9701 · 7.1
Chemical equilibria: reversible reactions, dynamic equilibrium flashcards
Revision flashcards for Cambridge 9701 Chemical equilibria: reversible reactions, dynamic equilibrium (syllabus 7.1). Flip, recall, then mark a real past-paper question.
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What is a reversible reaction?
A reaction that can proceed in both the forward and reverse directions. It is represented by the '⇌' symbol.
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Define 'dynamic equilibrium'.
A state in a closed system where the rate of the forward reaction is equal to the rate of the reverse reaction, and the concentrations of reactants and products remain constant.
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What are the two key conditions for a dynamic equilibrium to be established?
1. The reaction must be reversible. 2. The system must be closed (no matter or energy can enter or leave).
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What does 'dynamic' mean in the context of equilibrium?
It means that on the molecular level, both the forward and reverse reactions are still occurring continuously, even though there is no overall change in concentrations.
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State Le Chatelier's principle.
If a change of condition is applied to a system in equilibrium, the system will shift in a direction that opposes the change.
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How does increasing the concentration of a reactant affect the position of equilibrium?
The equilibrium shifts to the right (the product side) to use up the added reactant and counteract the change.
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For a gaseous reaction, how does increasing the pressure affect the position of equilibrium?
The equilibrium shifts to the side with the fewer moles of gas to reduce the pressure.
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How does increasing the temperature affect an exothermic reaction (ΔH is negative) at equilibrium?
The equilibrium shifts to the left (the reactant side) in the endothermic direction to absorb the added heat.
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What is the effect of a catalyst on the position of equilibrium?
A catalyst has no effect on the position of equilibrium. It increases the rate of both the forward and reverse reactions equally, so equilibrium is reached faster.
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Common Trap: Are the concentrations of reactants and products equal at equilibrium?
Not necessarily, and usually not. It is the *rates* of the forward and reverse reactions that are equal. The concentrations become *constant*, but are rarely equal.
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What are 'macroscopic properties' in the context of equilibrium?
Observable properties of the system as a whole, such as concentration, pressure, colour, and density. These properties are constant at equilibrium.