ch 8: aqueous solutions: chemistry of the … ch 8: aqueous solutions: chemistry of the hydrosphere...

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Ch 8: Aqueous Solutions:

Chemistry of the Hydrosphere

H2S + Cu2+ → CuS(s) + 2H+

(Fe, Ni, Mn also)

HS- + 2 O2 HSO4- + energy

(supports life)

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Figure taken from “Principles of Biochemistry”, 2nd Ed. By Lehninger, Nelson, and Cox

Chapter Outline

8.1 Solutions and Their Concentrations

8.2 Dilutions

8.3 Electrolytes and Nonelectrolytes

8.4 Acids, Bases, and Neutralization Reactions

8.5 Precipitation Reactions

8.6 Oxidation-Reduction Reactions

8.7 Titrations

8.8 Ion Exchange

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Concentration of Solutions - Molarity

The concentration of a solution is the amount of solute

present in a given quantity of solvent or solution.

Molarity (M) = liters of solution

moles of solute

Mass-to-mass ratios:

Mass-to-volume ratios:

Parts per million:

mg/kg solvent

mg/L solvent

1 g 106 g

= mg 103 g

= mg L

dH2O = 1.00 g/mL @ 25 oC

Sample Exercise 8.1: Converting Mass-per-

Volume Concentrations into Molarity

Vinyl chloride (C2H3Cl, MW = 62.49) is one of the most widely used industrial chemicals. It is also one of the most toxic, and it’s a known carcinogen. The maximum concentration of vinyl chloride allowed in drinking water in the U.S. is 0.002 mg/L. What is that concentration in moles per liter?

Plan: mg/L g/L mol/L = Molarity

0.002 mg

L x

1 g

1000 mg x

1 mol

62.49 g = 3.2 x 10-8

mol/L

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Sample Exercise 8.2: Converting Mass-per-Mass

Concentrations into Molarity

A water sample from the Great Salt Lake in Utah contains 83.6 mg Na+ per gram of lake water (soln). What is the molar concentration of Na+ ions if the density of the lake water is 1.160 g/mL? AW Na = 22.99

Plan: mg Na+ g mol

1 g soln mL L

dsoln = 1.160 g/mL

mol

L = M

83.6 mg Na+ x 1 g

1000 mg x 1 mol Na+

22.99 g = 3.636 x 10-3 mol Na+

1 g soln x 1 mL soln

1.160 g x 1 L

1000 mL = 8.6221 x 10-4 L

mol

L M =

8.6221 x 10-4 L

3.636 x 10-3 mol Na+

= = 4.22 M

Sample Exercise 8.3: Calculating the Quantity of

Solute Needed to Prepare a Solution

An aqueous solution called phosphate-buffered saline (PBS) is used in biology research to wash and store living cells. It contains ionic solutes including 10.0 mM Na2HPO42H2O (MW = 178.0). How many grams of this solute would you need to prepare 10.0 L of PBS?

Plan: mM M mol g

use the volume

MM

Useful equation: M x V = moles

x 1 mol

1000 mmol x

178.0 g

mol = 17.8 g 10.0 mmol L

x 10.0 L

M x V = moles

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Chapter Outline


8.2 Dilutions





8.7 Titrations

8.8 Ion Exchange

Dilution is the procedure for preparing a less concentrated

solution from a more concentrated solution.

Dilution

Add Solvent

Moles of solute

before dilution (i)

Moles of solute

after dilution (f) = MiVi MfVf =

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Dilution Equation: Mi Vi = Mf Vf

The solution used in hospitals for intravenous infusion—called physiological or normal saline - is 0.155 M NaCl. It may be prepared by diluting a commercially available standard solution that is 1.76 M NaCl. What volume of standard solution (Vi) is required to prepare 10.0 L of physiological saline?

Sample Exercise 8.4: Diluting Solutions

Collect data:

Mi = 1.76 M Mf = 0.155 M

Vi = ? Vf = 10.0 L

Mi Vi = Mf Vf

Vi = Mf Vf

Mi

Vi = (0.155 M )(10.0 L)

(1.76 M)

= 0.881 L = 881 mL

add

H2O

Add 881 mL of

1.76 M NaCl,

and then “dilute

to the mark”

(10.0 L)

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Chapter Outline


8.2 Dilutions





8.7 Titrations

8.8 Ion Exchange

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Strong Electrolytes:

• Nearly 100% dissociated

into ions

• Conduct current efficiently

Examples: ionic compounds such as

NaCl, strong acids like HCl

• NaCl(s) → Na+(aq) + Cl−(aq)

• HCl(aq) + H2O(l) H3O+ + Cl-(aq)

Figure from John J. Fortman, J. Chem. Ed. Vol. 71, No. 1, 1994, p. 27-28

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Weak Electrolytes:

• Only partially dissociate into

ions

• Slightly conductive

Example: weak acids such as

acetic acid

Hydronium Ion: H3O+

+

The higher the concentration of H3O+ in solution, the stronger the acid.

