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Acids and Bases Chapter 15. End-of-Chapter HW: See Next Page; Not Graded 2) Some Deadlines (OL = Online; Time = 11:50 pm unless noted): OL Practice4/5/2013 OL Math Review4/8/2013 OL Chemistry Review4/9/2013 Lab Safety Quiz4/11/2013 – Thursday, Noon OL HW 154/15/2013

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Acids and Bases Chapter 15

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Acids and bases chapter 15 l.jpg

Acids and BasesChapter 15

End-of-Chapter HW: See Next Page; Not Graded

2) Some Deadlines (OL = Online; Time = 11:50 pm unless noted):

OL Practice4/5/2013

OL Math Review4/8/2013

OL Chemistry Review4/9/2013

Lab Safety Quiz4/11/2013 – Thursday, Noon

OL HW 154/15/2013

OL available at: www.saplinglearning.com

3) Exam & Quiz over 15, 16 & Labs 1,2 in ~ 4 weeks

4) Download, print, read, do lab material for 1st lab (meet next wk)


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End-of-Chapter Problems: pp 656 - 662Chapter 15

Not turned in; will discuss in lecture after ch 15 & 16

1 3 4 7 10 through 23 29 33 35

39 41 53 57 59 61 65 67 71

75 77 81 87 93 103 105 107 116


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Review - Significant Figures (SF)

1. Definition:All digits known with certainty plus the first uncertain one. Assume last digit is in doubt by ± 1 unless told otherwise.

2. Zeros: Assume significant with one exception - not significant when left of first non-zero digit. Always report all SF including zeros. Can use underline to show last SF.

3. Examples: 0.0010030 = 5 SF 1.00 = 3 SF 1.013456 = 4 SF

4. Why Use? So that folks looking at your data will immediately know which digits are meaningful. One can be misled on your accuracy if significant figures are not used.

5. Significant Figures limited by:

a) Technique Used

b) Measuring Devices

c) Convention:Standard Deviation , Rel. Std. Dev., Error: Use 2 SF


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Review - Significant Figures continued

6. Use of SF in calculations:

a) Multiplication / Division: Answer can’t be more accurate than least accurate number. Smallest # of SF limits answer to that # of SF.

b) Addition / Subtraction: Set up problem & cut off answer at first doubtful digit. Answer can gain or lose SF.

7. Examples:1 . 0 3 4 2 7 6(7 SF)

- 0 . 0 1 1(2 SF)

1 . 0 2 3 2 7 6 = 1.023 (4 SF)

5.692 x 0.00220 x 1.0011

= 1.124946126 = 1.1 (2 SF)

9.06x10-2 x (3.333 - 3.21) (use addn/subtn rule 1st, then use mult/div rule)


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Review - Significant Figures, Accuracy & Precision

8. Logs & Inverse Logs: a log is the addn of exponent plus the log value of the #

Examples: Log 3.62x104 = 4.0000… + 0.558709 = 4.559 (gain SF)

Inverse log of 4.559 = 104.559 = 100.559 x 104.000 = 3.62 x104(lose SF)

Use the rule given in Sapling hints of chemistry review.

9. Grading: Ignoring SF in lab & lecture WILL lower grade.

10. Accuracy: Closeness of average value to true value; use RE(relative error) in ppt:

(true value – average exp value) x 1000 = RE in ppt (parts per thousand)

true value

11. Precision: Grouping of measurements; good precision = small grouping.

Precision usually reported as standard deviation, sd (units for sd are same as x).

sd = ± √∑(X - xi)2 / ( n-1) ∑= sum;X = ave; n = # runs; xi = each value

rsdin ppt = (sd x 1000) / X Convention: Report sd,rsd, & RE to 2 SF.

See burette lab for worked out example. Can use Excel for calculations.


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I. DefinitionsAcids & Bases

A. Arrhenius Acids and Bases

H+ + H2O H3O+Hydronium Ion

Note: 1) I will use H+ in place of H3O+; 2) [ ] = m/L = M

Acid: A substance which increases [H+] when dissolved in water.

Base: A substance which increases [OH-] when dissolved in water.

-The above concepts were

introduced by Svante Arrhenius

in his doctoral thesis in 1884 in

Sweden. His work was assumed

to be wrong, and he was given

the lowest possible passing grade;

awarded the Nobel Prize in 1903

for his work!


