How to Study Chemistry

What Goes Around

After so much advice on how to study chemistry, it is obvious a lot of "information" is not really helpful.

What You Usually Hear

There must be better advice than "to study more" , "remove distractions", "eat well", "make a schedule", and "get organized". Many schemes and systems rely on six different hi-lite colors, fold out notebook pockets, and a dreaded weekly planner.

What they all mostly lack is a real nuts and bolts list of techniques that help you in an actionable way .

Try Something New

Everyone has their own "best thing" that will help them master how to study. But that implies they have tried more than one thing rather than saying: "This is how I know how to study so it must be the way to study."

Every person is an individual with their own quirks. But don't imagine you must reinvent the wheel to find success. There are proven methods to succeeding. If what you are doing is not producing the results you want, then try something else. You would be surprised at the number of students who simply will not do that.

Find A Reason

Your Brain Decides

The single largest predictor of whether you do well is if you see a purpose to what you are studying. Your brain acts as a well tuned instrument to filter out whatever it thinks is irrelevant. If you have an attitude you do not really need to know or care about something , you will find cramming an uncomfortable experience; which may or may not be successful despite super human willpower.

When you find something interesting or believe this particular bit of studying has some real world advantage, remembering things and studying becomes far easier.

Think About This

In the case of studying chemistry, well over ninety percent of people on the board of directors in fortune 500 companies have a degree in chemistry or chemical engineering. The vast majority of new patents come from someone in the field of chemistry. No matter what your eventual field, your ambition will be well rewarded with at least some knowledge of chemistry.

Bear in mind also, the vast majority of Americans change their career path multiple times before they settle into a specific tract.

Treat It Like A Job

How Much time?

You can not succeed in studying without putting in a certain amount of effort. This does not mean you must put in hour after relentless hour with your nose to the grind stone. It means if you put in study time of at least two times the amount of time you spend in class studying outside of class, you should be studying enough.

For example if you have a class that meets MWF for one hour three times a week, this equals three hours per week you spend in class. That means you should study a minimum of six hours a week outside of class. Never more than nine hours a week.

Here is what this translates into in real world terms. If you take a typical class load of twelve credit hours, this means you would study twenty-four hours outside of class each week. Together the twelve hours and twenty-four hours comes to thirty-six hours of time you commit to academic pursuits.

This leaves evenings and weekends free for you to do whatever you want. Provided you actually study and produce academic results when you study. It also alerts you when you study a standard amount of time and do not produce the best results. This means you do not understand something crucial and need to seek help from the instructor or a tutor.

Don't Major In Minor

Another critical problem is when you choose what to study and for how long. Not all material is equally important. Treating everything like it is equally important blurs a hierarchy of information which you should pick up based on how something is taught.

The mere act of organizing material into major topics and subordinate topics will help you to retain as much information as possible.

All too often students spend an inordinate amount of time on problems which do not advance their preparation. If you must spend time on one type of problem or even one problem, budget your time so other information which needs attention does not fall behind. Don't walk into an exam with large parts of a topic neglected because you got stuck somewhere else.

Quality Control

Have A Specific Goal

Something which is also often neglected is finding out whether or not you have made progress as a result of the time you spend studying. You should begin every study time with specific goal in mind with a specific and measurable way to decide if you mastered given of skill.

What Kind of Goal

A goal means something specific you can measure.

It is not a goal to say you will study until you understand phase changes in water. This is too vague, and the term "understand" can mean almost anything.

Real Goals

When you set a real goal it means something like: "I will show my mastery of the phase changes of water by drawing a phase diagram of water as it changes from ice to water to steam. I will label the diagram correctly and carry out calculations which show how much energy is required for both gram and mole quantities of water to change phases

Do not consider you have "studied" the material simply because you spent a fixed amount of time looking at it, looking at flash cards, or anything which is passive or does not require you to put something in writing or solve a problem in writing.

The Product of Studying

A Paper Trail

You are looking for concrete physical evidence of progress. Do not rely on doing all this in your head. Thinking you understand something and really understanding something are two different things. One you can look at later if the exact details escape you. The other (the bad kind) evaporates after a remarkably short period of time.

When you have solved problems, have notes with explanations, and written explanations of graphs, you have something in your hand you can look at. Remember that as a professional anything you will be required to produce written reports, summaries, and briefs.

Test Yourself

The last piece to quality control is self-testing. Work on problems you anticipate will be of the same or similar difficulty on an exam. Be certain you can get them right in an appropriate period of time.

