Stress is a state of mental or emotional strain or tension resulting from adverse or demanding circumstances. In the context of 13.3 Shifting Equilibria: Le Châtelier's Principle, stress refers to the external factors that can disrupt the equilibrium of a chemical system.
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Stress on a chemical system can be introduced by changes in concentration, pressure, or temperature.
When a system at equilibrium is subjected to a stress, the system will shift in a direction that counteracts the stress and reestablishes equilibrium, as described by Le Châtelier's Principle.
The reaction quotient (Q) is a measure of the current state of a reaction and is compared to the equilibrium constant (K) to determine the direction of the shift.
If the stress increases the value of Q relative to K, the system will shift to decrease the value of Q and reestablish equilibrium.
Conversely, if the stress decreases the value of Q relative to K, the system will shift to increase the value of Q and reestablish equilibrium.
Review Questions
Explain how a change in concentration can act as a stress on a chemical system at equilibrium.
If the concentration of a reactant or product in a chemical system at equilibrium is changed, the system will shift to counteract that change and reestablish the equilibrium. For example, if the concentration of a reactant is increased, the system will shift to consume more of that reactant and produce more products, decreasing the reactant concentration back towards the equilibrium value. This shift is driven by the system's attempt to maintain the equilibrium constant (K) by adjusting the reaction quotient (Q) to be equal to K.
Describe how a change in temperature can act as a stress on a chemical system at equilibrium and predict the direction of the shift.
Temperature is a key factor that determines the equilibrium constant (K) of a reaction. If the temperature of a system at equilibrium is increased, the value of K will also change, disrupting the equilibrium. According to Le Châtelier's Principle, the system will shift in a direction that counteracts the temperature increase and reestablishes the equilibrium. Specifically, if the forward reaction is endothermic (absorbs heat), the system will shift to favor the forward reaction to consume the added heat and decrease the temperature. Conversely, if the forward reaction is exothermic (releases heat), the system will shift to favor the reverse reaction to produce heat and decrease the temperature.
Analyze how changes in the reaction quotient (Q) relative to the equilibrium constant (K) can drive a system to shift and reestablish equilibrium.
The reaction quotient (Q) is a measure of the current state of a reaction, while the equilibrium constant (K) represents the value of Q at equilibrium. When a stress is applied to a system at equilibrium, it causes Q to deviate from K. Le Châtelier's Principle states that the system will then shift in a direction that brings Q back towards K to reestablish the equilibrium. If Q is greater than K, the system will shift to decrease Q by consuming products and producing reactants. Conversely, if Q is less than K, the system will shift to increase Q by producing more products and consuming reactants. This shift continues until Q = K and the system returns to the equilibrium state.
A state of balance between opposing forces or actions, where the rate of forward and reverse reactions are equal, resulting in no net change in the concentrations of reactants and products.
A principle stating that when a system at equilibrium is subjected to a change in one of the factors (concentration, pressure, or temperature) determining the equilibrium, the system will shift in a direction that tends to counteract the change and reestablish the equilibrium.
Reaction Quotient (Q): The ratio of the concentrations of the products raised to their stoichiometric coefficients to the concentrations of the reactants raised to their stoichiometric coefficients at any given time during a reaction.