Chapter 6: Equilibrium Flashcards
What is the medical condition DKA? What will be a primary symptom of a patient with DKA? How does this relate to chemical equilibrium?
DKA refers to diabetic ketoacidosis. A patient with DKA, among other things, will have rapid shallow breathing. Their body is trying to rid itself of CO2 to compensate for metabolic acidosis. Reducing CO2 concentrations in the blood shifts the bicarbonate buffer system to the left, increasing pH (decreasing acidity).
What is the difference between chemical equilibrium expression (Keq) and the rate expression?
A chemical equilibrium expression represents the ratio of product concentrations to reactant concentrations at equilibrium, indicating the relative amounts of each species when a reaction has reached stability, while a rate expression describes the change in concentration of a reactant or product over time, showing how fast a reaction is proceeding at a given moment.
In equilibrium expressions, the exponents are equal to the coefficient in the balanced equation.
In rate laws, the exponent must be determined experimentally, and often do not equal the stoichiometric coefficients.
What is an irreversible reaction? Reversible?
Any irreversible reaction proceed in one direction only, the reaction goes to completion, and the maximum amount of product form is determined by the amount of limiting reagent initially present.
Reversible reactions are those in which the reactions can proceed in one of two ways: forward, or to the right toward the products, and reverse, or to the left toward the reactants.
What is dynamic equilibrium? What static equilibrium?
In a dynamic equilibrium, the forward and reverse reactions are still occurring, but they are going at the same rate; thus, there is no net change in the concentration of the products or reactants. That is great doodle and food doodle doodle doodle. Do you want food?
In static equilibrium, the forward and reverse reactions have stopped.
What is entropy? What are the units of entropy?
Entropy is the measure of the distribution of energy throughout a system or between a system and its environment. Entropy is also a measure of the disorder of a system.
For a reversible reaction at a given temperature, when will the reaction reach equilibrium regarding entropy and Gibbs free energy?
For a reversible reaction at a given temperature, the reaction will reach equilibrium when the systems entropy is at a maximum and Gibbs free energy of the system is at a minimum.
At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, entropy is at a maximum, and Gibbs free energy is at a minimum. This links the concepts of thermodynamics and kinetics.
What is the law of mass action?
For a generic reversible reaction, shown in the image, the law of mass action states that if the system is at equilibrium at a constant temperature than the following ratio is constant:
Derive the concept of Keq using rates of a forward and reverse reaction using the equation in the image. You won’t need to do this for the MCAT, but this is important for understanding and distinguishing rates and equilibriums. (Hint, use rate forward and rate reverse)
What is the expression for the equilibrium constant for the following reaction?
Describe the reaction quotient, Q.
Depending on the actual rates of the forward and reverse reactions, equilibrium might be achieved in microseconds or millennia. The reaction quotient serves as sort of a timer to indicate how far the reaction has proceeded towards equilibrium. The reaction quotient (Q) in reference to the equilibrium constant (Keq) will allow us to determine how a reaction will proceed, using a ratio of concentrations of products and reactions.
Knowing what the reaction quotient, Q, is, describe a reaction when Q is less than Keq, a reaction when Q is equal to Keq, and reaction when Q is great greater than Keq. Also, in terms of Delta G.
Describe a Gibbs Free energy versus reaction quotient graph. Describe what happens to Keq and deltaG when Q is less than, equal to, and greater, than Keq.
When Q < Keq, delta Gforward is less than zero, reaction proceeds forward.
When Q = Keq, deltaG = 0, equilibrium has been reached.
When Q > Keq, delta Greverse is less than zero, reaction proceeds reverse.
Do concentrations of pure solids and pure liquids appear in the equilibrium constant expression?
Concentrations of pure, solid solids and pure liquids do not appear in the equilibrium, constant expression. This is because the equilibrium expression is technically based on the activities of compounds, not concentrations. The activities of pure solids and liquids are defined to be 1.
For the purposes of the MCAT, there is a negligible difference between concentration and activity. Just remember that the activities of pure solids and liquids are defined to be 1, and therefore do not appear in the equilibrium constant expression.
Is the equilibrium constant temperature dependent?
The equilibrium constant (Keq) is characteristic of a particular reaction at a given temperature; therefore the equilibrium constant is temperature dependent.
What does a very large value of Keq indicate?
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, how was the equilibrium constant written for the reverse reaction?
If the equilibrium constant for a reaction written in one direction is Keq, the equilibrium constant for the reverse reaction is 1/Keq.
There are four characteristics of the law of mass action and equilibrium constant expressions. What are they?
The concentrations of pure solids and pure liquids do not appear in the equilibrium constant expression as the activities of pure solids and liquids are defined to be 1.
Keq is temperature dependent.
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.
When Keq equals one, what does that mean?
When Keq equals one, the concentrations of the products and reactants are equal.
When you take the square root of an exponent, how do you calculate that by hand? Provide an example.
When you take the square root of an exponent, divide the exponent in half. For example:
Considering the following reaction in the book:
Solve for the concentration of product (x).
If X amount of a is reacted, X amount of C and X amount of B have been produced at equilibrium, and the equilibrium concentration of A will be [1-x] M.
We can see that Keq is a very small number and can approximate that very little reactants have reacted considering that x is negligible.
3 moles of N2O4 is placed in a 0.5L container and allowed to reach equilibrium according to the following reaction. What is the equilibrium concentration of NO2, given Keq for the reaction is 6x10-6?
I need to figure out where they get 2x^2 in the products in the Keq formula. 2 moles of NO2 explains the exponent of 2. But where does the multiplication of 2 come from?
Given the following, determine the direction of the reaction and deltaG.
