Acids and BasesChapter 19

DHS Chemistry

I. Definition and Properties

A. Definitions of Acids & Bases

•There are many theories and definitions of acids and bases.

Bronsted-Lowry Definitions

•An acid is a molecule or ion that acts as a proton (hydrogen ion) donor.

HCl + H20 H30+ + Cl-

acid

Bronsted-Lowry Definitions

•A base is a molecule or ion that acts as a proton (hydrogen ion) acceptor.

HCl + NH3 NH4+ + Cl-

base

Arrhenius Definitions:

oAn acid produces hydronium (H30+) ions when dissolved in water.

Ex. HCl + H2O Cl- + H3O+

Arrhenius Definitions:

oA base produces hydroxide (OH- ) ions when dissolved in water.

Ex. NH3 + H2O NH4+ + OH-

Polyprotic acids• Acids that can contain multiple

hydrogens to donate.

• Match the terms with the probable acid

monoprotic H2SO4

diprotic H3PO4

polyprotic HCl

•Acids that contain 1 ionizable hydrogen are monoprotic acids.Ex. HCl, HBr

•Acids that contain 2 ionizable hydrogens are diprotic acids.Ex. H2SO4

•Acids that contain 3 ionizable hydrogens are triprotic/polyprotic acids.Ex. H3PO4

•Some substances can act as both an acid and a base – these substances are considered amphoteric.

EX: HCl + H20 H30+ + Cl- water as a base

acid base

NH3 + H20 NH4+ + OH- water as

an acid

base acid

H20 + H20 H30+ + OH- water as an

acid

pure water has H2O, H3O+, OH- & base

B. Properties(refer to your previous

notes!)

C. Common Acids and Bases

(refer to your previous notes! Make sure you have those

particular ones memorized, both their formula and their name!)

II. Strength of Acids and Bases

A. Ionization(refer to your previous notes!)

B. pH Scale

Hydronium vs Hydroxide

•Hydronium H3O+

– A hydrogen ion in water

– H+ + H2O H3O+

– H+ and H3O+ used interchangeably

– For acids

•Hydroxide OH-

– For bases

B. pH Scale•a measure of the number of hydronium or hydroxide ions is the pH scale

•it is based on the concentration of hydrogen and hydroxide ions in solution

•pH is defined as the negative logarithm of the hydrogen ion concentration

•[ ] = concentration (molarity)•notice pH is based on a log scale

•log(base 10) is the power to which 10 must be raised to equal that number.

pH = -log([H+]) or -log([H3O+])

pOH = -log ([OH-])

Logarithms

b n = x n = logb xLogarithm is an exponent

23 = 8

Then, 3 is called the logarithm of 8 with a base 2

3 = log28

Logarithms – Base 10

104 = 10,000

The logarithm of 10,000 with a base 10 is 4

4 = log1010,000

We’ll use this concept for pH

pH and pOH are related by the following:

14 = pH + pOH

For example, calculate the pH of the following:

0.001 M HCl

In calculator: -log(0.001)

(some calculators are a little different)

pH = 3

For example, calculate the pH of the following:

0.01 M NaOH

Because NaOH is a base, we’re calculating the pOH

pOH = -log(0.01)

pH = 12

pOH = 214 = pH + pOH2

For example, calculate the pH of the following:

3.2 x 10-9 M HNO3

pH = -log(3.2 x 10-9)

pH = 8.49

pH Scale

•the typical pH scale runs from 0 to 14 .

III. Titrations

A. Titrations•The concentration of an acid (or base) in solution can be determined by performing a neutralization reaction.

Recall neutralization…

C. Neutralization Reactions•The reaction of an acid with a base produces water and a class of compounds called salts.

HA + BOH B A + H2O

ex. HCl(aq) + NaOH(aq) NaCl (aq) +

H2O(l)

acid base salt water

•An indicator is used to show when neutralization has occurred.

•An indicator is a substance that forms different colors in different pH solutions. Phenolphthalein is a common indicator used in acid-base titrations. It will change from colorless in acidic environments to pink in basic environments.

