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- SCH3U1 -Acids and Bases

Sections 10.1

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Learning Goals

1. What is Arrhenius's definition of an acid? A base?  

2. What is the Brønsted-Lowry definition of an acid? A base?  

3. What do the terms "strong" and "weak" mean when applied to an acid or a base?  

4. How are common acids and bases named?  

5. How is an acidic anhydride formed? A basic anhydride?

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Acids General Properties• Acids are substances that exhibit the following

properties when dissolved in water: – Acids taste sour. (Do NOT taste chemicals!)– Acids produce a stinging sensation on the skin

when they enter an open wound. – Acids turn the color of the indicator dye litmus

from blue to red. – Acids react with many metals, such as magnesium,

zinc, and iron, to produce ionic compounds and hydrogen gas.

– Acids react with bases, thereby losing their acidic properties.

Table 10.1 Common Acids

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Bases General Properties

• Bases exhibit the following properties when dissolved in water:

– Bases taste bitter. (Do NOT taste chemicals!)

– Bases feel slippery or soapy on the skin.

– Bases turn the color of the indicator dye litmus from red (pink) to blue.

– Bases react with acids, thereby losing their basic properties.

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Table 10.1 Common Bases

Conductivity of Strong Acids & Bases?

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HCl = Strong Acid

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Acetic Acid = Weak Acid

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NaOH = Strong Base

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Ammonia = Weak Base

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Properties of Acids and Bases

• Taste

• Conductivity

• Feel

• Reaction with litmus paper

• Reaction with active metals

• Reaction with carbonate compounds

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Properties of Acids and Bases

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Arrhenius Theory

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Arrhenius Theory

How would the following acids dissociate in water according to Arrhenius?

What is the recognizable PATTERN?

HBr (aq) HClO4 (aq)

LiOH (aq) Ba(OH)2 (aq)

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Arrhenius Theory

• In 1887 the Swedish chemist Svante Arrhenius proposed:

– An acid is a substance that ionizes (breaks up into ions) in water to produce 1 or more H+ ions

– A base is a substance that dissociates in water to form 1 or more OH- ions

Let’s Use Arrhenius here…

• According to Arrhenius you have to have and H+ and an OH- in the base of an acid or base that is being dissociated to create an H+ or OH- in the products.

• This is not true!

• NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

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Limitations to Arrhenius Theory?

We know NH3 is a base!

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Limitations of Arrhenius Theory

• A free proton does not exist in water solutions. Polarity of water?

Hydronium Ion H3O+ – a hydrated proton,

H

H

O:

:

+ H+ H

H

O

:

H+

WaterHydrogen Ion

(proton) Hydronium Ion

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Brønsted-Lowry Theory

These shortcomings were overcome by a theory proposed independently, in 1923, by J. N. Brønsted (Denmark) and T. M. Lowry (London)

An acid is a proton donor A substance that gives up H+ (a proton).

A base is a proton acceptorA substance that accepts H+ (a proton).

TRANSFER OF A PROTON!!!

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Brønsted-Lowry Examples

Hydrogen Chloride in water is Hydrochloric acid

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Brønsted-Lowry Examples

Ammonia in water is a base

Conjugate Acid-Base PairsBy Brønsted-Lowry theory, the products of an

acid base reaction are also acids and bases.

An acid-base conjugate pair differs in structure only by a proton (H+):

CONJUGATE means “linked together”

• The conjugate acid of a species is that species plus a proton;

• The conjugate base of a species is that species minus a proton.

CH3COOH + H2O H3O+ + CH3COO-

acidacid basebase conjugateconjugateconjugateconjugate

acid base acid base

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Brønsted-Lowry Notes

Like Arrhenius a Brønsted-Lowry acid must have a H+.*So all Arrhenius acids are also Brønsted-Lowry acids

However, any negative anion species can be a Brønsted-Lowry base. (not only OH-)

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Conjugate Pairs Examples

Nitrous Acid

HNO2 (aq) + H2O H3O+ + NO2-

Conjugate Acid Base Pairs

The conjugate base of HNO2 is NO2-,

the species that remains after HNO2 loses a proton.

acidacid base base conjugate conjugateconjugate conjugate acid acid

basebase

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Conjugate Pairs Examples

Ammonia

NH3(aq) + H2O OH- + NH4+

Conjugate Acid Base Pairs

basebase acid acid conjugate conjugateconjugate conjugate base base acid acid

Notice acid-base pairs only differ by one proton (H+)

NH3(aq) + H3O+(l)

Arrhenius vs. Brønsted Lowry

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Strong Acids

• Acids that are completely ionized in water solution are called strong acids.

