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Example Questions
Example Question #1 : Biological Energetics
A biochemist is studying two metabolic reactions at constant temperature and pressure. Reaction 1 is found to have a G value of
. Reaction 2 has a
G value of
. Which statement is true about these reactions?
Reaction 1 and reaction 2 proceed at equally fast rates
None of the other answers are true
Reaction 1 and reaction 2 proceed at equally slow rates
Reaction 1 proceeds at a faster rate than reaction 2
Reaction 1 proceeds at a slower rate than reaction 2
None of the other answers are true
The G values given in the question relate to the reactions' thermodynamics. A negative
G value means that a reaction is thermodynamically spontaneous. A spontaneous reaction can occur without further energy input. This does not tell us anything about the reaction's rate (kinetics). A spontaneous reaction may be slow or it may be fast.
Example Question #1 : Biological Energetics
The rate of a slow metabolic reaction is sped up by the addition of a catalyst. Which of the following is a possible function of the catalyst?
Increase the energy of the transition state
Decrease the G value
Decrease the free energy required to reach the transition state
Increase the G value
Increase the activation energy
Decrease the free energy required to reach the transition state
A catalyst lowers the activation energy of a reaction. This could be accomplished through stabilization of the transition state by lowering its energy. A catalyst does not change the free energy difference between products and reactants, so no change in G of the reaction is observed.
Example Question #602 : Biochemistry
Consider the hydrolysis of a molecule of ATP, as shown by the reaction below:
If the pH of the solution in which this reaction is occurring were to be lowered, what effect would this have on the above reaction?
The equilibrium constant of the reaction will increase
A change in pH will have no effect on the reaction
The reaction will be pushed to the right
The equilibrium constant of the reaction will decrease
The reaction will be pushed to the left
The reaction will be pushed to the left
This question is asking us to determine what effect a pH change would have on the reaction in which ATP is hydrolyzed into ADP. First, we have to realize that if pH were lowered, that means we are dealing with a more acidic solution. A more acidic solution, in turn, means that we have an increased concentration of . So, in essence, the question is asking what effect an increased
concentration will have on the reaction. In the reaction, we can see that
is on the product side of the reaction (on the right side). Therefore, if we drive the
concentration up, the reaction will be pushed toward the left according to Le Chatelier's principle. Furthermore, it's important to note that changing the concentration of any of the reactants or products will not have an effect on the equilibrium constant,
. In fact, it is precisely because the equilibrium constant doesn't change that the reaction will shift to the left, so that the constant will remain just that, constant. Moreover, the only thing that can change an equilibrium constant is the temperature at which the reaction takes place.
Example Question #603 : Biochemistry
The standard free energy change for any chemical reaction becomes less favorable when __________.
increases and
increases.
increases and
decreases
decreases and
decreases.
temperature increases
decreases and
increases
increases and
decreases
The Gibbs free energy becomes less favorable, or more positive, as enthalpy
increases and entropy
decreases.
Example Question #51 : Homeostasis And The Biological Environment
For this reaction, at given reaction conditions.
Which of the following statements concerning the reaction is true?
More than one of these
Entropy increases as the reaction proceeds to the left
The mixture contains an equal amount of reactants and products
The mixture contains more reactants than products
The mixture contains more products than reactants
The mixture contains more products than reactants
Because for the reaction is negative, it is spontaneous and proceeds favorably to the right. Equilibrium does not occur when the concentrations of reactants and products are equal; it occurs when the rates of forward and reverse reactions are equal. At equilibrium
thus there is no net change in the concentration of either the reactants or the products.
Example Question #1 : Biological Energetics
Regarding formation of double stranded DNA from two complementary single strands, which of the following is true?
The formation of dsDNA is entropically favorable, but is enthapically unfavorable.
The formation of dsDNA is entropically and enthalpically unfavorable.
None of these
The formation of dsDNA is entropically unfavorable, but is enthalpically favorable.
The formation of dsDNA is entropically and enthalpically favorable.
The formation of dsDNA is entropically unfavorable, but is enthalpically favorable.
If two complementary single strands of DNA are put into a solution, they will spontaneously form dsDNA. This process results in a loss of heat from the system - demonstrating that it is an enthalpically favorable process. However, it is entropically unfavorable given the formation of a more ordered structure.
Example Question #2 : Biological Energetics
A person just eats a very large meal filled with starches and sugars. What is expected to happen to their insulin to glucagon ratio as a result? Assume the person is not diabetic.
The ratio will remain more or less constant
Insulin will levels will increase and glucagon levels will drop to zero
Glucagon will levels will increase and insulin levels will drop to zero
Insulin will increase relative to glucagon
Glucagon will increase relative to insulin
Insulin will increase relative to glucagon
Insulin is produced by pancreatic beta cells in response to a rise in blood sugar, which occurs after eating a meal rich in carbohydrates (sugars and starches). It causes other cells of the body to take up the blood sugar (primarily glucose) and use it for energy production, and it prompts the liver to store excess glucose as glycogen. Glucagon is produced when blood sugar is too low, and it blocks glycolysis and prompts liver cells to convert stored glycogen back to glucose and release it and also produce glucose through gluconeogenesis. This is always determined by the ratio of the levels of two hormones - if there is far more insulin than glucagon, its physiological effects will dominate. There is never absolutely zero of either hormone unless the individual is diabetic to such a degree that their cells cannot produce insulin.
Example Question #3 : Biological Energetics
Under what conditions will a reaction with a positive change in entropy and a positive change in enthalpy take place spontaneously?
This reaction will never take place spontaneously
When temperature is very high
When temperature is very low
When pressure is low
This reaction will always take place spontaneously
When temperature is very high
A negative change in Gibbs free energy means that a reaction will take place spontaneously. By using the equation:
We can see that if both entropy and enthalpy are positive, will only be negative when
is sufficiently high.
Example Question #601 : Biochemistry
What is reaction coupling?
Stopping a favorable reaction from occurring by pairing it with an unfavorable reaction
Pairing two reactions together in order to alter their chemical equilibriums
Combining two reactions in order to double the amount of product that will be created
Driving an unfavorable reaction forward by pairing it with a very favorable reaction
Pairing two unfavorable reactions together so that they both become favorable
Driving an unfavorable reaction forward by pairing it with a very favorable reaction
Reaction coupling is the pairing of one unfavorable reaction to another reaction that is favorable. The energetics of the favorable reaction drive the unfavorable one forward.
Example Question #602 : Biochemistry
Which of the following is true of a reaction that has reached equilibrium?
The reaction quotient (Q) is greater than the equilibrium constant (Keq)
The reaction quotient (Q) is less than the equilibrium constant (Keq)
There is no net movement between the products and reactants
The rate of the reaction is at its maximum
There is no movement at all between products and reactants
There is no net movement between the products and reactants
It is a common misconception that at chemical equilibrium, movement between the product and reactant sides of the equation has stopped. In reality, reactants and products are still converting back and forth to one another, however there is no longer any net movement from one side of the equation to the other. Also, Q=Keq at equilibrium.
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