You may find explaining concepts and problem solving techniques to a study partner or study group enhance how well you understand material.

March 12, 2019March 12, 2019

Ions: Predict Charge

The Periodic Table

You use the periodic table to assign standard charges to ions.

Main Group and Transition Metals

To assign charges to ions, you must recognize these rules apply to main group elements. The other elements correspond to transition metals and post transition metals. You can not transfer the same rules to transition metals.

The main group comprises IA(1)-IIA(2), and IIIA(13)-XVIIA(18).

Figure 1 shows the main group elements in white. The grayed out areas show the transition metals.

Periodic chart main group — **Figure 1**: Main group in white, transition elements grayed out

The ability to predict charges resides in the main group elements. Any elements between IB(3)-VIIIB(12) are transition metal elements. You can not predict the charge in the same simple straight forward way.

Metals and Nonmetals

In another way to see the periodic table, elements are divided into metals and nonmetals. Ionic compounds form from a metal and nonmetal (or nonmetal polyatomic ion). Both a metal and a nonmetal must be present.

You must also distinguish between metals and nonmetals.

You can do this when you divide the periodic table into metal and nonmetal elements, shown in Figure 2.

periodic table metals and non meteals — **Figure 2**: Regions of periodic table separated into metals and nonmetals

On the left side of Figure 2 the gray shaded area shows which elements are metals. Next to the metals on the right, a zig-zag region with a marbled texture shows metalloids. The far right of the periodic table shows white elements which correspond to nonmetals.

Metalloids are elements between metals and nonmetals. These elements have properties of both metals and nonmetals, but are not completely either one.

The Crucial Difference: Metals and Nonmetals

The way an ion forms from an element, whether it loses electrons or gains electrons depends on whether an element is a metal or nonmetal, Figure 3.

Ions and the periodic chart — **Figure 3**: Metals form cations and nonmetals form anions

Octet Rule

How many electrons (negative charges) does an atom lose to become a cation or gain to become an anion? The answer depends on how many valence electrons an element has when it is neutral.

The number of charges an element gains or loses, must in the end, equal eight valence electrons in its outer most electron shell. This is the same number of electrons in the valence electron shell of as the inert noble gases.

Whether an element gains or loses electrons in order to conform to the octet rules is a characteristic which makes an element a metal or nonmetal.

Figure 4 shows both a metal and nonmetal in a gray neutral state. Their path to obtain an octet of valence electrons are opposites. Nonmetals gain electrons to become anions. Metals lose electrons to reach a state where they mimic the valence electrons of the noble gases.

elements are metals lose electrons, nonmetals gain electrons — **Figure 4**: Metals lose electrons and nonmetals gain electrons

Metals Make Cations

When a metal forms an ion, it gives up its valence electrons and becomes a positively charge atom, cation.

For instance sodium has one valence electron. It gives up its electron because it is a metal. That gives it a +1 charge. The electron shell below its outermost electron shell has a full octet. Sodium has sacrificed its outer electron to resemble the electron configuration of neon.

Nonmetals Make Anions

In contrast to metals, nonmetals gain electrons. When a neutral atom gains electrons it becomes negative, an anion.

For example sulfur has six valence electrons in its outer most electron shell. That's two fewer electrons than argon. When sulfur gains two electrons to become an anion, it now has eight electrons in its valence shell. This makes the sulfide -2 anion have the same stable configuration as argon.

Charge and Columns

ion charge and periodic table — **Figure 5**: The charge of an ion can be predicted by its position on periodic table

Main group elements have predictable charges they assume when they become ions. The charge they most frequently assume depends on which column an element is found in. Figure 5 shows which elements assume which charge by the family of element.

Notice carbon and group 14 (IVA) does not have a charge assigned to it. This is because carbon can take a +4 or -4 charge depending on who its bonding partner is.

The other main group elements however follow predictable patterns when they form ions.

It Takes Two: Metal Plus Nonmetal

Bare in mind an element only becomes an ion under the influence of a near by complimentary element. Metals and nonmetals in their native state always have a charge of zero (or neutral).

For example sodium metal has a charge of 0. When sodium (Na) encounters a nonmetal like chlorine (Cl₂) which also has a charge of zero, the sodium loses and electron while chlorine gains an electron. That is the moment when an element takes on the identity of an ion.

In language you will often hear: "what charge does calcium have?" The element itself does not have a charge. The question stated more correctly would be: "What charge does calcium take when it becomes an ion due to its interaction with an element which can accept its electrons?"

