Equilibria Flashcards
Meaning of dynamic
Both forwards and backwards reactions occur simultaneously
Features of dynamic equilibrium
- Forward and backward reaction occur at equal rate.
2. Concentration of reactants and products remains constant
Le Chatelier’s Principle
Is change is imposed on a system at equilibrium, the equilibrium will shift to oppose the change.
Effect of increasing temperature on equililbrium
Equilibrium will shift to oppose the change favouring the endothermic reaction to reduce the temperature by absorbing heat. (IN u ENDO)
Effect of decreasing temperature on equililbrium
Equilibrium will shift to oppose the change favouring the exothermic reaction to increase the temperature by releasing heat.
If a questions asks about yield, you should talk about…
position of equilibrium
If a questions asks about rate, you should talk about…
frequency of collisions
Effect of increasing pressure on equilibrium
Equilibrium will shift to oppose the change favouring the reaction that decreases the number of gaseous molecules. Always state how many gaseous moles are on each side of the equation in your answer.
Effect of decreasing pressure on equilibrium
Equilibrium will shift to oppose the change favouring the reaction that increases the number of gaseous molecules. Always state how many gaseous moles are on each side of the equation in your answer.
Effect of increasing concentration of a substance on equilibrium
Equilibrium will shift to oppose the change favouring the reaction that removes that substance.
Effect of decreasing concentration of a substance on equilibrium
Equilibrium will shift to oppose the change favouring the reaction that produces that substance.
Effect of a catalyst on equilibrium
No effect on position of equilibrium as the rate of both forward and backward reactions are increased. Equilibrium will be reached faster.
Reaction and conditions for Haber Process
N2 + 3H2 --> 2NH3 forward reaction is exothermic T = 450 oC P = 200-1000 atm cat = Fe
Reasons for temp of Haber process
Forward reaction is exothermic so lower temp gives higher yield but slower rate as less frequent collisions. A compromise temperature is used
Reasons for pressure of Haber process
High pressure gives high yield and high rate but is more expensive to maintain so a lower pressure is used.
Contact process - reaction and conditions
- S(s) + O2(g) –> SO2(g)
- SO2(g) + 1/2 O2 (g) <=> SO3
Exothermic reaction
T = 450 oC,
P = 10 atm
cat = V2O5
Reasons for temp of Contact process
Forward reaction is exothermic so lower temp gives higher yield but slower rate as less frequent collisions. A compromise temperature is used
Reasons for pressure of Contact process
High pressure gives high yield and high rate but is more expensive to maintain so a lower pressure is used.
Production of methanol from CO. Reaction and conditions
CO(g) + 2H2(g) <=> CH3OH Exothermic reaction T = 400oC P = 50 atm Cat = chromium and zinc oxides
Reasons for temp of methanol production
Forward reaction is exothermic so lower temp gives higher yield but slower rate as less frequent collisions. A compromise temperature is used
Reasons for pressure of methanol production
High pressure gives high yield and high rate but is more expensive to maintain so a lower pressure is used.
Hydration of ethene to produce ethanol. Reaction and conditions
CH2=CH2(g) + H2O(g) <=> CH3CH2OH(g) Exothermic reaction T = 300 oC P = 70 atm Cat = conc. H3PO4
Reasons for temp of ethanol production
Forward reaction is exothermic so lower temp gives higher yield but slower rate as less frequent collisions. A compromise temperature is used
Reasons for pressure of ethanol production
High pressure gives high yield and high rate but is more expensive to maintain so a lower pressure is used.
High pressure also leads to unwanted polymerisation to polyethene
Why are catalysts used in industrial reversible reactions?
They speed up the rate of reaction allowing a lower temp to be used and so lower energy costs without having any affect on equilibrium and therefore yield.
How can overall yields of industrial reversible reactions be increased?
By recycling unreacted reactants back into the reactor.
Equation for Kc for general equation:
mA + nB <=> pC + qD
[A]^m x [B]^n
How to calculate unit of Kc
concentration of each reactant and product is measured in mol dm^-3. Look at the Kc equations and deduce the units
Unit of Kc in equation N2 + 3H2 <=> 2NH3
mol^-2 dm^6
Unit for Kc in equation H2 + Cl2 <=> 2HCl
no units
How to calculate Kc
Work out initial moles of each reactant and product. Work out moles of one substance at equilibrium. Work out the moles of remaining substances at eqbm by decreasing original reactant moles and increasing original product moles. Convert the moles to concentration. Calculate Kc
The reaction H2 + Cl2 <=> HCl was done in a 600 cm^3 container. There were initially 0.5 mol of H2 and 0.6 mol of Cl2. At eqbm there were 0.2 moles of HCl. Calculate Kc
0.196 no unit
In what circumstance do you not need to calculate concentration to calculate Kc
If Kc has no units because there are equal numbers of reactants and products.
In the Haber process there were initially 1.5 moles of N2 and 4 moles of H2 in a 1.5 dm^3 container. At eqbm 30% of the N2 had reacted. Calculate Kc
0.0927 mol^-2 dm^6
What effect does increasing pressure have on Kc
No effect
What effect does adding a catalyst have on Kc
No effect
What factors affect Kc?
ONLY changing temperature
What does a Kc >1 mean?
Equilibrium favours products
What does a Kc <1 mean
Eqbm favours reactants
What effect does changing concentration have on Kc
no effect
