Acids & Bases

Properties of Acids

Taste sour Contain H+ ion pH less than 7 React with bases to form a

salt and water React with some metals to

produce hydrogen gas

Properties of Acids

Turn litmus paper red Phenolphthalein is colorless in

the presence of an acid Bromothymol blue is yellow in

the presence of an acid Found in citrus fruits in the

form of citric acid

Properties of Acids

Found in soured milk and in sore muscles in the form of lactic acid

Found in vitamin C in the form of ascorbic acid

Found in carbonated beverages in the form of carbonic acid…that’s also what you exhale

Properties of Bases

Taste bitter Contain OH- ion pH greater than 7 React with acids to form a salt

and water React with organic material

Properties of Bases

Feel slippery because they immediately begin to dissolve the outer layer of skin tissue

Turn litmus paper blue Phenolphthalein is fuchsia in

the presence of a base Bromothymol blue is blue in

the presence of a base

Properties of Bases

Found in drain cleaners usually in the form of sodium hydroxide

Found in ammonia-based cleaners like Windex

Lye (NaOH) is used to make soaps

Classifying Acids and Bases

Svante Arrhenius—Swedish guy who put forth his definitions of acids and bases in 1884 at the age of 25Worked with our buddy van’t

HoffReceived 1903 Nobel Chemistry

Prize for electrolytic dissociation discoveries

Arrhenius Acids

are substances that will dissociate in water to yield hydrogen ion (H+)

Arrhenius Bases

are substances that will dissociate in water to yield hydroxide ion (OH-)


Johannes Brønsted (Danish) and Thomas Lowry (English) came up with a new way to classify acids and bases and their conjugates (pairs that have features in common but are opposites) in the 1920’snever received a Nobel for

furthering these acid/base concepts, and Arrhenius never accepted them!

Brønsted-Lowry Acid

A reactant that donates a proton in a chemical reactionThe proton is actually the

hydrogen ion…since a hydrogen atom has 1 proton and 1 electron and the ion with a 1+ charge indicates that it has lost an electron

Brønsted-Lowry Base

A reactant that accepts a proton in a chemical reaction

Brønsted-Lowry Conjugate Acid

A product thatis formed when a base accepts a

protonin the reverse reaction, will

donate a proton

Brønsted-Lowry Conjugate Base

A product thatis formed when an acid donates

a proton (what’s left after the donation occurs)

in the reverse reaction, will accept a proton


Around the same time that Brønsted and Lowry were devising their acid/base scheme, our buddy Gilbert Lewis (yep, the same guy who did the dots) came up with yet another method of classifying them…it’s a broader method than Arrhenius, Brønsted, or Lowry ever postulated

Lewis Acid

A reactant that accepts an electron pair

Lewis Base

A reactant that donates an electron pair

Example #1

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

Or

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

Example #1

HCl is an acidIt dissociates to yield H3O+

(hydronium ion), which is really water with an extra H+. (Arrhenius)

It donates a proton (H+) to water in the first reaction written. (Brønsted-Lowry)

Example #1

H2O is an baseIt accepts a proton (H+) from

HCl in the first reaction written. (Brønsted-Lowry)

Example #1

H3O+ is a conjugate acidIt is produced when the water

accepts a proton (H+) from HCl in the first reaction written. (Brønsted-Lowry)

In the reverse reaction, it will donate a proton (H+) to Cl- in the first reaction written. (Brønsted-Lowry)

Example #1

Cl- is a conjugate baseIt is produced when the HCl

donates a proton (H+) to water in the first reaction written. (Brønsted-Lowry)

In the reverse reaction, it will accept a proton (H+) from H3O+ in the first reaction written. (Brønsted-Lowry)

Example #2

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

(aq)

Example #2

NH3 is a baseIt accepts a proton (H+) from

water. (Brønsted-Lowry)

Example #2

H2O is an acidIt donates a proton (H+) to

ammonia. (Brønsted-Lowry)

Example #2

NH4+ is a conjugate acid

It is produced when the ammonia accepts a proton (H+) from water. (Brønsted-Lowry)

In the reverse reaction, it will donate a proton (H+) to OH-. (Brønsted-Lowry)

Example #2

OH- is a conjugate baseIt is produced when the water

donates a proton (H+) to ammonia. (Brønsted-Lowry)

In the reverse reaction, it will accept a proton (H+) from NH4

+. (Brønsted-Lowry)

Water—our special friend

Did you notice that it behaved as a base in the first example and as an acid in the second example?

