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) + H 2 O(l) Total Ionic Equation: H + (aq) + Cl - (aq) + Na + (aq) + OH - (aq) → Na + (aq) + Cl - (aq) + H 2 O(l) Net Ionic Equation: H + (aq) + OH - (aq) → H 2 O(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% CH 3 COOH (l)  H + (aq) + CH 3 COO - (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, NH 3 (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: NH 3 (aq) + H 2 O(l) → NH 4 + (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 IndicatorColour in Acid Colour in Base Colour in Neutral Solution red litmusredbluered blue litmusredblue phenolphthaleincolourlesspinkcolourless bromothymol blueyellowbluegreen

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