Bronsted-Lowry Bronsted-Lowry Acid – Base ReactionsAcid – Base Reactions

ChemistryChemistry

Bronsted – Lowry AcidBronsted – Lowry Acid Defined as a molecule or ion that is a Defined as a molecule or ion that is a

hydrogen ion donorhydrogen ion donor Also known as a proton donor Also known as a proton donor

because Hbecause H++ is a proton. is a proton. ExamplesExamples HCl – a “HCl – a “monoproticmonoprotic” acid” acid HH22SOSO44 – a “ – a “diproticdiprotic” acid” acid HH33POPO44 – a “ – a “triprotictriprotic” acid” acid These all dissociate in water to These all dissociate in water to

produce Hproduce H++ ions which can then react ions which can then react with a Bronsted-Lowry Base with a Bronsted-Lowry Base

Bronsted-Lowry BaseBronsted-Lowry Base

Defined as a hydrogen ion acceptor.Defined as a hydrogen ion acceptor. In an acid-base reaction the base In an acid-base reaction the base

“accepts” the hydrogen ion from the “accepts” the hydrogen ion from the acid.acid.

NHNH33 + H + H++ NH NH44++

NHNH33 is Bronsted-Lowry Base is Bronsted-Lowry Base

Conjugate AcidConjugate Acid The compound that is formed when The compound that is formed when

the B/L base gains a proton (Hthe B/L base gains a proton (H++).). NHNH44

++ is the conjugate acid of NH is the conjugate acid of NH33 ( a ( a base)base)

NHNH44++ acts as an acid in the reverse acts as an acid in the reverse

reaction.reaction.

Conjugate BaseConjugate Base The compound that is formed when a The compound that is formed when a

B/L acid gives away it’s proton (H+)B/L acid gives away it’s proton (H+) Cl- is the conjugate base of HCl (an Cl- is the conjugate base of HCl (an

acid)acid) Cl- acts as a base in the reverse Cl- acts as a base in the reverse

reaction reaction

The conjugate acid and bases are the acid and bases for the reverse reaction!

AmphotericAmphoteric - -

Definition: A substance that can act Definition: A substance that can act as an acid (proton donor) or a base as an acid (proton donor) or a base (proton acceptor)(proton acceptor)

Water (HWater (H22O) is amphoteric.O) is amphoteric.

HBr + HHBr + H22O O Br Br-- + H + H33OO++

NHNH33 + H + H22O O NH NH44++ + OH + OH--

Water acts as a base!

Water acts as an acid!

Strong vs. Weak Acids and Strong vs. Weak Acids and BasesBases

StrongStrong - Dissociate 100% - Dissociate 100% The following are the strong acids, all The following are the strong acids, all

the rest are considered weak. the rest are considered weak. (Memorize!)(Memorize!)

HCl – Hydrochloric acidHCl – Hydrochloric acid HBr – Hydrobromic acidHBr – Hydrobromic acid HI – Hydroiodic acidHI – Hydroiodic acid HNOHNO33 – nitric acid – nitric acid HH22SOSO44 – sulfuric acid – sulfuric acid HClOHClO44 – perchloric acid – perchloric acid

Strong BasesStrong Bases

Strong bases are Strong bases are soluble soluble hydroxides hydroxides such as NaOH, KOH, and LiOHsuch as NaOH, KOH, and LiOH

All the rest are considered weak.All the rest are considered weak.

WeakWeak – Only dissociate < 10% , – Only dissociate < 10% , They are equilibrium reactions.They are equilibrium reactions.

We use Equilibrium constants to We use Equilibrium constants to compare weak acid strength.compare weak acid strength.

Strong Acids and Bases have very Strong Acids and Bases have very largelarge equilibrium constants equilibrium constants KeqKeq

Weak Acids and Bases have Weak Acids and Bases have smallsmall equilibrium constants (<< 1)equilibrium constants (<< 1)

The equilibrium constant for an The equilibrium constant for an acid is called acid is called KaKa

HA HHA H++ + A + A--

The equilibrium constant for a The equilibrium constant for a base is called the base is called the KbKb

NHNH33 + H + H22O NHO NH44++ + OH + OH--

The larger the Ka value – the stronger the acid!

ExamplesExamples

AcidAcid Conjugate Conjugate

BaseBase

HNOHNO33 _____________ _____________

HCOHCO33-1-1 _____________ _____________

HPOHPO442-2- _____________ _____________

NO3-1

CO32-

PO43-

*Conjugate base is what is left after acid donates it’s H+ ion

BaseBase Conjugate AcidConjugate Acid

HCOHCO33-1-1 ____________________________

F F -1-1 ____________________________

HH22OO ______________ ______________

HPOHPO442-2- ____________________________

H2CO3

HF

H3O+

H2PO41-

* Conjugate acid is what you get after the base accepts the H+ ion.

Ka’s are used to predict which species Ka’s are used to predict which species will act as an acid and which as a base will act as an acid and which as a base in an acid base reaction. It also allows in an acid base reaction. It also allows you to predict which way an equilibrium you to predict which way an equilibrium reaction is favored. (with the products reaction is favored. (with the products or reactants)or reactants)

Remember:Remember: The reactant with the higher Ka acts as The reactant with the higher Ka acts as

the acid.the acid. Compare the acid and the conjugate Compare the acid and the conjugate

acid Ka’s, the one with the higher Ka acid Ka’s, the one with the higher Ka wants to dissociate more, so the wants to dissociate more, so the equilibrium will favor the reaction equilibrium will favor the reaction direction that allows it to dissociate!direction that allows it to dissociate!

Example 1Example 1

NHNH33 + H + H22OO

Compare Ka values to determine which acts Compare Ka values to determine which acts as an acid. Higher Ka value on chart is acid!as an acid. Higher Ka value on chart is acid!

Write the products (remove H+ from acid Write the products (remove H+ from acid and add it to the base)and add it to the base)

Compare the Ka values of the acid and Compare the Ka values of the acid and conjugate acid.conjugate acid.

Higher Ka wants to dissociate more, so the Higher Ka wants to dissociate more, so the equilibrium will be favored away from that equilibrium will be favored away from that acid.acid.

NHNH44+ has higher Ka so it wants to dissociate + has higher Ka so it wants to dissociate more, so Reactant side is favored!more, so Reactant side is favored!

NH4+ + OH-

Example 2Example 2

HCOHCO33-- + HPO + HPO44

2-2-

Example 3Example 3

HS- + HHS- + H22COCO33