Preparing for the American Chemical Society General Chemistry Exam (Part I)
Many college courses require you to take the first section of the American Chemical Society General Chemistry Exam as your final. Whether you are majoring in chemistry or not, the ACS exam may have you cowering in fear. Learn how to use a variety of resources to master everything you need to know about your first semester in General Chemistry.
Get Your Study Guide Early
The first thing you need to do is purchase the ACS official study guide. The book is just over 100 pages long and provides example questions along with explanations for the correct answer. It is separated into the following categories, each containing an impressive set of practice questions similar to the ones you will find on the exam.
- Atomic Structure
- Molecular Structure and Bonding
- States of Matter/Solutions
- Energetics (also known as thermochemistry or thermodynamics)
- Descriptive Chemistry/Periodicity
- Laboratory Chemistry
In many Gen Chem I courses, dynamics and equilibrium are not discussed, and they will not be reviewed in this article.
The exam focuses on remembering important constants and trends, and that is where a good memory and steady studying can boost your grade on this exam.
Isotopes are varied forms of an element that have different mass numbers.
It is almost guaranteed that the exam will contain an isotope question: for example, something like this:
How many protons are in the isotope 28Al?
It is important to remember that different isotopes of an element will not vary in the number of protons. The amount of protons will always be the atomic number, which in the case of aluminum (Al) is 13.
The number of electrons in 28Al, or any isotope of the pure element (aluminum metal), is also 13. The only way the amount of electrons will change is if there is a charge on the atom. An atom with a charge, called an ion, will have the charge written as a superscript. The aluminum ion Al3+, which has a charge of +3, would have 10 electrons. A positive charge means electrons are lost when the atom becomes an ion.
The number of neutrons is slightly trickier. You must subtract the atomic number from the atomic weight (mass number). In this case, that would be 28-13, which is 15. So 28Al has 15 neutrons. A good way to remember this is to think of neutrons as the "black sheep" of the atom. They have no charge, so it requires a little more effort to figure out how many of them there are.
Molecular Structure and Bonding
This topic gets a little tricky, especially if you are not good at remembering names.
Expect to see at least one question on the geometry of an atom. As the exam does not want you to waste unnecessary time on a simple task, it is likely that the Lewis Dot structure will already be done for you: now it is just a matter of knowing your stuff.
It is important to remember that lone electrons on the central atom of the structure count as a side of the figure. Many books will use a steric number to figure out the geometry, but this technique is rather involved for this exam and will not be discussed.
Number of sides, with no lone pairs:
2: the shape is Linear
3: the shape is Trigonal Planar
4: the shape is Tetrahedral
5: the shape is Trigonal Bipyramidal
6: the shape is Octahedral
Shape of a Molecule Versus Number of Sides
Number of sides (with no lone pairs)
Now, there are exceptions to these names if a lone pair is included in the figure. This article provides a complete list of all the names of these figures. It is also important to know the bond angles of these figures.
Another important topic is the shape of the separate orbitals. The s orbital has a sphere shape, the p is dumbbell shaped. The rest of the shapes and allowed quantum numbers are explained here.
There is not much to say about this topic, either you know it or you don't. This topic is used frequently in the test and you should have a solid knowledge of these three things:
1. How to find the empirical and molecular formula for a compound
2. How to find the percent composition of a compound
3. How to determine the mass of a compound yielded using a balanced equation
You will also need to know how to use Avogadro's number correctly (6.022 x 1023). Some questions may ask you to find the amount of atoms or molecules in something, in which case you need to know that there are 6.022 x 1023 molecules in one mole of something.
States of Matter/Solutions
There are two things that should be stressed regarding this topic.
1. The first is that you know what a phase diagram is and what it represents. It represents phase changes in an element or compound under different temperatures and pressures: the x axis is temperature and the y axis is pressure.
A phase diagram usually has a nice prong shape, with the middle being the liquid phase, the left side being the solid phase, and the bottom is the gas phase. It is also important to know the names of phase changes (sublimation, condensation, etc.)
The second thing that will likely show up on the exam regarding states of matter is the difference between a substance, a pure element, and homogeneous/heterogeneous compound. Usually this will appear as a series of representations of these types of matter, and it will ask you to choose the correct one. If you cannot visually tell between these things, it will be helpful to look at the link below.
The Difference Between Mixtures and Pure Substances
The most importance thing about energetics is knowing your equations and strategies!
and under constant pressure:
Also remember to keep your constants straight! Your value for the specific heat has units, which should match your other variables. Specific heat values will be given to you, of course.
You should also know how to calculate ΔH, which is done in several ways:
1. Hess's Law: If you don't remember, Hess's Law requires manipulation of several equations that are combined (along with respective ΔH) to calculate ΔH for a target reaction.
2. nΣProducts - nΣReactants, where n is the number of moles (given in a balanced equation) and respective ΔH values are given for the formation or decomposition of the compounds in the reaction.
How To Calculate Bond Energy
Though some courses will have covered electrochemistry in detail, others leave out that subject to save time. It will not be discussed here, but here is a link for more information.
There will be at least one redox-related question on the exam. Here are a few things to keep fresh in mind.
- How to determine oxidation numbers (remembering that certain elements, like oxygen, sulfur, hydrogen, and flourine have set oxidation numbers)
- How to determine reduced and oxidized elements in a reaction (and their agents!)
- How to properly balance a reaction performed in either basic or acidic solution (though this is less likely to appear, it is good to know if continuing on with chemistry)
And on that note, know the difference between a "solution" and a "solvent"! A solvent dissolves in a solute, and creates a solution.
This topic really tests your ability to remember closely related periodic trends, as well as specific traits. Here is a list of what you might see.
- Questions about physical properties of the transition metals. For example, transition metals usually turn vibrant colors when ionized.
- Questions about atomic radii. This is where you NEED to know the trend. The smaller elements are in the upper right corner while the largest are in the bottom left corner. Ions are tricky, this is where you need to compare the amount of protons in the atom and the amount of electrons. If an atom has more protons than electrons, then the nucleus is more effective at pulling electrons in, making it smaller.
- Questions about electronegativity. The trend here is, the smaller the atom, the more electronegative it is. This is also good to know if you get a question about polarity. There needs to be an uneven spread of polar bonds in a molecule in order to be polar.
1. Knowing your equipment. Sure, you know what a beaker is, but how about a mass spectrometer? (it separates atoms by size, by the way).
2. Knowing your significant figures. This is a huge deal in any science. If you don't know this by now, you better get going! You also need to be fknow how many significant figures common lab equipment can read to. A buret measures to two decimal places, by the way.
3. Knowing the difference between precision and accuracy.
Let's say your target number is 35.51.
If you get 35.81 and 35.80, that is precise, but not accurate.
If you get 35.90 and 35.70, that is accurate, but not precise.
4. You may also be asked to calculate percent error. The equation for this is:
absolute value (actual - theoretical) / actual value