Acids & Bases SCH3u/4C Ms. Richardson

Acids & Bases Acids and bases were originally defined by their properties: Acids Bases Liquid or solid Corrosive Sour taste Conducts electricity Reacts with most metals to produce H2 gas Reacts with carbonates to produce CO2 Liquid or base Bitter taste Reacts with acids Doesn’t react with metal Soapy/slippery feeling

The Arrhenius Theory Scientists began to realize that defining acids and bases based on their chemical behavior was more accurate. Svante Arrhenius (1859-1927) defined acids as substances that produce hydrogen ions (H+) in solution. HCl(aq)  H+(aq) + Cl-(aq) H2SO4(aq)  2H+(aq) + SO42-(aq) He defined bases as substances that produce hydroxide ions (OH-) in solution. NaOH(aq)  Na+(aq) + OH-(aq) Ca(OH)2 (aq)  Ca2+(aq) + 2OH-(aq)

Acid Base Indicators Acid-base indicators are chemicals that can be used to identify acids and bases by detecting the presence of H+ ions. Indicators Acid Colour Base Colour Blue Litmus Paper Red Blue Red Litmus Paper Bromothyl Blue Yellow Phenolphthalein Colourless Pink Methyl Orange

Limitations to the Arrhenius Theory The hydrogen ion is extremely reactive. As soon as the acid dissociates in water, H+ reacts with the water to form the hydronium ion (H3O+). H+ + H2O  H3O+ Hydrogen and hydronium ions are often used interchangeably as their chemical behavior is the same

Limitations to the Arrhenius Theory The dissociation of the base ammonia (NH3) does not result in the production of hydroxide ions. We can show the hydroxide ion produced by showing the reaction with water: NH3 + HOH  NH4+ + OH-

Strong Acids and Strong Bases Acids and bases that dissociate completely are considered to be strong acids/bases. Examples: H2SO4, HCl HCl + H2O  H3O+ + Cl- 1.0 mol/L 1.0 mol/L When an acid or base only partially dissociates, it is referred to as a weak acid/base. Example: Acetic acid dissociates at 5% CH3COOH + H2O  H3O+ + CH3COO- 1.0 mol/L 0.05 mol/L