Flashcards
Chemical Equilibria
Cards in this deck (25)
- 1is a state in which there is no change to both macroscopic and microscopic properties in a system or when a force acting on an object at rest are balancedStatic equilibrium
- 2refers to the state of a reversible rxn when both forward and backward rxns are taking place simultaneously at the same rate in a closed system at a constant temperature and there is no change in the conc of reactants and productsDynamic equilibrium
- 3• ratef = rateb • conc of rxts and pdts do not changewhen a reversible rxn has reached dynamic equilibrium
- 4Q = [C]^c[D]^d/[A]^a[B]^b C,D - products A,B - reactants a,b,c,d - stoichiometric coefficientsReaction quotient, Q
- 5Kc = [C]eqm^c[D]eqm^d / [A]eqm^a[B]eqm^bat dynamic equil, Q is a constant = Kc
- 6Kc = Kf/Kb• Qc < Kc (Kc is large) => forward rxn favoured
• Qc = Kc => system is at equilibrium
• Qc > Kc (Kc is small) => backward rxn favoured - 7Equilibrium constant for gases => KpKp = (PC)^c(PD)^d / (PA)^a(PB)^b
P = partial presurre - 8• PC = nc/nt x PT • PT = PA + PB + PC + PD• Homogeneous equilibrium - all substances involved are in the same phase(state)
• Heterogeneous equilibrium - substances involved are not in the same phase(state)
=> conc. of pure solid & pure liquid are constant -> omitted from Kc and Kp expression - 9To calculate Kc • write equation • ICE table • calculate amt of rxts and pdts • calculate concentration • write Kc expression and substituteICE table
• Initial amt
• Change in amt
• Equilibrium amt - 10is the fraction of reactants that has undergone dissociationDegree of dissociation, a
- 11a = amount dissociated / total initial amt• fraction or percentage
• increases with temperature - 12Kc/Kp not affected by: • changes in conc of rxt/pdt or addition/removal of rxt/pdt • changes in pressure (partial or total) • presence of absence of catalyst => Kc/Kp is constantKc/Kp is affected by changes in temperature
- 13states that if a system at dynamic equilibrium is subjected to a change which disturbs equilibrium, the system will respond in such way so as to counteract the change and re-establish the equilibriumLe Chatelier's Principle
- 14• Write equilibrium eqn • Write "According to Le Chatelier's Principle," • State -the change -how the position of equilibrium shifts left/right to counteract the effect of the change -whether foward or backward rxn is favoured • Write down the new observationTo explain equilibrium observation
- 15N2 + 3H2 <> 2NH3 According to Le Chatelier's Principle, when more H2 is added to the system, the position of equilibrium shifts right to reduce the increased concentration of H2. Forward rxn is favoured. At the same time, N2 is consumed and more NH3 is formed.Hence at the re-established eqm:
• [H2]new > [H2] old
• [N2]new < [N2]old
• [NH3]new > [NH3]old - 16Effect of pressure changes: (a)Changes in partial pressure • adding/removing gas • similar to that of changing conc PV=nRT P = (n/V)RT = [A]RT(b)Changes in total pressure/volume of system
• change by compression (increase in total pressure by decreasing volume) or expansion (decrease in total pressure by increasing volume) - 172SO2 + O2 <> 3SO3 {When total pressure is increase (volume decrease)} According to Le Chatelier's Principle, the equilibrium position shifts to the right to reduce the total pressure by reducing the total number of gaseous molecules present in the system.• Increase in pressure => favours rxn which produces fewer no. of moles of gas
• Decrease in pressure => favours rxn which produces more no. of moles of gas - 18•When there is no change in total no. of moles of gas in a rxt, then changes in total pressure has no effect on the position of equilibrium • When an inert gas is added to eqm mixture at constant volume, the total pressure of the system is increased. The partial pressures of each reacting species and the total pressure of the reacting species remains the same => no shift in eqm-
- 19Effect of temperature changes on equilibrium position: • Increase in temperature favours endothermic rxn • Decrease in temperature favours exothermic rxnCo(H2O)6^2+(aq) + 4Cl-(aq)
<> CoCl4^2-(aq) + 6H2O(l) ◇H>0
pale pink -> deep blue => violet solution
{When temperature is increased}
According to Le Chatelier's Principle, when the temperature is increased, the eqm position shifts right to absorb the additional heat energy, favouring the forward endothermic reaction. Hence, the resulting soln is deep blue.
The increase in temp causes the rates of both the forward and backward rxn to increase. Since the forward rxn is favoured, the ratef increases more than rateb. Hence, Kc increases. - 20Effect of Catalyst on Equilibrium position: • Catalyst increases the rate of reaction by providing an alternative pathway with lower activation energy, Ea. • lowers both forward Ea (Ef) and backward Ea (Eb) by the same extent • As the rates of both forward & backward rxn increase, eqm is attained faster • A catalyst has no effect on the composition of an eqm mixture. Hence, eqm position is not affected and Kc remains unchanged
- 21—◇G = ◇H - T◇S
• eqm will establish when both ◇H and ◇S have the same sign - 22—-
- 23Haber Process: Manufacture of Ammonia N2(g) + 3H2(g) <> 2NH3(g) ◇H=-92 kJmol-1 Temp: 450°C Pressure: 200 atm Catalyst: Finely divided iron• Temp: 450°C
By Le Chatelier's Principle, a low temp would favour the forward rxn, which is exothermic, to release more energy. Hence, obtain a higher yield of NH3. However, a low temp would slow down the rate of prod and cause equilibrium to be achieved slowly. Hence, a low temp is not economically effective. A high temp increases the rate of prod, but results in lower yield and high prod cost. Hence, a moderate temp of about 450 is used to speed up the rxn, although the % of NH3 produced at eqm is reduced. - 24• Pressure: 200 atm By Le Chatelier's Principle, a high pressure shift the equilibrium position to the right, where there is less gaseous species, to reduce the pressure. Hence, obtain a higher yield of NH3. However, high pressure would increase the cost and maintenance of equipments. Thus, an optimum pressure of 200 atm is used.•Catalyst: finely divided Fe with Al2O3 as promoter
Catalyst is added to increase the rate of rxn so that eqm is reached faster. Furthermore, a catalyst permits lower operating temperatures which is more economical. Finely divided iron(heterogeneous catalyst) is mixed with small amounts of promoters (such as K2O and Al2O3) to enhance the effectiveness of the Fe catalyst - 25•Continual removal of NH3 The removal of ammonia shifts the position of eqm to the right thereby increasing the yield of ammonia. This is achieved by cooling the rxn mixture to -50°C to liquefy the ammonia formed (b.p. of NH3(l) is -33°C)• Molar ratio of N2: H2 of 1:3
The molar ratio used is similar to that of the stoichiometric ratio to minimise excess
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- GCE 'A' Levels H2 Chemistry Flashcards on Chemical Equilibria, created by Sabrina N on 02/05/2017.
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