DAT Chemistry Tip: How To Solve Gibbs Free Energy Problems
Gibbs free energy problems are one of those tricky questions that will most likely be tested on your DAT exam.
Here’s a question to test your knowledge of Gibbs free energy:
Which of the following is true about a spontaneous, exothermic reaction?
A. ΔG is positive
B. ΔS is positive
C. ΔS is negative
D. ΔH is positive
Don’t stress out if you can’t answer this question right now. Once you understand the concepts given below, you will to be able to solve this problem on the DAT exam very easily.
And oh yeah, this question will most likely show up on your DAT test. So make sure you understand this concept!
So What Is Gibbs Free Energy?
This is what wikipedia says:
“In thermodynamics, the Gibbs free energy (IUPAC recommended name: Gibbs energy or Gibbs function; also known as free enthalpy[1] to distinguish it from Helmholtz free energy) is a thermodynamic potential that measures the “useful” or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure (isothermal, isobaric)”
It’s something we don’t need to know. Sure if you love chemistry so much, you might want to understand this and other information on wikipedia to satisfy your appetite, but for our purposes, let’s just focus on what’s important here.
→ Gibbs free energy ΔG, is the energy that’s available to do useful work for a chemical reaction. ΔG helps us determine whether a given reaction is spontaneous or not.
Change in Gibbs Free Energy:
ΔG = ΔH – T ΔS
You must memorize this equation!
Here’s a mnemonic: Goose Hunters Take Shot Guns
If ΔG < 0, reaction is spontaneous.
If ΔG = 0, reaction is at equilibrium.
If ΔG > 0, reaction is non-spontaneous.
So how can we know when ΔG < 0 or ΔG > 0 ?
Let’s look at the equation first: ΔG = ΔH – T ΔS
Here are the different scenarios:
- If ΔH is negative and ΔS is positive, ΔG will be negative. Thus, reaction is spontaneous.
- If ΔH is positive and ΔS is negative, ΔG will be positive. Thus, reaction is non-spontaneous.
- If ΔH is positive and ΔS is positive, ΔG will be negative at high temperatures (T) and ΔG will positive at low temperatures (T). Thus, the reaction can be spontaneous or non-spontaneous depending on temperature.
- If ΔH is negative and ΔS is negative, ΔG will be positive at high temperatures (T) and ΔG will negative at low temperatures (T). Thus, the reaction can be spontaneous or non-spontaneous depending on temperature.
The chart below summarizes the points written above:
ΔH | ΔS | ΔG |
---|---|---|
Negative (-) | Positive (+) | Negative (-) |
Positive (+) | Negative (-) | Positive (+) |
Positive (+) | Positive (+) | Negative (-) at high T and Positive (+) at low T |
Negative (-) | Negative (-) | Positive (+) at high T and Negative (-) at low T |
If ΔG < 0, then reaction is spontaneous. | ||
If ΔG = 0, then reaction is at equilibrium. | ||
If ΔG > 0, then reaction is non-spontaneous. |
So let’s review the question from the beginning of this post:
Which of the following is true about a spontaneous, exothermic reaction?
A. ΔG is positive
B. ΔS is positive
C. ΔS is negative
D. ΔH is positive
By looking at the chart, we see that only possible scenario is Answer choice B. For a reaction to be a spontaneous in a exothermic reaction (ΔH < 0), ΔS must be positive.
How is the DAT scored?
The DAT is scored on a scale of 1 to 30. However, it is not a linear scoring system.
A 20 on a section doesn’t mean you answered one third of the questions incorrectly, but in fact you may have answered 90% of the questions correctly.
Your score is based on the number of questions you answered correctly in comparison to others who answered questions correctly. It is then calculated through CDA’s proprietary formula.
Each section is scored individually and then groups of sections are scored collectively.
The standard score of 15 signifies average performance on a national basis.
You will receive 6 scores on the DAT:
Biology
Chemistry
Reading Comprehension
Total Science (TS)
Academic Average (AA)
Perceptual Ability Test (PAT)
TS = Total Science. Weighted Average from Biology and Chemistry.
AA = Academic Average. Weighted Average from Biology, Chemistry, Reading Comprehension and Perceptual Ability Test.
PAT = Perceptual Ability Test (angle discrimination, form development, cubes, orthographic projections and apertures)
Will You Ever Feel Ready For the DAT?
Quite often, students say that they don’t feel ready for the DAT a few days before the DAT exam. In many cases, they’ve been studying for months, so how can it be that after all the time spent studying, they don’t feel prepared?
Keep in mind that it’s very unlikely that you’ll ever know 100% of the material covered in all the resources you study from.
It doesn’t matter what level you’re at, and no matter how much you prepare for the DAT, you will see a few questions on the DAT that are unfamiliar. It’s just the nature of the beast and it’s okay not to feel ready!
It may help to know that everybody else feels the same way you do – most students don’t feel completely ready for the DAT!
However, do get your bearings straight.
The DAT is not as difficult as you may think. Keep in mind that for most pre-dental students, the DAT is simply a review of what you have already learned in your undergraduate courses. In fact, for biology majors, the exams you encountered in class are more difficult than the biology questions on the DAT.
Just as an aside, you should study for at least 2 – 3 months for the DAT. Do as many DAT practice questions as you can. Plan out your DAT study schedule, set goals and keep your notes organized.
Join A Pre-Dental Student Club
You should join the pre-dental student club at your University.
Usually, the pre-dental student clubs hold mock DAT exams. These mock DAT exams can help you prepare for the exam day since you first-hand experience what it feels like to be examined with a proctor in the room. This is a better option than practicing yourself because you won’t be able to ‘cheat’ or take regular breaks.
Another benefit is that pre-dental clubs invite Dental students to give tips on how they prepared for the DAT exam. They also share their experiences of being a Dental student and what you should expect when you get into Dental school.