Steps in a formal titration1. A measured volume

of a solution of unknown concentration (acid or base) is added to an Erlenmeyer flask.

2. A solution of known molarity (acid or base) is added to a buret.

3. Several drops of an indicator are added to the unknown solution

4. Measured volumes of a solution of known molarity (acid or base) are mixed into unknown solution until the indicator just barely changes to a different color.

• The solution of known concentration is called the standard solution. The standard solution is added using a buret.

• The process of adding a known amount of solution of known concentration to determine the concentration of another solution is called titration.

• The point at which the indicator changes color is the end point of the titration.

•This can also be done less “formally” using any volume measures. The results won’t be as accurate, but it gets you close.

Example: (drops, substitute for mL)

The end point

How to read a buret

B. Solving Titration Problems

Remember, in order for the solution to be neutral,

Moles H30+ = moles OH-

a = # of H+ ions in acidsb = # of OH- ions in basesM = Molarity (M)V = volume

aMAVA = bMBVB

(similar to dilutions)

Examples

1) Determine a and b for the following

a = __ for H2SO4 and

b = __ for Ca(OH)2

2

2

Example

2) It takes 26.23 mL of a 1.008 M NaOH solution to neutralize 35.28 mL of a monoprotic acid solution. What is its molarity?

1(MA)(35.28 mL) = 1(1.008M)(26.23 mL)

MA = 0.749 M

Example

3) If 15.50 mL of NaOH solution were neutralized with 23.40 mL of 0.533 M H2SO4, what is the concentration of the NaOH ?

2(0.533 M)(23.40 mL) = 1(MB)(15.50 mL)

MB = 1.61 M

Example

4) 25.00 mL of 0.720 M nitric acid is used to completely neutralize a 1.0 M NaOH solution. What volume of NaOH is present?

1(0.720 M)(25.00 mL) = 1(1.0 M)VB

VB = 18.00 mL

Ex: A 25 mL solution of nitric acid is completely neutralized by 18mL of 1.0M NaOH. What is the concentration of the acid solution?

Extra Example

Ex: A 25 mL solution of nitric acid is completely neutralized by 18 mL of 1.0M NaOH. What is the concentration of the acid solution?

MA = ?? MB = 1MVA = 25mLVB = 18mL

MAVA = MBVB rearrangedto solve for MA MA= MBVB

VA

(1M)(18mL) = 0.720M 25mL

Practice1. What is the molarity of

phosphoric acid if 15.0 mL of the solution is completely neutralized by 38.5 mL of 0.150 M Al(OH)3?

2. It takes 26.23 mL of a 1.32 x 10-3 M NaOH solution to neutralize a 45.56 mL of a triprotic acid solution. What is the concentration of the acid?

0.385 M H3PO4

MB = 2.53 x 104 M

Practice

3. What is the molarity of sodium hydroxide if 20.0 mL of the solution is neutralized by 28.0 mL of 0.60 M HCl?

4. How many mL of 0.45 M HCl must be added to 25.00 mL of 1.00 M Sr(OH)2 to make a neutral solution? VA = 111.00 mL

0.840 M NaOH

1. What is the molarity of phosphoric acid if 15.0 mL of the solution is completely neutralized by 38.5 mL of 0.150 M Al(OH)3?

H3PO4 Al(OH)3

MA = ?? MB = 0.150MVA = 15mL VB = 38.5mL

MAVA = MBVB rearranged to solve for MA MA= MBVB

VA

(0.150M)(38.5mL) = 0.385M H3PO4 15mL

3. What is the molarity of sodium hydroxide if 20.0 mL of the solution is neutralized by 28.0 mL of 0.60 M HCl?

HCl NaOHMA = 0.60M MB = ??VA = 28.0mL VB = 20.0mL

MAVA = MBVB rearranged to solve for MB MB= MAVA

VB

(0.60M)(28.0mL) = 0.840M NaOH 20.0mL