The first question is the more common one, but it usually means the second question. Just keep the idea straight that elements do not have charges, but ions of the elements do have charges.

February 22, 2019

Isotopes

More Than One Version Of Each Element

The Atomic Mass Mystery

Isotopes exist as the same element but with different mass numbers. All nuclei of any element must contain the same number of protons. They have different masses, because each isotope has a unique number of neutrons.

Examine the entries for each element on the periodic chart and you'll see the atomic mass of the elements include some fraction smaller than one. If neutrons and protons have masses of approximately one, alone it makes no sense to find fractions of one included in the mass.

Figure 1 shows an entry for carbon. Instead of finding 12.00 amu as you expect, the entry has 12.01. This results from the fact the atomic mass is composed of carbon atoms which have a mass of 12.00 amu and 13.00 amu . The atomic mass results from the weighted average of the carbon with a mass number of 12.00 and 99% abundant and carbon with a mass number of 13.00.

⦿ definition

Isotopes are atoms of the same element which have different mass numbers

carbon on periodic table — **Figure 1**: Essential information for each element

Atomic Mass

Weighted average mass of all the isotopes of a given element

Mass Number

Mass of single kind of isotope

One Size Fits All

Isotopes exist as atoms of the same element. Each isotope has a different total mass. Even though all isotopes of a single element must have the same number of protons, they differ from one another by the number of neutrons.

Consider only the nucleus when you are concerned with isotopes. Electrons play no part in isotopes or the atomic mass, Figure 2. Only protons and neutrons contribute to the mass of an isotope.

Focus on nucleus of atom — **Figure 2**: Focus on nucleus contains protons and neutron

Isotopes Are Useful

You find isotopes in many roles when they are put to use:

BNCT

Boron neutron capture therapy is used to treat tumors which might otherwise be difficult to operate on. It relies on ¹⁰B capturing neutrons. The new unstable ¹¹B undergoes α-decay. Then it produces a ⁷Li ion. The α-particle attacks the cancer cell form within the cell.

Radiometric Dating

The most important radiometric dating is carbon dating. It relies on the fact ¹⁴C decays to ¹⁴N, which has a half-life of about 5,200 years. Carbon dating becomes ineffective earlier than 50,000 years in the past.

Nuclear Energy

The most common isotopes used to produce nuclear energy are ²³⁵U and Pu²³⁹. Both must be extracted from the population of other isotopes of the same element before a sample becomes useful for nuclear reactions.

Medical Imaging

⁹⁹Te is the most common radio nuclide used for radioactive tracers. It finds use to examine brain function without resort to invasive methods.

This list is not exhaustive, but just the tip of the iceberg. It is intended to illustrate a variety of contributions made by isotopes and the need to purify isotopes.

Each Isotope Has A Name

In order to specify a specific isotope among a set of isotopes present for a specific element, you use isotope or nuclide notation. This provides a method to tell the difference between carbon with a mass of 12.00 and carbon with a mass of 13.00.

To identify each isotope among other isotopes of the same element, Figure 3 shows isotope notation.

isotopic symbol notation — **Figure 3**: Isotope symbol used to denote each unique isotope.

X denotes any possible element. There is no X on the periodic table.

Z means "charge" or the atomic number. As you know, the atomic number gives you the number of protons.

The superscript A stands for mass number; the sum of the protons and neutrons.

There are two major isotopes of chlorine, ³⁵₁₇Cl and ³⁷₁₇Cl. Notice the mass number is always a whole number. From the isotopic symbol you see for instance ³⁵Cl has 17 protons and 18 neutrons. You find the number of neutrons when you subtract the atomic number from the mass number.

Notice also, you can specify ³⁵Cl without the atomic number, because if you write Cl the only possible atomic number is 17. You should get used to the idea the atomic number and atomic symbol are the same thing. One automatically gives you the other.

February 15, 2019

Matter: Composition

Matter by Composition

You can describe matter based on composition. This means you describe collections of matter based on what makes it up. The first kind of description refers to whether or not you want to describe a pure substance or mixture.

Cultivate the skill of seeing the microscopic scale of particles when you see or hear about what you observe on the scale you see in ordinary life.

large scale, molecular scale — **water as we see it everyday and water the way it would look with a molecular magnifying glass**

Pictured above is how we see water in a glass and how we might see water if we pretend to have a magnifying glass strong enough to see individual water molecules.

Pure Substance or Mixture

The components of a pure substance can not be separated based on their different physical properties.

