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Equilibrium

Equilibrium. Brief Outline. What is reversible reaction? Examples of reversible reaction Dynamic Equilibrium Le Chatelier’s Principle The Haber Process. What is reversible reaction?.

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Equilibrium

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  1. Equilibrium

  2. Brief Outline • What is reversible reaction? • Examples of reversible reaction • Dynamic Equilibrium • Le Chatelier’s Principle • The Haber Process

  3. What 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).

  4. Reversibe Reactions HgO HgO HgO Hg HgO HgO HgO O2 HgO Hg HgO HgO Hg Hg HgO O2 2 HgO (s) 2 Hg (l) + O2 (g) 2 Hg (l) + O2 (g) 2HgO (g) Mercury and oxygen combine to form mercury oxide just as fast asmercury oxide decomposes into mercury and oxygen

  5. 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.

  6. Dynamic Equilibrium Open System (No Equilibrium) Evaporation Closed System Evaporation Liquid Gas Liquid Gas Liquid Gas (No Equilibrium) (Equilibrium) (No Equilibrium)

  7. Dynamic Equilibrium Dynamic equilibrium is the state in a reversible reaction in which the forward and reverse reactions are occurring at the same rate Ag + Cl - Ag + + Cl - AgCl (s) Cl - Ag + Chemical Equilibrium AgCl (s) Rate of Precipitation = Rate of Dissolving HC2H3O2 (aq) H+ + C2H3O2- Rate of Dissociation (ionization) = Rate of Association HC2H3O2 H + H + C2H3O2- HC2H3O2 C2H3O2-

  8. Dynamic 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) . Dynamic equilibrium vs Static Equilibrium

  9. Static Equilibrium Dynamic Equilibrium Dynamic equilibrium vsStatic Equilibrium

  10. Characteristics of Dynamic Equilibrium • The 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).

  11. Characteristics of Dynamic Equilibrium • The 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.

  12. H2 + I22 HI Synthesis of Hydrogen Iodide 2 HI H2+ I2Dissociation of Hydrogen Iodide 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.

  13. H2 + I2 2HI The 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. For more information, Click Here

  14. We have seen that when the rate forward equals the rate backwards in a chemical reaction, the system reaches a state of equilibrium. But, How far does the reaction go? What is the final concentration of reactants and products?

  15. The adjacent graph shows the changes in the reaction rates of the forward and backward reactions: A + B C + D • Initially (t = 0), [A] and [B] were maximum, while [C] and [D]were zero. • The rate of the forwardreaction decreasesas A and B are used up. • The rate of the reverse reactionincreases as C and D are formed. • Equilibrium is attained when the two rates become equal • [A], [B], [C], and [D] remain constant at equilibrium.

  16. 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? Chemical Equilibrium Consider this reaction:

  17. N2O42NO2 Low T High T Get time progression Check silberberg • The final state depends on: • The chemical nature of the reactants and products • The conditions of the system (temperature, pressure, volume).

  18. aA + bB cC + dD [C]c[D]d [A]a[B]b Kc = The Equilibrium Constant For a reaction of the type the following is a CONSTANT (at a given T) The equilibrium expression for this reaction would be Kc : equilibrium constant [C] and [D] : concentration of products [A] and [B] : concentration of reactants If Kcis known, then we can PREDICT concentrations of products or reactants

  19. Your Turn Equilibrium mix What is the equilibrium expression for the reaction? .

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

  21. Your Turn Equilibrium mix What is the equilibrium expression for the reaction? . .

  22. Your Turn Equilibrium mix . Keq ~ 22/(6 x 4) = 0.17

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

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

  25. PbCl2 (s) Pb2+ (aq) + 2 Cl−(aq) The Concentrations of Solids and Liquids Are Essentially Constant Therefore, the concentrations of solids and liquids do not appear in the equilibrium expression Kc = [Pb2+] [Cl−]2

  26. CaCO3 (s) CO2 (g) + CaO(s) As long as some CaCO3 or CaO remain in the system, the amount of CO2 above the solid will remain the same.

