Acids and Bases Lesson 2 Strong and Weak Bases

Review of Bronsted-Lowry Acids

FORMING HYDRONIUM IONS The proton (H+) has been transferred from the HCl molecule to a water molecule. form a hydronium (H3O+) ion and a Cl- ion.   This type of reaction is called ionization (because ions are being formed). Complete Question 1 on page 4 of your notes.

BRONSTED-LOWRY DEFINITION OF ACIDS AND BASES An acid is any substance which donates (gives) a proton (H+) to another substance. A base is any substance which accepts (takes) a proton from another substance. A Bronsted Acid is a proton donor A Bronsted Base is a proton acceptor

BRONSTED-LOWRY DEFINITION OF ACIDS AND BASES We see that the HCl is donating the proton and the water is accepting the proton. Therefore HCl is the Bronsted acid and H2O is the Bronsted base. HCl + H2O  H3O+ + Cl- acid base

BRONSTED-LOWRY DEFINITION OF ACIDS AND BASES Let’s look at another example: base acid acid base  

Bronsted Bases Weak Bases acids bases SO32- NH3 CN- Increasing strength CO32- PO43- Strong Bases- yellow Oxide O2- Amide NH2-

The two Bronsted strong bases The two Bronsted strong bases. They are found on the right bottom area of the acid chart pg. 485 H+ O2- + H2O ® OH- + OH- H+ NH2- + H2O ® NH3 + OH- Again, all bases make OH- ® means strong

Bronsted Weak Bases H+ F- + H2O ⇄ HF + OH- H+ C2O42- + H2O ⇄ HC2O4-

Arrhenius bases

Arrhenius Bases Strong Bases- yellow LiOH NaOH KOH RbOH CsOH FrOH Ra(OH)2 Sr(OH)2 Ba(OH)2

Weak Bases- most other metals Arrhenius Bases Be(OH)2 Zn(OH)2 AgOH

Strong Bases Strong bases completely ionize in water to produce OH- There are 11 Arrhenius strong bases - Group I and Group II hydroxides LiOH ® Li+ + OH- KOH ® K+ + OH- Sr(OH)2 ® Sr2+ + 2OH- Ra(OH)2 ® Ra2+ + 2OH- ® means strong Note all bases make OH-

Arrhenius Weak Bases A weak base is one that partially reacts with water to produce OH-. Be(OH)2(s) ⇄ Be2+ + 2OH- Zn(OH)2(s) ⇄ Zn2+ + 2OH-

  H+ C6H5O73- + H2O ⇄ HC6H5O72- + OH- H+ NH3 + H2O ⇄ NH4+ + OH- Whether you consider Arrhenius or Bronsted-Lowry, BASES act the same way: They all make OH- by accepting a proton from water!

Weak bases At equilibrium, use Use Kb Base ionization constant

Write the Kb expression for the following 3 weak bases [HF][OH-] Kb = [F-] [HC2O4-][OH-] Kb = [C2O42-] [H2CO3][OH-] Kb = [HCO3-]

AMPHIPROTIC SUBSTANCES Some substances (ex: H2O) are capable of acting as an ACID (when surrounded by a stronger base) OR acting as a BASE (when surrounded by a stronger acid). Substances that act as acids or bases are called amphiprotic.

AMPHIPROTIC SUBSTANCES Other amphiprotic substances: H2O H2PO4- HCO3-   + H+ - H+ Example: H3PO4 H2PO4- HPO42-

AMPHIPROTIC SUBSTANCES In general, amphiprotic substances… Have a negative charge and An easily removable hydrogen.  

AMPHIPROTIC SUBSTANCES Identify the acid and base in the reactants of the following reactions:   H2S + HCO3-  H2CO3 + HS- NH4+ + H2O  H3O+ + NH3 HCOOH + HSO3-  H2SO3 + HCOO-

CONJUGATE ACID-BASE PAIRS Conjugate acid – the species with one more proton (ex. HIO3) Conjugate base - the species with one less proton (ex. IO3- ) *There will always be 2 conjugate pairs

POLYPROTIC ACIDS The formula of an acid tells us how many protons (H+) the acid can donate.   An acid that can supply: ONE proton (ex: HCl) = monoprotic acid TWO protons (ex: H2SO4) = diprotic acid THREE protons (ex: H3PO4) = triprotic acid More than ONE proton = polyprotic acid

Write the acid base equilibria when this pair is mixed; HCN and F-