Mixtures on the other hand, contain more than one pure substance. Each substance within a mixture can be identified by separate physical properties.

pure substances and mixtures — **Figure 1**: Pure substances and mixtures

In Figure 1 the left column shows pure substances. In the right column you see pure substances when added together and no chemical change happens, they form a mixture.

The first mixture is an example of two compounds: water and carbon dioxide. When they add together, both still remain the same pure substance with the same physical properties. Carbon dioxide mixes with water in order to make carbonated water. You can separate the carbon dioxide from the water and find they remain unchanged.

The second example shows two elements mixed to together to form a mixture. In this case, when copper is mixed with tin in a three to one mixture, we obtain the alloy bronze.

Pure Substance

The key feature of a pure substance is that it possesses a fixed proportion of elements. This proportion is the same no matter what the size of the sample. The fixed proportion remains identical whether you find it in your back yard or on the dark side of the moon. It is a pure substance because it has the same proportion of elements as every other sample of the same pure substance.

Element

Elements are made of one and only one kind of atom. If there is only one kind of atom, then it has a one hundred percent proportion of only one element. An element cannot be broken down further by any chemical means. This also includes elements which are made of more than one atom like: diatomic elements, molecular elements, network solids, and metals.

Figure 2 shows examples of each way you might encounter an element in its wild natural habitat.

Compound

To qualify as a compound that is a pure substance, the compound contains more than one element. A compound also has a fixed proportion of elements. The compound is a specific compound because of its specific proportion of elements, (formula).

The fixed proportion of elements (sometimes called fixed composition) makes H₂O water, and H₂O₂ hydrogen peroxide. They make separate pure compounds because they have a different proportion of elements.

Each compound has a unique set of physical properties: boiling point, melting point, density, and refractive index. When you test and record the physical properties of a sample, you characterize a compound.

Because each pure compound has unique physical properties, you use this to separate each pure compound when more than one pure substance resides in a mixture.

Compounds Ionic or Molecular

You can divide compounds into two types: ionic or molecular. The distinction between these is important. Ionic and molecular compounds differ in their physical properties because they are either molecular or ionic.

On the all important microscopic level, you find ionic compounds are stacks of ions attracted to each other by plus being attracted to minus charges. This attraction goes in all directions and repeats without stop.

In contrast, a molecular compound like carbon dioxide holds together in the bulk phase by attraction between the molecules. The bonds between the atoms: carbon to oxygen hold only the pieces of each individual molecule together.

molecular versus ionic compounds — **Figure 4**: The atomic level difference between molecular and ionic substances

Figure 4 shows the contrast between sodium chloride, NaCl, and carbon dioxide, CO₂. The ions pack together in a hungry mass of opposite charges pressed together. Carbon dioxide has independent molecules loosely held together with weak forces between the molecules.

How loose?

Sodium chloride has a boiling point of 1465°C, compared to carbon dioxide which has a boiling point (sublimation point) of -78ºC.

Mixtures

A mixture of more than one substance forms a mixture because it has a variable composition. This means instead of one fixed proportion of elements, you can have a sample of air which has twenty percent oxygen or fifteen percent oxygen. It still remains a mixture of air.

A salt solution of water provides another common example where you might have a ten percent salt solution or a one percent salt solution. In both case you are referring to a salt solution though the examples do not have identical compositions.

Heterogeneous or Homogeneous Mixture

Mixtures themselves can have two possible compositions: homogeneous and heterogeneous. Homogeneous mixtures mean all the components of the mixture are evenly distributed throughout the body of the material. Heterogeneous mixtures where the components are thrown together without the requirement the mixture has uniform components.

The distinction between these two states of mixture plays into how a mixture can be separated into two or more pure substances.

To visualize the difference between types of mixtures on the molecular scale, Figure 5 shows how two typical mixtures might appear.

Two kinds of mixtures — **Figure 5**: Homogeneous versus heterogeneous mixture

The part of Figure 5 on the left shows an aqueous solution of sodium chloride. The chloride anions are green and the sodium cations ions are yellow. The important point to notice is that these ions are uniformly distributed through the entire solution. Every part of the mixture has an identical percentage of sodium chloride in the water.

The right side of Figure 5 shows a mixture of water and octane, (a liquid hydrocarbon). The lower layer has a clear separation between itself and the upper level. The mixture of octane and water do not have a uniform distribution.

Homogeneous

When a mixture has a uniform distribution of every component substance, the homogeneous mixture is often a solution.

