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The Concept of Equilibrium
Chemical equilibriumoccurs when a reactionand its reverse reaction proceed at the same rate.
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A System at Equilibrium
Once equilibrium is achieved, the
amountof each reactant and product
remains constant.
The Concept of Equilibrium
As a system approachesequilibrium, both the
forward and reverse
reactions are occurring.
At equilibrium, the forward
and reverse reactions are
proceeding at thesame rate.
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Equilibrium Can Be Reached from
Either Direction
It does not matter whether we start with N2and H2or whether we start with NH3, we will have the same
proportions of all three substances at equilibrium.
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What Does the Value of KMean?
If K>>1, the reaction isproduct-favored; productpredominates at equilibrium.
Equilibrium lies toward right(products)
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Figure 6-7 p215
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Changes in Temperature'.8g#m;l
#t8$-; t h'A8$-; '.'Ah8$-; t lg#m8.;
The Haber ProcessIf H2is added to the system, N2will be consumedand the two reagents will form more NH3.
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The Haber ProcessThe transformation of nitrogen and hydrogen into ammonia(NH3) is of tremendous significance in agriculture, where
ammonia-based fertilizers are of utmost importance.
Introduction to Acids and Bases Acids: taste sour
Bases: taste bitter and feel soapy.
Arrhenius definition of acids & bases
Acids - increase [H+] in water
Bases - increase [OH-] in water
BrnstedLowry
Acid: Proton donor
Base: Proton acceptor
Amphiprotic: can be either an acid or base
Lewis Acid/Base
Acid: electron pair acceptor
Base: electron pair donor
Chapter 7 Acids and Bases
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Acid-Base Rxns Are Proton-Transfer Rxns
Example: H3O++ OH
-!2 H
2O
Brnsted Defn of Acid & Base
Acid= proton (H+) donor
must have removable (acidic) proton
(Consider: HCl(aq) + H2O(l)!H3O+(aq) + Cl-(aq) )
Base= proton (H+) acceptor
Must have LP (nonbonding) electrons
(Consider: NH3(aq) + H2O(aq) NH4+(aq) + HO-(aq))
Amphiprotic ions or molecules (either donor or acceptor)
(Examples: H2O; HSO4-; HCO3
-)
The H+ion in water
H+called a proton (is a hydrogen atom with no
electrons.
H+interacts strongly with H2O to form clusters.
The simplest cluster is H3O+(aq). Larger clusters
are H5O2+(aq)and H9O4
+(aq).
H3O+is called the hydronium ion
H+(aq)& H3O+(aq)are used interchangeably to
represent the species giving rise to acidic
properties.
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Conjugate Acid-Base Pairs
Whatever is left of the acid after the proton is donated is called its conjugate base.
Whatever remains of the base after it accepts a proton is called a conjugate acid.
HA(aq)+ H2O(l) H3O+(aq)+ A
-(aq)
Rule:The stronger an acid the weaker its conjugate base.
Relative Strengths of Acids
Acids differ in H
+
donor ability
differ in strength
Three categories of H-containing substances:
Strong acids
react 100% with H2O to form H3O+
Weak acids
react 1-99% with H2O to form H3O+
in practice, most react about 1-5%
Compounds with negligible (or no) acidity
react 0% with H2O to form H3O+
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acid base conj. base conj. acid
Rule: Equilibrium position always favors transferof proton from strongest acid to strongest base
Proton-transfer rxns are extremely fast
If we know something about the strength of an
acid we also know something about the strengthof its conjugate base.
Equilibrium for proton-transfer rxns
HX + B- X- + HB
Acid and Base Strength
Strong acids are completelydissociated in water.
Their conjugate bases areextremely weak.
Weak acids only dissociatepartially in water. Their conjugate bases are
weak bases.
Substances with negligibleacidity do not dissociate inwater.
Their conjugate bases areexceedingly strong.
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Acidic, Basic & Neutral Solns
Neutral soln has [H3O+] = [OH
-]
[H3O+] = 1.0 x 10-7M
Acidic soln has [H3O+] > [OH
-]
Basic soln has [H3O+] < [OH
-]
The pH Scale:
pH = -log[H3O+]
or [H3O+] = 10-pH
Neutral soln has pH = 7.00