Acids and Bases

Svante Arrhenius

• In the 1880’s, Swedish chemist Svante Arrhenius developed a theory about electrolytes• His theory explained why solutions of

electrolytes conduct electricity• According to Arrhenius, when an

electrolyte dissolves, its ions dissociate which allows them to move freely in solution and conduct electricity

Conductivity of Ionic Compounds

Arrhenius Theory of Acids and Bases• Arrhenius soon modified his theory to include

acids and bases • Since acidic and basic solutions also conduct

electricity he concluded these solutions must also contain ions

Arrhenius Theory of Acids and Bases• According to Arrhenius, acids are substances that

ionize in water to produce one or more hydrogen ions

in water

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

(aq)

• According to Arrhenius, bases are substances that dissociate in water to form one or more hydroxide ions

in water

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

(aq)

Arrhenius Theory of Acids and Bases

Neutralization Reactions

The Arrhenius theory also explains neutralization reactions

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Total Ionic Equation:

H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) → Na+(aq) + Cl-(aq) + H2O(l)

Net Ionic Equation:

H+(aq) + OH-(aq) → H2O(l)

Acid Strength

• Based on the degree of ionization• Strong acids ionize completely in water to

form ions, and, therefore, are good conductors of electricity (e.g. hydrochloric acid)

100%HCl(g) -------- H+

(aq) + Cl-(aq)

Acid Strength

•Weak acids partially ionize in water to form ions, and, therefore, are poor conductors of electricity (e.g. acetic acid)

1.3%

CH3COOH(l) ------- H+(aq) + CH3COO-

(aq)

• Table 4 on pg. 365 in your textbook lists some strong and weak acids

Acid Strength

Base Strength

• A strong base dissociates completely in water 100%

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

(aq)

• All hydroxides of the alkali metals (Group 1) are strong bases• The alkaline earth metals (Group 2) below

beryllium in the periodic table are also strong bases

Base Strength

• Weak bases do not produce many hydroxide ions in water• Ammonia, NH3(aq) is a weak base

Why is ammonia a base?

When it is dissolved in water, it removes a hydrogen ion from water producing an ammonium ion and a hydroxide ion:

NH3(aq) + H2O(l) → NH4+(aq) + OH-(aq)

Strength Vs. Concentration

• The terms strong or weak refer to the ionization or dissociation of particles in water • The terms concentrated and dilute refer to

the amount of solute in a solvent• A dilute solution of a strong acid can be

more dangerous than a concentrated solution of a weak acid

Acid-Bases Indicators

• Are substances that change colour in the presence of an acid or a base

Indicator Colour in Acid

Colour in Base

Colour in Neutral Solution

red litmus red blue red

blue litmus red blue blue

phenolphthalein colourless pink colourless

bromothymol blue yellow blue green

The pH Scale

• Is used to describe the acidity or basicity of a solution based on the concentration of hydrogen ions in solution

• pH refers to the power (strength) of the hydrogen ions

The pH Scale

• An acidic solution has a [H+] of more than 1.0 x 10-7 mol/L, and so has a pH value of less than 7

• A basic solution has a [H+] of less than 1.0 x 10-7 mol/L H+, and so has a pH value of more than 7

• A neutral solution has a [H+] of 1.0 x 10-7 mol/L H+, and so has a pH value of 7

The pH Scale

• Note, the difference between any two consecutive pH units in the scale corresponds to a ten-fold difference in hydrogen ion concentration

• E.g. A solution with a pH of 4 has a hydrogen ion concentration that is ten times greater than that of a solution with a pH of 5