This is a very long page. Don't be afraid of this unwieldy collection of possible topics. Over the last classes, we’ll start at the top of this page and pursue the first two or three topics, after which your questions will direct me from there. It's very important for you to ask and/or submit questions to guide me.
Class #5 will start with an overview of chemistry, in two takes, one video, and one to read.
Overview of Chemistry
(what? now?)
Take 1: Here's the video:
This video has a few minor errors (listed in the text material below the video), but gives a very concise and clear overview of the ideas and activities that constitute chemistry. I think it might be more effective now, after you have encountered some of the basic terms and concepts, than it would have been on day 1.
I hope it stimulates questions!
Take 2: What do chemists do?
In chemical terms, molecular structure gives rise to function. Molecular structure is the basis of the properties of substances - their colors, textures, shapes, and tendencies to change. As we learn to make these connections, we are drawn into a world of imagination that enriches the world we see. We study chemistry because many interesting and important things we see -- the intricacy of a snow flake, the symptoms of an cancer sufferer -- are caused by things we cannot see, but must understand.
- purification: the separation of mixtures into "pure"* substances
- analysis: detecting and identifying substances (qualitative analysis), and measuring their quantities (quantitative analysis)
- structure determination: composing and testing structural models that fit all available data on a substance, in order to learn its molecular structure
- synthesis: making complex molecules of defined structure using reactions between simpler substances
- studying reactions, particularly their rates (kinetics) and energy changes (thermodynamics), in order to establish relationships between structure and reactivity.
* What Does Pure Mean?
No substance is completely pure.
Once more, for emphasis: no substance is completely pure.
In practice, a pure substance is one whose level of impurities is too low to have a detectable effect on what you are using the substance to do. The question is never whether there are impurities in a natural or synthetic substance. The question is how much. A liter of "pure" water, with impurities at less than one part per trillion (only 1 part in 1012 !) could still harbor trillions of impurity molecules in each liter. We are surrounded by impurities. There are likely to be atoms of every element in any sample taken on the earth, though most are at undetectable levels. There are likely to be some atoms of radioactive plutonium in our classroom.
But come to class anyway.
As scientists develop the technology to detect ever small numbers of atoms and molecules, it becomes more important to determine what level of a specific impurity actually is significant, because all imaginable impurities are probably present in some minuscule amount.
We will consider some simple equilibrium processes in class.
We will consider reactions in solution. First a few new terms. A solution is a mixture in which a solid, liquid or gaseous substance, the solute, is dissolved in a liquid, the solvent. In a aqueous solution, the solvent is water. Syrup is an aqueous solution of sugar. In other words, it is sugar (solute) dissolved in water (solvent).
(Think about the difference between dissolving and melting. Consider the melting of ice to produce liquid water, versus the dissolving of sugar in water to produce a solution.)
But even absolutely pure water (don't ask me where to find any) contains in each liter about about a hundred million billion (1 x 1017) positively charged hydrogen ions (H+) and the same number of negatively charged hydroxide ions (-OH).
Where do they come from? At any moment, any water molecule can fragment to produce these two ions. It's happening all the time, but only to a tiny fraction of the water molecules, according to this chemical equation:
The double arrow (<===>) is the best I can do with this page editor to represent an equilibrium reaction, a reaction that does not go to completion. Instead the reaction proceeds until the rate of the forward process (water --> ions) equals the rate of the reverse process (ions --> water), and then it's stuck there, at -- erm -- equilibrium, because water molecules are forming ions at the same rate that ions in solution happen to find each other and make water molecules.
In pure water, the number of hydrogen ions exactly equals the number of hydroxide ions because every water molecule that ionizes produces one of each ion, and every time a water molecule is formed in the reverse reaction, one of each ion is consumed. The equilibrium dissociation of water into hydrogen ions, H(+) and hydroxide ions, OH(-) strongly favors H2O, so the amount of the ions in pure water is very low. While the concentration of water is about 55 M, the concentration of H+ ions is about 0.0000001 M. or 1 x 10^(-7) M. Same for the hydroxide ions.
pH is a measure of the concentration of hydrogen ions in a water solution. pH of 7 means that the number of hydrogen ions equals the number of hydroxide ions, as in pure water. The pH of pure water at room temperature is 7. But I can shift the equilibrium to a new position by adding hydrogen ions in the form of a strong (HCl) or weak (acetic) acid. If I increase the concentration of hydrogen ions by ten-fold, the pH will be 6, and the concentration of hydroxide ions will be reduced by 10-fold. Such a solution is said to be acidic. A solution of pH 5 is 100 times more acidic than pure water, and a solution of pH 4 is 1000 times more acidic than pure water.
If I add hydroxide ions in the form of a strong (NaOH) or weak (ammonia) base, there will be more hydroxide ions than hydrogen ions, and the solution will be basic. A solution of pH 8 contains ten times as many hydroxide ions as does pure water, and ten times fewer hydrogen ions. A solution of pH 10 is 1000 times more basic than pure water.
