Chemistry 121/122Hydrogen Ions and Acidity

Hydrogen Ions from WaterWater is highly polar – what does that mean?

Water particles are in continuous motionIf they possess enough energy, a H+ can be

transferred from one water molecule to another

The particle that remains once the H+ leaves is OH-

The water molecule gaining the H+ becomes hydronium (H3O+)

H2O(l) + H2O(l) → H3O+(aq) + OH-

(aq)

Self-ionization of water

Another way of writing the ionization of water is:H2O(l) ↔ H+

(aq) + OH-(aq)

Hydronium and hydrogen ions can be written interchangeably

Both examples show water forming ionsThe equilibrium concentration for both hydrogen

and hydroxide ions is very small at 25°CEach is only 1 x 10-7 MWhen both [H+] and [OH-] are equal in

concentration, the solution is said to be neutral

Ion Product Constant for WaterIn any aqueous solution, as hydrogen ion

concentration increases, hydroxide ion concentration decreases

If hydrogen ions were to be added, the equilibrium would shift and the hydroxide ion concentration would decrease

The product of the hydrogen-hydroxide concentration is 1 x 10-14

In other words, the Kw = [H+][OH-] = 1 x 10-14

Sample Problem, p. 596

Complete 9, 10, p. 596

Acidic and Basic Solutions When acids form aqueous solutions, there is

more H+ ions than OH- (coming from the ionization of water)

The [H+] is greater than 1 x 10-7 M

Basic solutions are the oppositeThe hydroxide ion concentration is greater

than the hydrogen ion concentrationThe [OH-] is greater than 1 x 10-7 MThey are otherwise known as alkaline

solutions

Calculating pH (power of hydrogen)Ranges from 0 – 14Neutral solutions have a pH of 7The closer the pH is to zero, the more acidic

it is; the closer it is to 14, the more basic it is

To calculate pH from the hydrogen ion concentration,

pH = -log[H+]

Figure 19.9, p. 597

Calculating pOHpH + pOH = 14

pOH = -log[OH-]

Express concentrations in scientific notationWhen writing the pH or pOH, the

concentration of the solution is equivalent to the same number of digits after the decimal of the value itself

Ex. 1.0 x 10-5 = 5.00 (two digits in the base of the scientific notation – two digits after the decimal)

When is pH not a whole number?Most of the time…

As a result, it is not easy to make a simple mental calculation. Instead, use the logarithmic equation to convert from concentration to pH or pOH.

Ex. [H+] = 4.2 x 10-10MpH = -log[[4.2 x 10-10]

= -9.37675= 9.38

Questions 11-12, p. 599

Calculating [H+] from pHRearrange the equation pH = -log[H+]

[H+] = - antilog pH

Using Kw, in addition to pH/pOH, [H+]/[OH-], you can solve for any unknown

Sample Problem 19.3, p. 600

Sample Problem 19.4, p. 601

Questions 15-16, p. 601