notes- acids and bases

Notes-Acids and Bases

Acids and Bases

Arrhenius ACIDS – produces hydrogen ions in aqueous solutions, sour taste, low pH, and the fact that they turn litmus paper red

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

Arrhenius BASES – produces hydroxide ions in aqueous solutions, bitter taste, slippery feel, high pH, and the fact that they turn litmus paper blue

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

Arrhenius definition – limits the concept of a base

Bronsted – Lowry definition – gives a broader definition of a base Bronsted – Lowry ACID – a proton (H+) donor Bronsted – Lowry BASE – a proton (H+) acceptor

General Reaction –

HA (aq) + H2O (l) H3O+ (aq) + A- (aq)

Acid Base Conjugate Conjugate Acid Base

Conjugate Base – everything that remains of the acid molecule after a proton is lost

Conjugate Acid – the base with the transferred proton (H+)Conjugate Acid – Base Pair – two substances related to

each other by the donating and accepting of a single proton

proton donor

proton acceptor

Examples: Finish each equation and identify each member of the conjugate acid –base pair.

H2SO4 (aq) + H2O (l) HSO4-1

(aq) + H3O+ (aq)

Acid Base Conjugate Base

Conjugate Acid

CO32- (aq) + H2O (l) HCO3

-1(aq) + OH-

(aq)

Base Acid Conjugate Acid

Conjugate Base

The hydronium ion, H3O+, forms when water behaves as a base. This happens when the two unshared pairs of electrons on O bond covalently with the H+.

Water as an Acid and a Base

Amphoteric – a substance that can behave as either an acid or a base

- water is the most common amphoteric substance

Ionization of Water –

H2O (l) + H2O (l) H3O+ (aq) + OH- (aq)

In the shorthand form:

H2O (l) H+ (aq) + OH- (aq)

Ion-product constant – Kw refers to the ionization of water

Kw = [H+][OH-]At 25C, Kw = [H+][OH-] = [1.0 x 10-7] [1.0 x 10-7]

= 1.0 x 10-14

If [H+] increases, the [OH-] decreases, so the products of the two is still 1.0 x 10-14.

There are three possible situations –1. A neutral solution, where [H+] = [OH-]2. An acidic solution, where [H+] [OH-]3. A basic solution, where [H+] [OH-]

Example: Calculate [H+] or [OH-] as required for each of the following solutions at 25C, for each solution state whether it is neutral, acidic, or basic.

a. 1.0 x 10-5 M OH- b. 10.0 M H+

Kw = [H+][OH-]

1 x 10-14 = [H+][1.0 x 10-5 M]

[H+] = 1.0 x 10-9 M

BASIC

Kw = [H+][OH-]

1 x 10-14 = [10.0 M][OH-]

[OH-] = 1.00 x 10-15 M

ACIDIC

pH scalepH scale – because the [H+] in an aqueous solution is

typically small, logarithms are used to express solution acidity

pH = -log [H+] pOH = -log [OH-]

Graphing calculator Non graphing calculator1. Press the +/- key 1. Enter the [H+]2. Press the log key 2. Press the log key3. Enter the [H+] 3. Press the +/- key

Significant Figure Rule – The number of places to the right of the decimal for a log must be equal to the number of significant figures in the original number.

pH pH ScalScal

ee

Example – Calculate the pH or pOH

a. [H+] = 1.0 x 10-9 M b. [OH-] = 1.0 x 10-6 M

pH = - log [H+]

pH = - log (1.0 x 10-9 M)

pH = 9.00

pOH = - log [OH-]

pOH = - log (1.0 x 10-6 M)

pOH = 6.00

Example – Calculate the pH and pOH

if the concentration of OH-1 is 1.0 x 10-3 M

pOH = - log [OH-]

pOH = - log (1.0 x 10-3 M)

pOH = 3.00

pH = - log [H+]pH = - log (1.0 x 10-11 M)

pH = 11.00

Kw = [H+][OH-]

1 x 10-14 = [H+][1.0 x 10-3 M]

[H+] = 1.0 x 10-11 M

Since Kw = [H+][OH-] = 1.0 x 10-14 ,

pH + pOH = 14.00

Example - The pH of blood is about 7.4. What is the pOH of blood?

pH + pOH =14.00

7.4 + pOH = 14.00

pOH = 6.6

In order to calculate the concentration from the pH or pOH,

[H+] = 10-pH [OH-] = 10-pOH

Graphing calculator Non-graphing calculator1. Press the 2nd 1. Enter the pH

function, then log 2. Press the +/- key2. Press the +/- key 3. Press the

inverse3. Enter the pH log key

Example - The pH of a human blood sample was measured to be 7.41. What is the [H+] in blood?

[H+] = 10-pH

[H+] = 10-7.41

[H+] = 3.9 x 10-8 M

Example – The pOH of the water in a fish tank is found to be 6.59. What is the [H+] for this water?

[OH-] = 10-pOH

[OH-] = 10-6.59

[OH-] = 2.6 x 10-7 M

Kw = [H+][OH-]

1 x 10-14 = [H+][2.6 x 10-7 M]

[H+] = 3.8 x 10-8 M

How Do We Measure pH?

• For less accurate measurements, one can use– Litmus paper

• “Red” paper turns blue above ~pH = 8

• “Blue” paper turns red below ~pH = 5

– An indicator

How Do We Measure pH?

For more accurate measurements, one uses a pH meter, which measures the voltage in the solution.

Strong Acids

• seven strong acids are HCl, HBr, HI, HNO3, H2SO4, HClO3, and HClO4.

• These are, by definition, strong electrolytes and exist totally as ions in aqueous solution.

Strong Bases

• Strong bases are the soluble hydroxides, which are the alkali metal and heavier alkaline earth metal hydroxides (Ca2+, Sr2+, and Ba2+).

• Again, these substances dissociate completely in aqueous solution, strong electrolytes

Titration

A known concentration of base (or acid) is slowly added to a solution of acid (or base).

Titration

A pH meter or indicators are used to determine when the solution has reached the equivalence point, at which the stoichiometric amount of acid equals that of base.

Titration of a Strong Acid with a Strong Base

From the start of the titration to near the equivalence point, the pH goes up slowly.


Just before and after the equivalence point, the pH increases rapidly.


At the equivalence point, moles acid = moles base, and the solution contains only water and the salt from the cation of the base and the anion of the acid.


As more base is added, the increase in pH again levels off.

Neutralization

Neutralization Reaction =

Acid + Base Salt + Water

Salt – ionic compound containing a positive ion other than H+ and a negative ion other than OH-

Buffered solutions – resists a change in its pH even when a strong acid or base is added to it

- A solution is buffered in the presence of a weak acid and its conjugate base

pH and pOH Calculations

H + O H -

pH pO H

[O H -] = 1 x 10 - 1 4

[H + ]

[H + ] = 1 x 10 - 1 4

[O H -]

p O H = 14 - p H

p H = 14 - p O H

pOH

= -l

og[O

H- ]

pH =

-log

[H+

]

[OH

- ] = 1

0-pO

H

[H+

] = 1

0-pH

notes- acids and bases

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