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A2 chemistry 4.5

Edexcel chemistry - equilibria

QuestionAnswer
Define reversible reaction, equilibrium, dynamic equilibrium Reversible reaction goes in both directions @ same temperature so doesn't go to completion. Equilibrium - [reactants/products] remain constant. Dyanmic equilibrium - rates of forward + backward reactions same so no further change in [reactants/products]
What is the equilibrium yield? The % of reactant that is converted into product in a reversible reaction
What is Le Chatelier's principle? When the external conditions of a system in equilibrium are changed, the position of equilibrium shift to nullify the change
Write the Kc expressions for the following reactions: 1) MgCl2 + H2O -> Mg2+(aq) + 2Cl-(aq). 2) HCOOH + CH3OH -> HCOOCH3 + H2O 1) Kc = [Mg2+][Cl-]2. 2) Kc = [HCOOCH3][H2O]/ [HCOOH][CH3OH]
Describe the relationship between Kc and Q Kc is equilibrium constant, Q is reaction quotient. Q = Kc at equilibrium. Q> Kc means position of equilibrium lies to right, system converts more products into reactants. Q < Kc means equilibrium lies to left, system converts more reactants to product
Describe the steps for finding Kc Balanced equation, table: initial mol, delta mol, eq mol, concentration (mol/dm3) - use x for delta mol if not given, use v if volume isn't given. Write out correct Kc expression, sub in concentrations, find units for Kc (if none, state this)
Which of the following species would you exclude from a Kc calculation and why: CO2, H2O, NaCl, Br2? H2O if it is the solvent - solvent concentration remains relatively constant. NaCl unless if it is dissolved in water - solids are excluded. Br2 - liquids are excluded, no concentration
Find Kc for hydrolysis of ethyl ethanoate (acidic) -200g of ethyl ethanoate, 7g water, @ equilibrium 0.25 mol ethanoic acid 0.1905
Define partial pressure and mol fraction Partial pressure = pressure exerted by 1 gas in a gaseous mixture if it alone occupied the same volume under the same conditions, partial pressure = mol fraction x total pressure. Mol fraction = amount of gas/ total amount
Describe the steps for determining Kp BAlanced equation, table: intial mol, delta mol, eq mol, mol fraction, partial pressure (units), write out correct Kp equation, sub in partial pressures, find units of Kp
2NO2 (g) <-> 2NO (g) + O2 (g). Eq moles: 0.96 NO2, 0.04 NO, 0.02 O2. Kp = 6.8 x 10^-6 atm, what must total pressure have been?
How does ΔS total affect value of K ΔStotal = RlnK. As ΔStotal becomes more positive, K value increases. Endothermic: increase T = increase K. Exothermic: increase T = decrease K.
If 0<ΔStotal<200J/K/mol, then where does the position of equilibrium lie/ value of K? 1< K < 10^10. Position of equilibrium lies on the right, so reaction favours products
If ΔStotal < -200, what is value of K/where does equilibrium lie? K<10^-10. Position of equilibrium lies FAR to the left, reaction doesn't go
How does temperature affect K + position of equilibrium? Endothermic: increase T, K increases, Q<K, system converts more reactants to products (shift to right) so Q=K. Exothermic: increase T, K decreases, Q>K, system converts more products to reactants (shifts to left) so Q=K. Le Chatelier
How does temperature affect rate of reaction? Increase T increases rate of reaction (collision theory), rate increase more in endothermic direction due to high Ea
How does pressure affect K + position of equilibrium? Le Chatelier's principle: shifts position of equilibrium to side with fewer gas moles THEREFORE no change in value of K.
How does pressure affect rate of reaction? Increases rate of reaction (collision theory) equally in both directions
How does concentration affect K + position of equilibrium? No effect on value of K, but it changes the reaction quotient so Q doesn't equal K, the position of equilibrium shifts (Le Chatelier's principle: to the side that reverses the change) so Q=K again
How does concentration affect rate of reaction? Increases rate of reaction (collision theory)
How does catalysis affect K, position of equilibrium and rate of reaction? No effect on value of K or Q (position of equilibrium), but it increases rate of reaction equally in both directions so dynamic equilibrium is reached more quickly, although rate is limited by solid catalyst's active site availability
What are the desirable features of industrial processes and how are they achieved? High equilibrium yield (continually remove products), low cost (heat exchanger/catalysis/compromise pressure), high rate, high atom economy (recycle unreacted reactants)
State Haber process equation + explain conditions N2 + 3H2 -> 2NH3, exothermic. Compromise temperature, 400 degrees c - reasonable yield at an economically acceptable rate. Compromise pressure. Iron catalyst. Unreacted reactants pass through catalyst many times so atom economy is 98%
Created by: 11043
 

 



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