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Arrhenius DefinitionArrhenius Definition

An acid is a substance that increases the hydrogen (hydronium) concentration in awater solution.

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

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

Either equation is acceptable and H+(aq) orH3O+(aq) is a hydrated proton.

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H+ is very strongly hydrated in water because

of its small size and high positive chargedensity.

H+(aq) + H2O(l) H3O+(aq)

Arrhenius definitions are limited to aqueoussolutions.

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A base is a substance that increases the hydroxide ions in a water solution.

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

Remember that Arrhenius definitions arelimited to aqueous solutions.

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Bronsted-Lowry Theory (BLT)Bronsted-Lowry Theory (BLT)

An acid is a molecule or ion that donates aproton.

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

A base is a molecule or ion that accepts aproton.

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

(aq)

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The BLT of an acid and a base are not limitedto aqueous solutions.

When working with BLT, it is common to usethe terms conjugate acid and conjugate base.

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

base acid conjugate conjugate acid base

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H2SO4(aq) + H2O(l) H3O+(aq) + HSO4-(aq)

acid base conjugate conjugate acid base

HSO4-(aq) + H2O(l) H3O+(aq) + SO4

2-(aq) acid base conjugate conjugate acid base

HSO4- is called amphoteric or amphiprotic

because it can act as either a BLT acid or aBLT base depending on its chemicalenvironment.

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Here HSO4- is acting as the conjugate base of

H2SO4.

H2SO4(aq) + H2O(l) H3O+(aq) + HSO4-

(aq)

Here HSO4- is acting as the conjugate acid of

SO42-.

HSO4-(aq) + H2O(l) H3O+(aq) + SO4

2-

(aq)

Each acid has one more proton than itsconjugate base.

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Each base has one less proton than itsconjugate acid.

Two important points to remember:

The stronger the acid, the weaker its conjugate base.

The stronger the base, the weaker its conjugate acid.

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Acid-Base ReactionsAcid-Base Reactions

An acid-base reaction always proceedstoward the weaker acid and weaker base.

HClO4(aq) + H2O(l) H3O+(aq) + ClO4-

(aq)

stronger stronger weaker weaker acid base acid base

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When analyzing an acid-base reaction,remember that you can’t have it both ways.

ClO4- is too weak a base to compete with

thestronger base, H2O, to acquire the proton.

How do you know that ClO4- is such a

weak base?

Because HClO4 is one of the six strong acids.

When you have a strong acid such as HClO4, 100% ionization is assumed.

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If a molecule wants to completely ionize, why

would its anion want to undergo hydrolysis?

Similarly, H3O+ is too weak an acid tocompete with the stronger acid, HClO4, todonate a proton.

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Lewis Acids and BasesLewis Acids and Bases

An acid is a substance that accepts anelectron pair.

Al3+(aq) + 6H2O(l) Al(H2O)63+(aq)

The Al3+ cation has the empty orbitals 3s, 3px,

3py, 3pz, as well as the size to accommodated-orbitals.

Also, the Al3+ has a large positive chargedensity resulting in its interaction with watermolecules.

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The Al3+ cation acts as a Lewis acid and thewater with its two unshared pair of

electronsacts as a Lewis base (an electron pair

donor).

The hydrated Al3+ cation, Al(H2O)63+, can

nowbehave as an Arrhenius acid or aBronsted-Lowry acid.

Al(H2O)63+(aq) H+(aq) + Al(H2O)5(OH)2+

(aq)

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A base is a substance that donates anelectron pair.

Zn2+(aq) + 4OH-(aq) Zn(OH)42-

(aq)

The Lewis definition of acids and basesexpands the number of species that can beacids.

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Strength of Binary AcidsStrength of Binary Acids

The H – X bond strength is the most important

factor to consider when determining acidstrength in a group or family.

Consider the following bond enthalpies:

H – F 567 kJ mol-1 H – Cl 431 kJ mol-1 H – Br 366 kJ mol-1 H – I 299 kJ mol-1

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The bond enthalpies from the previous slideindicate that the strength of the H – X bonddecreases as the atomic radii of the

halogenincreases.

Longer bonds are generally weaker or less stable than shorter bonds.

Similarly, H2S is a stronger acid than H2O,Ka(H2S) > Ka(H2O).

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The H – X bond polarity is the most important

factor to consider when determining acidstrength in a period or series.

Because electronegativity increases from left to right in a period, the acid strength also increases proceeding

from left to right.

The acid strength increases from left to right in a period.

Ka(HF) > Ka(H2O) > Ka(NH3) > Ka(CH4)

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Strength of OxyacidsStrength of Oxyacids

When comparing oxyacids, there areadditional factors to consider.

– X – O – H│

│

As the electronegativity of elementX increases, the stronger the acid.

When the electronegativity of X increases, thepolarizability of the O – H bond increases.

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As more O terminal atoms are added to thecentral atom, X, the more the electron

densityis pulled from the O – H bond.

By adding more electronegative atoms to X,the acid strength is increased.

Cl – O – H < Cl – O – H < Cl – O – H <

O – Cl – O – H

│ │

││

│

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By adding more oxygens (the second mostelectronegative element) to the central

atom,Cl, the electron density shifts more towardsthe oxygens, making the O – H bond morepolarizable.

For oxoacids with the same number of O – H

bonds and the same number of oxygen atoms,

the acid strength will increase with anincrease of electronegativity of the centralatom.

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O – Cl – O – H > O – Br – O – H > O – I – O – H

│

│

│

│

│

│

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Oxoacid Wrap UpOxoacid Wrap Up

For an oxoacid, the H atom that ionizes isbonded to an O atom which in turn is bondedto a nonmetal atom.

The strength of any acid depends on howeasily the O – H bond is broken.

One deciding factor is the oxidation of the central atom.

The higher the oxidation number the stronger the acid.

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To increase the ionization, the electrondensity surrounding the O atom which isbonded to the ionizable H, should be as lowas possible.

To decrease the electron density around the

O atom:

Make the central atom more electronegative.

Add more O atoms to the central atom.

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A second deciding factor is theelectronegativity of the central atom.

The more electronegative the central atom, the stronger the acid.

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Extent of HydrolysisExtent of Hydrolysis

There are six strong acidsThere are six strong acids that completelyionize in water.

HCl, HBr, HI, HNO3, HClO4, H2SO4

When representing the ionization of theseacids, a single arrow is used and 100%ionization is assumed.

HClO4(aq) H+(aq) + ClO4-(aq)

The Ka of these acids is assumed to beinfinite.

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Most acids are weak and only partially ionize

in water.

HC2H3O2 (aq) + 2H2O(l) H3O+(aq) +

C2H3O2

-(aq)

Or alternatively

HC2H3O2(aq) H+(aq) + C2H3O2-(aq)

The Ka of these acids is small and can belooked up for each acid.

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Also note that with weak acids, a doublearrow is used and a dynamic equilibriumresults.

The most commonly encountered basesThe most commonly encountered bases that

completely dissociate in water are:

LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2

When representing the dissociation of thesebases, a single arrow is used and 100%dissociation is assumed.

LiOH(aq) Li+(aq) + OH-(aq)

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The Kb of these bases is assumed to beinfinite.

Most bases are weak and only partialionization takes place.

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

(aq)

The Kb of these bases is small and can belooked up for each base.

Note that in the case of a weak base, watermust be explicitly written as a reactant

unlikethe case of a weak acid.