Air makes a great example where all the gases in air are equally present throughout a room. Very few people choose their seat in a class room because they are afraid part of the room has less air than any other part of the room.

Heterogeneous

You label a mixture as heterogeneous if the components jumble together and are not uniformly distributed. This means the material consists of zones and regions. This includes materials like wood, concrete, and composite materials.

The Big Deal

When you follow a chain of questions about any substance, you will find it easy to correctly classify any lump of matter into its correct category.

Figure 6 gives a flowchart where you serially ask questions and find the correct composition of matter.

decision flowchart to classify compound — **Figure 6**: How to decide what kind of compound

If the materiel does not have a uniform distribution, then you have a heterogeneous mixture.

If you can separate the components of a sample by physical means: distillation, filtration, chromatography; then the sample is an homogeneous mixture.

If the sample can be decomposed to other pure substances, the your substance is a compound.

If all these questions result in "no" your final remaining option is that your sample is an element.

January 29, 2019

Chemistry Energy

Energy

The idea of energy is abstract and difficult to visualize. You can understand it better from the point that although you cannot measure energy directly, you can measure the effects of energy. The two most common visible effects of energy are work being done or a change in temperature.

Figure 1 shows how coal contains energy in the form of chemical bonds. When the bonds in coal reform to make carbon dioxide and water, radiant heat releases and heats water. The water boils and steam expands and causes a piston that moves and forces a wheel to move.

The energy to mechanically move the gear comes from energy within the bonds between atoms in the coal.

Work

By work, we mean a piston moves, an elevator is hauled up, or some type of visible force acts on a body to move it a distance.

Heat

A change in temperature means heat flows from a reservoir to an area which has less energy. It is principally the flow of heat chemists concern themselves with.

Thermal Energy

All matter contains a certain amount of energy from the motion of the atoms or molecules which make it. Even when particles are not in motion, they vibrate and rotate. Because the direction of vibration of one particle is independent from any other particle, the overall effect is that the vibrations and rotations occur at random. These random motions are what thermal energy are made of.

Two Kinds of Energy

Regardless of how the energy transfers from one place to another place, energy is divided into two forms: potential energy and kinetic energy.

Potential Energy

Potential energy is often referred to as the "energy of position". The idea is taken from the fact that if you consider a rock at a given height, when you drop the rock it accelerates from resting until it impacts the grounds.

Rocks and Heights

In Figure 2, on the left a rock perched fifty feet above the ground is held in a field (gravity). When you release the rock, the potential energy converts to kinetic energy. The rock strikes the ground and releases kinetic energy in the form of mechanical energy. (Even if you do not realize it, some heat energy is also released).

The energy released when the rock hits the ground was stored by the position above the ground the rock occupied. The higher the rock, the harder the rock strikes the ground. This means rocks which are higher have more potential energy than rocks at a lower height.

Molecules

The same idea can be extended to include chemical reactions. If burning wood heats a boiler that goes onto cause the expansion of steam to move a piston, then you can say the wood contained the potential energy, which when converted to heat, caused the piston to move.

But where is this potential energy? The potential energy comes from the chemical bonds between the atoms which form the wood. In the process of combustion chemical bonds break and reform with oxygen. The difference in potential energy between the bonds in wood the products of combustion are released as heat.

So chemists speak about chemical bonds as containing potential energy, since breaking or formation of bonds can be measured either in the form of mechanical work or a change in temperature.

Kinetic Energy

Kinetic energy is the energy of motion. You may well think in terms of translational motion. That is. motion which moves through space. However kinetic energy also refers to the fact atoms and molecules vibrate and rotate. This also contributes to the total kinetic energy of matter.

Chemical Potential

The most important conversion of potential energy to some other kind of energy is the potential energy in bonds which change as a result of the change of bonds between specific atoms. How atoms rearrange with new bonding partners determines whether they release or absorb energy, often in the form of heat.

combustion of hydrocarbon. — **Figure 3**: Reaction of a hydrocarbon and oxygen makes carbon dioxide and water

In an abstract sense, Figure 3 shows that a generic organic molecule reacts with oxygen and produces carbon dioxide and water. When the bonds between atoms in the organic molecule rearrange when they encounter molecules of oxygen, both molecules trade their bonds to form bonds between hydrogen and oxygen and carbon and oxygen.

Bond Change Changes Energy

The atoms go from one bond with a certain potential energy to another bond with another amount of potential energy stored in the bonds. The difference in potential energy before a chemical reaction and after a chemical reaction releases energy. In the case of hydrocarbons, the difference in energy is released as heat.