Chemistry Articles

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.

Article / Updated 07-26-2021

Covalent bonding is the sharing of one or more electron pairs. In many covalent bonding situations, multiple chemical bonds exist — more than one electron pair is shared. (In hydrogen and the other diatomic molecules, only one electron pair is shared.) Nitrogen is a diatomic molecule in the VA family on the periodic table. Nitrogen has five valence electrons, so it needs three more valence electrons to complete its octet. A nitrogen atom can fill its octet by sharing three electrons with another nitrogen atom, forming three covalent bonds, a so-called triple bond. The triple bond formation of nitrogen is shown in the following figure. Triple bond formation of nitrogen A triple bond isn’t quite three times as strong as a single bond, but it’s a very strong bond. In fact, the triple bond in nitrogen is one of the strongest bonds known. This strong bond is what makes nitrogen very stable and resistant to reaction with other chemicals. It’s also why many explosive compounds (such as TNT and ammonium nitrate) contain nitrogen. When these compounds break apart in a chemical reaction, nitrogen gas is formed, and a large amount of energy is released. Carbon dioxide is another example of a compound containing a multiple bond. Carbon can react with oxygen to form carbon dioxide. Carbon has four valence electrons, and oxygen has six. Carbon can share two of its valence electrons with each of the two oxygen atoms, forming two double bonds. These double bonds are shown in the following figure. The formation of carbon dioxide

Article / Updated 07-26-2021

Alkanes are not limited to staying in a line — they can have structures that branch. When naming a branched alkane, your first step is to find and number the longest chain. After you number the parent chain, if you want to name the branched alkane, you need to determine the names of all the substituents that stick off of the parent chain, and then order the substituents alphabetically in front of the parent chain. This figure shows an example of a branching alkane. As you can see, the alkane with the formula C4H10 can have two possible chemical structures, or isomers — one alkane that’s a straight chain (butane), and one alkane that’s branched (isobutane). The isomers of C4H10 Find the longest chain in a branched alkane The first, and potentially the trickiest, step in naming a branched alkane is to find the longest chain of carbon atoms in the molecule. This task can be tricky because, as a reader of English, you’re so used to reading from left to right. Often, however, the longest chain of carbons is not the chain that follows simply from left to right, but one that snakes around the molecule in different directions. Organic professors like to make the parent chain one that curves around the molecule and doesn’t necessarily flow from left to right, so you have to keep on your toes to make sure that you’ve spotted the longest carbon chain. The wrong and right ways to count the parent chain The longest carbon chain for the molecule shown here is seven carbons long, so the parent name for this alkane is heptane. Number the longest chain in a branched alkane Number the parent chain starting with the end that reaches a substituent first. A chain can always be numbered in two ways. For the molecule in this example, the numbering could start at the top and go down, or it could start at the bottom and go up. The correct way to number the parent chain is to start with the end that reaches the first substituent sooner. A substituent is organic-speak for a fragment that comes off of the parent chain. The right and wrong ways to number the carbons of the parent chain As you can see here, if you number from the top down, the first substituent comes at carbon number three; if you number from the bottom up, the first substituent comes at carbon number four. The correct numbering in this case, then, starts at the top and goes down.

