Flashcards in General Chemistry- Equilibrium Deck (69)
What is Le Chatelier's principle?
CO2(g) + H2O(l) H2CO3(aq) (H+)(aq) + (HCO3-)(aq)
What is important to not confuse?
The chemical equilibrium expression for the rate expression
Catalysts do what in regards to equilibrium and can't do what?
Catalysts make reactions go faster toward their equilibrium position, but that they can't actually change the equilibrium position or alter the value of keq
What are irreversible reactions?
the reaction proceeds in one direction only, the reaction goes to completion, and the maximum amount of product formed is determined by the amount of limiting reagent initially present.
What are reversible reaction?
The reaction can proceed in one of two ways: forward (toward the products or "to the right") and reverse (toward the reactants or "to the left")
Why do reversible reactions usually not proceed to completion.
The products can react together to reform the reactants.
In a reversible reaction, when the system is closed and no reactants are added or removed, the system will what?
The system will eventually settle into a state in which the rate of the forward reaction equals the rate of the reverse reaction and the concentrations of the products and reactants remain constant.
What is a dynamic equilibrium?
The forward and reverse reactions are still occurring--they haven't stopped, but they are going at the same rate; thus, there is no net change in the concentrations of the products or reactants
what is static equilibrium?
The forward and reverse reactions have stopped
What is the generic reversible reaction?
What does the generic reversible reaction show?
At equilibrium, the concentrations of A and B are constant (although not necessarily equal), and the reaction A --> B and B --> A continue to occur at equal rates.
How can equilibrium be though of as?
A balance between the forward and reverse reactions.
What is entropy?
The measure of the distribution of energy throughout a system or between a system and its environment.
For a reversible reaction at a given temperature, the reaction will reach equilibrium when what occurs?
The system's entropy--or energy distribution--is at a maximum and the Gibbs free energy of the system is at a minimum.
What does the law of mass action state for a generic reversible reaction:
aA + bB cC + dD?
If the system is at equilibrium at a constant temperature, then the following ratio is constant:
The law of mass action is actually related to what?
The expression for the rates of the forward and reverse reactions.
What are the rates of forward and reverse reactions based on the following one-step reversible reaction
2A B + C?
When rate(f)=rate(r), the system is what?
The rate expressions for the forward and reverse reaction equal each other and will look like what?
(kf)[A}^2 = (kr)[B][C] --> (kf)/(kr) = [B][C]/[A]^2
What can occur since (kf) and (kr) are both constants?
We can define a new constant (kc)
What is the equilibrium constant for a equilibirum equation of (kf) and (kr).
kc and the subscript c indicates that it is in terms of concentration. When dealing with gases, the equilibrium constant is referred to as (kp), and the subscript p indicates that it is in terms of pressure.
What is the new equation when using (kc) as the constant instead of (kr) or (kf)?
(kc)= keq = [B][C]/[A]^2
Are the concentrations of the reactants and products equal when the forward and reverse reaction reates are equal at equilibrium?
The concentrations of the reactants and products are not usually equal
If the concentrations of the reactants and products are not equal, are the forward and reverse reaction rate constants equal?
(kf) and (kr) are not usually equal to each other
What is the ratio of (kf) to (kr) in regards to (kc)?
(kc)= Keq= (kf)/(kr)
How do you find the equilibrium constant for the overall reaction when the reaction occurs with more than 1 step?
By multiplying the equilibrium constants for each step of the reaction.
The equilibrium constant for the overall reaction is equal to the concentrations of the products divided by the concentrations of the reactants in the overall reaction, with each concentration term raised to the stoichiometric coefficient for the respective species.
What would the equation for rate constant be, for the reaction aA + bB = cC + dD
(kc)= [(k1)(k2)(k3)/(k-1)(k-2)(k-3)] = [C]^c[D]^d/[A]^a[B]^b
The law of mass action defines what?
The position of equilibrium
Equilibrium is only achieved how?
What is the role of the reaction quotient?
To serve as a timer to indicate how far the reaction has proceeded toward equilibrium
What is the equation used to calculate the reaction quotient?
even though the reaction quotient equation and the equilibrium constant equation look exactly the same, what makes them different?
The information it provides is quite different.
The utility of Q is not the value itself but rather the comparison that can be made between Q at any given moment in the reaction to the known Keq for the reaction at a particular temperature.
What does it mean when Q
The forward reaction has not yet reached equilibrium
What are the results when Q
There is a greater concentration of reactants (and smaller concentration of products) than at equilibrium.
The Forward rate reaction is increased to restore equilibirum
What does it mean when Q = Keq?
The reaction is in dynamic equilibirum
What are the results when Q = Keq?
The reactants and products are present in equilibrium proportions.
The forward and reverse rates of reaction are equal
What does it mean when Q > Keq?
