Acid Base Theories

Properties of Acids and Bases

• Acids taste sour, bases taste bitter
• Both acids and bases conduct electricity due to presence of ions
• Bases feel slippery

Indicators

• Acids turn blue litmus paper red, bases turn red litmus paper blue
• Phenolphthalein is clear for pH levels under 8.2 and bright pink for pH levels above that
• Bromothymol blue is yellow for pH levels under 6 and pH levels over 7.6

Arrhenius theory of acids and bases When hydrogen chloride dissolves in water, it dissociates into hydrogen ions and chloride ions

HCl -> H${}^{+}$ + Cl${}^{-}$

When sodium hydroxide dissolves in water, it dissociates to form sodium ions and hydroxide ions.

NaOH -> Na${}^{+}$ + OH${}^{-}$

Bases have hydroxide ions floating around them but acids have hydrogen ions floating around them

Svante Arrhenius noticed a pattern when acids and bases dissociate in water

Acids dissociating in water produce H${}^{+}$

Bases produce OH${}^{-}$

The Arrhenius theory of Acids and Bases

• An acid is a substance that dissociates in water to produce one or more hydrogen ions, H${}^{+}$.
• A base is a substance that dissociates in water to form one or more hydroxide ions, OH${}^{-}$

Limitations to the Arrhenius theory

• In water, the hydronium ion is produced - not H${}^{+}$
• Assumes water is solvent always
• There are exceptions: NH${}_3$

We change theory to fit reality not change reality to fit theory

The bronstead theory of Acids and Bases Their theory overcame most of these problems

The Bronstead-Lowry theory of acids and Bases

• An acid is a substance from which a proton (H${}^{+}$ ion) can be removed. (a proton donor)
• A base is a substance that can remove a proton (H${}^{+}$ ion) from an acid. (a proton acceptor)

An acid-base reaction requires transfer of a proton

• Two molecules are related by the transfer of a proton are called a conjugate acid base pair
• The conjugate base of an acid is the particle that remains when a proton is removed from the acid
• The conjugate acid of a base is the particle that results when the base received the proton from the acid

HCl + H${}_2$O -> H${}_3$O${}^{+}$ + Cl${}^{-}$

HCl - acid Cl${}^{-}$ - conjugate base H${}_2$O - base H${}_3$O${}^{+}$ - conjugate acid

Strong and weak acids and bases and their properties

Strong Acids	Weak
Perchloric Acid HClO${}_4$	Title
Chloric Acid HClO${}_3$	Text
Hydrochloric Acid HCl
Hydrobromic Acid HBr
Hydroiodic Acid HI
Nitric Acid HNO${}_3$
Sulfuric Acid H${}_2$SO${}_4$

Strong Bases	Weak
Group 1A metal hydroxides	LiOH, NaOH, KOH, RbOH, CsOH
Heavy group 2A metal	Text

Acids / Base - Reactions

Neutralization Reactions - a double displacement reaction in which an acid and a base combine to form water and a salt

ex: HClO${}_4$ (aq) + NaOH (aq) -> NaClO${}_4$ (aq) + H${}_2$O (l)

acid base salt water

• When mole ratio amounts of strong acids and bases are reacted the result is a neutral solution of pH 7

Calculations involving neutralization reactions

If 15.5 mL of HCl (aq) just neutralizes 25.5 mL of a 0.15 M of Ca(OH)${}_2$ (aq), what is the molar concentration of the acid

Givens: V${}_{HCl}$ = 15.5 mL = 0.0155 V_C$$_a$$_($$_O$$_H$$_) = 25.5 mL = 0.0255 L C_C$$_a$$_($$_O$$_H$$_) = 0.15 mol / L

Unknown:

C_H$$_C$$_l = ?

2 HCl (aq) + Ca(OH)${}_2$ (aq) -> CaCl${}_2$ (aq) + 2H${}_2$O (l)

n_C$$_a$$_($$_O$$_H$$_) = C_C$$_a$$_($$_O$$_H$$_) * V_C$$_a$$_($$_O$$_H$$_) = 0.15 mol / L * 0.0255 L = 0.003825 mol

n_H$$_C$$_l = 0.003825 mol * 2 mol HCl / 1 mol Ca(OH)${}_2$ = 0.00765 mol

C_H$$_C$$_l = n_H$$_C$$_l / V_H$$_C$$_l = 0.00765 mol / 0.0155 L = 0.494 mol / L = 0.49 mol / L (2 sig digs)

titration - a process to determine the concentration of an acidic or basic solution by reacting it with a solution of known concentration. (as in the example above)

standard solution - the solution of known concentration (Ca(OH)${}_2$ in the above example)

equivalence point - the point in a titration when the moles of added solution is stoichometrically equal to the number of moles of standard solution. (in our example exactly when 15.5 mL of HCl was added)

end point - the point in a titration when the acid-base indicator changes color (an indicator is chosen so that the end point and equivalence point are the same)

Indicators - weak acids that are diffirent colors in their associated and disociated forms

ex. H in (aq) -> H${}^{+}$ (aq) + In${}^{-}$ (aq) color(1) (color 2)

ex.

• phenolphthalein
• bromothymol blue
• universal indicators
• litmus

Homework: p. 457 # 1 - 6 p. 462 # 7 - 12 p. 463 # 1 - 16 p. 466 # 1 - 10 p. 467 # 13 - 18 p. 470 # 1 - 16