Finally, two definitions: An acid is a source of hydrogen ions; hydrogen ion itself is considered to be an acid, the simplest one. A base is a consumer of hydrogen ions; hydroxide ion is the simplest base.
How is pH Measured?
The classic way is to use indicators, pigments that show different color in solutions of different pH. You can make an indicator at home, using pigments in red cabbage. Click HERE to see how.
Phenolphthalein is often the first pH indicating pigment that a first-year chemistry student encounters. See this entry at Wikipedia about how color changes in phenolphthalein come about.
All pH methods (actually all methods of scientific measurement) rely on calibration, which in the case of pH means using solutions of known pH to compare with unknowns. Calibration solutions can be made with specific pH values, by applying well-understood chemistry.
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Thermodynamics
Read a little about thermodynamics, at Wikipedia, just enough to see some of the most important terms.
Chemists use the Gibbs equation to talk about the forces that determine the position of an equilibrium, or the direction that a reaction with proceed spontaneously.
The Gibbs Equation: ∆G = ∆H – T ∆S (G is free energy; H is heat; S is entropy.)
A process can proceed spontaneously if ∆G is negative. For a process that gives off heat, ∆H is negative. For a process that increases the disorder of an isolated system, ∆S is positive.
Rubber Band Thermodynamics
Bring to class a rubber band, the thicker and heavier, the better. Before class, try this.
1) Touch the band to your lips to see what it feels like at room temperature.
2) Now stretch the band as far and as hard as you can. Immediately touch the stretched band to your lips. Can you detect a temperature change?
3) Keep the band stretched until it feels like it's back to room temperature. Then quickly let the band relax and immediately touch the relaxed band to your lips. Can you detect a temperature change?
If the band feels warm after one of the steps above, it means it is losing heat to the surroundings. If the band feels colder after one of the steps above, it means it is absorbing heat from its surroundings. (If it feels cold to your lips, then heat is flowing from your lips to the band.)
A heat pump moves heat from one place to another -- from a hot room to the outside of your house in summer, and from outdoors into a cold room in winter. Think about how you could use the rubber band as a makeshift heat pump. Sound crazy? Not practical, but not crazy.
In class, we will us a rubber band to give an example of a spontaneous process (contraction of a stretched band) and what forces (heat or entropy) drive the contraction.
Some Practical Applications of Chemical Knowledge
I) Refrigeration
How a Heat Pump Works (applies to refrigerators as well)
2) Solar cells and LED lights
VIDEOS on how solar cells work:
LED lights are exactly the same things as solar cells, but electricity applied to the external circuit forces reverse flow of electrons and holes, forcing them to combine, which means the electrons drop to lower energy levels and emit light.
Methods of Chemical Analysis
I. Chromatography, including lateral-flow or "rapid" COVID testing
Read or watch THIS about chromatography.
Here is an example of chromatography for separating the pigments (colored substances) in spinach leaves.
Spinach leaves were mashed up and stirred into acetone, which dissolves many of the leaf pigments, producing a spinach-leaf extract. A tiny drop of the solution is placed near the bottom of a glass plate coated with silica gel (the stationary phase of the chromatography). The plate is stood in a jar containing a small amount of a solvent or solvent mixture (the mobile phase). The solvent rises up the plate by capillary action, carrying pigments with it. Pigments that are more soluble in the solvent and/or less attracted to the silica gel move faster. Those less soluble and or more attracted to the gel move slower. The designer of this procedure has previously optimized (by trial and error) the mobile and stationary phases to give the best separation of all the pigments.
Methods of Chemical Analysis
II. Spectroscopy, including figuring out structures of molecules
Watch this video about spectroscopy:
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Epilogue: Teach Yourself Real Chemistry at Home?
Chemistry students who expect to excel in first-year college chemistry can expect to spend about 3 hours per week in class, 4 hours per week in lab, and as much as a dozen hours per week in private study. Much of what they learn they will actually teach themselves from the text, problem solving, and other sources.
If you want to follow the path of such a course, whose aim is to prepare students to use basic chemical ideas in succeeding courses, watch the whole Crash Course Chemistry series. Try watching about three episodes per week, and each time, think about what you would do to reinforce your learning and be able to solve chemistry problems, which is tantamount to using the ideas of chemistry to answer practical questions.
You can start right here.
Thank you for participating in this class.
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APPENDIX: Nernst Equation and pH
The Nernst equation allows calculation of electrochemical cell voltage (E) between two compartments having different ion concentrations. If the main ions present in the two compartments are hydrogen ions (H+), then the voltage (also called the potential) is related to the pH difference between the compartments. The relationship between voltage and pH is the last equation in this derivation.
Solving the last equation for pH,
pH = E / 0.059 mV, in which E is the measured voltage
So the chip in the pH meter displays the value E / 0.059, which is the pH.
