chemical equilibrium

1. Higher College of Technology
Department of Applied Science
Chemistry Section
CHEM1202/ Chemistry I
Chemical Equilibrium-Chapter 2

2. • Generally speaking, an equilibrium is a state
of balance.
• A static equilibrium is one in which there is
no motion.
2.1 Introduction
Static and Dynamic Equilibrium

3. • A dynamic equilibrium
is one in which there is
motion despite there
being no net change.
• A chemical equilibrium
is an example of a
dynamic equilibrium.

4. •A process in which
products can react back
to give reactants.
(OR)
•A reaction which can
take place in both
directions.
•A process in which
products cannot react to
give back reactants.
(OR)
•A reaction which can
proceed only in one
direction.
2.1 Introduction

5. Why only some
reactions are
reversible?
2.1 Introduction

6. Reversible Reactions
 Consider the reaction
 A + B C + D
Shows both reactions happen at the same time.
They are A + B C + D (Forward reaction
C + D A + B (Backward reaction)
2.1 – INTRODUCTION

7. As the reaction progresses
[A] and [B] decreases to a constant,
[C] and [D] increases from zero to a
constant.
When [A] + [B] and [C] + [D] are constant,
equilibrium is achieved.
At equilibrium , concentration of reactants is not equal to
concentration of products. Only the rates of reactions become equal.
2.1 – INTRODUCTION

8. Chemical equilibrium- Illustration
Consider decomposition of N2O4
2.1 – INTRODUCTION

9. Features of Equilibrium
 1. There is no change in the composition of
reaction mixture with time.

10. 2. Rate of forward reaction = Rate of backward reaction.
Features of Equilibrium
2.1 – INTRODUCTION

11. 2.2- The Equilibrium constant
For the reaction aA +bB cC +dD
Equilibrium constant K=[C]c[D]d
[A]a[B]b
Where a, b, c and d are the number of moles in the
balanced chemical equation.

12. 2.3- Characteristics of Equilibrium constant
1.The value of K is independent of the initial concentrations
of the reactants.
Here the term pressure is used instead of concentration.

13. 2.The value of K indicate the extent to which forward reaction
is complete at a particular temperature.
2.3- Characteristics of Equilibrium constant

14. 2.3- Characteristics of Equilibrium constant
Equilibrium constant can be written in two ways. Kp and Kc
Kc –for solutions and gases
Unit of equilibrium
concentration
is mol/dm3
Kp used only for gases
Unit of equilibrium
concentration is atm
or Pascal

15. • Examples:
N2(g) + 3H2(g) ⇄ 2NH3(g); Kc = [NH3]2
[N2][H2]3
PCl5(g) ⇄ PCl3(g) + Cl2(g); Kc = [Cl2]2 [PCl3]
[PCl5]
2.3-Equilibrium constant- Kc

16.  Consider the following reaction involving gases:
2SO2(g) + O2(g) ⇄ 2SO3(g)
Kp =
)(P)(P
)(P
O2
2
SO2
2
SO3
2.3-Equilibrium constant- Kp

17.  Consider the reaction: 2SO2(g) + O2(g) ⇄ 2SO3(g)
 Kc = and Kp =
 Assuming ideal behavior,
 where PV = nRT and P = (n/V)RT = [M]RT
 and PSO3 = [SO3]RT; PSO2 = [SO2]RT; PO2 = [O2]RT

18. In general, for reactions involving gases such
that,
aA + bB ⇄ cC + dD
where A, B, C, and D are all gases,
and a, b, c, and d are their respective
coefficients,
Kp = Kc(RT)Dn
and Dn = (c + d) – (a + b)
(In heterogeneous systems, only the
coefficients of the gaseous species are
counted.)