Equilibrium. Dynamic Equilibrium, Position of Equilibrium, Liquid-Vapor Equilibrium, Equilibrium Law January 2015

Transcription

1 Equilibrium Dynamic Equilibrium, Position of Equilibrium, Liquid-Vapor Equilibrium, Equilibrium Law January 2015

2 Equilibrium Review What is equilibrium? Features of equilibrium the rate of the forward reaction is equal to the rate of the reverse reaction It is dynamic It occurs in a closed system Concentrations of products and reactants remain CONSTANT There is no change in properties (color/density - dependent on concentration) Can be reached from either direction

3 The Position of Equilibrium Measured by the equilibrium constant (Kc ) Example - H2(g) + I 2(g) 2HI (g) When finding the Kc - the equilibrium constant is equal to the concentration of the products, divided by the concentration of the reactants

4 In General... aa + bb cc + dd equilibrium constant expression is: ([C] c [D] d )/ ([A] a [B] b ) = K c Kc can be found by substituting values of concentration of the products and reactants at equilibrium

5 Back to our Example: H2(g) + I 2(g) 2HI (g) [HI] 2 / ([H 2 ][I 2 ])

6 Another for fun!! Find the equilibrium expression for the following reaction: Cu 2+ (aq) + 4NH 3(aq) [Cu(NH 3 ) 4 ] 2+ (aq)

7 Magnitude of K c Think basic division... If your reaction produces a lot of products and leaves very little reactants behind, will K c be a high or low value? If your reaction does not produce much product and it stays in the reactant form, what will happen to K c?

8 Magnitude of K c H2(g) + I 2(g) 2HI (g) K c = 2 H2(g) + Br 2(g) 2HBr (g) K c = H2(g) + Cl 2(g) 2HCl (g) K c = If Kc >>1, the reaction is said to go to completion (very high conversion of reactants to products) If Kc <<1, the reaction hardly proceeds. The magnitude of Kc does NOT tell you how fast the reaction reaches equilibrium

9 Disruption of Equilibrium... Who do we need to consider when an equilibrium shifts? Le Chatelier s Principle a system at equilibrium when subjected to a change will respond in such a way as to minimize the change

10 Le Chatelier s Principle... Change in Concentration Like you didn t get enough of Ammonia last year... N2(g) + 3H 2(g) 2NH 3(g) Adding a reactant will cause a shift towards the products (making more ammonia), while using up some of the other reactant Removing a product will cause a shift toward the products, using up reactants in the process

11 Changes in Pressure An increase in pressure of a system, favours the side with the fewer number of gas particles A decrease in pressure will favour the side with more gas particles CO(g) + 2H 2(g) CH 3 OH (g) - which way will it shift with an increase?

12 Pressure and Concentration Changes in Pressure and Concentration will cause a different position of equilibrium to be established BUT!...the Kc will not change

13 Changes in Temperature Dependent on H - does the reaction release or absorb energy? Youtube vid! 2NO2(g) N 2 O 4(g) H = -24 kj/mol brown colorless

14 Changes in Temperature N2(g) + O 2(g) 2NO (g) H = +181 kj/mol What happens with an increase in temperature here? Unlike changes in pressure and concentration, if the temperature is changed in an equilibrium there will be a change in the K c value H T K c exothermic negative increase negative decrease endothermic positive increase positive decrease

15 Addition of a Catalyst What does a catalyst do in a reaction? lowers the activation energy Since the catalyst decreases the activation energy for both the forward and reverse reaction, it will have no effect on the position of equilibrium. Therefore, K c will not change.

16 Industrial Applications What aspects are important to creating a product in the manufacturing field? Yield - how much you make Rate - how long it takes to complete It would be really nice if you were able to get a 95% yield all the time for your manufacturing process, but if it took you several years to do it...would it be worth it?

17 The Haber Process N2(g) + 3H 2(g) 2NH 3(g) H = -93kJ mol -1 What information can you gather from this equation? All products and reactants are gases, fewer molecules on product side of the reaction Forward reaction is exothermic, reverse is endothermic

18 Applying Le Chatelier s to the Haber Process... Concentration Increase concentration of H2 & N 2 collect NH 3 as it is made Pressure Increase Pressure Temperature Catalyst Favoured by lower temperature, but too slow and it becomes uneconomical C is used Doesn t effect position of equilibrium, but can force the reaction to occur more quickly

19 Testing Hint... When discussing the Haber process, focus on the REASONS for choosing the conditions - explained using concepts of equilibrium AND kinetics

20 The Contact Process - Formation of Sulfuric Acid - H 2 SO 4 H 2 SO million tons are manufactured every year highest production of any chemical in the world Contact Process 3 simple reactions combustion of sulfur - S (s) + O 2(g) SO 2(g) Oxidation of sulfur dioxide - 2SO 2(g) + O 2(g) 2SO 3(g) Combination of SO 3(g) with water (really violent reaction, substitute is below) SO 3(g) + H 2 SO 4(l) H 2 S 2 O 7(l) + H 2 O (l) 2H 2 SO 4(aq)

21 The Contact Process - Formation of Sulfuric Acid - H 2 SO 4 It has been determined that the second step is the rate determining step - if Le Chatelier s is applied to that step, we can maximize yield 2SO 2(g) + O 2(g) 2SO 3(g) H = -196 kj mol -1 What can be done to increase the yield of this reaction?

