Gibbs Free Energy Worksheet Ap Biology Essays

Introduction to Gibbs Free Energy

In this article, we will cover Gibbs Free Energy. To adequately describe the energetic state of a system you need to include all the various functions relating to the variables of kinetic, potential, and internal energy. Many equations compromise the total potential and internal energies of a system, and calculating each one is an incredibly tedious process, if not downright impossible. So, a way to get around this obstacle is only to consider changes in a system. When we do so, we only need to consider those variables that are changing between the beginning and end of our reaction.

Thermodynamic Expression for Free Energy – Essential Knowledge 5.E.2.

The Gibbs Free Energy is a relative value that serves as a benchmark for calculating how much energy a reaction will consume to proceed, or generate while proceeding. Gibbs Free Energy change, or delta G (\Delta G) in short, is a thermodynamic calculation based on the changes in Entropy and Enthalpy of a system. Since these parameters are functions of temperature, they fall under the topic of thermodynamics. When these parameters are calculated under standard conditions (STP), they are referred to as standard changes in free energy, enthalpy, or entropy, respectively. The changes in each of these quantities are represented mathematically below.

\Delta { S }^{ o }=\Sigma { S }^{ o }products – \Sigma { S }^{ o }reactants

\Delta { H }^{ o }=\Sigma \Delta { H }_{ { f }^{ o } }products – \Sigma \Delta { H }_{ { f }^{ o } }reactants

\Delta { G }^{ o }=\Sigma \Delta { G }_{ { f }^{ o } }products – \Sigma \Delta { G }_{ { f }^{ o } }reactants

The standard change in any of the thermodynamic properties is calculated based on the sum of the property in the products minus the sum of the property in the reactants.

There are three different flavors of the equation for calculating the standard Gibbs Free Energy of a system. Standard entropy, enthalpy, and free energy constants are all related to each other by the expression below, where the absolute temperature T is in units of Kelvin. The standard change in entropy and enthalpy are both experimental values that would be provided for you on the AP Exam questions.

\Delta { G }^{ o }=\Delta { H }^{ o }-T\Delta { S }^{ o }

A negative change in delta energy means that energy is released from the system to the surroundings, and the reaction is therefore overall favorable. Therefore, reactions with a negative standard change in enthalpy and a positive standard change in entropy will always be thermodynamically favorable (i.e. spontaneous) at STP. Conversely, reactions that have a positive standard change in enthalpy and a negative standard change in entropy will always require a net input of energy from their surroundings, and will, therefore, be thermodynamically unfavorable (i.e. non-spontaneous) at STP.

When both the standard change in entropy and enthalpy have the same sign, the favorability of the reaction depends on the temperature at which the reaction takes place and the difference in values between the two variables. For example, a reaction with a very large positive standard change in enthalpy and only a small positive standard change in entropy will only be favorable at very high temperatures. The reverse is true for reactions with a small negative standard change in enthalpy but a large positive standard change in entropy. Qualitatively this can be summed up as

Value of \Delta { H }^{ o }
Value of \Delta { S }^{ o }
\Delta { H }^{ o }<0\Delta { S }^{ o }<0Spontaneous at low T
\Delta { H }^{ o }>0\Delta { S }^{ o }>0Spontaneous at high T
\Delta { H }^{ o }<0\Delta { S }^{ o }>0Always spontaneous
\Delta { H }^{ o }>0\Delta { S }^{ o }<0Never spontaneous


A multiple choice question on the AP Exam might look something like this:

Example 1

Natural gas is used to heat up many residential houses. The main component in natural gas is propane. Calculate the amount of free energy when methane combusts in oxygen at 25°C, given the following standard thermodynamic values.

\Delta { H }_{ { f }^{ o } }\left( { Kj }/{ mol } \right)
{ S }^{ o }\left( { J }/{ K\cdot mol } \right)
\Delta { G }_{ { f }^{ o } }\left( { kJ }/{ mol } \right)
{ C }_{ 3 }{ H }_{ 8 }\left( g,propane \right)-103.8269.9-23.49
C{ O }_{ 2 }\left( g \right)-393.509213.74-394.359
{ O }_{ 2 }\left( g \right)0205.1380
{ H }_{ 2 }O\left( g \right)-241.818188.825-228.572


The first thing we would need to do in this case is write out and balance the chemical reaction. The combustion of propane will be written out as follows:

{ C }_{ 3 }{ H }_{ 8 }+{ O }_{ 2 }\leftrightarrow C{ O }_{ 2 }+{ H }_{ 2 }O

We can now balance the equation to take into account how many moles of compounds are on each side of the equation. There must be three times as many moles of carbon dioxide as moles of propane since each molecule of propane has three carbon atoms. This reaction will also create four molecules of water for every molecule of propane since propane contains eight hydrogen atoms. The total number of oxygen atoms in three molecules of carbon dioxide and four molecules of water is an even number. Therefore, we do not need to adjust the molar coefficient of propane.

