Introduction to Chemical Equilibrium Many Chemical Reactions Are

Introduction to Chemical Equilibrium

Many Chemical Reactions Are Reversible • A chemical reaction is reversible when the reverse also happens as well as the forward reaction.

Aspirin Equilibrium I • The ancient Greeks chewed willow bark to relieve headaches. Willow bark contains salicyclic acid that relieves headaches but upsets stomach linings.

Aspirin Equilibrium II • To reduce stomach irritation, salicyclic acid is reacted with acetic acid to produce acetylsalicyclic acid (ASA), the active ingredient in aspirin. • With time, old ASA will revert to salicyclic acid and irritate the stomach again. • This illustrates that many reactions go forward and reverse.

A Definition of Equilibrium • Chemical equilibrium is a state in which the forward and reverse reactions are happening at the same rates.

Equilibrium Requires a Closed System • For equilibrium to occur, products must be retained for the reverse reaction (rather than escaping to the surroundings). • A closed system temporarily prevents substances from leaving or entering so that equilibrium can occur. Actually, NO system is ever closed since energy and matter can NOT be totally prevented from entering or leaving. • An Open System : Products leaving

The Equilibrium of NO 2 with N 2 O 4 • The gas, N 2 O 4 is colourless while the gas NO 2 is brownish-red. • Temperature affects the equilibrium. Adding heat makes more NO 2 while removing heat makes more N 2 O 4.

The Equilibrium of NO 2 with N 2 O 4 • One molecule of colourless N 2 O 4 can break apart into two molecules of reddish-brown NO 2 or the reverse reaction can take place.

Facts About Equilibrium • 1. Temperature affects equilibrium. • 2. A new equilibrium is established at a new temperature. • 3. When a system is at equilibrium, no macroscopic (visible) changes occur.

Temperature Changes and Equilibrium • When energy is added, equilibrium shifts in the direction of the endothermic reaction (equation 1 below). • When energy is removed, equilibrium shifts in the direction of the exothermic reaction (equation 2 below).

Temperature Changes and Le. Chatelier’s Principle • In this equilibrium, it seems that the system “tries” to undo what you as an experimenter do (One way of expressing Le Chatelier’s Principle). • When you add heat to the system, it tries to use up the heat energy you supply (the endothermic reaction becomes more active). The heat the system uses up is not enough to offset the heat added, though. • When you remove heat from the system, the system responds by making more heat through the exothermic reaction. The heat the system makes is not enough to offset the heat removed, though.

Spontaneous Reactions • A spontaneous reaction occurs by itself, without outside assistance. • Generally exothermic reactions are spontaneous and endothermic reactions are NOT spontaneous. Another way of saying this is to say that minimum enthalpy is favoured.

Facts showing that Enthalpy alone does NOT explain why reactions are spontaneous • Some endothermic reactions DO occur spontaneously. For example “chemical cold packs” in which an inner container is broken to release a reactant that spontaneously undergoes an endothermic reaction (absorbing heat or chilling its surroundings). • Exothermic reactions do not necessarily go to completion (the reverse reaction may proceed against the exothermic reaction).

The Other Factor Determining Spontaneous Reactions : Entropy • Entropy is a measure of the randomness in a system or substance. • Entropy is directly related to the number of ways that energy can be distributed within a system or substance.

Entropy = Disorder

More on Entropy • Entropy increases from solids to liquids to solutions to gases (these are states or phases of matter).

Reactions Favoured by Entropy • The direction of a reaction favoured by entropy is the side which has THE MOST PARTICLES (MOLES) OF THE MOST RANDOM PHASE. Randomness increases from solids to liquids to solutions to gases • The dissolving of ammonium nitrate in water is an endothermic reaction, but IS spontaneous. Why? By dissolving, the entropy increases to offset the rise in enthalpy.

Entropy and Enthalpy Together Determine Reaction Progress • 1. When both entropy and enthalpy point to the products, the reaction will be spontaneous. • 2. When both entropy and enthalpy point to the reactants, the reaction will not occur (nonspontaneous) • 3. When entropy and enthalpy point in opposite directions, equilibrium will occur.

Entropy and Enthalpy Together Determine Reaction Progress • 1. When both entropy and enthalpy point to the products, the reaction will be spontaneous. • 2. When both entropy and enthalpy point to the reactants, the reaction will not occur (nonspontaneous) • 3. When entropy and enthalpy point in opposite directions, equilibrium will occur.

Entropy and Enthalpy Together Determine Reaction Progress • 1. When both entropy and enthalpy point to the products, the reaction will be spontaneous. • 2. When both entropy and enthalpy point to the reactants, the reaction will not occur (nonspontaneous) • 3. When entropy and enthalpy point in opposite directions, equilibrium will occur. • 4 Au(s) + 3 O 2 (g) + 162 kj 2 Au 2 O 3 (s)

End of Equilibrium Presentation