Chapter 16: Equilibrium
in Acid-Base Systems
16.2b: Predicting Acid-Base Reactions
Predicting Rxtn. Equilibria
 B-L theory does not explain why certain
protons are attracted more than others
 we must look at relative strengths of acids
and bases
 both collision-reaction and Bronsted-Lowry
theory deal with proton transfer
 we use a combination of concepts to help us
predict which acid-base rxtn. is most
dominant and observable in a system where
multiple reactions can be taking place
Predicting Rxtn. Equilibria
con’t
 Collision-rxtn theory
 in an A-B system
collisions of all entities
are always happening
 however the only
significant rxtn is that
between the SA and SB
 other proton transfer
has a negligible effect
on the system
 Bronsted-Lowry theory
 a proton will only transfer if
an acid collides with a
base that is a stronger
proton attractor
 a proton can transfer
multiple times until with the
strongest base
 once the transfer from SA
to SB has occurred, proton
remains
Guide for predicting reaction
Restrictions for proton transfer in
aqueous solution
 H3O+ is strongest acid
Assume unless otherwise informed
 A-B system eqtn. represents
entity in aqueous sol’n, all
a single proton transfer
+
SA are written as H3O
between entities in a 1:1
 OH- is strongest base entity
mole ratio
in aqueous sol’n,
 the SA and SB are both
exceptions are ionic oxides
present and reacting in
where the oxide ion can be
approx. equal concentrations
written as OH (K2O)
 no entity can react as a
refer to acid-base strength table
base if it is a weaker base
in data booklet and LSM 16.2a
than water, conj. base of
summary 2 for 5 step method
SA are not bases
Relative equilibrium positions
Products favoured
SA
>50%
<--->
+
SB
Relative equilibrium positions
Rectants favoured
SB
+
SA
<50%
<--->
Example
 Predict the products and direction in which
the rxtn. between sulphate ions and acetic acid
will proceed.
SO4 2- + CH3COOH <---> ? + ?
1)List all entities
SO4 2- , CH3COOH , H20
2)Label as B-L acid/base according to A-B strength table
SO4 2- , CH3COOH , H20
B
A
A,B
3)Identify SA and SB present
CH3COOH -- SA
SO4 2- -- SB
Example con’t
4)Write eqtn. showing transfer of one proton from SA
to SB and predict conjugates
SO4 2- + CH3COOH <---> HSO4- + CH3COOSB
SA
CA
CB
5)Predict approx. position of the equilibrium
-since SO42- is above CH3COOH, the equilibrium is to
the left and the reactants are favoured
- <50% is used over the equilibrium arrows
<50%
SO4 2- + CH3COOH <----> HSO4- + CH3COO-
Homework
 Textbook p.731 #9,12,13,15 p. 735 #3,4
 LSM 16.2a summary 2