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Arrhenius Acids

- Strong acids completely ionize in water to release H+ (H3O+) and an anion; know the names & formulas of the strong & weak acids & bases.

Strong Acids:HClO4 H2SO4 HI HBr HCl HNO3

HI + H2O H3O+ + I-or HI H+ + I-(100%)

Notes:

1) Caution: Strong acids react with proteins, carbohydrates and fats.

2) Caution: HClO4 and HNO3 can react explosively with organics.

3) Only the first ionization is 100% with H2SO4

4) Important: Know the Strong & Weak acids/Bases, Formulas & Names.

5) For oxy acids like HNO3 , H2SO4 & HClO4the acidic H is on an O atom.


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Arrhenius Acids

- Weak acids only partially ionize in water; < 5% ionization.

Weak Acids:HC2H3O2 HF H3PO4 HCN H2S HNO2 NH4+ HClO

HC2H3O2 H+ + C2H3O2- & NH4+NH3 + H+(~ 1% ionized)

Note: - only the H attached to the O in acetic acid is released; H’s attached to C are NOT acidic; most organic acids are weak acids.

- pH, conductivity & van’t Hoff factor (i) measurements, theoretical calculation allow one to determine the % ionization.

- Know the names & formulas of the above weak acids.


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Arrhenius Bases

Strong bases ionize 100 % in water and weak bases only partially ionize.

Examples of Strong Bases: LiOH NaOH KOH Ca(OH)2 Sr(OH)2 Ba(OH)2 Na3PO4

Examples of Weak Bases: NH3 CO3-2 HCO3- CN- C2H3O2- F-

NaOH Na+ + OH-(strong, 100 % Ionized)

NH3 + H2O NH4+ + OH-(weak, ≤1 % Ionized)

F- + H2O HF + OH-(weak, ≤1 % Ionized)

Notes: 1) Salts of weak acids (like C2H3O2- & F-) are weak bases. These are usually found as group I salts: NaF, KF, LiF, NaC2H3O2, etc.

2) Bases are much more damaging than acids to proteins & triglyceride fats. Eye damage can occur on contact; even with dilute bases.

3) Important; know the strong & weak bases/acids & their names.


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Arrhenius Acids & Bases

- H+ and OH- are in dynamic equilibrium in water; changing the M of one will change the M of the other.

- The following ALWAYS takes place in water regardless of any other chemicals present:

H2O H+ + OH-

M in pure water = 56 10-7 10-7

- [H+] & [OH-] = 1.00x10-7M for pure, neutral water at 25oC

- Notes:

1) Le Chatelier’s Principle predicts effect of adding additional H+ or OH- to H2O.

2) Acid + Base react completely to yield a salt plus water.

Example: 2HCl + Ca(OH)2 ----) CaCl2 + 2H2O ;

Heart & driving force of this reaction (NIE): H+ + O-H- ----) H-O-H (H2O)


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B. Brønsted-Lowry (Brønsted) Acids and Bases

- The Arrhenius definition does not apply when water is not present; so, the Brønsted concept was generated.

Acid:A proton donorNote: Proton = H+

Base:A proton acceptor

Example: HCl (g) + NH3 (g) -----) NH4Cl (s)

H+(aq) + :NH3(aq) -----) NH4+(aq)(Net Ionic Eqn in H2O)

Notes: 1) HCl is the proton donor & NH3 is the proton acceptor

2) The products are called conjugates.

3) The Brønsted definition includes all Arrhenius acids & bases plus additional ones (more general).


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B. Brønsted-Lowry Acids and Bases

Older Bronsted

Younger Bronsted

Lowry


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B. Brønsted & Lewis Acids and Bases

Examples:HF + HCO3- H2CO3 + F-

HF = Acid F- = Conjugate BaseHCO3- = Base H2CO3 = Conjugate Acid

CH3NH2 + HI ------) CH3NH3+ + I-

Which is conjugate acid & which is conjugate base?

I- = conjugate base CH3NH3+= conjugate acid

C. Lewis Acids and Bases

- The most general acid-base definition is based on electrons and is called the Lewis definition (Also gave us: Lewis structures, covalent bonds, “photons”).