{ C }_{ 3 }{ H }_{ 8 }+5{ O }_{ 2 }\leftrightarrow 3C{ O }_{ 2 }+4{ H }_{ 2 }O

To find the Gibbs free energy, we must calculate the total change in entropy and multiply that by the temperature in Kelvin. To do so, we will multiply the molar coefficient of the compound from the balanced chemical equation by the thermodynamic value provided above.

\Delta { S }^{ o }=\Sigma { S }^{ o }products – \Sigma { S }^{ o }reactants

\Delta { S }^{ o }=[(3\times 213.74)+(4\times 188.825)]-[(269.9)+(5\times 205.138)]

\Delta { S }^{ o }=100.93{ J }/{ K\cdot mol }

Then convert the units of temperature from degrees Celsius to units Kelvin.

Temperature (K) ={ 25 }^{ o }C+273=298K

Finally, we will replicate the process outlined for the change in entropy and repeat it for finding the standard change in enthalpy of the combustion.

\Delta { H }^{ o }=\Sigma \Delta { H }_{ { f }^{ o } }products – \Sigma \Delta { H }_{ { f }^{ o } }reactants

\Delta { H }^{ o }=[(3{ \times }^{ - }509)+(4{ \times }^{ - }241.818)]-[(^{ - }103.8)+(5\times 0)]

\Delta { H }^{ o }=^{ - }2,046\cdot { kJ }/{ mol }

Now that we have calculated all the necessary variables, we plug the values into the expression for the standard change in free energy. BUT WAIT! We have to make sure that all the units to match… in this case, we will convert the units of standard change in entropy to kilo Joules.

\Delta { G }^{ o }=\Delta { H }^{ o }-T\Delta { S }^{ o }=\left( ^{ - }2,046\cdot { kJ }/{ mol } \right) - [(298K)(0.10093{ J }/K\cdot mol)]

\Delta { G }^{ o }=^{ - }2,076\cdot { kJ }/{ mol }

The big takeaway here is to remember to multiply the standard thermodynamic value by the molar coefficient of the compound as determined by the balanced chemical equation.

Equilibrium Expression – Essential Knowledge 3.A.2.

The expression for free energy can take on two other forms, depending on the context. If we are dealing with equilibrium problems, then the equilibrium expression for free energy will be used, where R is the Gas constant, and K is the equilibrium expression for a chemical reaction.

\Delta { G }^{ o }=-RT\ln { K }

As you may recall, equilibrium is like the energetic low-point in the system, and can be expressed as a fraction of the theoretical concentrations of products and reactants when the reaction has reached this low point.

{ O }_{ 2 }(g) + { C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 }(s) = { CO }_{ 2 }(g) + { H }_{ 2 }O(g) + q

Above is the unbalanced reaction for the combustion of glucose in oxygen to water and carbon dioxide. Balancing this reaction is no different than balancing any other reactions: we place the compounds side by side, determine how many moles of each reactant would be required for the reaction to go to completion, and how many moles of the product would be generated. In this case, we know that carbon-containing products will be generated in multiples of six, and hydrogen-containing products in multiples of twelve. Using the lowest-possible multiple, we find that the reaction balances simply to:

6{ O }_{ 2 }(g) + { C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 }(s) = { 6CO }_{ 2 }(g) + 6{ H }_{ 2 }O(g) + q

We will then use the balanced reaction to write out the equilibrium expression. For a reaction in the form:

aA + bB = cC + dD

The general scheme for writing out the equilibrium expression, including the molar amounts of each compound, is shown below. The concentration of each gaseous compound is expressed as the partial pressure, { P }_{ A }. If we know the total pressure at which the reaction is taking place, we can then calculate the molar fraction of the gas in the air or vice versa.

