Dimensional Analysis in Chemistry

 

 

One of the first road blocks you might find when you start chemistry is how to do dimensional analysis problems.

 

 

Dimensional analysis problems do not lend themselves to memorization. The first factor of frustration comes when being able to solve a problem does not boil down to finding which equation to use.

 

Now you must learn to figure out each problem. However this is not as hard as it looks. 

The fundamental operation in all dimensional analysis problems is to put the given unit in the denominator and the unit to convert to into the numerator. The rest of this method merely extends this idea, Figure 1.

The strategy is to put the given unit and quantity to the left. The unit you want to have the quantity converted to has a ratio to the original unit. Arrange the ratio so the given unit appears in the denominator of the of the ratio. The unit you want appears in the numerator. As you carry out the computation, the units of the original quantity cancel out.

 

 

dimensional analysis basic operation
Figure 1 : Dimensional analysis is done with one basic operation.

 

 

First step : what kind of problem is this?

 

There are only two types of problems: 

 

one unit to another unit, or one ratio to another ratio, Figure 2.

 

This never changes. The problem follows one pattern or the other pattern. Once you know which pattern a problem follows, you can identify the both the specific question and where to begin the problem.

 

 

two kinds of problems
Figure 2: All dimensional analysis problems are unit conversions or ratio conversions

 

Unit to Unit

 

In the case of unit conversion problems, convert the known units to units of an unknown quantity with the pattern in Figure 3:

 

symbolic unit to unit conversion
Figure 3: An example of a two step equation to convert one unit to another

 

Instead of specific units, the units are shown as symbols in shapes. When you start with a square with dots, put the square with dots in the denominator with a different unit in the numerator: right pointed triangle. 

 

Ratio to Ratio

 

In contrast, ratio to ratio conversions follow the following kind of pattern, Figure 4.

 

ratio to ratio conversions
Figure 4 : dimensional analysis used to convert a given ratio to another ration

 

Like the previous example, symbols take the place of numerical values and units. Once again, find a ratio with the unit you want to change. Arrange the ratio so the unit you want to change appears in the denominator.

 

To change the bottom unit, find a ratio with the unit that  in the denominator. Place the unit to change in the numerator.

 

The initial ratio and final ratio used to make the conversion might not match. Insert any ratios needed until the units match.

 

When you break down dimensional analysis into smaller tasks, you will find it easier to tackle.

 

Step by Step

 

Approach each problem systematically. To solve problems, use a known process which gives proven results, Figure 5.

 

The steps break down into smaller self-contained steps. Once you find the question and given information, you know what kind of problem you have. Find the needed ratios in the problem statement or look the ratios up.

 

 

steps to solve conversion problems
Figure 5: Sequence of steps used to solve unit conversion problems

 

    1. Identify the Question

      The key to any question is to understand what question the problems asks. If the answer takes the form of a unit, then you have a unit to unit conversion. If you have a ratio as the final answer, then you know you will start the problem with a ratio.


    2. Find the Start

      Based on whether the final answer is a unit problem or ratio problem, you can spot the start of the conversion to see what ratio or unit remains to convert.

    3. Find the Ratios

      Once you know where the problems starts and ends, the rest of the ratios you need are in the problem statement or you are able to look up any needed ratios.


      You can find common ratios in your textbook or on the internet. These ratios include: miles to kilometers, psi to mm Hg, atm to mm Hg, inch to cm, or  gallons to liters.

       

    4. Arrange the First and Last Ratio

      Take the ratio which alters the unit in the denominator and arrange them so the numerator unit changes.

      Place the last ratio with the unit in the numerator so the desired unit is either canceled or ends up in the denominator.

    5. Add Middle Ratios

      Match any needed conversions until the units for the first conversion and last ratio match the units you need to carry out the first and last conversion. 

    6. Compute


      Calculate the numerical values associated with each unit. Check your work. If you have a correct answer, you can cancel any unit which appears in a ratio which is anywhere in both a numerator or denominator.

 

 

 

 

 

 

 

 

Chemistry of Solids

adamantine structure of quartz

 

 

Solid State Chemistry

 

 

Solid state chemistry is overlooked in general chemistry.  It would be an under statement to say solid state chemistry is the substance of civilization. Many of the most interesting and practical uses of chemistry fall into the category of solid state chemistry.

 

Periodic Table

 

The quickest method to make a good guess as to what type of solid comes from a  formula, use the periodic table. Elements are divided into two areas: metals and nonmetals, Figure 1.

 

 

Metals on the left, nonmetals on the right
Figure 1: Metals in gray, nonmetals in red, metalloids in blue, inert elements in beige

 

Metals are shown in gray: main group metals, transition metals, rare earth metals, and post transition metals. They donate electrons to form positive ions in ionic compounds. Metals form alloys with other metals.

 

Nonmetals are pale red. When nonmetals interact with metals, they add electrons  and become negative ions. 

Nonmetals also form bonds with other nonmetals through sharing electrons. This forms molecules or other times forms molecular or network solids.

 

Metalloids have the properties of both metals and nonmetals. This means the element behaves as both a metal which forms an ion, a metal that exists in metallic bonds, or form covalent bonds with other nonmetals.

 

 

Types of Solids

 

Ionic Materials

Ionic solids are composed of positive and negative ions arranged in borderless arrays. Positive ions come from metal elements that release electrons to become cations. Negative ions result when nonmetal elements add electrons, which become anions.

