MCAT Content / Principles Of Bioenergetics / Bioenergetics Thermodynamics 1d

Bioenergetics/Thermodynamics

Topic: Principles Of Bioenergetics

Bioenergetics refers to the concept of energy flow through living systems while thermodynamics deals with heat and temperature, and their relation to energy, work, radiation, and properties of matter.

Free energy called Gibbs free energy (G) is usable energy or energy that is available to do work. Consider the hypothetical reversible reaction in which reactants A and B react to form products C and D. This equilibrium can be shown below, where the lower case letters represent the coefficients of each substance.

The rates of the forward and reverse reactions are the same at equilibrium, and so the concentrations of all of the substances are constant. Since that is the case, it stands to reason that a ratio of the concentrations for any given reaction at equilibrium maintains a constant value. The equilibrium constant (Keq) is the ratio of the mathematical product of the concentrations of the products of a reaction to the mathematical product of the concentrations of the reactants of the reaction. Each concentration is raised to the power of its coefficient in the balanced chemical equation. For the general reaction above, the equilibrium constant expression is written as follows:

The concentrations of each substance, indicated by the square brackets around the formula, are measured in molarity units (mol/L).

The value of the equilibrium constant for any reaction is only determined by experiment. As detailed in the above section, the position of equilibrium for a given reaction does not depend on the starting concentrations, and so the value of the equilibrium constant is truly constant. It does, however, depend on the temperature of the reaction. This is because equilibrium is defined as a condition resulting from the rates of forward and reverse reactions being equal. If the temperature changes, the corresponding change in those reaction rates will alter the equilibrium constant. For any reaction in which a Keq is given, the temperature should be specified.

Le Chatelier’s principle is an observation about chemical equilibria of reactions. It states that changes in the temperature, pressure, volume, or concentration of a system will result in predictable and opposing changes in the system in order to achieve a new equilibrium state.

According to Le Chatelier’s principle, adding additional reactant to a system will shift the equilibrium to the right, towards the side of the products. By the same logic, reducing the concentration of any product will also shift the equilibrium to the right. The converse is also true. If we add an additional product to a system, the equilibrium will shift to the left, in order to produce more reactants. Or, if we remove reactants from the system, equilibrium will also be shifted to the left.

Thus, according to Le Chatelier’s principle, reversible reactions are self-correcting; when they are thrown out of balance by a change in concentration, temperature, or pressure, the system will naturally shift in such a way as to “re-balance” itself after the change.

VARIABLE To Increase the Rate of Reaction To Increase Yield of Reaction
Concentration/Pressure Increase Smaller number –> Bigger Number=Decrease
Bigger number –> Smaller Number= Increase
Temperature Increase Depends on reaction – look at the type of reaction
Exothermic = Decrease
Endothermic = Increase
Catalyst Add Makes reaction faster only
Surface Area Increase Makes reaction faster only
Amount of Reactants/Products In equilibrium Increase reactants
Decrease products (by removing them)

Free energy, called Gibbs free energy (G), is usable energy or energy that is available to do work. Every chemical reaction involves a change in free energy, called delta G (∆G). The change in free energy can be calculated for any system that undergoes a change, such as a chemical reaction. To calculate ∆G, subtract the amount of energy lost to entropy (denoted as ∆S) from the total energy change of the system. This total energy change in the system is called enthalpy and is denoted as ∆H. The formula for calculating ∆G is as follows, where the symbol T refers to the absolute temperature in Kelvin (degrees Celsius + 273): G=ΔH−TΔS.

  • Δr Go (rxn) = sum of the DGof (products) minus the sum of the DGof (reactants)
IF Δr G   is the change (or reaction) is
< 0 (free energy decreases) spontaneous
> 0 (free energy increases) non-spontaneous
= 0 at equilibrium

If energy is released during a chemical reaction, then the resulting value from the above equation will be a negative number. In other words, reactions that release energy have a ∆G < 0. A negative ∆G also means that the products of the reaction have less free energy than the reactants because they gave off some free energy during the reaction. Reactions that have a negative ∆G and, consequently, release free energy, are called exothermic reactions. exothermic means energy is exiting the system. These reactions are also referred to as spontaneous reactions because they can occur without the addition of energy into the system.

If a chemical reaction requires an input of energy rather than releasing energy, then the ∆G for that reaction will be a positive value. In this case, the products have more free energy than the reactants. Thus, the products of these reactions can be thought of as energy-storing molecules. These chemical reactions are called endothermic reactions; they are nonspontaneous. An endothermic reaction will not take place on its own without the addition of free energy.

The significance of the sign of a change in Gibbs free energy parallels the relationship of Keq when examining spontaneous and nonspontaneous reactions

If K, the reaction is nonspontaneous in the direction written.

If = K, the reaction is in a state of equilibrium.

If K, the reaction is spontaneous in the direction written.

 


Practice Questions

 

MCAT Official Prep (AAMC)

Practice Exam 4 B/B Section Passage 6 Question 32



Key Points

• The rates of the forward and reverse reactions are the same at equilibrium, and so the concentrations of all of the substances are constant.

• The equilibrium constant (Keq) is the ratio of the mathematical product of the concentrations of the products of a reaction to the mathematical product of the concentrations of the reactants of the reaction.

• The position of equilibrium for a given reaction does not depend on the starting concentrations, and so the value of the equilibrium constant is truly constant. It does, however, depend on the temperature of the reaction.

• Every chemical reaction involves a change in free energy, called delta G (∆G).

• To calculate ∆G, subtract the amount of energy lost to entropy (∆S) from the total energy change of the system; this total energy change in the system is called enthalpy (∆H ): ΔG=ΔH−TΔS.

• Le Chatelier’s principle can be used to predict the behavior of a system due to changes in pressure, temperature, or concentration.

• Le Chatelier’s principle implies that the addition of heat to a reaction will favor the endothermic direction of a reaction as this reduces the amount of heat produced in the system.

• Increasing the concentration of reactants will drive the reaction to the right while increasing the concentration of products will drive the reaction to the left.

• Endothermic reactions require an input of energy; the ∆G for that reaction will be a positive value.

• Exothermic reactions release free energy; the ∆G for that reaction will be a negative value.


Key Terms

Equilibrium: a condition resulting from the rates of forward and reverse reactions being equal.

Equilibrium constant (Keq): the ratio of the mathematical product of the concentrations of the products of a reaction to the mathematical product of the concentrations of the reactants of the reaction.

Exothermic reaction: a chemical reaction where the change in the Gibbs free energy is negative, indicating a spontaneous reaction

Endothermic reaction: a chemical reaction in which the standard change in free energy is positive, and energy is absorbed

Gibbs free energy: the difference between the enthalpy of a system and the product of its entropy and absolute temperature

Reversible reaction: a chemical reaction where  a reactant can turn into a product and back again

Le Chatelier’s principle: states that changes in the temperature, pressure, volume, or concentration of a system will result in predictable and opposing changes in the system in order to achieve a new equilibrium state



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