Chemistry Articles

Cheat Sheet / Updated 11-08-2022

Chemistry covers all kinds of stuff. Sometimes you might not be sure where to start when you are first given a set of problems and told to go forth and succeed. Sometimes it’s converting metric units, writing ionic formulas, naming covalent compounds, balancing reactions, or dealing with extensive and intensive properties. This Cheat Sheet is designed to give you some help on a few of the trickier things you might encounter so that when you are done looking it over you can go forth and succeed!

Article / Updated 10-26-2022

A conversion factor uses your knowledge of the relationships between units to convert from one unit to another. For example, if you know that there are 2.54 centimeters in every inch (or 2.2 pounds in every kilogram or 101.3 kilopascals in every atmosphere), then converting between those units becomes simple algebra. It is important to know some common conversions of temperature, size, and pressure as well as metric prefixes. Conversion factor table The following table includes some useful conversion factors. Using conversion factors example The following example shows how to use a basic conversion factor to fix non-SI units. Dr. Geekmajor absentmindedly measures the mass of a sample to be 0.75 lb and records his measurement in his lab notebook. His astute lab assistant, who wants to save the doctor some embarrassment, knows that there are 2.2 lbs in every kilogram. The assistant quickly converts the doctor’s measurement to SI units. What does she get? The answer is 0.34 kg. Let’s try another example. A chemistry student, daydreaming during lab, suddenly looks down to find that he’s measured the volume of his sample to be 1.5 cubic inches. What does he get when he converts this quantity to cubic centimeters? The answer is 25 cm3. Rookie chemists often mistakenly assume that if there are 2.54 centimeters in every inch, then there are 2.54 cubic centimeters in every cubic inch. No! Although this assumption seems logical at first glance, it leads to catastrophically wrong answers. Remember that cubic units are units of volume and that the formula for volume is Imagine 1 cubic inch as a cube with 1-inch sides. The cube’s volume is Now consider the dimensions of the cube in centimeters: Calculate the volume using these measurements, and you get This volume is much greater than 2.54 cm3! To convert units of area or volume using length measurements, square or cube everything in your conversion factor, not just the units, and everything works out just fine.

Article / Updated 09-27-2022

Studying the elements of the periodic table is vital for understanding organic chemistry. So that you don't have to memorize each element, they're grouped together by their properties.

Step by Step / Updated 09-27-2022

When elements combine through chemical reactions, they form compounds. When compounds contain carbon, they’re called organic compounds. The four families of organic compounds with important biological functions are

Cheat Sheet / Updated 04-08-2022

Chemistry II is more than fires and smelly explosions. Chemistry II is more about solving calculations. In fact, Chemistry II has a lot more calculations and math than your Chemistry I class did. In your Chemistry II class, you need to master several formulas so you can calculate different mathematical problems, ranging from kinetics, different types of equilibrium, thermochemistry, and electrochemistry. This Cheat Sheet can serve as a quick reference to how to solve kinetics, thermodynamics, and different types of equilibrium problems.

Cheat Sheet / Updated 02-24-2022

You won't get very far in your study of organic chemistry without the periodic table of elements and an understanding of the common functional groups (or reactive centers) that dictate how most of a compound's chemical reactions occur.

Cheat Sheet / Updated 02-17-2022

Solving chemistry problems is a great way to master the various laws and calculations you encounter in a typical chemistry class. This Cheat Sheet provides some basic formulas, techniques, and tips you can refer to regularly to make solving chemistry problems a breeze (well, maybe not a breeze, but definitely easier).

