Acids and Bases

pH Scale pH scale – numbered from 0 to 14, measures acidity and alkalinity (how acidic or basic a solution is) pH scale – numbered from 0 to 14, measures acidity and alkalinity (how acidic or basic a solution is) Neutral

Strengths of Acids and Bases The lower the pH the STRONGER the acid The lower the pH the STRONGER the acid More hydrogen ions (H + )in solution More hydrogen ions (H + )in solution The higher the pH the STRONGER the base The higher the pH the STRONGER the base More hydroxide ions (OH - ) in solution More hydroxide ions (OH - ) in solution

I. Acid-Base Models A) Arrhenius Acids and Bases 1. Acids – substances contain hydrogen and form H+ ions in aqueous solution Example - HCl H + (aq) + Cl - (aq)

2. Bases – substances contain hydroxide (OH – ) ions and form OH – ions in aqueous solution NaOH Na + (aq) + OH – (aq)

Problem with this model – some acids and bases don’t fit Arrhenius definitions Problem with this model – some acids and bases don’t fit Arrhenius definitions Example: Is NH 3 an acid or a base? It is not likely that NH 3 will lose a H +, but it will produce OH – ions in water, like this: It is not likely that NH 3 will lose a H +, but it will produce OH – ions in water, like this: NH 3 + H 2 O NH 4 +1 (aq) + OH –1 (aq) NH 3 + H 2 O NH 4 +1 (aq) + OH –1 (aq)

B) Brønsted-Lowry Acids and Bases 1. Acids – hydrogen ion (proton) donors 2. Bases – hydrogen ion (proton) acceptors ; do not necessarily contain OH – NH 3 + H 2 O NH 4 +1 (aq) + OH –1 (aq)

II. Auto-Ionization of Water In pure water, 2 out of every billion molecules transfer a proton and break apart into ions according to this reaction: HOH  H + + OH -

B) Ion-product constant for water (K w ) K w = [H + ] ∙ [OH - ] = 1.0 x M 2 this value is a constant that can be used to determine acidity or alkalinity (basic) of a solution this value is a constant that can be used to determine acidity or alkalinity (basic) of a solution In neutral solutions, [H + ] = [OH - ] In neutral solutions, [H + ] = [OH - ] in acidic solutions, [H + ] > [OH - ] in acidic solutions, [H + ] > [OH - ] in basic solutions, [OH - ] > [H + ] in basic solutions, [OH - ] > [H + ]

Ion Concentration Examples [H + ] [OH - ] = 1 x M 2 1. What are the ion concentrations of a M solution of HCl? is the solution an acid or base? is the solution an acid or base? 2. What are the ion concentrations of a M solution of KOH? Acid or base?

pH Calculations pH stands for power of Hydrogen 1. pH = – log[H + ] Example: What is the pH of a solution with a hydrogen ion concentration of 3.5 x M? pH = – log [H + ] pH = -log[3.5 x ] pH = 8.46

How do I get back from pH to ion concentrations? [H + ] = antilog (-pH) What’s antilog? 2 nd function of log key (10 x )

[H + ] = antilog (–pH) [H + ] = antilog (–pH) Example: Determine the concentration of hydrogen ions in a solution with a pH of 3.5 Example: Determine the concentration of hydrogen ions in a solution with a pH of 3.5 [H+] = antilog (–3.5) [H+] = 10 (–3.5) [H+] = 3.16 x M

What is redox? Any chemical process in which elements experience a change in charge is an oxidation-reduction reaction (redox reaction) Any chemical process in which elements experience a change in charge is an oxidation-reduction reaction (redox reaction) Redox reactions involve a transfer of electrons. Redox reactions involve a transfer of electrons. Oxidation – involves losing electrons Oxidation – involves losing electrons Reduction – involves gaining electrons Reduction – involves gaining electrons LEO the lion says GER LEO the lion says GER

Rules for Assigning Charges All unbonded elements have a charge of 0 All unbonded elements have a charge of 0 Oxygen is always a -2 (for this class) Oxygen is always a -2 (for this class) Fluorine is always a -1 Fluorine is always a -1 Hydrogen is always +1 (for this class) Hydrogen is always +1 (for this class) Alkali metals are always +1 Alkali metals are always +1 Alkaline earth metals are always +2 Alkaline earth metals are always +2 Aluminum is always +3 Aluminum is always +3 Neutral compounds have a charge of 0 Neutral compounds have a charge of 0 The sum of the charges of the elements in a polyatomic ion equals the charge of the polyatomic ion. The sum of the charges of the elements in a polyatomic ion equals the charge of the polyatomic ion.

Example Identify if the following equations are redox reactions or not. For each redox reaction, determine which element is oxidized and which element is reduced. Identify if the following equations are redox reactions or not. For each redox reaction, determine which element is oxidized and which element is reduced. Ca(OH) HCl → CaCl H 2 O Ca(OH) HCl → CaCl H 2 O CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O(g) CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O(g) 2 Al(s) + 3 CuCl 2 (aq) → 2 AlCl 3 (aq) + 3 Cu(s) 2 Al(s) + 3 CuCl 2 (aq) → 2 AlCl 3 (aq) + 3 Cu(s)

Neutralization When a strong acid reacts with a strong base to form a salt and water. When a strong acid reacts with a strong base to form a salt and water. Example: Example: 2 NaOH + H 2 SO 4  Na 2 SO H 2 O

Precipitation Reactions Precipitate – solid that is produced when two solutions are mixed together Precipitate – solid that is produced when two solutions are mixed together These reactions are double-replacement reactions (switch dance partners) that form solids These reactions are double-replacement reactions (switch dance partners) that form solids Example: Example: 2 KI (aq) + Pb(NO 3 ) 2 (aq)  PbI 2 (s) + 2 KNO 3 (aq) 2 KI (aq) + Pb(NO 3 ) 2 (aq)  PbI 2 (s) + 2 KNO 3 (aq)

Titration Problems Titration – using a solution of known concentration to determine the concentration of an unknown solution Titration – using a solution of known concentration to determine the concentration of an unknown solution Equivalence Point – the point when there are equal amounts of moles of the acid and base Equivalence Point – the point when there are equal amounts of moles of the acid and base Indicator – substance that changes color when the reaction is complete, end point. Indicator – substance that changes color when the reaction is complete, end point.