AP Chem Acids/Bases

AP Chem Acids/Bases

Thursday, April 12, 2012

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When asked to give conjugate base or acid of a species, remember:

• Conjugate acid is simply the parent substance plus one proton (H+)• Conjugate base is simply the parent

substance minus one proton (H+)

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What is the conjugate base of each acid?

• HClO4

– minus one H+ ClO4-

• H2S– minus one H+ HS –

• PH4+

– minus one H+ PH3

• Why the change in overall Charge? Losing +1 charge = more negative charge (actually a reduction taking place)

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What is the conjugate acid of each base?

• CN-

– plus one H+ HCN

• SO42-

– plus one H+ HSO4-1

• H2O– plus one H+ H3O+

• Gaining +1 causes the charge to become more positive (oxidation happening)

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Relative Strengths of Acids and Bases

Some acids are better proton donors that others. STRONG acids completely transfer their protons to water, creating completely dissociated ions in solution.

Their conjugate bases have a negligible tendency to protonate (gain a proton) in aqueous solution.

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Weak Acids only partly dissociate in water.

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Inverse Relationship between strength of acid and conjugate bases

• The stronger an acid, the weaker its conjugate base.

• The stronger the base, the weaker its conjugate acid.

• Reactions involving strong species go nearly to completion; little or no reverse reaction.

• Keq is very large.

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Page 618

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Strong Acids and Bases

• Called strong because they are STRONG ELECTROLYTES, existing entirely as ions in aqueous solutions.

• Seven most common strong acids (memorize these)

–HCl, HBr, HI, HNO3, HClO3, HClO4 (monoprotic) and H2SO4 (diprotic)

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http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/016_STRONGWEAK.MOV

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There are also relatively few strong bases.

• Includes soluble hydroxides (there aren’t many)– Group IA (alkali metals) LiOH, NaOH, KOH, RbOH,

CsOH– Heavier members of Group IIA (alkaline earth

metals) Ca(OH)2 , Sr(OH)2,, Ba(OH)2

• Completely ionize in water.

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Stronger Acid

Stronger Base

Weaker Base

Weaker acid

Chemical Change

Chemical Change

Proton Transfer

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Predicting Acid-Base Reactions

• The stronger acid will always transfer a proton to the stronger base.

• The weaker acid and base are more stable – less reactive.

• The stronger acid will always transfer a proton to the stronger base, yielding the weaker acid and base as favored species at equilibrium.

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In every acid-base reaction, equilibrium favors transfer of proton to the stronger base.

Which direction (forward or reverse) is most likely??

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

• A B CA CB• H2O is a stronger base than Cl- (check the table), so H2O

acquires the proton from HCl.

• HC2H3O2(g) + H2O(l) H3O+ (aq) + C2H3O2- (aq)

• A B CA CB • C2H3O2

- is a stronger base than H2O. Therefore, acetate, C2H3O- gains (or abstracts) the proton from H3O+

.

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Predict whether the equilibrium lies forward or reverse:

• HSO4-(aq) + CO3

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

- (aq)

• A B CB CA– Find the relative strengths of the two bases

• CO32- is a stronger base than SO4

-2

• CO32- will get the proton preferentially to become HCO3

-.

• Equilibrium will lie to the right (forward)

• Another approach: Examine Acid and CA• HSO4

- is a stronger acid than HCO3-; gives up a proton

while the weaker one keeps its proton. (forward)

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Key to remember

• The reaction favors –Consumption of the stronger acid and

stronger base and–Formation of the weaker acid and

weaker base

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Predict whether the equilibrium lies predominately to the left or right.

• HPO42- (aq) + H2O(l) H2PO4

-(aq) + OH-(aq)

• B A CA CB• OH- is a stronger base than HPO4

2-

– Reverse

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

• A B CB CA• OH- is a stronger base than NH3

– Forward.

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Write the net ionic equation for the reaction between hydrofluoric acid, HF,

and the sulfite ion, SO32-, and predict

which side will be favored at equilibrium.

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HF(aq) + SO32- (aq) F- (aq) + HSO3

1- (aq)

• A B CB CA

• Refer to the acid-base strength table and identify strength.– HF is stronger acid than HSO3

-

– SO32- is stronger base than F-

• Equilibrium favors the side with the weaker acid and base. Forward reaction favored.

• “The weak shall inherit the test tube.”