Unit 4: Equilibrium, Acids & Bases Part 2: Acids and Bases

Unit 4: Equilibrium, Acids & Bases

Part 2: Acids and Bases

Bronsted Lowry Acids and Bases Autoionization of Water pH Strong Acids and Bases Weak Acids and Bases Ionization Constants Buffers Titrations Lewis Acids and Bases

Review

Arrhenius Acid: Substance that increases the concentration of H+ ions when dissolved in water

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

Arrhenius Base: Substance that increases the concentration of OH- ions when dissolved in water.

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

H2O

H2O

Review

The Arrhenius definition of acids and bases is limited to aqueous solutions

Two other common definitions for acids and bases.Bronsted-Lowry acids and basesLewis acids and bases

Bronsted-Lowry Acids & Bases

Bronsted-Lowry Acidany substance that can transfer a

proton (H+ ion) to another substance a proton donor

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

A Bronsted-Lowry acid must have a hydrogen that can be lost as H+

B-L acid


Bronsted-Lowry Baseany substance that can accept a

proton (H+ ion) from another substance

A proton acceptor

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

A B-L base must have an atom with a lone pair of electrons that can form a new bond to a hydrogen ion.

B-L acid B-L base


The H+ ion interacts strongly with the nonbonding pairs of electrons on water molecules, forming the hydronium ion

The hydronium ion is responsible for the characteristic properties of aqueous solutions of acids.

H+ + O

H

H H O

H

H

hydronium ion


H3O+ is a more realistic depiction of the hydrogen ion in solution but for convenience we often use H+ (aq) to depict the hydrated hydrogen ion.

HCl (g) + H20 (l) H3O+ (aq) + Cl- (aq)

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


Some substances like water are amphotericCapable of acting as either an acid or a

base

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

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

base

acid


Examples of other amphoteric substances include:NaHCO3

NaH2PO4

NaHSO4

Alcohols such as ethanol:

H C

H

H

C

H

H

O H

In any acid-base equilibrium, both forward and reverse reactions involve proton transfers.

HNO2 (aq) + H2O (l) NO2 – (aq) + H3O

+ (aq)


Forward Reaction:B-L acid =B-L base =

Reverse Reaction:acid =base =

The reaction between a BL acid and base produces a new acid (the conjugate acid) and a new base (the conjugate base).

HNO2 (aq) + H2O (l) NO2 – (aq) + H3O

+ (aq)

HNO2 and NO2- are called a conjugate acid-

base pair.

H2O and H3O+ are also a conjugate acid-base

pair.


BL acid conjugate base

BL base conjugate acid


Conjugate acid:The acid formed when a base gains a

proton Conjugate acid of H2O

H3O +

Conjugate acid of SO4 2-

HSO4 -

NOTE: The conjugate acid is always shown on the product side.


Conjugate base:The base formed by removing a proton

from an acid Conjugate base of H2O

OH -

Conjugate base of H2SO4

HSO4 -

NOTE: The conjugate base is always shown on the product side.


Conjugate acid-base pair:An acid and a base that differ only in

the presence or absence of a single proton HNO2 and NO2

-

H3O+ and H2O HCO3

- and CO32-

NH4+ and NH3


Example: Identify the acid, base, conjugate acid and conjugate base for the following reaction.

HSO4– (aq) + CO3

2- (aq) SO42- (aq) + HCO3

- (aq)

Bronsted-Lowry Acids & BasesExample: Identify the BL acid, BL base, conjugate acid and conjugate base in the following reactions.

NaH + CH3CH2OH CH3CH2O- Na+ + H2

CH3C CH + NaNH2 NH3 + CH3C C - Na+

In any acid-base equilibrium, both forward and reverse reactions involve proton transfers.

HNO2 (aq) + H2O (l) NO2 – (aq) + H3O

+ (aq)


How can we predict the position of the chemical equilibrium?

baseacid Conjugateacid

Conjugatebase

Bronsted Lowry Acids & Bases The relative strengths of the acid and

the conjugate acid can be used to predict the position of the equilibrium.

Equilibrium favors the formation of the weaker acid.

The stronger acid more effectively loses a proton than its conjugate acid

Bronsted Lowry Acids & Bases

Every substance can be categorized as either:strong acid

ionizes completelyweak acid

ionizes partially solutions contain mixture of acid molecules, hydronium ion, and conjugate base

negligible acidity no tendency to form H+ in solution

The seven most common strong acids:HCl hydrochloric acidHBr hydrobromic acidHI hydroiodic acidHNO3 nitric acidHClO3 chloric acidHClO4 perchloric acidH2SO4 sulfuric acid


You must know these acids by name and formula.


Examples of common weak acids:acetic acid, citric acid, phosphoric acid

Examples of substances with negligible acidity:CH4

H2

OH –

NH3


An inverse relationship exists between the strength of an acid and its conjugate base or between a base and its conjugate acid.

Strong acids form very weak conjugate bases (usually have negligible basicity).

Weak acids form stronger (but still fairly weak) conjugate bases

Substances with negligible acidity form very strong bases.

Bronsted-Lowry Acids and Bases

The stronger the acid, the weaker the conjugate base

The stronger the base, the weaker the conjugate acid

Example: Does the following acid/base equilibrium favor the reactants or products?


HF (aq) + HSO4- (aq) H2SO4 (aq) + F- (aq)

Unit 4: Equilibrium, Acids & Bases Part 2: Acids and Bases

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Transcript of Unit 4: Equilibrium, Acids & Bases Part 2: Acids and Bases