1. Sec. 18.1: Acids & Bases: An Introduction
Ch. 18: Acids & Bases
Sec. 18.1:
Acids & Bases: An Introduction

2. Objectives
Identify the physical & chemical properties of acids & bases.
Classify solutions as acidic, basic, or neutral.
Compare the Arrhenius, Brønsted-Lowry, and Lewis models of acids & bases.

3. Properties Acids Taste sour May sting or burn on contact
Turn blue litmus paper pink
Are electrolytes
React with most metals to form H2 gas*
React with carbonates to form CO2 gas**
Bases
Taste bitter
Feel slippery
Turn pink litmus paper blue
Are electrolytes

4. Metal* & Carbonate** Acid Reactions
Recall that acid and metal reactions are single replacement reactions:
Mg + HNO3  ?
Al + H2SO4  ?
Recall that acid and carbonate reactions are double replacement reactions with the immediate decomposition of H2CO3:
HBr + CaCO3  ?
HCl + KHCO3  ?

5. Aqueous Solutions
All aqueous solutions contain H+ and OH- ions because water does dissociates slightly. The process is called self-ionization.
H2O H+ and OH-
If [H+] = [OH-], the solution is neutral. It is not acidic or basic. Pure water is neutral because [H+] has to equal [OH-].
In an acidic solution, the [H+] > [OH-].
In a basic solution, [OH-] > [H+].

6. The Hydronium Ion
Since H+ ions will bond to water molecules in a solution, the solution does not really contain H+ ions. It contains H3O+ ions.
We will use the symbols H+ and H3O+ interchangeably to represent a hydrogen ion in a solution.

7. The Arrhenius Model
An acid is a substance that contains hydrogen and produces H+ ions in aqueous solution.
A base is a substance that contains a hydroxide group and produces OH- ions in aqueous solution.

8. Examples An Arrhenius acid A Arrhenius base
HCl --> H+(aq) + Cl-(aq)
HCl is an acid.
When dissolved in water, the resulting solution is acidic.
A Arrhenius base
NaOH --> Na+(aq) + OH-(aq)
NaOH is a base.
When dissolved in water,
the resulting solution is basic.

9. Brønsted-Lowry Model
Two chemists independently proposed a new model of acids & bases - one that would recognize that some bases (like NH3, for example) did not contain hydroxide yet produced OH- ions in water.

10. Brønsted-Lowry Model NH3 + H2O  NH4+ + OH-
According to Bronsted-Lowry, ammonia was considered a base:
NH3 + H2O  NH OH-

11. Brønsted-Lowry Model An acid is a hydrogen ion (proton) donor.
A base is a hydrogen ion (proton) acceptor.
Look at this general equation:
HX (aq) + H2O H3O+ + X-
the acid the base

12. Practice Problems
Identify the acid and base in the following reactions:
H3O+ + OH-  H2O + H2O
HCl + NH3  NH4+ + Cl-
S-2 + H2O  HS- + OH-
HS- + H2O  S-2 + H3O+
H2O + HC2H3O2  C2H3O H3O+
C2H3O H3O+  H2O + HC2H3O2

13. Brønsted-Lowry Model HX (aq) + H2O H3O+ + X-
On accepting the H+ ion, H2O becomes
H3O+ (which is an acid). WHY?? Because in the reverse reaction, the H3O+ donates its H+ ion to X-.
On donating its H+ ion, HX becomes X- (which is a base). WHY?? Because in the reverse reaction, the X- accepts the H+ ion.

14. Brønsted-Lowry Model
The forward and reverse reactions are BOTH reactions of an acid and base.
The acid and base that react in the reverse reaction are called the conjugate acid and base.
The conjugate acid is the substance produced in the forward reaction when the base accepts H+ from an acid.
The conjugate base is the substance leftover after the acid has donated H+ to a base in the forward reaction.

15. Practice Problems
Identify the conjugate acid and the conjugate base in the reactions from before:
H3O+ + OH-  H2O + H2O
HCl + NH3  NH4+ + Cl-
S-2 + H2O  HS- + OH-
HS- + H2O  S-2 + H3O+
H2O + HC2H3O2  C2H3O H3O+
C2H3O H3O+  H2O + HC2H3O2

16. Brønsted-Lowry Model HX (aq) + H2O H3O+ + X-
The hydronium ion is the conjugate acid of the base water.
The X- ion is the conjugate base of the acid HX.
A conjugate acid-base pair are two substances that are related to each other by the donating & accepting of a H+ ion.

17. Examples HCO3- & H2CO3 are a conjugate acid-base pair.
H2O & OH- are also a conjugate acid-base pair.

18. Examples

19. Water
Water (& other substances) that can act as both an acid & a base are said to be amphoteric.
HF + H2O H3O+ + F-
BASE
NH3 + H2O NH OH-
ACID

20. Practice Problems
Identify the conjugate acid-base pairs in the following reactions:
HSO H2O H3O+ + SO4-
CO H2O HCO OH-
NH OH NH3 + H2O
OH- + HC2H3O C2H3O2- + H2O

21. Monoprotic and Polyprotic Acids
An acid that can donate only one hydrogen ion is a monoprotic acid.
Acids that can donate more than one hydrogen ion are polyprotic acids.
See Table 1 on p Which acids are monoprotic? polyprotic?

22. The Lewis Model
According to the Lewis model, a Lewis acid is an electron-pair acceptor and a Lewis base is an electron pair donor.
The Lewis model includes all the substances classified as Brønsted-Lowry acids and bases and many more.