Write the equilibrium constant expression for the following reactions.
For each of the following, determine if the amount of reactant A that has converted to product of equilibrium will be negligible compared to the starting concentration of A.
Calculate the following:
First step in solving this is to write the equation for Keq.
Note that the equation does not include product D because D is a solid.
Next, using the initial concentrations for A and B and X for the amount that has reacted, plug into the equation for Keq.
The value of x (being so small) is equal to both the equilibrium concentration of C and the amount of A and B that have reacted. The approximation that x is negligible compared to the initial concentrations of A and B is valid.
What is the bicarbonate buffer system? What happens when acidity is too high in the body such that for DKA (what is DKA BTW?). How would the respiratory system with reduced acidity? Describe.
The bicarbonate buffer system is a system in the body that helps adjust acidity and is a fine example of Le Chatelier’s principle. If a patient is experiencing diabetic ketoacidosis, acidity in the blood will increase (decrease pH). The bodies reaction to this would be rapid shallow breathing in an attempt to exhaust more carbon dioxide, thus shifting the bicarbonate buffer system to the left, ultimately increasing pH of the blood (decreasing acidity). Conversely, if the blood was too basic (high pH), respiration would slow, causing a shift to the right in the bicarbonate buffer system, decreasing pH.
How do changes in concentration impact a chemical reaction?
When reactants or products are added, or removed from a reaction equilibrium, the reaction is moved from its minimum energy state resulting in Qc not equaling Keq.
If reactants are added (or products removed) Qc < Keq and the reaction will spontaneously move in the forward direction increasing the value of Qc until Qc=Keq.
If products are added (or reactants removed), Qc>Keq, and the reaction will spontaneously move in the reverse direction, decreasing the value of Qc until Qc=Keq.
How do changes in pressure (and volume) stress a chemical reaction, and what is the response?
Consider:
When a system is compressed, it’s volume decreases, and it’s total pressure increases. The increase in total pressure is associated with an increase in the partial pressure of each gas in the system, and this results in the system no longer being in the equilibrium state such that Qp does not equal Keq.
The system will move forward or in reverse, always toward which ever side has the lower total number of moles of gas. Recalling that this is a consequence of the ideal gas law, which tells us that there is a direct relationship between the number of moles of gas and the pressure of a gas.
What is enthalpy?
Enthalpy is a state function and is the sum of a thermal dynamic systems internal energy and the product of its pressure and volume.
Consider the following reaction. Is it exothermic or endothermic? What will cause a shift to the right? What will cause a shift to the left?
delta H is negative
Describe kinetic and thermal dynamic control of a reaction.
Reactants may undergo two different sets of reactions. At lower temperatures with a smaller heat transfer, a kinetic product is formed. At higher temperatures with larger heat transfer, a thermodynamic product is formed.
A reaction is found to stop just before all reactants are converted to products. Which of the following could be true about this reaction?
C
This scenario likely describes the situation which reaction has reached equilibrium very far to the right with high product concentration and low reactant concentration. This reaction must be reversible because the reaction did not proceeded all the way to the right. Any reaction and equilibrium has equal forward and reverse rates of reaction.
What is the equilibrium expression for the reaction:
D
Recall that pure solids and liquids do not appear in the equilibrium expression; thus, Keq has no denominator because the only reactant is solid cuprous sulfate. This could also be called Ksp because a solid is dissociating into ions and solution. The correct Keq should have [Cu+] squared because its stoichiometric coefficient is two.
Acetic acid dissociate in solution according to the following equation:
B
Adding sodium acetate increases the number of acetate ions present. According to Le Chatelier’s principal, this change will push the reaction to the left, resulting in a decrease in the number of free H+ ions. Because pH is determined by the hydrogen ion concentration, a decrease in the number of free protons will increase the pH. An acids Ka will remain constant under a given temperature and pressure, eliminating C and D.
Consider the following reaction:
If this reaction were exothermic, what effect would decreasing the temperature have on the equilibrium?
Increasing the pressure (or decreasing the volume) would increase the formation of a product because there are fewer moles in the product than there are in the reactants.
An exothermic reaction produces heat, you can, and you can think of heat as being a product in the chemical reaction. Therefore decreasing the temperature is effectively the same as decreasing the products and would favor product formation, resulting in an increase in the forward reaction rate with a concomitant decrease in the reverse reaction rate.
Which of the following actions does not affect the equilibrium position of a reaction?
B
The equilibrium of a reaction can be changed by several factors such as adding or subtracting heat, increasing the reactant concentrations, changing the volume of reactant.
Adding a removing a catalyst would change the reaction rates, but it would not change where the equilibrium lies.
C
Ka is equal to the ratio of products to reactants, with each species raised to its stoichiometric coefficient. A compound with a Ka greater than 10^-7 contains more cations than HA- and ions at equilibrium, which makes it an acid. This means that the compound and question is likely to react with a compound that is basic. Of the four answers choices, NH3 is the only base.
Consider the following two reactions:
Recall that one of the properties of the law of mass action is that if the equilibrium constant for a reaction written in one direction is Keq, the equilibrium constant for the reverse reaction is the inverse, 1/Keq.
Noticed that reaction one and reaction to are the same reaction. Reaction one in the forward, reaction two in the reverse. The answer is 1/0.1, which is 10.
Which of the following statements best describes the effect of lowering the temperature of the following reaction:
A
The negative deltaH indicates that the reaction is losing heat (producing heat) meaning that heat is a product of this reaction, meaning that it is exothermic. Lowering the temperature of an exothermic reaction has the same effect on the reaction as if to remove products. Removing products would cause C and D to increase.
Can think of the reaction as this:
