Chemical Equilibrium (OCR A Level Chemistry A): Flashcards
Exam code: H432
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Define dynamic equilibrium
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Cards in this collection (20)
Define dynamic equilibrium
A dynamic equilibrium is a state in a closed system where the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant.
Define Le Chatelier's principle
Le Chatelier's principle states that if a change is made to a system in dynamic equilibrium, the position of equilibrium shifts to counteract that change.
Define equilibrium constant (Kc)
The equilibrium constant (Kc) is a constant that relates the equilibrium concentrations of reactants and products for a given reaction at a specific temperature, with products in the numerator.
True or False?
At dynamic equilibrium, the concentrations of reactants and products are equal.
False.
At dynamic equilibrium, concentrations remain constant but are not necessarily equal — the ratio depends on the position of equilibrium for that particular reaction.
How does increasing temperature affect the position of equilibrium in an exothermic reaction?
The equilibrium shifts to the left (towards reactants) to absorb the excess energy, favouring the endothermic reverse reaction.
What is the only factor that changes the value of Kc for a reaction?
Only a change in temperature alters the value of Kc — changes in concentration, pressure or catalyst have no effect on Kc.
Why can dynamic equilibrium only be reached in a closed system when gases are involved?
In an open system, gaseous products escape to the surroundings, preventing the reverse reaction from occurring and so equilibrium cannot be established.
True or False?
Adding a catalyst to a system at dynamic equilibrium shifts the position of equilibrium to the right.
False.
A catalyst increases the rate of both the forward and reverse reactions equally — it has no effect on the position of equilibrium, only on how quickly equilibrium is reached.
True or False?
Increasing the concentration of a reactant in an equilibrium mixture increases the value of Kc.
False.
Kc only changes with temperature — adding more reactant shifts the equilibrium position but the value of Kc remains unchanged.
At dynamic equilibrium, the rate of the .......... reaction equals the rate of the .......... reaction, and the concentrations of all species remain .......... .
At dynamic equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of all species remain constant.
Increasing the pressure on a gaseous equilibrium mixture shifts the position of equilibrium towards the side with .......... moles of gas, in order to .......... the pressure.
Increasing the pressure on a gaseous equilibrium mixture shifts the position of equilibrium towards the side with fewer moles of gas, in order to reduce the pressure.
For the reaction aA + bB ⇌ cC + dD, the equilibrium constant expression is Kc = .......... divided by .......... .
For the reaction aA + bB ⇌ cC + dD, the equilibrium constant expression is Kc = [C]c[D]d divided by [A]a[B]b.
What symbol is used to represent a reversible reaction in a chemical equation?
Two opposing half-arrows, ⇌, indicate that the reaction can proceed in both the forward and reverse directions.
Why is a compromise temperature of 400–450 °C used in the Haber process rather than a lower temperature?
At lower temperatures, the rate of reaction is too slow for equilibrium to be reached in a reasonable time, despite the higher yield of ammonia that a lower temperature would favour.
What does a very large value of Kc (>>1) indicate about the position of equilibrium?
A very large Kc indicates the equilibrium lies to the right, meaning the reaction mixture contains mostly products at equilibrium.
True or False?
A reaction carried out entirely in solution can reach equilibrium in an open flask.
True.
When all species are in solution, negligible material is lost through evaporation, so a closed system is effectively maintained and equilibrium can be established.
True or False?
Increasing the concentration of a reactant shifts the position of equilibrium to the right.
True.
Adding more reactant increases its concentration, so the equilibrium shifts to the right to oppose this change and reduce the concentration of the added reactant.
True or False?
Kc has the same units for every equilibrium reaction.
False.
The units of Kc depend on the form of the equilibrium expression — they vary with the stoichiometry of the reaction, and Kc is sometimes dimensionless when units cancel.
What catalyst is used in the Contact process, and what does it produce?
Vanadium(V) oxide is used as a catalyst to convert SO2 and O2 into SO3, which is then used to produce sulfuric acid.
Why are solids excluded from the Kc expression?
The concentration of a solid is constant and does not change during the reaction, so it is incorporated into the value of Kc and omitted from the equilibrium expression.
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