Article / Updated 12-17-2021

Chiral molecules usually contain at least one carbon atom with four nonidentical substituents. Such a carbon atom is called a chiral center (or sometimes a stereogenic center), using organic-speak. Any molecule that contains a chiral center will be chiral, with the exception of a meso compound (see below for how to identify these). For example, the compound shown here contains a carbon atom with four nonidentical substituents; this carbon atom is a chiral center, and the molecule itself is chiral, because it's nonsuperimposable on its mirror image. A chiral center You need to be able to quickly spot chiral centers in molecules. All straight-chain alkyl group carbons (CH3 or CH2 units) will not be chiral centers because these groups have two or more identical groups (the hydrogens) attached to the carbons. Neither will carbons on double or triple bonds be chiral centers because they can't have bonds to four different groups. When looking at a molecule, look for carbons that are substituted with four different groups. See, for example, if you can spot the two chiral centers in the molecule shown here. A molecule with two chiral centers Because CH3 and CH2 groups cannot be chiral centers, this molecule has only three carbons that could be chiral centers. The two leftmost possibilities, identified in the next figure, have four nonidentical groups and are chiral centers, but the one on the far right has two identical methyl (CH3) groups and so is not a chiral center. The chiral centers in a long molecule How to identify molecules as meso compounds A meso compound contains a plane of symmetry and so is achiral, regardless of whether the molecule has a chiral center. A plane of symmetry is a plane that cuts a molecule in half, yielding two halves that are mirror reflections of each other. By definition, a molecule that's not superimposable on its mirror image is a chiral molecule. Compounds that contain chiral centers are generally chiral, whereas molecules that have planes of symmetry are achiral and have structures that are identical to their mirror images. The plane of symmetry in meso compounds For example, cis-1,2-dibromocyclopentane (shown in the first figure) is meso because a plane cuts the molecule into two halves that are reflections of each other. Trans-1,2-dibromocyclopentane, however, is chiral because no plane splits the molecule into two mirror-image halves. Now look at the mirror images of these two molecules in the second figure to prove this generality to yourself. The mirror images of achiral (meso) and chiral molecules Even though the cis compound has two chiral centers (indicated with asterisks), the molecule is achiral because the mirror image is identical to the original molecule (and is, therefore, superimposable on the original molecule). Molecules with planes of symmetry will always have superimposable mirror images and will be achiral. On the other hand, the trans stereoisomer has no plane of symmetry and is chiral. In organic chemistry, you need to be able to spot planes of symmetry in molecules so you can determine whether a molecule with chiral centers will be chiral or meso. For example, can you spot the planes of symmetry in each of the meso compounds shown in the last figure? Some meso compounds How to Identify the Diastereomers of a Molecule When more than one chiral center is present in a molecule, you have the possibility of having stereoisomers that are not mirror images of each other. Such stereoisomers that are not mirror images are called diastereomers. Typically, you can only have diastereomers when the molecule has two or more chiral centers. The maximum number of possible stereoisomers that a molecule can have is a function of 2n, where n is the number of chiral centers in the molecule. Therefore, a molecule with five chiral centers can have up to 25 or 32 possible stereoisomers! As the number of chiral centers increases, the number of possible stereoisomers for that compound increases rapidly. For example, the molecule shown here has two chiral centers. A molecule with two chiral centers Because this molecule has two chiral centers, it can have a total of 22, or 4, possible stereoisomers, of which only one will be the enantiomer of the original molecule. Enantiomers are stereoisomers that are mirror images of each other. Because both chiral centers in this molecule are of R configuration, the enantiomer of this molecule would have the S configuration for both chiral centers. All the stereoisomers of this molecule are shown in the next figure. Those molecules that are not enantiomers of each other are diastereomers of each other. The four stereoisomers of a molecule with two chiral centers

Article / Updated 09-17-2021

In chemistry, you can add and subtract extreme numbers by using exponential notation, and expressing your numbers as coefficients of identical powers of 10. To wrestle your numbers into this form, you may need to use coefficients less than 1 or greater than 10. Adding with exponential notation To add two numbers by using exponential notation, you begin by expressing each number as a coefficient and a power of 10. In this example, you add these numbers, by following these steps: Convert both numbers to the same power of 10. Add the coefficients. Join your new coefficient to the shared power of 10. Subtracting with exponential notation To subtract numbers in exponential notation, you follow the same steps but subtract the coefficients. Here’s an example: 0.0743 – 0.0022 To perform the subtraction, follow these steps: Convert both numbers to the same power of 10. Subtract the coefficients. 7.43 – 0.22 = 7.21 Join your new coefficient to the shared power of 10. Now try a few practice questions. Practice questions Add the following: Use exponential notation to subtract the following: 9,352 – 431 Answers and Explanations The correct answer is Because the numbers are each already expressed with identical powers of 10 (in this case, 10–6), you can simply add the coefficients: 398 + 147 = 545 Then join the new coefficient with the original power of 10. The correct answer is (or an equivalent expression). First, convert the numbers so each uses the same power of 10: Here, you’ve picked 10², but any power is fine as long as the two numbers have the same power. Then subtract the coefficients: 93.52 – 4.31 = 89.21 Finally, join the new coefficient with the shared power of 10.

Article / Updated 07-26-2021

Many of the things we deal with in life are related either directly or indirectly to electrochemical reactions. The Daniell cell is an electrochemical cell named after John Frederic Daniell, the British chemist who invented it in 1836. A galvanic or voltaic cell is a redox reaction that produces electricity. The following diagram shows a Daniell cell that uses the Zn/Cu²⁺ reaction. This reaction may be separated out so that you have an indirect electron transfer and can produce some useable electricity. Galvanic cells are commonly called batteries, but sometimes this name is somewhat incorrect. A battery is composed of two or more cells connected together. You put a battery in your car, but you put a cell into your flashlight. A Danielle cell In the Daniell cell, a piece of zinc metal is placed in a solution of zinc sulfate in one container, and a piece of copper metal is placed in a solution of copper(II) sulfate in another container. These strips of metal are called the cell’s electrodes. The electrodes act as a terminal, or a holding place, for electrons. A wire connects the electrodes, but nothing happens until you put a salt bridge between the two containers. The salt bridge, normally a U-shaped hollow tube filled with a concentrated salt solution, provides a way for ions to move from one container to the other to keep the solutions electrically neutral. With the salt bridge in place, electrons can start to flow. Zinc is being oxidized, releasing electrons that flow through the wire to the copper electrode, where they’re available for the Cu²⁺ ions to use in forming copper metal. Copper ions from the copper(II) sulfate solution are being plated out on the copper electrode, while the zinc electrode is being consumed. The cations in the salt bridge migrate to the container containing the copper electrode to replace the copper ions being consumed, while the anions in the salt bridge migrate toward the zinc side, where they keep the solution containing the newly formed Zn²⁺ cations electrically neutral. The zinc electrode is called the anode, the electrode at which oxidation takes place, and is labeled with a “–” sign. The copper electrode is called the cathode, the electrode at which reduction takes place, and is labeled with a “+” sign. This cell will produce a little over one volt. You can get just a little more voltage if you make the solutions that the electrodes are in very concentrated.