Transcript Slide 1

th
25 :
Friday, April
“A” Day
Monday, April 28th: “B” Day
Agenda
Hand back/go over Ch. 14 Tests
Begin chapter 15: Acids & Bases
15.1: “What Are Acids & Bases?”
Strong/weak acid, strong/weak base, Bronsted-Lowry
acid/base, conjugate acid/base, amphoteric species
Homework:
Concept Review: “What Are Acids and Bases?”
Ch. 14 Tests
Chemical Equilibrium
Class
3A
4B
Average
(out of 50)
Percentage
Chapter 15: Acids and Bases
Vinegar, many fruit juices, colas and some other
soft drinks are all acidic.
These substances can be recognized as acidic by
their tart, sour, or sharp taste.
(Of course, NEVER taste anything in the lab!)
Acid Solutions Conduct Electricity
Acids are electrolytes, so their solutions in water
conduct electric current.
Like other electrolytes, hydrogen chloride
dissociates to produce ions.
HCl (g) + H2O (l)  H3O+ (aq) + Cl− (aq)
The hydronium ion, H3O+, is able to transfer charge
through aqueous solutions much faster than other
ions do.
The result is that acid solutions are excellent
conductors of electricity.
Acids React With Many Metals
All metals that are above hydrogen in the activity
series react with acids to produce hydrogen gas.
The reaction is caused by the hydronium ion,
H3O+, present in the solution.
2 H3O+ (aq) + Zn (s)  2 H2O (l) + H2 (g) + Zn2+ (aq)
Acids Generate Hydronium Ions
(H3O+)
Because acids are electrolytes, they can be
classified as strong or weak based on how much
they dissociate into their ions.
Strong acid: an acid that ionizes completely in a
solvent.
Nitric acid, HNO3, is a strong acid:
HNO3 (l) + H2O (l)  H3O+ (aq) + NO3- (aq)
No HNO3 molecules are present in a solution of
nitric acid. All of the HNO3 dissociates into ions.
Acids Generate Hydronium Ions
+
(H3O )
Weak acid: an acid that releases few
hydrogen ions in aqueous solution
When a weak acid is dissolved in water, only a
small fraction of its molecules are ionized at any
given time.
Hypochlorous acid, HOCl, is a weak acid.
HOCl (l) + H2O (l)
H3O+(aq) + ClO- (aq)
The arrows in this equation indicate equilibrium.
In some acids, a single molecule can react
to form more than one hydronium ion.
 Sulfuric acid, H2SO4, has 2 hydrogen atoms that
can ionize:
The first one ionizes completely, and the other
ionizes partially as a weak acid.
Some Strong and Weak Acids
Arrhenius Definition of an Acid
The presence of a large number of hydronium
ions identifies an aqueous solution as acidic.
Swedish chemist, Svante Arrhenius, was among
the first to recognize this fact.
In 1890, he proposed that an acid be
defined as any substance that, when
added to water, increases the
hydronium ion concentration.
Bases
Bases are also electrolytes. Unlike acids, which
are usually liquids or gases, many common bases
are solids.
Solutions of bases are slippery to the touch.
The slippery feel comes because bases react
with oils in your skin, converting them into soaps.
This property of attacking oils and greases makes
bases useful in cleaning agents.
Bases Generate Hydroxide Ions
(OH-)
Because bases are electrolytes, they too can be
classified as strong or weak based on how much
they dissociate into ions.
Strong base: a base that ionizes completely in a
solvent.
Sodium hydroxide is a strong base:
NaOH (s)  Na+ (aq) + OH− (aq)
Bases Generate Hydroxide Ions
(OH-)
Weak base: a base that releases few
hydroxide ions in aqueous solution.
Ammonia is a typical weak base:
 NH4+ is the ammonium ion. Because NH3 is a
weak base, most of the NH3 molecules remain
un-ionized.
Arrhenius Definition of a Base
Both strong and weak bases generate
hydroxide ions when dissolved in water.
This property is the basis of the Arrhenius
definitions of a base.
Alkali: a base that is very soluble
in water. These solutions are said
to be “basic” or “alkaline”.
Two Problems With Arrhenius
Definitions
1. Arrhenius definitions are limited to aqueous
solutions only.
2. Arrhenius definition cannot classify
substances that sometimes act as acids and
sometimes act as bases.
Bronsted-Lowry Classification
In 1923, Danish chemist Johannes Brønsted
proposed a broader definition of acid.
The same year, British scientist Thomas Lowry
happened to make exactly the same proposal
independently.
Their idea was to apply the name acid to any
species that can donate a proton. (H+)
Bronsted-Lowry Acids Donate Protons
Bronsted-Lowry acid: a substance that donates a
proton to another substance.
A proton is a hydrogen atom that has lost its
electron.
It is a hydrogen ion and can be represented as H+.
HCl is a Bronsted-Lowry acid because it donates a
proton, H+, to water.
Bronsted-Lowry Bases Accept Protons
Bronsted-Lowry base: a substance that
accepts a proton.
Ammonia, NH3, is a Bronsted-Lowry base
because it accepts a proton from water when it
dissolves in water.
Bronsted-Lowry Recap
In the language of Brønsted and Lowry, an acidbase reaction is very simple: one molecule or ion
passes a proton to another molecule or ion.
Whatever loses the proton is an acid, and
whatever accepts the proton is a base.
+
H
Conjugate Acids and Bases
Conjugate acid: the acid that forms when a base
gains a proton.
Conjugate base: the base that forms when an
acid loses a proton.
Base
Acid
Conjugate Acid
Conjugate Base
Always in pairs:
Base/conjugate acid
Acid/conjugate base
Conjugate Acid-Base Pairs
Amphoteric Species
Are Both Acids and Bases
Some species are both an acid and a base and
can both donate and accept protons.
Such species are described as amphoteric.
Amphoteric: describes a substance, such as
water, that has the properties of an acid and
the properties of a base.
Homework
Because the section review is very similar to the
concept review (and I like the questions in the
concept review better), you only have to
complete the concept review for homework
today…
You’re welcome!
Next Time:
Sec. 15.1 quiz
**Have fun and be safe if you’re
going t prom!**