K = \frac { { [A] }^{ a }{ [B] }^{ b } }{ { [C] }^{ c }{ [D] }^{ d } }

Electrochemical Free Energy – Essential Knowledge 6.D.1.

If on the other hand, the topic is electrochemistry, then the Nernst Equation should be substituted into the expression, and the value of Faraday’s constant (F) and the standard reduction potential of the cell ({ E }^{ o }) should be used. The amount of free energy, in this case, depends on the number of moles of the compound in solution and is represented by the letter n.

\Delta { G }^{ o }=-nF{ E }_{ cell }^{ o }; where F = Faraday’s Constant = 96,485{ 96,485 coulombs }/{ mol }{ e }^{ - },

{ e }^{ o } = Standard Reduction Potential

n = number of moles of electrons

The { E }_{ cell }^{ o }, or standard reduction potential, is an experimentally-determined value of the electrical potential in volts (V) across the two half reactions. The standard reduction potential for the hydrolysis of hydrogen gas into protons and electrons is defined as 0. Since n must be a positive integer and F is a constant, any values of { E }_{ cell }^{ o } that are positive will give a negative \Delta { G }^{ o }, so the reaction spontaneous in the direction it is written. On the other hand, if { E }_{ cell }^{ o } is negative, then \Delta { G }^{ o } is positive, and the reaction as written becomes unfavorable and non-spontaneous.

Keep this diagram in mind when trying to figure out the relationship between free energy, standard reduction potential, and the equilibrium concentration of products and reactants in a reaction. It’s important to remember that { E }_{ cell }^{ o } measures the potential across the reactions going from right to left. Half reactions with a negative { E }_{ cell }^{ o } value are spontaneous in the direction they are written since a negative value of { E }_{ cell }^{ o } results in a negative \Delta { G }^{ o }.

Featured Image Source

Let’s put everything into practice. Try this AP Chemistry practice question:

Looking for more AP Chemistry practice?

Check out our other articles on AP Chemistry.

You can also find thousands of practice questions on lets you customize your learning experience to target practice where you need the most help. We’ll give you challenging practice questions to help you achieve mastery of AP Chemistry.

Start practicing here.

Are you a teacher or administrator interested in boosting AP Chemistry student outcomes?

Learn more about our school licenses here.

Dear Parents and Students,

All of my notes, calendars and tutorials are posted on SCHOOLOGY.  The information posted on this webpage is derived from the older curriculum and can be considered as extra information available for students who desire extra help.  Please sign in to your SCHOOLOGY ACCOUNT to receive this year's notes and lessons by clicking on the following link:   




    •  Sasek Students:   88.8% Passed
    •   National Pass Rate:    63.2%
    • Congratulations on your amazing results.  I am SO Proud of you!!



Welcome Students. The First section lists a ton of study sites for you to use as a reference throughout the year with practice AP exams and quizzes.  After the photos section is each unit's information, in order with support materials.



  Please feel free to email me @  if you have any questions.


STUDY TOOLS FOR YOU! (ALL Individual Unit Information begin after photos)



  -Lab Tutorials and Pre-lab Animations and Questions
      (Lab Bench AP Lab Tutorials)
 2)  AP BIOLOGY FLASH CARDS (all terms from each chapter!!)
        (chapters are off by one...our chapter 24 is their chapter 23)


 3)  BIOCOACH ACTIVITIES (helps review major concepts from each unit with quiz)



  4) PRACTICE TESTS AND QUIZZES FOR EACH UNIT!!! (check this out; worth doing!!




















   11)  Review AP Exams from REVIEW BOOKS

       - CampaignID=9248


  12) Cummulative list of ALL VOCAB TERMS/ALL CHAPTERS:   see how many you know!!




  1. Hamster Research Links
  • Hamster Genetics  (use this link for genetic info: we have Dwarf Campbell Russians...follow "Genetics" links to determine which traits are dominant).
  • Additional Hamster Genetics  (Hamster Genetics Direct Link)

AP Biology Dunes Succession Field Study, MILLER DUNES/PAUL H. DOUGLAS CENTER in Gary, Indiana. (September 23, 2011)

(AP Biology Macroinvertebrate Field Study, Bartel Grasslands and Frankfort Square
October, 2010)




(AP Biology Students at Indiana Dunes, Oct. 1, 2012 and Prairie, Yankee Woods

and Bartel Grasslands, November 20, 2012)