Acid = Electron pair acceptor

Base = Electron pair donor


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C. Lewis Acids and Bases

Example:AlCl3 + Cl- -----) AlCl4-

AlCl3 accepts a pair of electrons = Acid

Cl- donates a pair of electrons = Base

1)The above rxn is an acid base rxn in the Lewis sense; AlCl3 & Cl- are Lewis acids & bases but not Brønsted or Arrhenius acids & bases.

2) Many organic reactions use non-polar solvents, and non-polar Lewis acids such as AlCl3BF3FeCl3 are more soluble than polar acids in the organic solvents.

3) Lewis definition includes all Arrhenius & Bronsted acids & bases.


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II. Acid - Base Strengths (Table 15.2, Pg 631)

  • For two acids that are 100% ionized in water (like HI and HBr), one uses a solvent other than water (like acetic acid) to compare their strengths. Acetic acid as a solvent cuts down on acid ionization (Le Chatlier’s Principle).

    - Table 15.2 is a partial list of the strengths. Organics are left off. Organic acids such as: HC2H3O2 (acetic), HCO2H (formic), HC4H7O2 (butyric), and HOC6H5 (phenol) are weak acids. Organic bases like N(C2H5)3 or CHO2- are weak bases.

    O OO

    CH3COH HCOH CH3CH2CH2COH -OH

    - Organic acids are generally weaker than inorganic acids. Some organic bases like :CH3- are much stronger than inorganic bases.


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II. Acid - Base Strengths

- Two related factors influence acid strength: (EN = electronegativity)

1) Bond Polarity- The more polar the bond, the stronger the acid.

- HCl (∆EN = 0.9) > > H2S (∆EN = 0.4)

2) Bond Strength- The weaker the bond, the stronger the acid.

- HI = strong & HF = weak acid. HI bond weaker than HF bond.

- Bond strength is the larger of the two factors.

a) Which is the strongest acid: H-O-ClO3 or H-O-ClO2?

b) Which is the strongest acid: FCH2-CO2H orCH3-CO2H?

Answers: a) HClO4 b) FCH2-CO2H (both polarity & bond strength factor in)


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III. Calculating & Measuring [H+], [OH-]

A. Autoionization of Water - The following is always true in water:

1H2O 1H+ + 1OH-M H2O = 55.5 m/L ≈ constant

Keq = [H+]1[OH-]1 = [H+][OH-]

[H2O]1 [55.5]

Keq x [55.5] = Kw = 1.00x10-14 = [H+][OH-]

1.00x10-14 = [H+][OH-]

Notes: 1) The above equilibrium expression is always true for water at 25oC; the Kw value does change a little with temperature.

2) H2O drops out of all equilibrium expressions when water is the solvent.

3) You need to memorize this Kw value; assume T = 25oC for all problems.


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III. Calculating & Measuring [H+], [OH-]

H2O H+ + OH-(equilibrium reaction)

1.00x10-14 = [H+][OH-](equilibrium expression)

- This expression is true regardless of what is present in the water.

- If we let X = M of H2O that dissociates, then X = [H+] = [OH-] at equilibrium.

1.00x10-14 = [X] [X] = [X]2 X = √1.00x10-14 = 1.00x10-7

x = [H+] = [OH-] = 1.00x10-7M (this is called a neutral solution)

- If add another source of acid or base to water, then 1.00x10-14 = [H+][OH-]is still true; but, [H+] & [OH-] will no longer be equal; can calculate new values of [H+] & [OH-] with equil. expression.


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B. Addition of Strong Acid or Base to Water

1) If one adds 0.10 mole of HI to 1.0 L of water, then what is the [H+]?

- Strong acids ionize 100% The M of H+ = M of the strong acid for HI.

- 0.10 M HI yields 0.10 M H+ in water; [H+] = 0.10 M

2) What will be the [OH-] in 0.10 M HI? Note: [H+] = 0.10 M

- Need to use: 1.00x10-14 = [H+] [OH-] from: H2O H+ + OH-

1.00x10-14 = [0.10] [OH-] [OH-] = 1.0x10-13M

3) What are the [H+] & [OH-] in 1.0 x 10-2M NaOH ?

- NaOH ionizes 100%. 1.0 x 10-2M NaOH yields 1.0x10-2 M OH-

1.00x10-14 = [H+] [OH-] 1.0x10-14 = [H+] [1.0x10-2] [H+] = 1.0x10-12M

Note: The following summarizes the acidity scale (based on M of H+) :

[H+] > 1.0x10-7M = Acidic [H+] = 1.0x10-7M = Neutral [H+] < 1.0x10-7M = Basic


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B. Addition of Strong Acid or Base to Water continued

- General Rule: The M of H+ or OH- is the M of the strong acid or strong base.

