Title: Lesson 4 Strong and Weak Acids and
Bases
Learning Objectives:
– Understand the difference between strong and weak acids and bases
– Complete an experiment to explore the difference in properties of
strong/weak acids and bases
Refresh

100 cm3 of a NaOH solution of pH 12 is mixed with 900
cm3 of water. What is the pH of the resulting solution?
A.
B.
C.
D.
1
3
11
13
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
Strong acids example:
Good proton donor. Dissociation reactions go to completion,
their conjugate bases are not readily able to accept a proton.
Conjugate base Clshows virtually no
basic properties


Weak acids example:
Poor proton donors. Equilibria of dissociation reaction lies to
the left, their conjugate bases are readily able to accept a proton.
Conjugate base
CH3COO- is a
stronger base than
Cl-
Acid dissociation favours the production of weaker conjugate bases.
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Bases follow a similar pattern…
Conjugate acid Na+
shows virtually no
acidic properties


Strong base example:
Good proton acceptor. Dissociation reaction goes to
completion, their conjugate acids do not readily donate
protons.
Conjugate acid
NH4+ shows
virtually no acidic
properties


Weak base example:
Poor proton acceptor. Equilibria of dissociation reaction
lies to the left, their conjugate acids readily donate
protons.
Base dissociation favours the production of weaker conjugate acids.
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Strong Acids
Strong Bases
Weak Acids
Weak Bases
(only six; know 1st
three for IB)
(Grp 1 hydroxides & carboxylic and
barium hydroxide)
carbonic acids
ammonia and
amines
H2SO4,
LiOH,
CH3COOH,
C2H5NH2,
sulfuric acid*
lithium hydroxide
ethanoic acid
ethylamine
and other organic acids
and other amines
HNO3,
NaOH,
H2CO3,
NH3,
nitric acid
sodium hydroxide
carbonic acid
ammonia
Note CO2(aq) =
H2CO3(aq)
Note NH3(aq) =
NH4OH(aq)
HCl,
KOH,
H3PO4,
hydrochloric acid
potassium hydroxide
phosphoric acid
HI,
Ba(OH)2,
hydroiodic acid
barium hydroxide
HBr,
hydrobromic acid
HClO4,
Try to remember the main strong
acids and bases… Then you can
probably deduce any others you
come across will be weak…
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Strong, Concentrated and Corrosive
In everyday English, strong and concentrated are often used interchangeably. In
chemistry, they have distinct meanings:
 strong: completely dissociated into ions
 concentrated: high number of moles of solute per liter (dm3) of solution
 corrosive: chemically reactive
Similarly, weak and dilute also have very different chemical meanings:
 weak: only slightly dissociated into ions
 dilute: a low number of moles of solute per liter (dm3) of solution

It is possible for an acid or base to be strong but in a dilute solution, or weak
and present in a concentrated solution.
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Strong, Concentrated and Corrosive
Similarly, weak and dilute also have very different chemical
meanings:
 weak: only slightly dissociated into ions
 dilute: a low number of moles of solute per liter (dm3)
of solution
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Strong and Weak Acids


Strong Acids:
 HA(aq)  H+(aq) + A-(aq) … The acid fully dissociates into ions

For example: HCl(aq)  H+(aq) + Cl-(aq)

Includes: hydrochloric, sulfuric, phosphoric, nitric

Strong acids have weak conjugate bases
Weak Acids:
 HA(aq)  H+(aq) + A-(aq) … The acid only partially dissociates into ions
 For example: HF(aq)  H+(aq) + F-(aq)
 Includes: hydrofluoric, ethanoic, carbonic
 Weak acids have strong conjugate bases
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Strong and Weak Bases


Strong Bases:
 BOH(aq)  B+(aq) + OH-(aq) …..the base fully dissociates into ions

For example: NaOH(aq)  Na+(aq) + OH-(aq)

Includes: group (I) hydroxides, barium hydroxide

Strong bases have weak conjugate acids
Weak Bases:
 BOH(aq)  B+(aq) + OH-(aq) …..the base only partially dissociates into ions
 For example: NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)
 Includes: ammonia, amines
 Weak bases have strong conjugate acids
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Simulation

http://phet.colorado.edu/en/simulation/acid-base-solutions
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Distinguishing between strong and weak
acids and bases

The equilibrium has a profound effect on the properties of the
acid/base

Compared with strong acids of the same concentration, weak
acids will:






Have lower electrical conductivity (less mobile ions in from the same
moldm-3)
React more slowly (reactions depend on the concentration of H+
ions)
pH is higher (lower concentration of H+ ions)
Change pH more slowly when diluted
However, they neutralise the same volume of alkali
Weak bases follow a similar pattern
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Solutions
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Experiment

Complete the experiment here, investigating strong/weak
acids and bases

When you give explanations, focus on reasons in terms of
the equilibrium rather than just ‘because it is strong/weak’
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Key Points

Strong acids/bases dissociate fully into ions

Weak acids/bases only partially dissociate, forming an
equilibrium

The strong/weak character has a significant effect on the
chemical properties
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