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Acids & BasesPart I: Some Definitions

Dr. C. Yau

Spring 2014

Jespersen Chap. 16

Sec 1 & 2

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Different Definitions of Acid/Base

Arrhenius Definition:

An acid is a substance that increases the H3O+ concentration when placed in water.

A base is a substance that increases the OH- concentration when placed in water.

NaOH (s) Na+(aq) + OH-(aq)

HCl (g) + H2O (l) H3O+(aq) + Cl-(aq)

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Different Definitions of Acid/Base

Brønsted-Lowry Definition:

An acid is a proton donor.

A base is a proton acceptor.

O H

H Cl O H

H....

..

....

: : :: :+ .. + Cl_

..

_

proton donor

proton acceptor

Typically a Brønsted base has a lone pair to attract the proton.

H O H H Cl H O H

H

..

....

....

: : :+ .. + Cl_

..

+

water acting as a base

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Brønsted-Lowry DefinitionsWhy do we need new definitions?The Arrhenius definitions were based on

placing substances in water, but not all reactions are in water.

e.g. NH3 (g) + HCl (g) NH4Cl (s)This is an acid-base reaction and yet no

H3O+ nor OH- ions are involved.However, proton transfer is involved.Write the Lewis structures for the

reaction above and draw in curved arrows to show the flow of electrons.

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Brønsted-Lowry Acids/Bases

Organic carboxylic acids have –COOH group attached. Acetic acid has the structure…

Recognize all the different ways of writing its formula:

HC2H3O2 CH3COOH CH3CO2H

Remember: -COOH does NOT mean C-O-O-H

H C C

H

H

O

O HWhich H will be donated as a proton?

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Write the equation using Lewis structures and curved arrows to show the reaction of acetic acid with ammonia.

Which is the proton donor, which is the proton acceptor? Which is the proton being transferred?

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Conjugate Acids & Conjugate Bases

We will abbreviate "Brønsted-Lowry" to just "Brønsted."

Write the equation for the reaction of acetic acid and ammonia using molecular formulas: HC2H3O2 + NH3

HC2H3O2 + NH3 C2H3O2- + NH4

+

acid base

How is the acetate a base? and NH4+ an acid?

conjugate conjugate base acid

(This is ammonium acetate.)

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conjugate base conjugate acid

HCl

NH3

HC2H3O2

CN-

HF

Identify the Conjugate Partner for Each

The clue is to remember what the functions of Brønsted acids and bases are.

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Conjugate Acid-Base Pairs

Write the equation for the reaction of formic acid with water using molecular formulas:

HCO2H + H2O

Identify which is the acid, base, conjugate acid, conjugate base.

Give the conjugate acid-base pairs:

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Conjugate Acid-Base Pairs

Example 16.1 p. 744

What is the conjugate base of nitric acid and what is the conjugate acid of the hydrogen sulfate ion?

Do Pract Exer 1, 2, 3 on p. 744

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Conjugate Acid-Base Pairs

Example 16.2 p. 745

The anion of sodium hydrogen sulfate reacts as follows with the phosphate ion. Identify the 2 conjugate acid-base pairs.

HSO4 (aq) + PO4

3 (aq) SO42 (aq) + HPO4

2 (aq)

Do Pract Exer 4, 5, 6 p. 745.

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Amphoteric SubstancesAn amphoteric substance is one that can

act either as an acid or a base.Water is such a substance.Although it is exists primarily as a

molecule, there are some ions present due to the self-ionization of water:

HOH + HOH H3O+ + OH-

Write the equation for water reacting with ammonia.

Write the equation for water reacting with hydrochloric acid.

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Amphoteric SubstancesWrite the equation for the reaction of the

bicarbonate ion with hydroxide.

Write the equation for the reaction of the bicarbonate ion with hydronium ion.

Why is bicarbonate considered amphoteric?

Do Pract Exer 7, 8 p. 746

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Acid and Base StrengthsFirst recall what an acid is supposed to

do…. proton donor… gives away a proton.

A strong acid is one that can willingly "gives away" its proton.

HCl + H2O H3O+ + Cl-

A weak acid is one that "hangs on" to its proton.

HC2H3O + H2O H3O+ + C2H3O2-

100%

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Acid and Base StrengthsRecall what a base is supposed to do…

proton acceptor…. grabs up protons.

A strong base is one that "greedily grabs" protons.

NH2- + H2O NH3 + OH-

A weak base is one that does not particularly want to grab a proton.

Cl- + H2O HCl + OH-

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H3O+ and OH-

H3O+ is the strongest acid that can exist in water.

Note that mineral acids (nonorganic acids) such as HCl, HNO3, H2SO4, are stronger acids, but as soon as they are placed in water, they dissociate 100% and no longer exist in water as such.

They become H3O+ and the corresponding anions.

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H3O+ and OH-

OH- is the strongest base that can exist in water.

O2- is a stronger base but as soon as it goes into the water, it is converted to OH- and does not exist as O2-.

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Relationship between strengths of acid and conjugate base

HC2H3O + H2O H3O+ + C2H3O2-

Identify the acid, base, conjugate acid, conjugate base.Is the acid strong or weak?Is its conjugate base strong or weak?Is the base strong or weak?Is the conjugate acid strong or weak?

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Relationship between strengths of acid and conjugate base

HC2H3O + H2O H3O+ + C2H3O2-

The pattern we see is…A weak acid produces a strong conjugate base.A strong acid produces a weak conjugate base.A weak base produces a strong conjugate acid, & a strong base produces a weak conjugate acid.

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Relationship between strengths of acid and conjugate base

HC2H3O + H2O H3O+ + C2H3O2-

Another pattern we see is that the position of the equilibrium lies in favor of the weaker acid and base.

REMEMBER: Equilibrium goesfrom Strong acid to Weaker acid.

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Example 16.3 p. 749In this rxn, will the position of equilibrium lie

to the left or the right, given the fact that acetic acid is known to be a stronger acid than the hydrogen sulfite ion?

HSO3(aq) + C2H3O2

(aq) HC2H3O2 (aq) + SO32 (aq)

Do Pract Exer 9, 10 p. 750

Which is the stronger acid?

Which “acids” are we talking about?

How can we tell which is the stronger acid?

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