Ionic compound showing array
Figure 2: Representation of simple ionic compound with cations in blue and anions in red

The attraction between positive and negative charges is very strong. Ionic compounds act as solids at room temperature.

Ionic solids make up important materials like metal oxides used to extract metals, electrolytes in batteries, and essential nutrients like sodium chloride, (table salt).

 

Molecular Materials

 

Molecular compounds form when nonmetal elements bond to nonmetal elements. They form molecules. The molecules are modular basic units held together with covalent bonds. Covalent bond result from two nonmetal elements that share valence electrons. This allows each element to possess eight electrons in their outer-most electron shell.

Examples of molecules
Figure 3: Examples of simple molecules: water, ammonia, and methane

The solid however, is not held together by covalent bonds. The bulk material forms from interactions between the molecules. The interactions which bind molecules are called intermolecular interactions

structure of two molecular solids
Figure 4: Methane and water when frozen, show the structure of two molecular solids

Molecules take the form of solids, liquids, or gases. Among cohesive forces which enforce the structure of solids, intermolecular forces are the weakest.

Figure 4 shows two solid forms of simple molecules.

Ice, which is composed of water molecules, has an enforced higher order structure like diamonds. The dotted lines between water molecules are interactions between a partially positive charge on hydrogen with a partially negative charge on oxygen. 

 

In contrast, methane, CH4, has much weaker intermolecular attractions because methane is nonpolar. When methane does become solid, it solidifies at a much lower temperature than water: 0ºC for water versus -182ºC for methane.

 

The reason these two molecules have such different melting points is because water molecules have much stronger intermolecular interactions than methane.

 

Metallic Materials

 

When metal elements bond to other metal elements, metal-metal bonds result. You can understand many of the physical properties you remember when you think of a ‘metal based on the way metal atoms form the bulk material you know as a metal, Figure 5.

 

Atomic model of metal
Figure 5: Metal composed of atoms (blue) and a medium of freely moving valence electrons (red arrows)

The metal atoms (blue) sit inside a matrix of  shared valence electrons. The electrons are shown by bent red arrows. The arrows show the electrons move constantly and hop from one positive metal nuclei to another.

The background pink inside shows an overall negative charge. This allows the total charge of the metal to be neutral.

 

Network Solid Materials

 

Network solids hold together with covalent bonds between nonmetal atoms, Figure 6.

 

network solid
Figure 6: atoms held together by covalent bonds in an infinite array

⦿ Covalent Molecules and Covalent Networks

It is important not to confuse covalent molecules with a covalent network of atoms. 

  • Molecules come from internal covalent bonds. The molecules hold together with intermolecular forces
  • Covalent networks form from covalent atoms throughout the bulk of the material.

 

Organization of Solid

 

The basic structural unit of the silicate ion, (SiO4-4) is shown in Figure 7. The properties of various materials you know: quartz crystal, glass, and fiberglass have different properties. This happens because the way the individual silicates are arranged in space.

The structure is tetrahedral with a silicon atom in the center while oxygen atoms with a (-1) charge occupy each point of the tetrahedron.

 

molecular silicate structure
Figure 7: Tetrahedral structure of silicate anion

 

Crystalline Materials

 

When solids form a regular repeating structure, they solidify as crystals. If you examine a piece of quartz, you see the rock has a regular geometric shape, Figure 8.

 

quartz crystal
Figure 8: Regular geometric structure of quartz crystal.

Notice the outer walls of the crystal have a hexagonal shape. Its faces also have a clean and regular face. These types of geometric faces are only possible when there is an underlying regular structure.

The tetrahedral silicates connect with each other on the molecular level in a regular six member ring structure fused together. That builds the hexagonal structure you see when you look at a crystal, Figure 9.

 

adamantine structure of quartz
Figure 9: Silica quartz has a rigid crystalline structure

 

 

This arrangement is called “adamantyl”. It is common among other covalent networks like elemental carbon, and phosphorous pentoxide, P2O5. This arrangement is one of the most stable arrangements in nature.

 

 

 

 

Amorphous Materials

 

Heat silicates until quartz (or more often sand) turns into a liquid. As the melt cools, you are rewarded with glass. Though glass also forms from silicate anions, because it cooled too fast to form an orderly structure, the silicates attach to each other any way they happen to find each other, Figure 10.

 

 

glass structure
Figure 10: Silicate glass is composed of silicates with no long range order

 

Composite Materials

 

If you physically combine two materials on a bulk scale, the result is a composite. You may already know about fiberglass, Figure 11. The name “fiberglass” is a bit misleading. There are fibers made of glass. However the familiar structural material is made out of strands of glass fiber embedded in a plastic.

 

 

composite structure
Figure 11: Fiberglass is made of glass fibers embedded in a polymer support.

The blue cylinders represent strands of glass fibers. The gray polymer area can be made of plastics like: polyethylene, (PE), polyvinyl chloride, (PVC), or polypropylene.

The World of Solids Waits

The most important kinds of solids that make your world fall into a few types based on how a solid is organized at the microscopic level. Each category deserves a topic in its own right. However the most basic ideas makes the best place to start.

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.

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.

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.

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.

 

 

Types of Pure Substances

 

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.

 

 

kinds of elements
Figure 2: Elements occur in a variety of forms

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.

 

 

 

compound ionic or molecular

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.

 

 

Two kinds of mixtures

 

 

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

 

Chemistry Energy

chemical 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.

heat and work
Figure 1: Inter-conversion of heat and work

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.

 

kinetic and potential energy
Figure 2: Potential energy is energy of position and kinetic energy is energy of motion

 

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.