Acid-Base Reactions - Definitions Assume the acids and bases are in water. acid base Arrhenius: donates H+ donates OH– Bronsted-Lowry donates H+ accepts H+ (classic: NH3) Lewis: electron pair acceptor electron pair donor

Acid Dissociation + 1- HCl Cl Conjugate base Acid Conjugate pair H When a strong acid dissolves in water, the proton that is released is transferred to a water molecule that acts as a proton acceptor or base. Resulting H3O+ ion is called the hydronium ion. Bases that do not contain the hydroxide ion accept a proton from water, so small amounts of OH– are produced—water acts like an acid by donating a proton. Substances that can behave as both an acid and a base are said to be amphoteric. Products of an acid-base reaction are also an acid and a base. The acid product is the conjugate acid of the base, and the base product is the conjugate base of the acid. All acid-base reactions involve two conjugate acid-base pairs. Conjugate base Acid Conjugate pair Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 280

Conjugate Acid-Base Pairs Use Brønsted–Lowry Definitions of Acid & Base conjugates base conj. acid HCl + H2O H3O+ + Cl- acid conj. base conjugates Two species that differ by only a proton constitute a conjugate acid-base pair. 1. Conjugate base has one less proton than its acid; A– is the conjugate base of HA 2. Conjugate acid has one more proton than its base; BH+ is the conjugate acid of B • In the reaction of HCl with water, HCl, the parent acid, donates a proton to a water molecule, the parent base, forming Cl–; HCl and Cl– constitute a conjugate acid-base pair. • In the reverse reaction, the Cl– ion in solution acts as a base to accept a proton from H3O+, forming H2O and HCl; H3O+ and H2O constitute a second conjugate acid-base pair. Any acid-base reaction must contain two conjugate acid-base pairs, which in this example are HCl/Cl– and H3O+/H2O • HCl (aq) + H2O (l)  H3O+ (aq) + Cl– (aq) parent acid parent base conjugate acid conjugate base HCl + H2O H3O+ + Cl- acid base CA CB

Conjugate Acid-Base Pairs conjugates acid base NH3 + H2O NH41+ + OH- base acid conjugates base acid CA CB NH3 + H2O NH41+ + OH-

Conjugate Acid-Base Pairs: Direction of Reaction The reaction proceeds in the direction such that the stronger acid donates its proton to the stronger base. acid base CA CB HCl + H2O H3O1+ + Cl-

Amphoteric: A substance that can act as either an acid or a base. 1. Water can act as an acid: 2. Water can act as a base: H+ H+ S Water is amphoteric.

Zwitterion: A neutral molecule with both positive and negative electrical charges. acid cong. base base conj. acid Water is amphiprotic: it can act as an acid by donating a proton to a base to form the hydroxide ion, or as a base by accepting a proton from an acid to form the hydronium ion, H3O+ Structure of the water molecule 1. Polar O–H bonds and two lone pairs of electrons on the oxygen atom 2. Liquid water has a highly polar structure example: amino acid (glycine)

Strong Acids & Bases Strong Acids Strong Bases HCl HBr Group 1 HI LiOH HNO3 HClO3 H2SO4 Group 1 LiOH NaOH KOH (Rb & Cs) Heavy Group 2 Ca(OH)2 Sr(OH)2 Ba(OH)2 Strong acids & bases completely dissociate (e.g., HCl -> H+ + Cl–) Weak acids & bases don’t “ “ (e.g., HF H+ + F – )

Types of Acid-Base Reactions Types: (1) strong acid + strong base (2) weak acid + strong base (3) strong acid + weak base [(4) weak acid + weak base] {treated separately with titration} (N.B., general formulas for acid = HB dissociates into H+ and B–)

Strong Acid + Strong Base strong acid + strong base (using HCl and NaOH as examples) Both completely dissociate in water before the reaction occurs. HCl + NaOH  H+(aq) + Cl–(aq) + Na+(aq) + OH–(aq) net ionic equation: H+(aq) + OH–(aq)  H2O(l) (Na+ and Cl– are spectator ions) (neutralization reaction)

acetic acid, HC2H3O2, and NaOH as an example. HC2H3O2 + NaOH  ? Weak Acid + Strong Base acetic acid, HC2H3O2, and NaOH as an example. HC2H3O2 + NaOH  ? (A) Two reactions occur: (1) weak acid: HC2H3O2 H+ + C2H3O2– (reversible reaction) (2) strong base: (NaOH is completely dissociated so it is Na+ + OH–) NaOH  Na+ + OH– (S) overall reaction: Na+ + OH– + H+ + C2H3O2–  Na+ + H2O + C2H3O2– (B) net ionic equation: HC2H3O2 + OH–  C2H3O2– + H2O(l) (Na+ is a spectator ion.)

Practice Write the net ionic equation for sulfuric acid and barium hydroxide: H2SO4 + Ba(OH)2 strong acid strong base 2H+ + 2OH– -> 2H2O

Practice Ex. 4.3: Write the net ionic equation for the following reaction: a) Hypochlorous acid + barium hydroxide weak acid strong base HClO2 + OH– -> ClO2– + H2O

Strong Acid + Weak Base HCl and NH4 as examples HCl + NH3  Two reactions occur: (1) weak base: NH3 + H2O NH4+ + OH– (reversible reaction) (2) strong acid: HCl  H+ + Cl– Overall Reaction: NH3 + H2O + H+ + Cl– NH4+ + H2O + Cl– (H2O from H+ + OH–) (Cl– is a spectator ion) net ionic equation: H+ + NH3  NH4+

Practice Ex. 4.3: Write the net ionic equation for each of the following: a) Hypochlorous acid + barium hydroxide weak acid strong base 2H+ + 2OH– -> 2H2O H+ + OH– -> H2O

Acid-Base Titration Commonly used to determine the concentration of a solution. https://youtu.be/wQ98RfLJ5Uk?list=PLnEQfEiSza7A2yiSG8sun5YGxJ1sg0Rwe

Acid-Base Titration Titration: Measuring the concentration of an unknown using the volume and concentration of a standard solution of known concentration. Objective: To determine the point at which the reaction is complete. Equivalence point.

Acid-Base Titration What is the molar mass of H2X of 29.45 mL of 0.187 M NaOH are required to neutralize a solution prepared by adding 0.242 g of H2X to enough water to make 25.00 mL of solution? Plan: Determine number of moles of H2X Then you can calculate the mass, molar mass, and even volume or molarity. number of moles NaOH moles OH– mole H2X MM mole H2X (Then 25.00 mL volume is irrelevant.)

Acid-Base Titration What is the molar mass of H2X of 29.45 mL of 0.187 M NaOH are required to neutralize a solution prepared by adding 0.242 g of H2X to enough water to make 25.00 mL of solution? mol H2X: MM H2X: