Wednesday, May 23, 2012

The First Law of Thermodynamics

We're already familiar with the first law of thermodynamics, even if we are not aware of it. The first law of thermodynamics is simply the law of conservation of energy, reworded for systems that involve thermodynamics. It says:

"In a thermodynamic process, the increment in the internal energy of a system is equal to the difference between the increment of heat accumulated by the system and the increment of work done by it."

This is merely saying that, when energy seems to be lost during work, it is merely converted into heat. This in turn is important to realize because it means that the energy within the universe is a constant--energy cannot be created or destroyed, merely converted from one form to another.

It is also important to realize that, when systems are in thermal equilibrium, that there is no net heat transfer. (Thermal equilibrium merely means that two systems are at the same temperature.)

There are two different types of reactions that can take place--an endothermic reaction and an exothermic reaction. In an endothermic reaction, heat is added to the system, and in an exothermic reaction, heat is taken away from the system.

Now that we have the basics of thermodynamics, we can look at how they apply to a chemical system. A common example of this is considering what happens in a piston chamber. Heat is added to the system, causing the gas within the chamber to expand. This pushes the piston down, causing work to be done. Here, we can see that, when heat was added to a system, the internal energy of the system increased--because both the heat and the work done increased.

Another example of this can be found in my acquaintance Damien's experiment. Basically, they created a super-saturated solution heated to the boiling point, then cooled it and allowed it to crystallize. As it did this, the energy from the motion that was within the liquid was released as heat. Again, we can see that the energy did not disappear--it was converted into heat.

Tuesday, May 22, 2012

Acids and Bases

Look at the beginning of the Wikipedia page for "Acid," and you'll notice the extremely non-helpful definition:

"An acid (from the Latin acidus/ac─ôre meaning sour) is a substance which reacts with a base."

Ok...so that's great. But what is an acid really? To answer that question, we have to look at the structure of water. Everyone knows the chemical formula for water--H2O. Of course, that means that one water molecule has two hydrogen atoms bonded with one oxygen. However, those bonds can be broken, and the atoms can shift locations. Water molecules can exchange protons with each other because the oxygen has a higher electronegativity than the hydrogen atoms.When this exchange occurs, then one water molecule becomes positively charged (H3O+) and the other negative (OH-).

What does this have to do with acids? Well, when the compound HCl is added to water, the same process occurs. HCl separates into a hydrogen ion (H+) and a chlorine ion (Cl-). These hydrogen ions create the positively charge water molecules (H3O+). This in turn causes the solution to become more acidic because the ratio of H3O+ to OH- is shifted in favor of the hydronium ions (H3O+). A base, on the other hand, does the opposite. It doesn't add protons to the system; instead, it takes protons away and shifts the balance in favor of the hydroxide ions.

Acids have a reputation for being bad--after all, they can burn through your skin, right? However, some gentle acids include soda and rainwater, going all the way down the pH scale to not-so-gentle acids like stomach acid. Bases, on the other hand, can be just as severe in reaction--they include ammonia and Drano.

Acididy and basicity are measured on the pH scale. The pH scale corresponds to the concentration of hydronium ions in a solution. It's actually just the absolute value log of the concentration of hydronium ions in a solution. For example, in pure water, the concentration of hydronium ions is 1x10^-7. Since the absolute value of the log of 1x10^-7 is 7, the pH of water is 7.

Titration is a technique that allows us to determine the concentration of an unknown reagent by using a known concentration of another reagent that reacts with the other reagent. Titration can be used to determine of unknown acids or bases. It's called titration because the reagent with the known concentration is called the titrant. (That comes from a Latin word, but let's not go into that.) The two solutions are allowed to react, and once the reaction has completed (the "equivalence point" has been reached), all of the titrant added to the unknown solution and the unknown solution have reacted. Here, we can find that:

(Normality of titrant)(Volume of titrant needed) = (Normality of unknown)(Volume of unknown)

(Normality is a measure of a solution's ability to release ions into another solution. For example, in an acid reaction, the normality of H2SO4--sulfuric acid--is 2, because there are 2 moles of H+ per mole of sulfuric acid.)

Now, because the titrant is a known solution, we know its normality, and we can measure how much of it was needed to finish the reaction. We also can measure the volume of the unknown. This leaves us with one unknown--the normality of the unknown reagent. Because there is only one unknown in this equation, we can easily solve for it and determine the concentration of the reagent.

Beer's Law Lab

In this lab, we measured the absorbance of five different concentrations of a solution and used these data to find the unknown concentration of a solution. This was done by using a calorimeter, which directs red light through the solution into a photocell, which measures how much light passed through the solution. The data from the calorimeter were collected and analyzed through Logger Pro.

I created five different solutions of known concentration. The first had a concentration of .08 M, the next, .16 M, third, .24 M, fourth, .32 M, and the fifth, .4 M. After calibrating the calorimeter, I measured the absorbance of each solution and obtained the following results:


From this, we can see that the concentrations followed a relatively consistent line (deviations can be explained by measurement errors). Because the absorbance is linear when compared to concentration, we can use this line to predict the concentration of an unknown substance by measuring its absorbance.

