Chapter 18 – Acids, Chapter 18 – Acids, Bases, and SaltsBases, and Salts

Arrhenius (ah-ray-nee-Arrhenius (ah-ray-nee-uhs) definition uhs) definition

Acid – substance that dissociates Acid – substance that dissociates in water to produce hydrogen ions in water to produce hydrogen ions (H(H++))

Base – a substance that Base – a substance that dissociates in water to produce dissociates in water to produce hydroxide ions (OHhydroxide ions (OH-- ions) ions)

Bronsted – Lowry Bronsted – Lowry definitiondefinition

Acid – is any substance that Acid – is any substance that donates a hydrogen ion (a proton)donates a hydrogen ion (a proton)

Base – any substance that Base – any substance that accepts hydrogen ionsaccepts hydrogen ions

This is a broader definitionThis is a broader definition

A hydrogen ion is a A hydrogen ion is a single protonsingle proton

Substances that can accept a proton Substances that can accept a proton (H(H++ ion) have a nonbonding pair of ion) have a nonbonding pair of electrons that are used by the Helectrons that are used by the H++ ion. (think electron-dot structures)ion. (think electron-dot structures)

HCl + HHCl + H22O O → H→ H33OO++ + Cl + Cl- -

Acid base hydronium ionAcid base hydronium ion

Show the electron-dot of this processShow the electron-dot of this process

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

base acid conjugate base acid conjugate conjugateconjugate

acidacid basebase

Show electron-dot of this process Show electron-dot of this process

Strong acid – near Strong acid – near complete dissociationcomplete dissociation

HCl + HHCl + H22O O →→ H H33OO++ + Cl + Cl--

Acid base conjugate Acid base conjugate conjugateconjugate

acidacid basebase

Weak acid – very little Weak acid – very little dissociationdissociationHCHC22HH33OO22 + H + H22O HO H33OO++ + +

CC22HH33OO22--

99.6%99.6% .4% .4%

Strong Base (lime CaO)Strong Base (lime CaO)

CaO + HCaO + H22O O →→ Ca Ca+2+2 + 2 OH + 2 OH--

Show electron – dot of this processShow electron – dot of this process

CaCa+2+2 is called a spectator ion is called a spectator ion because it does not participate in because it does not participate in the reaction (it just “watches” the the reaction (it just “watches” the water and oxygen ion react.water and oxygen ion react.

For strength of conjugate acid-For strength of conjugate acid-base pairs see page 608base pairs see page 608

Note – HNote – H22O is a weak acid and a O is a weak acid and a weak base.weak base.

Diprotic Acid – two Diprotic Acid – two acidic protons per acidic protons per moleculemoleculeHH22SOSO44 + H + H22O O → H→ H33OO++ + HSO + HSO44

-1-1

Acid base conj. acid conj. BaseAcid base conj. acid conj. Base

HSOHSO44-1 -1 + + HH22O O → H→ H33OO++ + SO + SO44

-2-2

Acid base conj. acid conj. BaseAcid base conj. acid conj. Base

The Acid Dissociation The Acid Dissociation ConstantConstant

HAHAaqaq + H + H22OOll H H33OO++aqaq + A + A--

aqaq

This is an equilibrium – The forward reaction This is an equilibrium – The forward reaction equals the rate of the backward reaction equals the rate of the backward reaction so the concentration of all ions remains so the concentration of all ions remains constant (Kconstant (Keq eq is equilibrium constant)is equilibrium constant)

[H[H33O] [AO] [A--]]

KKeqeq = -------------- = --------------

[HA] [H[HA] [H22O]O]

[H[H33O] [AO] [A--]]

KKeqeq = -------------- = --------------

[HA] [H[HA] [H22O]O]

There is so much more H2O than anything There is so much more H2O than anything else, the water concentration does not else, the water concentration does not change much (% change in [Hchange much (% change in [H22O])O])

[H[H33O] [AO] [A--]]

KKeqeq [H [H22O] = --------------O] = --------------

[HA][HA]

Water can be included as part of the Water can be included as part of the constant, now called acid dissociation constant, now called acid dissociation constant Kconstant Kaa

[H[H33O] [AO] [A--]]

KKaa = -------------- = --------------

[HA][HA]

The greater the Ka, the stronger the The greater the Ka, the stronger the acidacid

See Ka values on page 610.See Ka values on page 610.

Base dissociation Base dissociation constantconstant

B + HB + H22O O HBHB++ + OH + OH--

[HB[HB++] [OH] [OH--]]

KKbb = -------------- = --------------

[B][B]

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

[NH[NH44++] [OH] [OH--]]

KKbb = -------------- = 1.8 x 10 = -------------- = 1.8 x 10-5-5

[NH[NH33]] (from page (from page 611)611)

See example problem page 612See example problem page 612

Assign practice problems 1,2 page Assign practice problems 1,2 page 613613

Acid-Base Properties Acid-Base Properties of Saltsof Salts

Salts from strong acids and strong Salts from strong acids and strong bases – will be neutralbases – will be neutralNaOH + HCl NaOH + HCl → NaCl + H→ NaCl + H22OO

NaCl is neutralNaCl is neutral Salts from strong acids and weak bases – will be Salts from strong acids and weak bases – will be

slightly acidicslightly acidic NHNH33 + HCl → NH + HCl → NH44ClCl NHNH44Cl is slightly acidic Cl is slightly acidic

NHNH44Cl + HCl + H22O HO H33O + NHO + NH33 + Cl + Cl--)) (Ka = 5.6 x 10(Ka = 5.6 x 10-10-10))

Salts from weak acids and strong Salts from weak acids and strong bases – will be slightly basicbases – will be slightly basic

2NaOH + H2NaOH + H22COCO33 → Na→ Na22COCO33 + +

2H2H22OO

NaNa22COCO3 3 is slightly basicis slightly basic

COCO33-2-2 + H + H22O HCOO HCO-- + OH + OH--

K Kbb = 1.8 x 10 = 1.8 x 10-4-4

Acidic HydrogenAcidic Hydrogen

Not all hydrogen can dissociate in Not all hydrogen can dissociate in water.water.

HCl + HHCl + H22O O → H→ H33OO++ + Cl + Cl--

The negative ion (anion) left The negative ion (anion) left behind when Hbehind when H++ leaves must be leaves must be stable with the negative charge. stable with the negative charge. (high electronegativity) Halogens (high electronegativity) Halogens work wellwork well

Stabilization through Stabilization through resonance (the resonance (the “spreading out” of a “spreading out” of a bond or charge)bond or charge) Alcohols are slightly acidic; but Alcohols are slightly acidic; but

organic acids are more so organic acids are more so (because of resonance)(because of resonance)

Compare electron dot of an Compare electron dot of an alcohol to the electron dot alcohol to the electron dot showing resonance of a showing resonance of a carboxylic acid. carboxylic acid.

Resonance helps stabilize the Resonance helps stabilize the negative chargenegative charge