Acid-Base ReactionsCh. 15

Acid-Base Reactions• Neutralization reactions

– pH is changed

• Produce a salt and H2O– Salts are ionic compounds

• 2 types of Acids– Strong and Weak

• 2 types of Bases– Strong and Weak

• 4 possible combinations of Acids and Bases– Strong A + Strong B– Strong A + Weak B– Weak B + Strong B– Weak B + Weak A

Strong Acid + Strong Base• HCl + NaOH • Double replacement reaction• Both compounds completely

dissociate HCl Cl- + H+

NaOH Na+ + OH-

• Complete equation (aq) can be written:

H++Cl-+Na++OH-Cl-+Na++H2O• Spectator Ions– Ions that do not take part in the

reaction

Net Ionic Equations

1) Write Complete Ionic Equation All ionic compounds are shown as free ions

2) Remove Spectator Ions Ions not directly evolved in the rxn.3) Balance the remaining rxn.

Strong Acid + Strong Base• What is the net ionic equation:

HCl(aq) + NaOH(aq)

1) H+(aq) +Cl-

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

(aq) Cl-(aq) +Na+

(aq) +H2O(aq)

2) H+(aq) +OH-

(aq) H2O(l)

• All strong acid and strong base reactions have this as a base net ionic equation

KOH(aq)+HNO3(aq)KNO3(aq)+H2O(l)

K++OH-+H++NO3- K++NO3

-+H2O(l)

OH-+H++ H2O(l)

Strong Acid + Weak Base• HCl(aq) + Al(OH)3(s)

• Weak bases wont completely dissociate– Cannot write them as ions on reaction side of net

ionic equation

H+(aq)+Cl-

(aq)+Al(OH)3(s)Al+3(aq)+Cl-

(aq)+H2O(l)

H+(aq)+Al(OH)3(s)Al+3

(aq)+H2O(l)

• End solution is slightly acidic• What about NH3?– Considered a weak base but has no OH-

– Does not produce water– HCl(aq)+NH3(aq)NH4Cl(aq)

H+(aq)+Cl-

(aq)+NH3(aq)NH4++Cl-

(aq)

H+(aq))+NH3(aq)NH4

+

Weak Acid + Strong Base• HC2H3O2(aq) + NaOH(aq) • Weak acid wont completely dissociate– Wont breakdown into ions on reaction side

HC2H3O2(aq) + Na+(aq)+OH-

(aq)Na+(aq)+C2H3O2

-(aq)+ H2O(l)

HC2H3O2(aq) +OH-(aq)C2H3O2

-(aq)+ H2O(l)

• End solution is slightly basicWeak Acid + Weak Base???Not clear???• Both the acid and base are so unreactive there is

little change• Not common reaction type in nature

Bronsted-Lowery Acids and Bases• Acids produce H+ ions when added to water• Bases produce OH- ions when added to water

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

(aq)

H2O(l)+NH3(aq)NH4+

(aq)+OH-(aq)

• Water can act as an acid or a base• Acid= Any compound that releases H+

• Base= Any compound that takes H+

• Conjugate Acid/Base– Weak acid or base produced from an acid-base reaction

ACID

BASEACID

BASE Conjugate acid

Conjugate acid

Conjugate base

Conjugate base

Regulating pH• Living things interact

with acids and bases all the time; their pH must be regulated

• Buffer– Solution that adjusts to

the addition of acids and bases to slowly change the pH

– Free OH- and H+ ions• Weak acic/base + salt of

that acid/baseNaOH + HC2H3O2H2O + NaC2H3O2

NaC2H3O2 and HC2H3O2

Add a strong base:HC2H3O2(aq)+NaOH(aq)

C2H3O2-(aq)+H2O(l)

HC2H3O2(aq)+OH-(aq)

C2H3O2-(aq)+H2O(l)

Add a strong acid:NaC2H3O2(aq)+HCl(aq)

HC2H3O2(aq)+NaCl(l)

C2H3O2-(aq)+H+

(aq)+

HC2H3O2(aq)

Buffers in the Blood• Blood must keep a pH of 7.4 to allow

the best exchange of CO2 and O2

• Blood buffer is HCO3-/H2CO3

• Add Base: H2CO3+OH-HCO3-+H2O

• Add Acid: HCO3-+H+H2CO3

• What happens when you take in too much CO2?– H2CO3 increases making blood more acidic

• What kind of blood pH results in yawning?– Acidic blood; body needs to release large

amount of CO2 by taking in large amount of O2

Antacids• Compound controls acidic pH levels by adding base• 2 types:– Hydroxide Antacids• Low solubility in water• Release OH- to neutralize H+

• Milk of Magnesia (Mn(OH)2)– Carbonate Antacids• XCO3 or XHCO3; react with HCl

• CaCO3 + 2HClCaCl2 + H2CO3

Breaks down to CO2 and

H2O

Titrations• Test to determine the molarity of

an acid or a base– Find the Standard Solution

• Process:– Standard solution of an acid/base is

slowly added to an acid/base of unknown molarity

– When the unknown acid/base is neutral, the [H+]=[OH-

• How do we know the Standard solution is neutral?

• pH Indicators

– Volume of acid/base used gives us molarity

– MAVA = MBVB

• End point• Equivalence

point• Neutral point

Titrations practice• If 15.0 mL of 0.50 M NaOH is used to neutralize 25.0 mL

of HC2H3O2, what is the molarity of the acid solution?

NaOH + HC2H3O2 H2O + NaC2H3O2; 1:1 ratio

• MAVA = MBVB

• MA= MBVB/VA = (0.50 M)(15.0 ml)/25.0 ml = 0.30 M• If 25.0 mL of a standard 0.05 M HCl solution is required

to neutralize 20.0 mL of a solution of Sr(OH)2, what is the concentration of the base?

2 HCl + Sr(OH)2 ® SrCl2 + 2H2O

• MAVA = 2 MBVB

• MB = MAVA/2VB = (0.05 M)(25.0 ml)/(2)(20.0 ml) = 0.03 M

Titrations practice• What types of acid-base titrations do these graphs

show?

Titrations practice• Graphs shows titration of 0.5 M NaOH with 50ml of an

unknown acid. After titration NaBr salt crystals were isolated from the solution.

a) What is the acids used? Is it strong or weak?

b) what is the concentration of the acids used?HBr: NaOH + HBr NaBr + H2O; strong

MA= MBVB/VA

= (0.5M)(50ml)/35ml = 0.7 M