The forward reaction has exceeded equilibirum
What are the results when Q > Keq?
There is a greater concentration of products (and smaller concentration of reactants) then at equilibrium.
The reverse rate of reaction is increased to restore equilibrium.
What happens when any reaction has not yet reached the equilibrium state or Q > Keq?
They will continue spontaneously in the forward direction (consuming reactants to form products) until the equilibrium ratio of reactants and products is reached.
What happens when any reaction has reached the equilibrium state?
They will continue to react in the forward and reverse directions, but the reaction rates for the forward and reverse reactions will be equal, and the concentrations of the reactants and products will be constants, such that Q = Keq
What happens when any reaction is beyond the equilibrium state, indicated by Q > Keq?
The reaction will proceed in the reverse direction (consuming products to form reactants) until the equilibrium ratio of reactants and products is reached again
Further movement in either the forward or reverse direction after a reaction is at equilibrium will be what?
What are the properties of the law of mass action?
The concentrations of pure solids and pure liquids do not appear in equilibrium constant expression.
Keq is characteristic of a particular reaction at a given temperature.
The larger the value of Keq, the farther to the right the equilibrium position.
If the equilibrium constant for a reaction written in one direction is Keq, the equilibrium constant for the reverse reaction is 1/Keq.
Why do the concentrations of pure solids and pure liquids not appear in the equilibrium constant expression?
The equilibrium expression is based on the activities of compounds, not concentrations; the activities of pure solids and liquids are defined to be 1. There is a negligible difference between concentration and activity.
What does the Le Chatelier principle state?
If a stress is applied to a system, the system shifts to relieve that applied stress. Regardless of the form the stress takes, the reaction is temporarily moved out of its equilibrium state.
why does the Le Chatelier principle exist?
The principle exists either because the concentrations or partial pressures of the system are no longer in the equilibrium ratio
Or because the equilibrium ratio itself has changed as a result of a change in the temp. of the system. The reaction then responds by reacting in whichever direction will result in a reestablishment of the equilibrium.
What results when changing the concentration of either a reactant or product?
Qc not equal to Keq
What is the simple way of putting the Le Chatelier's principle?
The system will always react in the direction away from the added species or toward the removed species.
Only chemical reactions that involve what, can be affected by changes in the system's _________ and _______.
At least one gaseous species
Pressure and volume
Why are liquids and solids not affected by changes in pressure and volume?
They are essentially incompressible
what happens when a system is compressed?
It's volume decreases and its total pressure increases
The increase in total pressure is associated with an increase in what?
The partial pressure of each gas in the system
What is the ideal gas law?
Tells us that there is a direct relationship between the number of moles of gas and the pressure of the gas.
In regards to the ideal gas law, if one increases the pressure of a system, what happens?
It will respond by decreasing the total number of gas moles, thereby decreasing the pressure.
Note that this scenario assumes that the volume of the system was decreased and then held constant while the system returned to its equilibrium
In regards to the ideal gas law, if one expands the volume of a system, what happens?
The total pressure and the partial pressures decrease. The system is no longer in its equilibrium state and will react in the direction of the side with the greater number of moles of gas in order to restore the pressure.
What is the result of changing the temperature of a system?
It does not change the reaction quotient, Qc or Qp, but a change in Keq.
The change in temperature does not cause the concentrations or partial pressure of the reactants and products to change immediately, so Q immediately after the temperature change is the same as before the temperature change. Thus, because Keq is now a different value, Q no longer equals Keq, The system has to move in whichever direction allows it to reach its new equilibrium state at the new temperature.
How is the direction to each equilibrium determined if the temperature of the system is changed?
It is determined by the enthalpy of the reaction.
If a reaction is endothermic (/\H > 0), heat functions as what?
If a reaction is exothermic (/\H
The reaction A (aq) + 2B (g) C(g) + heat, will shift to the right if what happens?
A or B is added
C is removed
The pressure is increased or the volume is reduced
The temperature is reduced
The reaction A (aq) + 2B (g) C(g) + heat, will shift to the left if what happens?
C is added
A or B are removed
The pressure is reduced or the volume is increased
The temperature is increased
What is the range of kinetic and thermodynamic control common on the MCAT?
Range from Metabolic reactions requiring high energy phosphate molecules such as ATP to the effects of temperature and solvents on enzyme activity.
What are reactants?
Starting material at a certain energy level
What are the two different sets of reactions?
When is a kinetic product formed?
At lower temperatures (with smaller heat transfer)
When is a thermodynamic product formed?
At higher temperatures (with large heat transfers)
Why are kinetic products sometimes called fast products?
Free energy that must be added for the kinetic pathway is lower than that of the thermodynamic pathway and often form faster.
The free energy of the thermodynamic product is what compared to the kinetic product?