Unit 7: Topic 9 - Introduction to Le Châtelier’s Principle Flashcards
What does Le Châtelier’s principle state?
Le Châtelier’s principle states that if a system in equilibrium is disturbed, the system will shift to neutralize the disturbance and re-establish equilibrium. A system will shift away from an addition and shift towards removal. If a chemical reaction has reached equilibrium and more reactants are added to the system, the system will shift away from the reactants (forming more products) to re-establish equilibrium.
A chemical reaction has reached equilibrium. The temperature of the system is increased. Using Le Châtelier’ principle, how would the system react to this disturbance?
If the reaction is exothermic, then heat is released, as in A + B –> C + D (∆H = -250 kJ/mol). In this case, we can consider heat as a product off the reaction, as it is released to the surroundings: A + B –> C + D + ∆H. Therefore, an increase in temperature is effectively an increase in the amount of product: Le Châtelier’s principle states that the system should shift opposite to this change, so there should be a shift towards the reactants. If the reaction were endothermic, an increase in temperature would cause a shift towards the products.
A chemical reaction reaches equilibrium. The pressure of the system is increased. Using Le Châtelier’ principle, how would the system react to this disturbance? What if the volume increases?
When dealing with a change in pressure, we only need to consider gaseous products and reactants. The system will respond to an increase in pressure by shifting towards the side that lowers pressure. The side that lowers pressure has FEWER moles of gas.
According to Boyle’s law, an increase in volume will decrease pressure. Therefore, the system will shift towards the side with more moles of gas to increase pressure.
The pH of a system is given by pH = -log[H+] = -log[H3O+]. For the dissociation of the acid HF, given by HF(aq) + H2O(l) –> F-(aq) + H3O+(aq), what would happen to the pH if we add more HF(aq)? What if we add more water?
Increasing the concentration of HF shifts the reaction towards the products. [F-] and [H3O+] will increase. Because [H3O+] increases, the pH decreases. Adding H2O doesn’t shift the system towards either reactants or products because H2O is a liquid. However, adding H2O will still dilute the acidic solution (decrease [H3O+]) and thereby increase pH.
