Chapter 9.9, Problem 3QQ

Interpretation Introduction

Interpretation:

From the given options, the effect of temperature change for the following reaction has to be chosen.

    ${\text{H}}_{\text{2}}{\text{(g)+F}}_{\text{2}}\text{(g)}\underset{}{\to }\text{2HF(g)+heat}$

Concept Introduction:

Le Chatelier’s principle:

If some forces applied, the system at equilibrium will get disrupted.  This change in equilibrium can be due to the change in pressure or temperature.  The change in reactant concentration can also disrupt the equilibrium.  Over time, the forward and backward reaction become equal and will attain a new equilibrium.  The equilibrium will shifts to right, if more products are formed and the system will shifts to left, if more reactants are formed.

The principle states that if some stress is applied to the system at equilibrium, the system will adjust itself in a direction which reduces the stress.

There are four types of stress or changes which affects the system:

Concentration Changes:

Addition of reactant or product or removal of reactant or product from a system at equilibrium will affects the equilibrium.  If some reactant is added to a system at equilibrium, then the equilibrium will shifts to the product side, so that the added reactant get consumed.  If product is added then the equilibrium will shift towards left side.

Example:

${\text{N}}_{\text{2}}{\text{(g)+3H}}_{\text{2}}\text{(g)}\stackrel{}{\rightleftharpoons }{\text{2NH}}_{\text{3}}\text{(g)}$

If ${\text{N}}_{\text{2}}$ is added then the equilibrium will shift towards right.

If ${\text{NH}}_{\text{3}}$ is added then the equilibrium will shift towards left.

If ${\text{N}}_{\text{2}}$ is removed then the equilibrium will shift towards left

Temperature Changes:

Heat is one of the products in exothermic reaction and heat is used up in endothermic reaction.

Consider an exothermic reaction;

    ${\text{H}}_{\text{2}}{\text{(g)+F}}_{\text{2}}\text{(g)}\underset{}{\rightleftharpoons }\text{2HF(g)+heat}$

If heat is added up, then the reaction will shift to left so that the amount of heat will decrease.

Lowering the temperature will make the reaction to shift towards right.

Consider an endothermic reaction;

    ${\text{Heat+2CO}}_{\text{2}}\text{(g)}\underset{}{\rightleftharpoons }{\text{2CO(g)+O}}_{\text{2}}\text{(g)}$

Increase in temperature will shift the reaction towards right.  If heat is added up, then the reaction will shift towards right.

Pressure Changes:

Only the gaseous reactants and products get affected by the pressure change.

Consider the reaction:

${\text{2H}}_{\text{2}}{\text{(g)+O}}_{\text{2}}\text{(g)}\underset{}{\to }{\text{2H}}_{\text{2}}\text{O(g)}$

3 moles of reactant gives 2 moles of product.

Increase in pressure will shift the reaction towards the side which have fewer molecules.