Acids & 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 (H 3 O + ) 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 H 2 O is the Bronsted base. HCl + H 2 O  H 3 O + + Cl - acid base

BRONSTED-LOWRY DEFINITION OF ACIDS AND BASES Let’s look at another example: NH 3 + H 2 O  NH OH - base acid acid base

Bronsted Bases Strong Bases- yellow Oxide O 2- Amide NH 2 - PO 4 3- Weak Bases SO 3 2- CN - CO 3 2- NH 3 Increasing strength acidsbases

The two Bronsted strong bases. They are found on the right bottom area of the acid chart -p 334 H+H+ H+H+ O 2- + H 2 O  OH - NH H 2 O  NH 3 Again, all bases make OH -  means strong + OH -

Bronsted Weak Bases H+H+ H+H+ H+H+ + OH - HFHF+ H 2 OF-F- + OH - HC2O4-HC2O4- ⇄ + H 2 OC 2 O OH - H 2 CO 3 ⇄ + H 2 OHCO 3 - ⇄

Arrhenius bases

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

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

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

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

Whether you consider Arrhenius or Bronsted- Lowry, BASES act the same way: They all make OH - by accepting a proton from water! NH 3 + H 2 O ⇄ NH OH - C 6 H 5 O H 2 O ⇄ HC 6 H 5 O OH - H+H+ H+H+

Weak bases At equilibrium, use Use Kb –Base ionization constant

Write the Kb expression for the following 3 weak bases Kb= [HF][OH - ] [F - ] [HC 2 O 4 - ][OH - ] [C 2 O 4 2- ] Kb= [H 2 CO 3 ][OH - ] [HCO 3 - ]

AMPHIPROTIC SUBSTANCES Some substances (ex: H 2 O) 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: H 2 O H 2 PO 4 - HCO H + - H + Example: H 3 PO 4 H 2 PO 4 - HPO 4 2-

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: H 2 S + HCO 3 -  H 2 CO 3 + HS - NH H 2 O  H 3 O + + NH 3 HCOOH + HSO 3 -  H 2 SO 3 + HCOO -

CONJUGATE ACID-BASE PAIRS Conjugate acid – the species with one more proton (ex. HIO 3 ) Conjugate base - the species with one less proton (ex. IO 3 - ) *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: H 2 SO 4 ) = diprotic acid –THREE protons (ex: H 3 PO 4 )= triprotic acid –More than ONE proton= polyprotic acid

WRITE THE BRONSTED-LOWRY ACID-BASE EQUILIBRIA WHEN THIS PAIR IS MIXED: HCN and F -

Homework: Hebden: pg 115 #10, pg 117 #11-12, pg 119 #13-14, pg 121 #16-19