Acid-Base Definitions

Type Acid Base

Arrhenius H+ producer OH– producer

Brønsted-Lowry H+ donor H+ acceptor

Lewis electron-pair acceptor electron-pair donor

● A chemist named Svante Arrhenius came up with a way to define acids and bases in 1887

● Arrhenius Acid - A substance that releases hydrogen ions (H+) in a solution. ● Arrhenius Base - a substance that releases hydroxide ions (OH-) in a solution. ● Chemists use the word "dissociated" to describe the breakup of a compound.

Brønsted-Lowry - These two chemists from Denmark and England looked at acids as donors and bases as acceptors.

● What were they donating and accepting? Hydrogen ions.● Brønsted-Lowry acid: a hydrogen ion (H+) donor. ● Brønsted-Lowry base: an hydrogen ion acceptor.

● Scientists use something called the pH scale to measure how acidic or basic a liquid is.

● Low pH = acidic = lots of H+

● High pH = basic = lots of OH-

When ammonia dissolves in water, hydrogen ions are transferred from water to

ammonia to form ammonium ions and hydroxide ions.

Is there an acid or a base?

Important Note:

● In an aqueous solution, hydrogen ions are not actually present. Instead, the hydrogen ions are joined to water molecules as hydronium ions.

● A hydronium ion (H3O+) is the ion that forms when a water molecule gains a

hydrogen ion.

HCl + H--2O → H

-+ + Cl-

● Acid

CH3COOH + H

2O → CH

3COO- + H-

● Acid

NaOH + H2O → Na+ + OH-

● Base

NH3 + H

2O → NH

4+ + OH-

● Base

A chemist named Lewis offered a third way to look at acids and bases. Instead of

looking at hydrogen ions, he looked at pairs of electrons.

● Lewis Acid: accept pairs of electrons. ● Lewis Base: donate pairs of electrons.

A conjugate acid is the ion or molecule formed when a base gains a hydrogen ion.

•NH4

+ is the conjugate acid of the base NH3.

A conjugate base is the ion or molecule that remains after an acid loses a hydrogen ion.

•OH– is the conjugate base of the acid H2O.

•The ammonia molecule and the ammonium ion are a conjugate acid-base pair.

•The water molecule and the hydroxide ion are also a conjugate acid-base pair.

Amphoteric - A substance that can act as either an acid or a base. Example: Water.

–In the reaction with hydrochloric acid, water accepts a proton and is therefore a

base.

–In the reaction with ammonia, water donates a proton and is therefore an acid.

● Scientists use something called the pH scale to measure how acidic or basic a liquid is.

● Low pH = acidic = lots of H+

● High pH = basic = lots of OH-

For aqueous solutions, the product of the hydrogen-ion concentration and the hydroxide-ion concentration equals 1.0 × 10−14

[H+] + [OH-] = 1.0 × 10-14 = 0.00000000000001

Concentration of H+ Concentration of OH-

ion-product constant for water (Kw

) - The product of the concentrations of the hydrogen ions and the hydroxide ions.

Kw

= [H+] × [OH−] = 1.0 × 10−14

Using this we know that ● Neutral solutions are half [H+] and half [OH-]

○ [H+] = (1.0 x 10-7) = 0.0000001 ○ [OH-] = (1.0 x 10-7)

● Acidic solutions have more [H+]. ● Basic solutions have more [OH-].

If the [H+] in a solution is 1.0 × 10−5M, is the solution acidic, basic, or neutral? What is the [OH−] of this solution?

● [H+] = 1.0 × 10−5M ● Is this less than or greater than 1.0 × 10−7M? ● 1.0 × 10−5M = 0.00005 M● 1.0 × 10−7M = 0.0000001 M● Greater! Thus the solution is acidic.

If the [H+] in a solution is 1.0 × 10−5M, is the solution acidic, basic, or neutral? What is the [OH−] of this solution?

● Rearrange - Kw

= [H+] × [OH−]● [OH-] = K

w / [H+]

● Kw

= 1.0 x 10-14 M

● [H+]=1.0 x 10-5 M● [OH-] = (1.0 x 10-14 M) / (1.0 x 10-5 M) = 1.0 x 10-9 M

How is pH defined - the negative logarithm of the hydrogen-ion concentration.

pH = −log[H+]

In pure water OR a neutral solution, the [H+] = 1 × 10−7M, and the pH is 7.

● This means....● pH = −log(1 × 10−7) = −(log 1 + log 10−7)● pH = −(0.0 + (−7.0)) = 7.0● If the [H+] of a solution is greater than 1× 10−7M, the pH

is less than 7.0.● If the [H+] is less than 1× 10−7M, the pH is greater than

7.0.

If a solutions [H+] = 0.0010M or 1.0 × 10−3M, what is the pH?

● pH = -log[H+] ● pH = −log(1 × 10−3) ● pH = -(log 1 + log 10-3) ● pH = -(0 + (-3)) ● pH = 3

The pH of an unknown solution has a pH of 6.35. What is the hydrogen-ion concentration of the solution?● pH = -log[H+] ● 6.35 = -log[H+] ● [H+] = antilog( -6.35) ● [H+] = 10(-6.35) = 0.000000447 = 4.47 x 10-7 M ● [H+] = 0.000000447 = 4.47 x 10-7 M

What is the pH of a solution if [OH−] = 4.0 × 10−11M?

● First use Kw = [H+] × [OH−] to find the [H+].● Then use pH = -log[H+] to find the pH. ● Rearrange - Kw = [H+] × [OH−]● [H+] = Kw / [OH-] ● Kw = 1.0 x 10-14 M (always!)

● [H+]= 1.0 x 10-14 M / 4.0 x 10-11 M● [H+] = 0.25 x 10-3 M = 2.5 x 10-4 M

What is the pH of a solution if [OH−] = 4.0 × 10−11M?

● [H+] = 0.25 x 10-3 M = 2.5 x 10-4 M ● Now plug into pH = -log[H+] ● pH = -log(2.5 x 10-4 M)● pH = 3.6

What is the pH of a solution if [OH−] = 4.0 × 10−11M?

● How can we check this???● pH = 3.6 ● A solution in which [OH−] is less than 1 × 10−7M is

acidic because [H+] is greater than 1 × 10−7M.