1. Acids & Bases Part I: Some Definitions
Jespersen Chap. 16
Sec 1 & 2
Dr. C. Yau
Fall 2014 1

2. Different Definitions of Acid/Base
Arrhenius Definition:
An acid is a substance that increases the H3O+ concentration when placed in water.
A base is a substance that increases the OH- concentration when placed in water.
NaOH (s) Na+(aq) + OH-(aq)
HCl (g) + H2O (l) H3O+(aq) + Cl-(aq) 2

3. Different Definitions of Acid/Base
Brønsted-Lowry Definition:
An acid is a proton donor.
A base is a proton acceptor.
water acting
as a base
proton donor
proton acceptor
Typically a Brønsted base has a lone pair to attract the proton. 3

4. Brønsted-Lowry Definitions
Why do we need new definitions?
The Arrhenius definitions were based on placing substances in water, but not all reactions are in water.
e.g. NH3 (g) + HCl (g) NH4Cl (s)
This is an acid-base reaction and yet no H3O+ nor OH- ions are involved.
However, proton transfer is involved.
Write the Lewis structures for the reaction above and draw in curved arrows to show the flow of electrons.

5. Brønsted-Lowry Acids/Bases
Organic carboxylic acids have –COOH group attached. Acetic acid has the structure…
Recognize all the different ways of writing its formula:
HC2H3O2 CH3COOH CH3CO2H
Remember: -COOH does NOT mean C-O-O-H
Which H will be donated as a proton?

6. Write the equation using Lewis structures and curved arrows to show the reaction of acetic acid with ammonia.
Which is the proton donor, which is the proton acceptor? Which is the proton being transferred?

7. Conjugate Acids & Conjugate Bases
We will abbreviate "Brønsted-Lowry" to just "Brønsted."
Write the equation for the reaction of acetic acid and ammonia using molecular formulas: HC2H3O2 + NH3
HC2H3O2 + NH C2H3O2- + NH4+
acid base
How is the acetate a base? and NH4+ an acid?
(This is ammonium acetate.)
conjugate conjugate
base acid

8. Identify the Conjugate Partner for Each
The clue is to remember what the functions of Brønsted acids and bases are.
conjugate base
conjugate acid
HCl
NH3
HC2H3O2
CN- HF 8

9. Conjugate Acid-Base Pairs
Write the equation for the reaction of formic acid with water using molecular formulas:
HCO2H H2O
Identify which is the acid, base, conjugate acid, conjugate base.
Give the conjugate acid-base pairs:

10. Conjugate Acid-Base Pairs
Example 16.1 p. 744
What is the conjugate base of nitric acid and what is the conjugate acid of the hydrogen sulfate ion?
Do Pract Exer 1, 2, 3 on p. 744

11. Conjugate Acid-Base Pairs
Example 16.2 p. 745
The anion of sodium hydrogen sulfate reacts as follows with the phosphate ion. Identify the 2 conjugate acid-base pairs.
HSO4 (aq) + PO43 (aq)  SO42 (aq) + HPO42 (aq)
Do Pract Exer 4, 5, 6 p. 745.

12. Amphoteric Substances
An amphoteric substance is one that can act either as an acid or a base.
Water is such a substance.
Although it is exists primarily as a molecule, there are some ions present due to the self-ionization of water:
HOH HOH H3O OH-
Write the equation for water reacting with ammonia.
Write the equation for water reacting with hydrochloric acid.

13. Amphoteric Substances
Write the equation for the reaction of the bicarbonate ion with hydroxide.
Write the equation for the reaction of the bicarbonate ion with hydronium ion.
Why is bicarbonate considered amphoteric?
Do Pract Exer 7, 8 p. 746

14. Acid and Base Strengths
First recall what an acid is supposed to do…. proton donor… gives away a proton.
A strong acid is one that can willingly "gives away" its proton.
HCl + H2O H3O Cl-
A weak acid is one that "hangs on" to its proton.
HC2H3O + H2O H3O C2H3O2-
100%

15. Acid and Base Strengths
Recall what a base is supposed to do…
proton acceptor…. grabs up protons.
A strong base is one that "greedily grabs" protons.
NH2- + H2O NH OH-
A weak base is one that does not particularly want to grab a proton.
Cl H2O HCl + OH-

16. H3O+ and OH- H3O+ is the strongest acid that can exist in water.
Note that mineral acids (nonorganic acids) such as HCl, HNO3, H2SO4, are stronger acids, but as soon as they are placed in water, they dissociate 100% and no longer exist in water as such.
They become H3O+ and the corresponding anions.

17. H3O+ and OH- OH- is the strongest base that can exist in water.
O2- is a stronger base but as soon as it goes into the water, it is converted to OH- and does not exist as O2-.

18. Relationship between strengths of acid and conjugate base
HC2H3O + H2O H3O C2H3O2-
Identify the acid, base, conjugate acid, conjugate base.
Is the acid strong or weak?
Is its conjugate base strong or weak?
Is the base strong or weak?
Is the conjugate acid strong or weak?

19. Relationship between strengths of acid and conjugate base
HC2H3O + H2O H3O C2H3O2-
The pattern we see is…
A weak acid produces a strong conjugate base.
A strong acid produces a weak conjugate base.
A weak base produces a strong conjugate acid, & a strong base produces a weak conjugate acid.

20. Relationship between strengths of acid and conjugate base
HC2H3O + H2O H3O C2H3O2-
Another pattern we see is that the position of the equilibrium lies in favor of the weaker acid and base.
REMEMBER: Equilibrium goes
from Strong acid to Weaker acid.

21. Example 16.3 p. 749
In this rxn, will the position of equilibrium lie to the left or the right, given the fact that acetic acid is known to be a stronger acid than the hydrogen sulfite ion?
HSO3(aq) + C2H3O2 (aq) HC2H3O2 (aq) + SO32 (aq)
Do Pract Exer 9, 10 p. 750

22. Which is the stronger acid?
Which “acids” are we talking about?
How can we tell which is the stronger acid?