H2O

HCl(g) -> H+(aq) + Cl-(aq)

In 0.0010 mol/L HCl(aq)[H+] = 0.0010 mol/L[Cl-] = 0.0010 mol/L[HCl] = 0 mol/L

Square Brackets indicates concentration

All the HCl is dissociatedin solution

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Weak Acids• Acids that are only partially ionized in

aqueous solution are called weak acids.

In 1.00 mol/L CH3COOH(aq) only about 1% of the molecules ionize, most of it remains as acetic acid molecules

H2O

CH3COOH(aq) H+ (aq) + CH3COO-

(aq)

In 1.000 mol/L CH3COOH (aq)

[H3O+] or [H+] = 0.010 mol/L [CH3COO-] = 0.010 mol/L[CH3COOH] = 0.99 mol/L

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Monoprotic Acids

• One ionizable H atom per molecule– Hydrochloric Acid HCl– Hydrofluoric Acid HF

– Nitric Acid HNO3

– Hydrocyanic Acid HCN

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Polyprotic Acids

• Diprotic AcidsTwo ionizable H atoms per molecule

– Sulfuric Acid H2SO4

– Carbonic Acid H2CO3

• Triprotic AcidsThree ionizable H atoms per molucule

– Phosphoric Acid H3PO4

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Not all Hydrogens are Acidic

• None of the hydrogens in methane (CH4) are given up in acidic solution

• Only one hydrogen in acetic acid (C2H4O2) is acidic, that is why it is often written as CH3COOH

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How to tell the acidic hydrogens

1. We write a molecular formula with ionizable H atoms first.– HNO3 , H2SO4 , and H3PO4  

– HC2H3O2,

Ionizable Hydrogens

Non-Ionizable Hydrogens

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How to tell the acidic hydrogens

2. In organic chemistry, we often use formulas that show the ionizable hydrogen atoms last.

Example carboxylic acids. Acetic acid CH3COOHFormic acid HCOOH

Propionic acid CH3CH2COOH

Butyric acid CH3CH2CH2COOH

In each of these, only the H atom on the O atom is ionizable.

Ionizable Hydrogens

Non-Ionizable Hydrogens

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Common Strong Acids

Hydrochloric acid HCl(aq)

Hydrobromic acid HBr(aq)

Hydriodic acid HI(aq)

Nitric acid HNO3(aq)

Sulfuric acid H2SO4(aq)  

Perchloric acid HClO4(aq)

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Common Bases• Bases produce OH- ions in aqueous solution

(Arrhenius definition)– Group 1 and 2 cations with hydroxide ions

• Examples:

NaOH Sodium Hydroxide (also known as lye)

KOH Potassium Hydroxide

Ca(OH)2 Calcium Hydroxide (a.k.a. slaked lime)

H2O

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

(aq)

Strong bases are completely ionize in water:Strong Bases

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Strong BasesAlkali metal hydroxides

 Lithium hydroxide LiOH (aq)

Sodium hydroxide NaOH (aq)

Potassium hydroxide KOH (aq)

Rubidium hydroxide RbOH (aq)

Cesium hydroxide CsOH (aq) Alkaline earth hydroxides

 Calcium hydroxide Ca(OH)2 (aq)

Strontium hydroxide Sr(OH)2 (aq)

Barium hydroxide Ba(OH)2 (aq)

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Weak Bases

Bases that are only partially ionized in aqueous solution are called weak bases.

H2O

NH3 (g) + H2O NH4+

(aq) + OH- (aq)

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Ammonia as a proton acceptor

• Lone pair can be used to accept proton