Acids and Bases

Solutions• homogeneous mixtures in which

one substance is dissolved into another

• the “solute” dissolves in the “solvent”

• example: Kool-Aid - water is the solvent, the drink mix is the solute

Molarity• Molarity of a solution is equal to the

number of moles of solute divided by the number of liters of solution

• M = mol/L• New symbol [square brackets]

[H+] = molarity of H+ ions• The larger the molarity, the more

concentrated the solution

Concentrated• In a concentrated solution, the

amount of solute is large compared to the amount of solvent it is dissolved in

• Ex: juices, detergents

Dilute• In a dilute solution, there is

much more solvent than solute• Solutions are “diluted” by

adding more solvent• 0.5M HCl is more dilute than

2.0M HCl

Electrolytes• Substances that, when dissolved in

water, produce aqueous solutions that will conduct electricity

• Strong electrolytes release many ions• Many ionic compounds

• Weak electrolytes release few ions

Properties of acids• React with most metals to produce

H2(g)

• react with carbonates to produce CO2

• taste sour• damage living tissues• pH 0 - 7• neutralize bases

Common acids• Acid formulas – start with H• HCl – hydrochloric acid• H2SO4 – sulfuric acid• HNO3 – nitric acid• H3PO4 – phosphoric acid• HC2H3O2 – acetic acid

• Also written CH3COOH

What is an acid?• Many definitions are used• Arrhenius acid: a substance

that produces H+ ions in water

• Then, H2O + H+ H3O+

• H3O+ = hydronium ion

Acid StrengthCompare the difference in these two

statements:

1) The more H+ ions in the water, the more acidic the solution

2) The more H+ ions a compound produces, the stronger the acid

Acid Strength• Strong acids release all of their H+ ions

• [strong acid] = [H+]• Strong acids are strong electrolytes

• Weak acids hold on to most of their H+ ions • [weak acid]>>>[H+]• Weak acids are weak electrolytes• Weak acids reach equilibrium with

“neutralization” products

Don’t get confused!• A solution of a strong acid can

be less acidic that a solution of a weak acid!

• IF: the strong acid solution is very dilute and the weak acid is concentrated!

Properties of bases• React with fats and oils to produce

soap• feel slippery• taste bitter• damage living tissues• pH 7 - 14• neutralize acids

What is a base?• Commonly called “antacids”• Arrhenius base:

• a substance that produces OH- (hydroxide) ions when added to water

Common basesThere are three common varieties of bases:

1) Hydroxide compounds (OH-) ex: NaOH, Ba(OH)2

2) Carbonates (CO32-) and

bicarbonates (HCO3-)

ex: Na2CO3, NaHCO3, CaCO3

3) Ammonia (NH3) and amines

Hydroxide bases

• Release hydroxide ions directly into the water

• NaOH(s) Na+(aq)+OH-

(aq)

Carbonates and bicarbonates

• React with water to produce hydroxide ions

• CO32- + H2O HCO3

- + OH-

• HCO3- + H2O H2CO3+OH-

Ammonia and amines

• React with water to produce hydroxide ions

• NH3 + H2O NH4+ + OH-

Chemical indicators• Phenolphthalein

• acids = colorless

• bases = pink

• Litmus• acids = red

• bases = blue

Autoionization of water Water molecules can react with

each other H2O + H20 H3O+ + OH-

At 25ºC, [H3O+] = [OH-]

[H2O] is a constant

Kw = [H3O+] [OH-] = 1x10-14

Let’s use [H+] instead of [H3O+] Pure water is neutral

[H+] = [OH-]

If [H+] > [OH-], the solution is acidic

If [H+] < [OH-], the solution is basic

pH scale• Used to indicate the acidity or

“basicity” of a solution• tells how strongly acidic a solution is -

NOT how strong an acid is!• Think pH as “parts H+”• the lower the pH, the more H+’s, the

more acidic the solution

pH calculation

pH = -log [H+]

[H+] = 10-pH

pH scale The pH is the measurement

of how many H+’s are in the water – NOT a measure of if the H+’s came from a “strong” or “weak” acid!!!

pH scale• 0 - 2

• strongly acidic• 2 - 4

• moderately acidic

• 4 - 7• weakly acidic

• 14 - 12• strongly basic

• 12 - 10• moderately

basic• 10 - 7

• weakly basic

pOH calculation

pOH = -log [OH-]

[OH-] = 10-pOH

pH & pOH relationship

In pure water at 25°C: [H+] = 1x10-7M

[OH-] = 1x10-7M

Therefore, [H+] x [OH-] = 1x10-14

And pH + pOH = 14

Arrhenius Neutralization

Works for the reaction of a strong acid with a strong base

Remember – acid (or base strength) has to do with how much of the acid (or base) ionizes in water, not directly how many H+ or OH- are produced

100% ionization = “strong”

Arrhenius NeutralizationHydroxide base – general form• Acid + Base Salt + H2O

• what’s actually happening?• H+ + OH- H2O

• anion from acid + cation from base = salt

Arrhenius Neutralization

Examples with a hydroxide base

NaOH + HCl NaCl + H2O

3H2SO4 + 2Al(OH)3 Al2(SO4)3 + 6H2O

Arrhenius Neutralizationcarbonate base – general form• Acid + Base Salt + H2O + CO2

• what’s actually happening?• 2H+ + CO3

2- H2CO3

• H2CO3 H2O + CO2

• anion from acid + cation from base = salt

Arrhenius Neutralization

Examples with a carbonate base

CaCO3 + 2 HCl CaCl2 + H2O + CO2

H2SO4 + NaHCO3 Na2SO4 + H2O + CO2