- Exception: Above not true when M of the strong acid (or base) is < 10-6M.

Why? Are two sources of H+ in water; one from H2O (~10-7) & one from acid.

[H+]total = [H+]H2O + [H+]StrongAcid [H+]total = (≈10-7) + [H+]S Acid

Example: What will be the [H+] if one adds 1.0x10-9 moles of HI to 1.0 L of water?

Will the [H+] be 1.0x10-9M; No, [H+] = (1.0x10-7+ 1.0x10-9) = 1.0x10-7

C. pHNote: “p” = mathematical operator = take -log

Need a more convenient scale than [H+] for measuring acidity; the [H+] scale is too large – from about 10o to 10-14 in water; can compact using the pH scale.

pH = - log [H+] - log [10-x] = x

ln is to the base e;log is to the base 10; Note: ln or log of 1.000 = 0.000

1) What is pH of: 1.0 x 10-4M HCl?1.0 x 10-5M HCl? 5.0 x 10-5M HCl?

4.005.00-log (5.0 x 10-5) = 4.30


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C. pH continued

2) if take -log of both sides of: 1.00x10-14 = [H+] [OH-]

(note: can use either equation)14.00 = pH + pOH

Example: What is pH if pOH = 11.0?pH = 14.00 – pOH = 3.0

3) Will have to calculate [H+] from pH. With your calculator, use:

10x or inverse log or antilog functions

Examples: ( use: [H+] = 10 –pH )

-What is [H+] if the pH is 5.50?[H+] = 10-5.50 = 3.2 x 10-6

-What is [H+] if the pH is 9.38?[H+] = 10-9.38 = 4.2 x 10-10

4) Can check acidity with either a pH meter or with an Indicator


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C. pH and Significant Figures

  • M to pHExample: Convert 4.5x10-3M H+ to pH (gain SF)

    -log 4.5x10-3 = -log 4.5 + -log 10-3 = -0.653 + 3.000 = 2.347 = 2.35

    MpH

    4.6x10-32.3372 3SF (gain a SF)

    4.5x10-32.3468

    4.4x10-32.3565

  • pH to MExample: Convert pH of 2.34 to M H+(lose SF)

    - antilog 2.34 = -antilog 3.00 + antilog 0.66 = 10-3 x 4.57 = 4.6x10-3

    - inverse log 2.34 = 10-2.34 = 4.6x10-3

    pHM

    2.334.68x10-3

    2.344.57x10-34.5±0.1 (2 SF) (lose a SF)

    2.354.47x10-3


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pH Electrodes

- pH meter is a device used to

measure potential developed by a set

of electrodes (reference and glass).

Et = Eglass+ Eref Et converted to pH

- The surface of the electrode has

special glass which attracts H+ & sets

up a potential difference across glass

Membrane (Eglass). Eref = constant

-Needs to be calibrated with standard

buffers. Up to ± 0.01 pH accuracy.

- Typical combination electrode ($100

to $1000). Will discuss in more detail

in chapter 19 (end of May).


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Acidity Scale

MScale: AcidicNeutralBasic

10-1 ………… 10-7 ………..... 10-14

pH Scale: 1 ……….. 7 …..… 14

pH < 7 (acidic)pH = 7 (neutral) pH > 7 (basic)


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Indicators

Indicators – organic compounds whose color varies with acidity. Can coat paper with several indicators and specific color will tell pH to within ± 1 pH. Predict result of mixing MR & TB.

This mix is my choice.


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Things to Know

  • Significant Figures & how to use

  • Accuracy, Precision, SD RSD, RE - what are & calculate

  • Three acid/base definitions & associated terms

  • Weak & Strong Acids & Bases; Formulas & Names

  • Water Equilibrium Reaction & Equilibrium Expression

  • Calculation of [H+] & [OH-] ( M ) of strong acids or bases

  • Calculation of pH & pOH from [H+] or [OH-]

  • Calculation of [H+] or [OH-] from pH or pOH

  • Acidity Scale

  • Measuring Acidity – Indicators or pH meter


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