Relationship between free energy change and reaction quotient.

These equations are the beating heart of chemistry.

In our earlier discussion of Thermochemistry, we reviewed the concepts of internal energy change and enthalpy change. This equipped us to describe the changes in a substance as a thermochemical system. We learned how to compare the products and reagents in a chemical reaction in terms of the allotment of energy between the system and its surroundings. If the system completely changed from Reagents A to Products B, does heat flow in or out? At this stage, we are ready to seek to understand spontaneity in chemical transformations. What is the availability of energy in a chemical system for drive a reaction forward? We are describing the free energy in a chemical system, energy that when it is expended during chemical tranformation increases the entropy of the universe. Free energy is expended until the equilibrium state for the system is achieved. At equilibrium, heat flows between the system and its surroundings become microscopically reversible. The forward direction is just as likely as the reverse because the entropy associated with heat flows in either direction is the same.

On the MCAT, you probably will not have too many direct chemical thermodynamics questions, but the topic is the scaffolding on which much else is built. I believe chemical thermodynamics to be one of the most important subject areas in science. Most students find chemical thermodynamics to be a very abstract subject. This is unfortunate. I think this happens because of the way that chemical thermodynamics is approached in undergraduate general chemistry, without physics underneath and biology above. However, if you approach chemical thermodynamics with a concrete basis in force and energy, you can relate the internal energy changes driving heat flow and entropy change to a fundamental understanding of the particle level in substances, and when you bring chemical thermodynamics to your understanding of biochemistry, your appreciation of life processes will become much more coherent and much more interesting.

WikiPremed Resources

Chemical Thermodynamics & Equilibrium Practice Items
Problem set for Chemical Thermodynamics & Equilibrium in PDF format

Answer Key
Answers and explanations

Chemical Equilibrium Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for Chemical Thermodynamics & Equilibrium
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals


Understand the basis for enthalpy change in internal energy change and thermodynamic work under the First Law of Thermodynamics. Develop a facility for predicting the direction of heat flow in a chemical reactions.

Work on your intuitive ability to compare the degree of order between reagents and products in chemical reactions so that you can better understand entropy change.

Be able to predict the entropy change for phase transitions, changes in volume at constant temperature, changes in temperature at constant pressure, and changes in pressure at constant temperature.

Comprehend the relationship of the entropy change of the system and the enthalpy change (heat flow) in determining the availability of energy to drive a reaction spontaneously.

Be able to describe microscopic heat flows in a chemical system at equilibrium.

Appreciate the purpose of the convention of standard-state conditions. Be able to apply the standard free energy change to determine the free energy for various reaction quotients.

Understand the difference between an equilibrium constant and other reaction quotients in terms of free energy.

Understand the effects of changing concentrations, external pressure, and temperature on an equilibrium mixture through the qualitative application of Le Chatelier's principle.

Suggested Assignments

Review the basic terms for chemical thermodynamics & the equilibrium state using the question server. Complete the fundamental terms crossword puzzle. Here is the solution to the puzzle.

Perform the practice items for chemical thermodynamics & equilibrium. Here is the answer key for the practice items.

In ExamKrackers Chemistry. Read pp. 131-140. Perform practice items 81-88 on pg. 141. (Reading and practice items for thermochemistry and some chemical thermodynamics concepts). Read pp. 142-148. Perform practice items 89-96 on pg. 149.

Review the web resources for chemical thermodynamics & equilibrium.

Review the web resources for states of matter.

Conceptual Vocabulary for Chemical Thermodynamics & Equilibrium

Equilibrium constant
The equilibrium constant is the reaction quotient describing the state in which the chemical activities or concentrations of the reactants and products have no net change over time.
Spontaneous process
A spontaneous process is a chemical reaction in which a system releases free energy and moves to a lower, more thermodynamically stable, energy state.
Chemical thermodynamics
Chemical thermodynamics is the mathematical study of the interrelation of heat and work with chemical reactions or with a physical change of state within the confines of the laws of thermodynamics.
Thermodynamic free energy
The term thermodynamic free energy is a measure of the amount of work that can be extracted from a system.
Endergonic reaction
An endergonic reaction (also called an unfavorable reaction or a nonspontaneous reaction) is a chemical reaction in which the standard change in free energy is positive.
Exergonic reaction
An exergonic reaction is a chemical reaction where the variation of Gibbs free energy is negative.
Le Chatelier's principle
Le Chatelier's principle states that if a chemical system at equilibrium experiences a change in concentration, temperature, volume, or total pressure, the equilibrium will shift in order to partially counter-act the imposed change.
Gibbs free energy
The Gibbs free energy is a thermodynamic potential which measures the useful or process-initiating work obtainable from an isothermal, isobaric thermodynamic system.
Dynamic equilibrium
A dynamic equilibrium occurs when two reversible processes proceed at the same rate.
Reaction quotient
The reaction quotient is a quantitative measure of the extent of reaction, the relative proportion of products and reactants present in the reaction mixture at some instant of time.
Entropy of mixing
The entropy of mixing is the change in the entropy when two different chemical substances or components are mixed.
Activity in chemistry is a measure of how different molecules in a non-ideal gas or solution interact with each other, extending the idea of concentration to more complex systems.
Activity coefficient
An activity coefficient is a factor used in thermodynamics to account for deviations from ideal behaviour in a mixture of chemical substances.
Thermodynamic potential
Thermodynamic potentials are parameters associated with a thermodynamic system and have the dimensions of energy which describe the capacity for spontaneous change in a thermodynamic system when it is subjected to certain constraints.
Chemical potential
The chemical potential of a thermodynamic system is the amount by which the energy of the system would change if an additional particle were introduced, with the entropy and volume held fixed.
Fugacity is a measure of chemical potential in the form of 'adjusted pressure' directly related to the tendency of a substance to prefer one phase over another.
Thermodynamic limit
The thermodynamic limit is reached as the number of particles in a system approaches infinity and the thermodynamic behavior of a system is asymptotically approximated by the results of statistical mechanics.
Helmholtz free energy
The Helmholtz free energy is a thermodynamic potential which measures the useful work obtainable from a closed thermodynamic system at a constant temperature.
Gibbs paradox
The Gibbs paradox, also known as mixing paradox, involves the discontinuous nature of the entropy of mixing.
Gibbs' phase rule
The Gibbs' phase rule relates the number of phases present in equilibrium, the number of degrees of freedom, pressure and composition of the phases present, and the number of chemical components required to describe the system.
Advanced terms that may appear in context in MCAT passages
Non-equilibrium thermodynamics
Non-equilibrium thermodynamics is a branch of thermodynamics concerned with studying time-dependent thermodynamic systems, irreversible transformations and open systems.
Dissipative system
A dissipative system is a thermodynamically open system which is operating far from thermodynamic equilibrium in an environment with which it exchanges energy, matter and/or entropy.
Debye-Hückel equation
The Debye-Hückel limiting law provides a method for obtaining activity coefficients for solutions that contain ionic solutes.
Van't Hoff equation
The Van't Hoff equation in chemical thermodynamics relates the change in temperature to the change in the equilibrium constant given the enthalpy change.
Gibbs-Duhem equation
The Gibbs-Duhem equation in thermodynamics describes the relationship between changes in chemical potential for components in a thermodynamical system, showing that in thermodynamics intensive properties are not independent but related.