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Non-electrolytes

Substances in which no ionization

occurs. There is no flow of electrical

current through the solution

Examples: molecular or covalent

compounds such as ethanol

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Chapter Outline


8.2 Dilutions





8.7 Titrations

8.8 Ion Exchange

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Acids

Have a sour taste. Vinegar owes its taste to

acetic acid. Citrus fruits contain citric acid.

React with certain metals to produce hydrogen gas.

React with carbonates and bicarbonates to produce carbon

dioxide gas

Cause color changes in plant dyes (litmus paper).

2HCl (aq) + Mg (s) MgCl2 (aq) + H2 (g)

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

Aqueous acid solutions conduct electricity.

Limestone

dissolving in acid =

cave

Most metals

dissolve in acids to

produce H2

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Strong Acids and Bases

Strong Acids/Bases:

• Completely ionized

in aqueous solution

(i.e., strong electrolytes)

• Memorize the 6 strong

acids

• All other acids are weak

KNOW!

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Have a bitter taste.

Feel slippery. Many soaps contain bases.

Bases

Cause color changes in plant dyes (litmus paper).

Aqueous base solutions conduct electricity.

A Brønsted acid is a proton donor

A Brønsted base is a proton acceptor

acid base conjugate

acid

conjugate

base

For every acid there is a conjugate base, and for every

base there is a conjugate acid (conj. acid-base pairs).

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Water: Acid or Base? Water as Base:

HCl(aq) + H2O(l) → H3O+(aq) + Cl−(aq)

Water as Acid:

NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq)

Amphiprotic: Acts as an Acid or Base.

Preface: Double-Displacement Reactions Many of the reactions in this chapter follow this pattern:

AB + CD → AD + CB (+) (-) (+) (-) (+) (-) (+) (-)

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Acid-Base (Neutralization) Reactions

acid + base salt + water

HCl(aq) + NaOH(aq)

H2SO4(aq) + 2 KOH(aq)

HOH(aq) + NaCl(aq)

2 HOH(aq) + K2SO4(aq)

Net Ionic Equations

1. Write the balanced molecular equation.

2. Write the ionic equation showing the strong

electrolytes completely dissociated into cations and

anions.

3. Cancel the spectator ions on both sides of the ionic

equation

Tips:

• Do not break apart solids, liquids (such as H2O), and gases (e.g. CO2)

• Do not break apart weak electrolytes (such as weak acids, e.g.

HC2H3O2)

• Carbonic acid, H2CO3, decomposes into CO2 and H2O

• An acid such as H2SO4 breaks apart into 2H+(aq) + SO42-, not H2

2+ +

SO42-

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H2SO4(aq) + 2 KOH(aq) 2 H2O(aq) + K2SO4(aq)


2. Write the ionic equation showing the strong electrolytes

completely dissociated into cations and anions.

3. Cancel the spectator ions on both sides of the ionic equation

strong strong strong Not a strong

electrolyte!!

2 H+ SO42- 2 K+ 2OH- 2 H2O 2 K+ SO4

2-

2 H+ + 2 OH- 2 H2O

H2O H+ + OH- Net ionic equation:

Write the net ionic equation describing the reaction that takes place when acid rain containing sulfuric acid reacts with a marble statue (calcium carbonate) -

Sample Exercise 8.5: Writing the Net Ionic

Equation for a Neutralization Reaction


2. Write the ionic equation showing the strong electrolytes completely dissociated

into cations and anions.

*generally speaking, CaSO4 is insoluble in water, but not in strong acid. More on

this later when we study solubility rules.

H2SO4(aq) + CaCO3(s) H2O(l) + CO2(g) + CaSO4(aq)

strong solid strong* Not strong

electrolytes!!

2H+(aq) + SO42-(aq) + CaCO3(s) H2O(l) + CO2(g) + Ca2+(aq) + SO4

2-(aq)

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Sample Exercise 8.5: Writing the Net Ionic

Equation for a Neutralization Reaction

3. Cancel the spectator ions on both sides of the ionic equation

2H+(aq) + SO42-(aq) + CaCO3(s) H2O(l) + CO2(g) + Ca2+(aq) + SO4

2-(aq)

2H+(aq) + CaCO3(s) H2O(l) + CO2(g) + Ca2+(aq)

Net ionic equation:

ch 8: aqueous solutions: chemistry of the … ch 8: aqueous solutions: chemistry of the hydrosphere...

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