Lecture 3 Flashcards
(36 cards)
What happens to the value of Q at equilibrium?
The value of Q stops increasing but the reaction has not gone to completion; reactant molecules are still present.
What occurs at the point where Q stops increasing?
The reaction is at equilibrium, with the conversion of N2O4 into NO2 happening simultaneously with the conversion of NO2 back into N2O4.
What is the equilibrium constant denoted as?
The equilibrium constant is denoted as Kc.
How is Kc defined in terms of concentrations?
Kc is defined as the ratio of the concentrations of products to reactants, raised to the power of their stoichiometric coefficients.
What is the equilibrium expression for the reaction N2O4(g) ⇌ 2NO2(g)?
Kc = [NO2]^2 / [N2O4]
Note that the subscript ‘e’ signifies the concentrations at equilibrium.
What happens when Q < K?
Reactants are converted into products until equilibrium is reached.
What happens when Q > K?
Products are converted back into reactants until equilibrium is reached.
Does the equilibrium constant K depend on starting concentrations?
No, K only depends on temperature and not on the starting concentrations.
What is the correct format for writing equilibrium constant expressions?
Always place products on the top line and reactants on the bottom, raising concentrations to the powers of their stoichiometric coefficients.
What is the equilibrium constant expression for the reaction CH4(g) + H2O(g)?
Kc = [products] / [reactants]
Always ensure products are on the top line and reactants on the bottom.
What is the formula for the equilibrium constant K?
Product raised to stoichiometric powers divided by reactants raised to theirs.
Pure solids and pure liquids do not appear in the expression.
What does the value of K indicate?
The extent of reaction at equilibrium, generally given at 25°C.
Example: For 2H2(g) + O2(g) = 2H2O(g), K = 9.1 x 10^80 indicates the reaction goes to completion.
What does a large K value signify?
The position of equilibrium lies far to the right, indicating the reaction goes essentially to completion.
What does a small K value signify?
The position of equilibrium lies far to the left, indicating the reaction barely begins when equilibrium is established.
Example: For N2(g) + O2(g) = 2NO(g), K is small.
What is the rule regarding pure solids and liquids in K expressions?
Pure solids and pure liquids never appear in an equilibrium constant expression because their concentration is constant.
Provide an example of a K expression.
For the reaction 2 NaHCO3(s) = Na2CO3(s) + H2O(g) + CO2(g), Kc = [H2O][CO2].
Note that H2O here is gas so it is included.
How do you calculate Kc for a given equilibrium?
For the equilibrium N2O4(g) ⇌ 2NO2(g), Kc = 0.0292 mol/L at 298K given the concentrations.
[N2O4] = 0.0116 mol/L.
What is the unit of Kc?
Kc has no units.
How does a system at equilibrium respond to changes in amounts of reactants?
A system at equilibrium will move to counteract the disturbance and restore equilibrium.
This is determined using Le Chatelier’s Principle.
How does a system at equilibrium respond to changes in pressure?
A system at equilibrium will move to counteract the disturbance and restore equilibrium.
This is determined using Le Chatelier’s Principle.
What principle can be used to determine the response of a system at equilibrium to disturbances?
Le Chatelier’s Principle states that if a system at equilibrium is disturbed, it will move in such a way to counteract the disturbance and restore equilibrium.