A substance that can behave as either an acid or a base is called, amphoteric or amphiprotic.

Example #3

H

H—N—H + H+ H—N—H

H H

Example #3

NH3 is a baseIt donates a pair of electrons to

H+. (Lewis)

Example #3

H+ is an acidIt accepts a pair of electrons

from NH3. (Lewis)

Autoionization of Water

Every acid and base will dissociate in water…even water (since it’s amphoteric)!

2H2O (l) H3O+ (aq) + OH- (aq)

orH2O (l) H+ (aq) + OH- (aq)


Usually, the 2nd reaction is the one we will use since H3O+ is just water with an extra H+.

Write the K expression for the 2nd reaction, keeping in mind that we only include gaseous and aqueous phases.


K = [H+][OH-]note that water is not included

because it is a liquid

This expression is known as the Kw, or equilibrium constant for water, expression K w = [H+][OH-]


The value of Kw is 1 x 10-14 M2 at 25°C.

This is a small K value.

If the temperature changes, so does the value of Kw


Make an equilibrium chart for the dissociation, or autoionization, of water.


[H2O] [H+] [OH-]

Initial -- 0 0

Change

Eq.


[H2O] [H+] [OH-]

Initial -- 0 0

Change -- +x +x

Eq.


[H2O] [H+] [OH-]

Initial -- 0 0

Change -- +x +x

Eq. -- x x


Determine the equlibrium concentrations of both the hydrogen ion and the hydroxide ion by plugging into the Kw expression.

1 x 10-14 M2 = [x][x]


1 x 10-14 M2 = x2

1 x 10-7 M = x

[H+] = 1 x 10-7 M

[OH-] = 1 x 10-7 M


Because the H+ and OH- concentrations are equal, the solution is neutral.

If [H+] > [OH-], then the solution is an acid.

If [H+] < [OH-], then the solution is a base.


Since every acid or base dissociation we will entertain occurs in water, then the Kw expression is applicable to any of these dissociations.


Thus, if you know the [H+] concentration of a solution, you can determine the [OH-] concentration.

And, if you know the [OH-] concentration of a solution, you can determine the [H+] concentration.

pH

Represents the “power of hydrogen”

Calculated by taking the opposite of the logarithm of the hydrogen ion concentration…

pH = -log [H+]

pH

Calculate the pH of water at 25°C knowing that the [H+] is 1 x 10-7 M.

pH = -log [1 x 10-7]

pH = 7

pH

If you already know the pH of a solution, then you can find the [H+] using:

[H+] = 10-pH

So,[H+] = 10-7

[H+] = 1 x 10-7 M

pOH

Represents the “power of hydroxide”

Calculated by taking the opposite of the logarithm of the hydroxide ion concentration…

pOH = -log [OH-]

pOH

Calculate the pOH of water at 25°C knowing that the [OH-] is1 x 10-7 M.pOH = -log [1 x 10-7]

pOH = 7

Ooh, ah!

So, the sum of pH and pOH for all aqueous solutions will be 14.pH + pOH = 14

Example #4—show all work

Find the pH, pOH, and [OH-] and state whether the solution is acidic, neutral or basic if the hydrogen ion concentration is3.48 x 10-4 M.

pH = 3.46pOH = 10.54[OH-] = 2.87 x 10-11 Macidic


Find the pOH, [H+], and [OH-] and state whether the solution is acidic, neutral or basic if the pH is 9.84.

pOH = 4.16[H+] = 1.45 x 10-10 M[OH-] = 6.91 x 10-5 Mbasic


Find the pH, [H+], and [OH-] and state whether the solution is acidic, neutral or basic if the pOH is 12.7.

pH = 1.30[H+] = 5.01 x 10-2 M[OH-] = 1.99 x 10-13 Macidic


Find the pH, pOH, and [H+], and state whether the solution is acidic, neutral or basic if the[OH-] is 5.26 x 10-2 M.

pH = 12.7pOH = 1.28[H+] = 1.90 x 10-13 Mbasic

Acids & Bases

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