Equilibrium

EquilibriumBrief OutlineWhat is reversible reaction?Examples of reversible reactionDynamic EquilibriumLe Chateliers PrincipleThe Haber Process

2What is reversible reaction?A reversible reaction is a chemical reaction in which the products can be converted back to reactants under suitable conditions. The reaction involves two reactions.There is a "forward" reaction and a mirror image "reverse" reaction.A reversible reaction is shown by the sign ( ) a half-arrow to the right (forward reaction) and a half-arrow to the left (reverse reaction).

3HgOHgOHgOHgOHgOHgOHgOHgO2HgHgHgOHgHgOO2HgO2 HgO (s) 2 Hg (l) + O2 (g) 2 Hg (l) + O2 (g) 2HgO (g)Reversibe Reactions Mercury and oxygen combine to form mercury oxide just as fast as mercury oxide decomposes into mercury and oxygen Upon heating, mercury (II) oxide decomposes to mercury (Hg) and oxygen (O2) [ Equation 1]:Under the same conditions, mercury (Hg) and oxygen (O2) recombine form mercury (II) oxide again [Equation 2]:Mercury and oxygen combine to form mercury oxide just as fast as mercury oxide decomposes into mercury and oxygen

4 2 HgO (s) 2 Hg (l) + O2 (g) Both reactions continue to occur, but there is no net change in the composition of the system.The amounts of mercury(II)oxide, mercury(Hg), and oxygen (O2) remain constant. There is a state of equilibrium between two chemical reactions.

6Dynamic EquilibriumEvaporation(No Equilibrium)EvaporationLiquid Gas(No Equilibrium)(No Equilibrium)Liquid GasLiquid Gas(Equilibrium)Closed SystemOpen SystemDynamic EquilibriumAg + + Cl - AgCl (s)ChemicalEquilibriumAg + Cl -Cl - Ag +AgCl (s)Rate of Precipitation = Rate of DissolvingHC2H3O2 (aq) H + + C2H3O2 -Rate of Dissociation (ionization) = Rate of AssociationHC2H3O2H + C2H3O2 -H +HC2H3O2C2H3O2 -Dynamic equilibrium is the state in a reversible reaction in which the forward and reverse reactions are occurring at the same rateDynamic equilibrium vs Static EquilibriumDynamic Equilibrium:Child ascending escalator at the same rate the escalator descends. At the balance point (the equilibrium position), the child and escalator are moving at the same rate in opposite directions.Static Equilibrium:Children on see-saw.At the balance point (the equilibrium position), there is no movement of the children or the see-saw (the opposing processes) .

8Dynamic equilibrium vs Static Equilibrium

Dynamic Equilibrium

Static Equilibrium 9Characteristics of Dynamic EquilibriumThe concentrations of the reactants and products (macroscopic properties) remain the same but the reactions don't stop!The reactants are still reacting to form the product and the product is still being converted back to the reactants (microscopic processes).

10Characteristics of Dynamic EquilibriumThe rate at which the reactants change into products is exactly equal to the rate at which the products change back to the original reactants. Thus it appears as if the reaction has stopped, but in actual fact, it is still going on. Dynamic equilibrium can only be achieved in a closed reaction (i.e an enclosed reactor)A closed system is one in which there is absolutely no loss/gain of material to/from the surroundings. An open system may allow matter to escape or to enter.

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These two sets represent the same chemical reaction system, but with the reactions occurring in opposite directions. Most importantly, note how the concentrations of all the components are identical when the system reaches equilibrium. 2 HI H2 + I2 Dissociation of Hydrogen Iodide H2 + I2 2 HI Synthesis of Hydrogen Iodide12The equilibrium state is independent of the direction from which it is approached. Whether we start with an equimolar mixture of H2 and I2 (left) or a pure sample of hydrogen iodide (shown on the right, using twice the initial concentration of HI to keep the number of atoms the same), the composition after equilibrium is attained (shaded regions on the right) will be the same. H2 + I2 2HIFor more information, Click Here13How far does the reaction go? What is the final concentration of reactants and products?We have seen that when the rate forward equals the rate backwards in a chemical reaction, the system reaches a state of equilibrium. But,

The adjacent graph shows the changes in the reaction rates of the forward and backward reactions:A + B C + DInitially (t = 0), [A] and [B] were maximum, while [C] and [D] were zero. The rate of the forward reaction decreases as A and B are used up.The rate of the reverse reaction increases as C and D are formed. Equilibrium is attained when the two rates become equal

[A], [B], [C], and [D] remain constant at equilibrium.15Chemical Equilibrium

If NO2 is reddish brown and N2O4 is colorless:What is happening here? What properties are changing?What is happening over time? After a long time?Consider this reaction:The final state depends on: The chemical nature of the reactants and products The conditions of the system (temperature, pressure, volume).Get time progressionCheck silberberg

N2O4 2NO2Low T High TThe Equilibrium ConstantThe equilibrium expression for this reaction would beKc = [C]c[D]d[A]a[B]baA + bBcC + dD

For a reaction of the type

the following is a CONSTANT (at a given T)If Kc is known, then we can PREDICT concentrations of products or reactantsKc : equilibrium constant[C] and [D] : concentration of products[A] and [B] : concentration of reactants18Your Turn

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Equilibrium mix

What is the equilibrium expression for the reaction?Tier 1

Keq ~ 92/(1 x 1) = 81Your TurnUA GenChemYour Turn.

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Equilibrium mix

What is the equilibrium expression for the reaction?Tier 1Your Turn.

Equilibrium mixKeq ~ 22/(6 x 4) = 0.17What Does the Value of K Mean?If K >> 1, the reaction is product-favored; product predominates at equilibrium.

23What Does the Value of K Mean?If K >> 1, the reaction is product-favored; product predominates at equilibrium.If K