  27. What Are the Equilibrium Expressions for these equilibria? • 2SO2(g) + O2(g) 2SO3(g) • Fe3+(aq) + SCN-(aq) [Fe(SCN)]2+(aq) • 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

  28. Example Give the expression of the equilibrium constant N2 (g) + 3 H2 (g) 2 NH3 (g) Given [N2]=0.1M,[H2]=0.125M, [NH3]=0.11M, find the equilibrium constant for the reaction.

  29. Example Find the relationship between K1 and K2, the equilibrium constants of these two reactions: 2A + 2B 2C K1 C A + B K2 What can you deduce about the equilibrium constant for the forward and backward reaction?

  30. Example The following reaction is an esterification reaction producing ethyl ethanoate. CH3CO2H(l) + C2H5OH(l) CH3CO2C2H5(l) + H2O(l) The value of Kc at 250C is 4.0. This equilibrium can be approached experimentally from the opposite direction. What is the value for Kc for the reaction, the hydrolysis of ethyl ethanoate. CH3CO2C2H5(l) + H2O(l) CH3CO2H(l) + C2H5OH(l)

  31. Example The following reaction takes place at 460ºC, where the equilibrium constant K has a value of 85. SO2(g) + NO2(g) NO (g) + SO3 (g) At a certain moment, the concentrations of the reactant and products were measured to be: [SO2] = 0.04, [NO2] = 0.5M, [NO] = 0.3M, [SO3] = 0.02M (a) Is this system at equilibrium? (b) If not, in which direction must the reaction go to reach equilibrium?

  32. Le Chatelier’s Principle • states that when a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise the change. • The possible changes in conditions include • Changes in concentration o f either the reactants or products. • Changes in pressure for gas phase reactions • Changes in temperature • Presence of a catayst • Has industrial significance as it allows chemists to alter the reaction conditions to produce an increased amount of product, hence increase the profitability of the chemical process.

  33. Concentration • A change in the concentration of any component in the equilibrium mixture will result in the shift in position of the equiibrium. Example • If one of the products is removed from the mixture, the equilibrium will shift to the right to replace the substance removed. In the reaction below, what happens when • water is removed / added ? • ethanol , C2H5OH is added / removed ? CH3CO2H(l) + C2H5OH(l) CH3CO2C2H5(l) + H2O(l)

  34. Pressure • The equilibrium positions of certain gas phase reactions are affected by changes in external pressure. • These are reactions where there is a change in the amount (number of moles) of substance during the course of the reaction. Example • An increase in pressure will shift the position of the equilibrium towards the side that involves fewer molecules. In the reaction below, what happens when pressure is • increased ? • decreased ? N2O4(g) 2NO2(g) colourless brown

  35. Temperature • A increase in temperature will always favour the endothermic process and will shift the equilibrium towards that direction. Lowering the temperature will favour the exothermic process. • A change in temperature will alter the Kc. Example In the reaction below, what happens when the temperature is • increased ? • decreased ? N2O4(g) 2NO2(g) ΔHƟ = +57kJmol-1 colourless brown

  36. Example Draw a table showing how the position of equilibrium in the reaction A,B and C would be affected by the followng changes: • Increased temperature • Increased pressure Reaction A : interconversion of oxygen and ozone 3O2(g) 2O3(g) ΔHƟ = +284 kJmol-1 Reaction B : the reaction between sulfur dioxide and oxygen in the presence of a platinium/rhodium catalyst. SO2(g) + 2O3(g) SO3(g) ΔHƟ = -197 kJmol-1 Reaction C : the reaction between hydrogen and carbon dioxide. CO2(g) + 2H2(g) CO(g) + H2O(g) ΔHƟ = +41 kJmol-1

  37. Catalyst • A catalyst has no effect on the position of an equilibrium at a particular temperature as increases the rate of the forward and the reverse reaction equaly. • It decreases the time it takes the system to reach equilibrium.

  38. Conditions affecting the position of Equilibrium An equilibrium system opposes changes

  39. Ammonia and the Haber Process

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