kc Flashcards
Explain what would happen to the pressure as the system was allowed to reach equilibrium.
(Pressure) decreases AND fewer molecules/moles
The chemist repeated the experiment at the same temperature with 1.00 mol SO2 and an excess of O2. The gas mixture was still compressed to a volume of 250 cm3.
State and explain, in terms of Kc, how the equilibrium yield of SO3 would be different from the yield in the first experiment.
Kc
Kc does not change (with pressure/ concentration)
Comparison of conc terms with more O2
[O2]/concentration of oxygen is greater
OR denominator/bottom of Kc expression is greater
QWC: yield of SO3 linked to Kc
(Yield of) SO3 is greater/increases
AND
numerator/top of Kc expression is greater/increases
The chemist heats the mixture to a higher temperature at constant pressure.
Explain whether the value of Kc would be greater, smaller or the same.
Kc is smaller AND (forward) reaction is exothermic OR ∆ H is negative
The chemist increases the pressure of the mixture at constant temperature.
Explain whether the value of Kc would be greater, smaller or the same.
Kc is the same AND Kc is temperature dependent OR Kc is not changed by pressure
What does your value of Kc suggest about the position of equilibrium in this experiment?
IF Kc from (iii) < 1 equilibrium to left/towards reactants
OR
IF Kc from (iii) > 1 equilibrium to right/towards
products
The chemist increases both the temperature and the pressure of the equilibrium mixture. The mixture is left to reach equilibrium again. What is the effect, if any, on the value of Kc?
Kc has decreased
AND
∆H is negative OR (forward) reaction is exothermic
Explain why it is difficult to predict what would happen to the position of equilibrium after these changes in temperature and pressure.
Effect of T and P on equilibrium
(increased) temperature shifts equilibrium to left
AND
(increased) pressure shifts equilibrium to right
AND
fewer (gaseous) moles on right-hand side
Overall effect on equilibrium
Difficult to predict relative contributions of two
opposing factors
The experiment is repeated but the pressure in the container is doubled.
Explain in terms of Kc the effect on the concentrations of NO2 and N2O4 when the mixture has reached equilibrium.
Effect on Kc Kc does not change (with pressure) Comparison of conc terms after increase in pressure [NO2] 2 increases more than [N2O4] OR concentration (term) on bottom (of Kc) increases more that concentration (term) on top (of Kc) Changes in concentrations linked to Kc (amount /concentration of) N2O4 increases AND (amount /concentration of) NO2 decreases AND to maintain/restore K
The chemist carries out the reaction between hydrogen and nitrogen monoxide at a higher pressure. Explain, with a reason, what happens to the initial rate of reaction.
(initial) rate increases
AND
more frequent collisions OR more collisions per
second/time
The research chemist doubled the pressure of the equilibrium mixture whilst keeping all other conditions the same. As expected the equilibrium yield of ammonia increased.
Explain in terms of le Chatelier’s principle why the equilibrium yield of ammonia increased.
Equilibrium shifts to right OR Equilibrium towards ammonia Right hand side has fewer number of (gaseous) moles
The research chemist doubled the pressure of the equilibrium mixture whilst keeping all other conditions the same. As expected the equilibrium yield of ammonia increased.
Explain in terms of Kc why the equilibrium yield of ammonia increased.
Kc does not change Increased pressure increases concentration terms on bottom of Kc expression more than the top OR system is now no longer in equilibrium top of Kc expression increases and bottom decreases until Kc is reached
Suggest another process that might be used in the future to obtain hydrogen gas for the Haber process.
Electrolysis of water
OR H2O H2 + ½O2