Article / Updated 07-26-2021

Water molecules have unusual chemical and physical properties. Water can exist in all three states of matter at the same time: liquid, gas, and solid. Imagine that you’re sitting in your hot tub (filled with liquid water) watching the steam (gas) rise from the surface as you enjoy a cold drink from a glass filled with ice (solid) cubes. Very few other chemical substances can exist in all these physical states in this close of a temperature range. Water’s unique properties Following are some of the unique properties of water: In the solid state, the particles of matter are usually much closer together than they are in the liquid state. So if you put a solid into its corresponding liquid, it sinks. But this is not true of water. Its solid state is less dense than its liquid state, so it floats. Water’s boiling point is unusually high. Other compounds similar in weight to water have a much lower boiling point. Another unique property of water is its ability to dissolve a large variety of chemical substances. It dissolves salts and other ionic compounds, as well as polar covalent compounds such as alcohols and organic acids. Water is sometimes called the universal solvent because it can dissolve so many things. It can also absorb a large amount of heat, which allows large bodies of water to help moderate the temperature on earth. Water has many unusual properties because of its polar covalent bonds. Oxygen has a larger electronegativity than hydrogen, so the electron pairs are pulled in closer to the oxygen atom, giving it a partial negative charge. Subsequently, both of the hydrogen atoms take on a partial positive charge. The partial charges on the atoms created by the polar covalent bonds in water are shown in the following figure. Polar covalent bonding in water Intermolecular forces Water is a dipole and acts like a magnet, with the oxygen end having a negative charge and the hydrogen end having a positive charge. These charged ends can attract other water molecules. This attraction between the molecules is an intermolecular force (force between different molecules). Intermolecular forces can be of three different types: London force (or dispersion force). This weak type of attraction generally occurs between nonpolar covalent molecules, such as nitrogen, hydrogen, or methane. It results from the ebb and flow of the electron orbitals, giving a weak and brief charge separation around the bond. Weak dipole-dipole interaction. This intermolecular force occurs when the positive end of one dipole molecule is attracted to the negative end of another dipole molecule. It’s much stronger than a London force, but it’s still pretty weak. Extremely strong dipole-dipole interaction. This force occurs when a hydrogen atom is bonded to one of three extremely electronegative elements — O, N, or F. These three elements have a very strong attraction for the bonding pair of electrons, so the atoms involved in the bond take on a large amount of partial charge. This bond turns out to be highly polar — and the higher the polarity, the more effective the bond. When the O, N, or F on one molecule attracts the hydrogen of another molecule, the dipole-dipole interaction is very strong. This strong interaction is called a hydrogen bond. The hydrogen bond is the type of interaction that’s present in water, as shown in the following illustration. Hydrogen bonding in water Water molecules are stabilized by these hydrogen bonds, so breaking up (separating) the molecules is very hard. The hydrogen bonds account for water’s high boiling point and ability to absorb heat. When water freezes, the hydrogen bonds lock water into an open framework that includes a lot of empty space. In liquid water, the molecules can get a little closer to each other, but when the solid forms, the hydrogen bonds result in a structure that contains large holes. The holes increase the volume and decrease the density. This process explains why the density of ice is less than that of liquid water (the reason ice floats). The structure of ice is shown below, with the hydrogen bonds indicated by dotted lines. The structure of ice

Article / Updated 07-26-2021

An easy way to find the R / S configuration of a molecule with more than one chiral center is with a Fischer projection. A Fischer projection is a convenient two-dimensional drawing that represents a three-dimensional molecule. To make a Fischer projection, you view a chiral center so that two substituents are coming out of the plane at you, and two substituents are going back into the plane, as shown here. Then the chiral center becomes a cross on the Fischer projection. Every cross on a Fischer projection is a chiral center. Creating a Fischer projection Fischer projections are convenient for comparing the stereochemistries of molecules that have many chiral centers. But these projections have their own sets of rules and conventions for how you can rotate and move them. As shown in the figure below, the two main ways to rotate a Fischer projection are as follows: You can rotate a Fischer projection 180 degrees and retain the stereochemical configuration, but you cannot rotate a Fischer projection 90 degrees. You can rotate any three substituents on a Fischer projection while holding one substituent fixed and retain the stereochemical configuration. Working with Fischer projections Here’s how to determine the configuration of a chiral center drawn in a Fischer projection: First, you prioritize each of the substituents using the Cahn–Ingold–Prelog prioritizing scheme. According to the Cahn–Ingold–Prelog prioritizing scheme, the highest priority goes to the substituent whose first atom has the highest atomic number. (For example, Br would be a higher priority than Cl, because Br has a larger atomic number.) Then, you put the fourth priority substituent on the top, and draw a curve from the first- to the second- to the third-priority substituent. If the curve goes clockwise, the configuration is R; if the curve goes counterclockwise, the configuration is S. To get the number-four priority substituent at the top of the Fischer projection, you have to use one of the two allowed moves diagramed in the second figure. (You can make a 180-degree rotation, or you can hold one substituent fixed and rotate the other three.) Two examples of the determination of the configuration from Fischer projections are shown here. Determining R and S configurations from Fischer projections