfer) pipet is designed to measure one volume accurately.

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Figure 4.12: Dilution Procedure (a) A measuring pipet is used to transfer 28.7mL of 17.4 M acetic acid solution to a volumetric

flask. (b) Water is added to the flask to the calibration mark. (c) The resulting solution is 1.00 M acetic acid.

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Eelectrophoresis

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偵測器注入端

白金電極

高電壓 (KV)

電解質緩衝溶液 電解質緩衝溶液

資料處理系統

偵檢端

毛細管

毛細管電泳 (Capillary Electrophoresis-CE)儀器結構簡圖

毛細管 I.D. 25-100 m

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= r /

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+

-

n

EOF

-+

+

n

-Net

毛細管電泳的向量圖

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Photograph of the micro-device with attached transfer capillary

CE-in-a-Chip

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-TAS (micro-TAS) concept:

Miniture -Total Chemical Analysis system.

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Types of Solution Reactions

Precipitation reactionsAgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

Acid-base reactionsNaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)

Oxidation-reduction reactionsFe2O3(s) + Al(s) Fe(l) + Al2O3(s)

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Figure 4.13: When yellow aqueous potassium chromate is added to a colorless barium nitrate solution, yellow barium chromate precipitates.

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Figure 4.14: Reactant Solutions: (a) Ba(NO3)2(aq) and (b) K2CrO4(aq)

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Figure 4.15a,b: The reaction of K2CrO4 and Ba(NO3)2(aq).

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Figure 4.15c: The reaction of K2CrO4 and Ba(NO3)2(aq). (cont'd)

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Figure 4.16: Precipitation of silver chloride by mixing solutions of silver nitrate and potassium chloride. The K+ and NO3- ions remain in solution.

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Figure 4.17: The reaction of KCl(aq) with AgNO3 to form AgCl(s).

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Table 4.1 Simple Rules for Solubility of Salts in Water

1. Most nitrate (NO3) salts are soluble.

2. Most alkali (group 1A) salts and NH4+ are soluble.

3. Most Cl, Br, and I salts are soluble (NOT Ag+, Pb2+, Hg22+)

4. Most sulfate salts are soluble (NOT BaSO4, PbSO4, HgSO4, CaSO4)

5. Most OH salts are only slightly soluble (NaOH, KOH are soluble, Ba(OH)2, Ca(OH)2 are marginally soluble)

6. Most S2, CO32, CrO4

2, PO43 salts are only slightly soluble.

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Describing Reactions in Solution

1. Molecular equation (reactants and products as compounds)

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

2. Complete ionic equation (all strong electrolytes shown as ions)

Ag+(aq) + NO3(aq) + Na+(aq) + Cl(aq)

AgCl(s) + Na+(aq) + NO3(aq)

3. Net ionic equation (show only components that actually react)

Ag+(aq) + Cl(aq) AgCl(s)

Na+ and NO3 are spectator ions.

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Stoichiometry Steps for reactions in solution.

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Performing calculations for acid-base reactions.

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Key Titration Terms

Titrant - solution of known concentration used in titration

Analyte - substance being analyzed

Equivalence point - enough titrant added to react exactly with the analyte (Stoichiometric point)

Endpoint - the indicator changes color so you can tell the equivalence point has been reached.

Indicator- a color substance with its color change to mark the endpoint of titration.

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Figure 4.18: The titration of an acid with a base.

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Figure 4.19: The reaction of solid sodium and gaseous chlorine to form solid sodium chloride.

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Sum of oxidation states = 0 in compounds

Sum of oxidation states = charge of the ion

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Figure 4.20: A summary of an oxidation-reduction process, in which M is oxidized and X is reduced.

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Balancing by Half-Reaction Method

1.Write separate reduction, oxidation reactions.

2. For each half-reaction:

Balance elements (except H, O)

Balance O using H2O

Balance H using H+

Balance charge using electrons

3. If necessary, multiply by integer to equalize electron count.

4. Add half-reactions.

5. Check that elements and charges are balanced.

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Balancing Oxidation-Reduction Reactions

Cr2O72-(aq) + SO3

-(aq) Cr3+(aq) + SO4

2-(aq)

How can we balance this equation?

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Method of Half Reactions

Cr2O72-(aq) 2Cr3+(aq)  

SO3-(aq) SO4

2-(aq)

How many electrons are involved in each half reaction?

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6e- + Cr2O72-(aq) 2Cr3+(aq)

 

SO3-(aq) SO4

2-(aq) + 2e-

How can we balance the oxygen atoms?

Method of Half Reactions (cont.)

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6e- + Cr2O72-(aq) Cr3+(aq) + 7H2O

 

H2O + SO3-(aq) SO4

2-(aq) + 2e-

How can we balance the hydrogen atoms?

Method of Half Reactions (cont.)

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This reaction occurs in an acidic solution.

14H+ + 6e- + Cr2O72-(aq) Cr3+(aq) + 7H2O

 

H2O +SO3-(aq) SO4

2-(aq) + 2e- + 2H+

How can we balance the electrons?

Method of Half Reactions (cont.)

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14H+ + 6e- + Cr2O72-(aq) Cr3+(aq) + 7H2O

 

3[H2O +SO3-(aq) SO4

2-(aq) + 2e- + 2H+]

Cr2O72-(aq) + 3SO3

-(aq) + 8H+(aq)

2Cr3+(aq) + 3SO42-(aq) + 4H2O(l)

Method of Half Reactions (cont.)

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Half-Reaction Method - Balancing in Base

1. Balance as in acid.

2. Add OH that equals H+ ions (both sides!)

3. Form water by combining H+, OH.

4. Check elements and charges for balance.

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