Slide 1
Steady composition can hide activity
Chemical equilibrium is a dynamic balance. Forward and reverse reactions continue, but at equal rates, so the overall composition stays steady.
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Imagine molecules switching between two forms. An unchanged count of each form does not require every molecule to remain unchanged; equal numbers can cross in opposite directions during the same interval.
Slide 2
Equal rates are not equal amounts
At equilibrium, reactant and product amounts need not be equal. Their balance depends on the reaction and the conditions.
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A system may contain mostly products while reverse events still match forward events. Different rate constants and concentrations can produce equal rates from very unequal populations.
Slide 3
An equilibrium constant describes the balance
For a specified reaction and temperature, the equilibrium constant relates activities of products and reactants. It is not a measure of reaction speed.
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A strongly product-favoured reaction may proceed slowly if its activation barrier is high. Thermodynamics describes the favoured balance; kinetics describes how quickly the system approaches it.
Slide 4
A disturbance can shift composition
Adding or removing a component can temporarily unbalance the forward and reverse rates. The mixture then changes toward a new equilibrium.
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Removing a product can allow more reactant to convert into it. The equilibrium constant remains the same at the same temperature, even though the final amounts change in response to the disturbance.
Slide 5
Temperature can change the constant
Changing temperature can alter which equilibrium composition is favoured. A catalyst instead changes how quickly equilibrium is approached.
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Pressure changes matter for gas equilibria when the reaction changes the number of gas particles. The response must be derived from the actual reaction rather than a rule that every compression has the same effect.
Sources
- Chemical Equilibrium: USNCO Coaching
American Chemical Society
- DOE Explains: Catalysts
US Department of Energy