iii - (7) acids and bases

8/2/2019 III - (7) Acids and Bases

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ACIDS and BASES


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ACIDS

The word acid comes from the Latin word acidus whichmeans sour

Properties of Acids

Acids have a sour taste

Vinegar: 5% acetic acid (CH3COOH)

Citrus fruits: citric acid

Sour milk: lactic acid


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Acids cause color changes in plant dyes

Basis: Litmus paper test

Acids usually react with metals to produce hydrogen gas

2HCl (aq) + Zn (s) ZnCl2 (Aq) + H2 (g)

Acids react with carbonates and bicarbonates producingcarbon dioxide gas

HCl (aq) + NaHCO3 (aq) NaCl (aq) + H2O (l) + CO2 (g)


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Acids in aqueous solution conduct electricity: Electrolytes

Strong acids: strongly conduct electricityWeak acids: weakly conduct electricity

Acids neutralize bases

weak electrolyte strong electrolyt


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Bases

Properties of Bases

Bases have bitter taste

Bases have a slippery or soapy feel

Detergents and soaps: bases


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Bases causes color changes to plant dyes

Strong bases usually react with metals to produce hydrogen gas

2KOH (aq) + Zn (s) K2ZnO2 (aq) + H2 (g)


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Bases conduct electricity

Strong bases: strong electrolytesWeak bases: weak electrolytes

Bases neutralize acids


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THEORIES on ACIDS and BASES

Arrhenius Theory Bronsted-Lowry Trhoery Lewis Theory


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Arrhenius Theory of acids and bases

Svante Arrhenius (1859-1927)

Characterized acids and bases based on their interaction with water

Acid

Substance that

produces H+ orH3O

+ whendissolved in water

Bases

Substances thatproduce OH- whendissolved in H2O


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HCl: Arrhenius acid

HCl + H2O H3O+

(aq) + Cl-

(aq)

NH3: Arhenius base

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


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CH3COOH

CH3COOH (l) + H2O (l) CH3COO- (aq) + H3O

+ (aq)

NaOH

NaOH + H2O Na+ (aq) + OH- (aq) + H2O (l)

HX HX + H2O H3O+ (aq) + X- (aq)

B B + H2O OH-(aq)


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Why do acids and bases conduct electricity?

They conduct electricity

because they form IONSwhen in aqueousconditions

HOWEVER:

The Arrhenius concept isonly limited to solutions in

water

It fails to account acid-basereactions in the gaseous phaseor in other solvents


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Bronsted-Lowry Theory

Johannes Nicolaus Bronsted (1879-1974)Thomas Lowry (1874-1936)

Acid any substance that can donate a proton (or H+)

Base any substance that can accept a proton (H+)

Acids: proton donors

Bases: proton acceptors

Acids and bases: can be molecules or ions


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CH3COOH + H2O

CH3COO-

(aq) + H3O+

(aq)



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CH3COOH (aq) + NH3 (aq) CH3COO- (aq) + NH4

+ (aq)

Consider the reverse reaction

CH3COO- (aq) + NH4

+ (aq) CH3COOH (aq) + NH3 (aq)

CONJUGATE ACID/BASE PAIR

-an acid and a base related with proton transfer


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CONJUGATE ACID-BASE PAIRS

Acid-conjugate base Base-conjugate acid

CH3COOH + H2O CH3COO- (aq) + H3O

+ (aq)

Consider the reverse reaction

CH3COOH

CH3COO-

H2O

H3O+

acid1

acid2

base1

base2


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HCO3- (aq) + H2O CO3

2- (aq) + H3O+ (aq)

Identify:

HCN (aq) + H2PO42- CN- (aq) + H3PO4 (aq)

HPO42- (aq) + H2CO3 (aq) H2PO4

2- (aq) + HCO3- (aq)


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Conjugate acid base pair

Acid-conjugate base

HCl/ Cl-

CH3COOH/ CH3COO-

HBr/ Br-

Base-conjugate acid

NH3/ NH4+

HCO3- / H2CO3

H2O as an acid

H2O as a base


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What is the conjugate base pair of:

H2CO3HCO3

-

HIHSO4

-

HNO3HPO4

2- What is the conjugate acid pair of:

HCO3-

SO4

2-

H2PO4-HPO4

2-

HSO3-


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Monoproticacids

HCl H+

+ Cl-

HNO3 H+ + NO3

-

CH3COOH H+ + CH3COO

-

DiproticacidsH2SO4 H

+ + HSO4-

HSO4- H+ + SO4

2-

Triproticacids

H3PO4 H+ + H2PO4

-

H2PO4- H+ + HPO4

2-

HPO42- H+ + PO4

3-


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Amphiprotic substances

Substances that can act either as an acid or a base

H2O


+ (aq)


HCO3-


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Lewis Theory

Gilbert Newton Lewis (1875-1946)

Based on electron sharing rather than proton transfer

Accounts for acidic and basicsubstances that may behaves asacids and bases but do notinvolve H+

Lewis Acid a substance that accepts a pair of electrons

Lewis Base a substance that donates electrons


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+ (aq)



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Lewis Acids and Bases

N H

H

H

acid base

F B

F

F

+ F B

F

F

N H

H

H

No protons donated or accepted!


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Theory Acids Bases

Arrhenius Yields H+ or H3O+

in aqueoussolutions

Yields OH- inaquoeussolutions

Bronsted-Lowry Proton (H+)donors

Protonacceptors

Lewis Electron-pairacceptors

Electron-pairdonors


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HOW ARE THE STRENGTHS OF ACIDS AND BASES MEASURED?

pH scale

Recall: H2O amphiprotic

H2O + H2O H3O+ (aq) + OH- (aq))

pH gives us an idea on how basic or acidic a substances is

pH = 7 neutralpH < 7 acidpH >7 base

pH = - log [H+]


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H2

O (l) H+ (aq) + OH- (aq)

The Ion Product of Water

Kc=[H+][OH-]

[H2O][H2O] = constant

Kc[H2O] = Kw= [H+][OH-]

The ion-product constant(Kw) is the product of the molar

concentrations of H+ and OH- ions at a particular temperature.

At 250C

Kw= [H+][OH-] = 1.0 x 10-14

[H+] = [OH-]

[H+] > [OH-]

[H+] < [OH-]

Solution Is

neutral

acidic

basic


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pH A Measure of Acidity

pH = -log [H+]

[H+] = [OH-][H+] > [OH-]

[H+] < [OH-]

Solution Is

neutralacidic

basic

[H+] = 1 x 10-7

[H+] > 1 x 10-7

[H+] < 1 x 10-7

pH = 7pH < 7

pH > 7

At 250C

pH [H+]


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pOH = -log [OH-]

[H+][OH-] = Kw= 1.0 x 10-14

-log [H+] log [OH-] = 14.00

pH + pOH = 14.00


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Calculate the pH of a 0.10 M HCl solution:



pOH?

pOH?

pOH?


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Calculate the pH of a 0.10 M NaOH solution:

Calculate the pH of a 0.008 M KOH solution:

Calculate the pH of a 1.8 x 10-6 M NaOH solution:

iii - (7) acids and bases

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