22 Liquid - Vapour Equilibrium Vapor Pressure Jan 2014

23 Vapor Pressure - the pressure exerted by a vapor on its liquid Remember Table H from last year??? Rate of Evaporation vs. Rate of condensation Where is equilibrium established?

24 Factors influencing vapor pressure Does NOT depend on: surface area of liquid volume of either the liquid or the vapor Equilibrium may be achieved more quickly, but the position of the equilibrium will not be changed. ie. 50 C will have a vapor pressure of 29.5kPa no matter the size of the container it is in

25 Factors influencing vapor pressure Does depend on: Temperature Nature of the substance

26 Temperature and Vapor Pressure As shown in the picture, the section in blue contains the particles that have enough energy to escape the liquid and become a vapor What happens to the graph as the temperature increases?

27 Temperature and Vapor Pressure The temperature/vapor pressure relationship is NOT a linear one in other words, the vapor pressure increases higher temperature What happens when the vapor pressure of a liquid equals the atmospheric pressure? The liquid boils! The bubbles forming inside the container can overcome the pressure above the liquid and reach the surface

28 Boiling Point Since the boiling point depends on external pressure... In Hilton, where we are approximately at sea level, the atmospheric pressure is 1.00 x 10 5 Pa and water boils at about 100 C How long does it take to soft boil an egg? On the top of Mt. Everest (the world s highest point), the pressure is 2.64 x 10 4 and water boils at 69 C. How long will it take to soft boil that same egg? Denver? High Altitude Recipes? Pressure Cookers?

29 The Cooling Effect... What happens as you get out of the shower in the morning? (especially in the winter!) Why does this occur? Highest energy molecules of water are evaporating first, leaving the ones with lower kinetic energy behind

30 Vapor Pressure & The Nature of the Substance What does the nature of the substance have to do with vapor pressure? IMFs! Dipole-Dipole Hydrogen Bonding Van Der Waals

31 IMFs and Vapor Pressure Substances with stronger IMFs... Have a lower vapor pressure than other substances at the same temperature Does this affect boiling point? Normal Boiling Points Water C Ethanol C Why is this true?

32 More on Vapor Pressure Volatile - substances with high vapor pressure, the liquid quickly evaporates have weak intermolecular forces low boiling points Usually have only weak VDW forces, non-polar molecules

33 Enthalpy of Vaporization - the energy needed to convert one mole of a substance in its liquid state into one mole of a gas - 298K X(l) X (g) H = enthalpy of vaporization - kj mol -1 Is this process endothermic or exothermic? X(g) X (l) - conensation Is this process endothermic or exothermic? - (hint: it s the opposite!)

34 In Summary... Stronger IMFs higher enthalpy of vaporization lower vapor pressure higher boiling point Weaker IMFs lower enthalpy of vaporization higher vapor pressure lower boiling point

35 The Equilibrium Law Calculating Equilibrium law from known concentrations Jan 2014

36 Example... Hydrogen can be prepared by the combination of carbon monoxide and 500 C. At equilibrium, the concentrations in the reaction mixture were found to be: CO mol dm -3 H mol dm -3 H mol dm -3 CO mol dm -3 What is a good place to start?...

37 Another... Sometimes, you may only get data on the initial condition and some small piece of data on the equilibrium... Write the Balanced Equation Write Values for each of the concentrations for initial, change and equilibrium Initial - what was originally in the flask (assume [product] = 0 unless stated otherwise) Change - represents the amount that reacts to reach equilibrium (subtract reactants, add products - as same ratio in the reaction) Equilibrium - can be calculated ([equilibrium] = [initial] +/- the change

38 Example... A Student placed 0.20 mol of PCl3(g) and 0.10 mol of Cl 2(g) in a 1 dm 3 flask at 350 C. The reaction, which produced PCl 5(g) was allowed to come to equilibrium at which time, it was found that the flask contained 0.12 mol of PCl 3(g). What is the value of K c in this reaction?

39 Another... The oxidation of NO to form NO2 occurs during the formation of smog. When 0.60 mol of NO was reacted with 0.60 mol O 2 in a 2 dm 3 container at 500 C, the equilibrium mixture was found to contain 0.20 mol of NO 2. Calculate the equilibrium constant for the reaction at this temperature.

40 Calculating Concentrations from the Equilibrium constant The reaction CO(g) + 2H 2(g) CH 3 OH (g) has K c = at 350K. If the concentrations at equilibrium are: [CO] = mol dm -3 [H2 ] = mol dm -3 What is the equilbrium concentration of CH3 OH?

41 More! It gets more difficult if we need to calculate equilibrium concentrations given the K c and initial concentrations... The equilibrium constant, Kc, for the reaction: SO3(g) + NO (g) NO 2(g) + SO 2(g) was found to be 6.78 at 400K. If the initial concentrations of NO and SO 3 were both 0.03 mol dm -3, what would be the equilibrium concentration of each component?

42 When K c is very small... What happens to the concentrations of the reactants and products? (Basically, which way is the reaction favoring?) The change in concentration of the reactants is very small (close to zero) [reactant] initial = [reactant] equilibrium

43 K c is very small... The thermal decomposition of water has a very small value of Kc. At 1000 C, K c = 7.3 x for the reaction: 2H2 O (g) 2H 2(g) + O 2(g) A reaction is set up at this temperature with an initial concentration of 0.10 mol dm -3. Calculate [H 2 ] at equilibrium. Rule of thumb - Use this approximation if K c < 10-3