INTRODUCTION UNIT:  Themes/Experimental Design & Statistics

a) Chapter 1 Scientific Inquiry/Data Analysis

  • Emergence Video NOVA                                                                                                                                                                                

b)  StatisticsTutorials (Khan Academy) 


c) Statistics Labs with M&M's

d)  Practice Test Questions (Click on the

Unit I:  Biochemistry (Chapters 2-5)

Chapter 2:  Basic Chemistry Review 
 -Chapter 2 NOTES

 -Chapter 2 Powerpoint Notes
 -Chapter 2 Homework Study Guide
 -Chapter 2 Activity
- Emergent Properties Video
-Animations and Tutorials  (bonding, atomic number, ect)      

 -Bozeman Biology Tutorials                                                                                                                                                                  


  -Chapter 2 Practice Test   (click on chapter; select maximum question number 0f 40 )                                                                                                                                                                                                                                                                                    
Chapter 3:  Properties of Water
  • -Water Powerpoint
  • -Chapter 3 Notes Water Chemistry
  •  -Chapter 3 Homework Study Guide
  • -Water Properties Activity (Book)
  • -Bozeman Biology Tutorials                                                                                                                                                                                                           
- Chapter 3 Animations 
  • -pH Animation
  • -pH Tutorial
  • -Buffer Animation
  • - Lecture on Acid/Base/Buffers and pKa
  • -Cat drinking water
  • -Dog drinking water 


    • Chapter 3 Practice Test   (click on chapter; select maximum of 40 )                                                                                                                                                                                                       


Chapter 4:  Carbon Chemistry


Chapter 5 Macromolecules


Carbs Notes

Carbs Study guide

Lipid Notes

Lipid Studyguide

Protein Notes

Protein Study Guide

Nucleic Acids Notes

Nucleic Acid Study Guide



Extra Worksheets

Worksheet 4/5.1 Identify Macromolecules

Worksheet 4/5.2 Macromolecules

Chart Cheat Sheet for Macromolecules
Amino acids chart

ANIMATIONS Of Biological Molecules (Lipids, Proteins, Carbs, Nucleic Acids)
macromolecules animation
step by step biomolecules tutorial with quiz
types of chemical reactions animation
hydrolysis/dehydration synthesis animation 1
hydrolysis/dehydration synthesis animation 2
You tube animation of biomolecules (good one)
Protein Folding (know this one1)
Protein Folding and pH Animation
Protein Folding Again
Advanced Protein Folding
Amino Acids and pH animation

Review Self-Quizzes
Chapter 5 Practice quiz (Choose Chapter)
Functional groups Quiz Interactive
DNA vs. RNA quiz

Unit II:  Cell Energy  (Chapters 8-10)


Chapter 8 Thermodynamics/Enzymes



  1. Gibbs Energy and Thermodynamics Online Tutorials
  1. ENZYMES Animations and Tutorials


  1. Enzyme Lab #2:  One of the "Big 13" 





Chapter 9 Respiration


Chapter 10 Photosynthesis  

         look at all of the photosynthesis quizzez and take them...use the animations for photosynthesis under "Web Links" as

         a wonderful tutorial.  These worksheets and quizzez will be used in class as study tools. 



Unit III: Ecology Unit (CHAPTERS 51-55)

  • Chapter 51 Animal Behavior


  • Chapter 50 and 52:  Introduction to the Biosphere and Population Ecology


  • Chapter 52 Video Lecture:  Population Ecology   
      • Watch Population Ecology:  Unlimited Resources
      • Watch Population Ecology:  Limited Resources
      • Watch Population Ecology:  r versus K
      • Watch Pop. Ecol:  Intraspecific competition
      • Watch Pop. Ecol:  Interspecific competition
      • Watch Pop. Ecol:  Predation
      • Watch Pop. Ecol:  Niches and ALSO symbiosis!!



Chapter 53:  Community Ecology 


Semester 1 Ecology Review Question Answers (1-44)

Sem1 Ecology Sample Questions Review Answers


Unit IV:   Cell Biology Ch. 6,7,11,12


  • Cell Video Lectures Cell Techniques   (if you can't find video directly through link, search under my mindbites)


  • Cell Video Lectures, Cell Structure and Organelles

Chapter 7 Cell Membranes/Transport


Leave a Reply

Your email address will not be published. Required fields are marked *