Chemical equilibrium
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Chemical Equilibrium. Many chemical reactions are reversible and the reaction will proceed until chemical equilibrium is reached. Characteristics of Chemical Equilibrium. The rate of the forward reaction occurs at the same rate as the reverse reaction.

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Chemical Equilibrium

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Chemical Equilibrium


  • Many chemical reactions are reversible and the reaction will proceed until chemical equilibrium is reached.


Characteristics of Chemical Equilibrium

  • The rate of the forward reaction occurs at the same rate as the reverse reaction.

  • Concentrations of reactants and products remain constant at equilibrium.

  • Attainment of equilibrium is not immediate and often requires an extended period of time.


Equilibrium Constant

  • Because the concentration of reactants and products are constant (not equal), we can calculate this ratio. Consider:

    aA + bB  cC + dD

    where the lowercase letters represent the coefficients of the balanced equation.


Equilibrium constant, Keq, is found by:

products

Keq = [C]c[D]d

[A]a[B]b

  • Square brackets, [ ], indicate molar concentrations.

  • Each concentration is raised to the power of its balancing coefficient.

reactants


  • Product concentrations are in the numerator while reactant concentrations are in the denominator.

  • The expression for Keq is also known as the law of chemical equilibrium.


Values of Keq

  • The value of the equilibrium constant tells us something about the position of the equilibrium.

  • If Keq is very large, the reaction favors the formation of products (most of the reactants will be converted to products).

  • If Keq is very small, the reaction favors reactants (very little reactants are converted to products).


  • If Keq is between 0.01 and 100, neither reactants nor products are strongly favored.


Once a reaction has reached chemical equilibrium, it will remain at equilibrium as long as it is undisturbed.


Henri LeChatelier

Le Châtelier’s Principle

Shifts in Equilibrium


Le Châtelier’s Principle

“If a change is imposed, the system will adjust to counteract the effect of the change and then establish a new equilibrium.”


3 Types of Stress

  • 1) CHANGE IN CONCENTRATION OF REACTANTS OR PRODUCTS ( no change to Kc)

  • 2) CHANGE IN TEMPERATURE (Kc will change)

  • 3) CHANGE IN PRESSURE (GASES ONLY)

    (no change to Kc)


1) Changes in Concentration


Consider the reaction at equilibrium:

N2(g) + O2(g) 2NO(g)

  • If we add more O2, the equilibrium will shift to the right, meaning O2 will react with N2 to produce more NO.

  • The result is that the concentration of O2 decreases, counteracting the effect of the change.


In other words,

  • a reaction will shift away from a substance added and toward a substance removed from an equilibrium mixture.

  • What would happen to: 3H2 + N2 2NH3

  • if we added H2?

    Shift to the right

    b) if we added NH3?

    shift to the left

    c) if we removed N2?

    shift to the left


Adding or removing substances causes a shift in the equilibrium, but it does not change the value of Keq.


2) Changes in Temperature


When the temperature is raised, the reaction shifts in the direction away from the energy term.

Example 1: Temperature increased for this reaction will shift in the direction of the products:

2SO3(g) + 197kJ 2SO2(g) + O2(g)

Kc is increased

Example 2: Temperature increased for this reaction will shift in the direction of the reactants:

C2H4(g) + 3O2(g)  2CO2(g) + H2O(g) + 1250.9kJ

Kc is decreased


  • When the temperature is lowered, the reaction shifts in the direction toward the energy term.

    Example 1: Temperature decreased for this reaction will shift in the direction of the reactants:

    2SO3(g) + 197kJ 2SO2(g) + O2(g)

    Kc is decreased

    Example 2: Temperature decreased for this reaction will shift in the direction of the products:

    C2H4(g) + 3O2(g)  2CO2(g) + H2O(g) + 1250.9kJ

    Kc is increased


  • The value of Kcdoes change when the temperature is changed.


Given the reaction:

PCl5 PCl3 + Cl2+ energy

  • If the temperature is increased, what effect will that have on:

  • the position of the equilibrium?

    shifts to the left

  • the value of Kc

    decreases


3) Changes in Pressure


  • An increase in pressure on a gaseous system always favors a shift towards the side with the lowest total number of gaseous particles and vice versa.

  • Example: Pressure increased for this reaction will shift in the direction of the reactants:

    2SO3(g)  2SO2(g) + O2(g)

    (2 molecules)(3 molecules)


To increase pressure:

Decrease volume of container (at a constant temp)

To decrease pressure:

Increase volume of container (at a constant temp)


No Effect:

  • If the total number of molecules in reactants and products is equal, pressure changes will not affect the system.

  • If a non-reacting (inert) gas is added to the reaction container


SUMMARY:

  • Changes in concentration: a reaction shifts away from a substance added and toward a substance removed

  • Changes in temperature: when temperature is raised, the reaction shifts away from the energy term and vice versa

  • Changes in pressure: when pressure is increased on a gaseous system, the reaction shifts towards the side with the lowest total number of gaseous particles and vice versa.


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