Acids and pH

• Acids form H+ ions when they dissolve in water.

• An H+ ion is a hydrogen atom that has lost it’s electron.

• In other words, it is a PROTON.

• Therefore, acids are PROTON DONORS

H1

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ionisation

• When an acid is dissolved in water the H+ ions dissociate from the acid.

• This dissociation is referred to as ionisation• The strength of the acid is related to the level

of ionisation

Strong and weak acids

The pH scale is used to rank the strength of acids and bases.

Strong acids and bases completely dissociate (separate into ions) in water.

HCl + H2O H+(aq) + Cl−(aq)

Weak acids and bases only partially dissociate in water.

H2CO3 + H2O H⇄ 3O+(aq) + HCO3−(aq)

A closer look at strong and weak acids

• In weak solutions, only some of the hydrogen ions dissociate

Don’t confuse strong and weak acids with concentrated and dilute solutions

Concentrated solutions• Have a greater number of

particles

• Strong acids have more ions dissociated

Dilute solutions• Have fewer particles

• Weak acids have less ions dissociated

Images from: http://tg103.wikispaces.com/Taylor+S

• pH = potential of Hydrogen• All acids and bases have a pH that shows the concentration

of hydrogen ions [H+]• Because the pH is a ratio of H+ ions to OH- ions, we can

calculate the pH of acids and bases• For aqueous solutions, the product of H+ and OH- must

equal 1x10-14

• [H+][OH-] = 1 x 10-14

• Neutral [H+] = [OH-] • Acid [H+] > [OH-] • Base [H+] < [OH-]

pH Scale

To calculate pH

• [H+] is the molarity of the acid in the solution• pH = -log[H+] • You'll need to be given (or calculate) the

concentration of acid.

Example

• To calculate the pH of a solution in which [H3O+] = 0.042mol/L using a calculator:– Key in 0.042– Press the log key and note that -1.377 is displayed– Mentally change the sign (because of the –log)– The solution has a pH of 1.38

Example 2

What is the pH of a solution that has a hydrogen ion concentration equal to 0.003 M?

pH = −log [H+] pH = −log (0.003) pH = 2.52

Example 3

For strong acids, the concentration of hydrogen ions (H+) is approximately equal to the concentration of the acid itself.

What is the pH of a 0.5 M solution of HCl?

pH = −log [H+] pH = −log (0.5) pH = 0.3

[H+] = [HCl] [H+] = 0.5

Example 4

What is the pH of a 0.00140 M solution of H2SO4?

For strong acids, that has two hydrogen ions (H+) you need to double the concentration of the acid before doing the calculation.

[H+] = [H2SO4] [H+] = 2 x 0.00140 = 0.00280

pH = −log [H+] pH = −log (0.00280) pH = 2.55