We determined the absorbance of the solutions to be as follows:

Unknown 1: .186
Unknown 2: .551
Unknown 3: .367

By plotting these points on the line of best fit, we can see that the concentrations of each respective substance is approximately:

Unknown 1: .155 M
Unknown 2: .365 M
Unknown 3: .26 M

ChemTech Project

Here's the writeup from my ChemTech Project on a commonplace radio system.

Enjoy!

Thursday, March 15, 2012

Silver/Copper Replacement Lab

Purpose: To determine how much silver is contained within one gram of silver nitrate by using a single-replacement reaction.  

Hypothesis: If 1 gram of silver nitrate is combined with an excess of copper, then approximately 0.6 grams of silver will be produced in the reaction.

Materials:

This experiment requires silver nitrate to use as a reactant, copper to use as a reactant, distilled water, a test tube, some filter paper, and a funnel beaker.  

Procedure:

1) Weigh out approximately 1 gram of silver nitrate. Add this to the test tube along with distilled water.
2) Cut a 30 cm piece of copper wire, coil it, and add it to the test tube.
3) Seal the test tube and wait a day.
4) Place the funnel over the beaker, and cover the funnel with the filter paper. Pour the contents of the test tube into the funnel, and spray all the silver precipitate off the wire using distilled water. Allow the funnel and beaker to drip.
5) Separate the silver from the copper wire and measure the weight of each.

Data: Before reaction: 1 g silver nitrate 3.49 g copper After reaction: 0.35 g silver 3.193 g copper

Conclusion: Again, it is possible to use stoichiometry to determine how much silver should be produced. Looking at the balanced equation

2AgNO3 + Cu ----> 2Ag + Cu(NO3)2

we can see that there is a 2:1 ratio of moles of silver nitrate to moles of copper. The molar mass of silver nitrate is about 170 moles, and the molar mass of copper is about 63.5 moles. Solving using the same techniques used in the previous post, it can be shown that about .6 grams should be produced. The experiment itself had a comparatively small percent yield, so only .35 grams were produced.

Baking Soda and Vinegar Lab

Baking Soda and Vinegar Lab

Purpose: To determine the amount of baking soda necessary in terms of a ratio to vinegar to complete a chemical reaction with vinegar with the products of water, carbon dioxide, and sodium acetate.

Hypothesis: A ratio exists such that, with baking soda and vinegar as the reactants, it is possible to define a chemical reaction that uses all of the available reactants and yields carbon dioxide, water, and sodium acetate as the products.

(I'm such a mathematician.)

Materials: A beaker to complete the reaction in, a stirring rod to stir, and baking soda and vinegar to use as the reactants.


Procedure:

1) Measure 1 gram of baking soda and put it in the bottom of the beaker.
2) Measure and record an estimated amount of vinegar and add it to the beaker with the baking soda.
3) Allows the reactants to react and occasionally stir the beaker.
4) When reaction finishes, check to see if any baking soda or vinegar remains. If so, add more of the substance that was used up until you completely use up both reactants.
5) Record the amount of vinegar required to completely react with 1 gram of baking soda.

Data: When we completed the lab, we found approximately 14.29 grams of vinegar to be sufficient in using up all of the vinegar.

Conclusion:

It is possible to solve for the amount of vinegar necessary in the reaction. Start by balancing the equation:

NaHCO3 + HC2H3O2 -----> CO2 + H2O + NaC2H3O2

This shows us that there should be one mole of acetic acid to one mole of baking soda. The molar mass of acetic acid (HC2H3O2) is about 60 grams per mole, and the molar mass of baking soda (NaHCO3) is about 84 grams per mole. Since we have a 1:1 ratio of acetic acid to baking soda, we can solve for how many grams of acetic acid are necessary for this equation to work with one gram of baking soda--it's about .7 g. Since vinegar is a 5% aqueous solution of acetic acid, we should multiply this by twenty to learn that it should be necessary to use slightly over 14 grams (the error comes from my rounding for ease of calculation). The experiment needed slightly more than this, because the percent yield of the equation was less than 100%.

Popcorn Lab

Purpose: To determine what percent of a popcorn kernel is water.

Hypothesis: Once popcorn is heated, it will lose some of its water mass to steam and have less mass afterwards.

Materials: This experiment will require a beaker in which to pop the popcorn, popcorn kernels to pop, oil to pop them in, and aluminum foil and a bunsen burner to pop the popcorn with.


Procedure:

1) Fill the bottom of the beaker with the oil, then cover the top of the beaker with aluminum foil. Poke holes in the aluminum foil. Weigh the beaker with oil. Record weight.

2) Add approximately 25-30 popcorn kernels to the beaker. Weigh the beaker with oil and popcorn. Record weight.

3) Place beaker over flame of bunsen burner to pop. Once popped, allow to cool, then weigh beaker with popped kernels.

Data:


(Conveniently hotlinked from my friend Steven's blog.)

Conclusion: From these data, we can see that the popcorn kernels lost mass after they were popped, meaning that some of their mass went to the water that evaporated and was captured by the aluminum foil. Precisely, they lost approximately 15% of their mass, suggesting that popcorn kernels are about 15% water.