Chemlab of Ages
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Period 3
When HCl was added to a solution of saturated salt water, grains of the NaCl immediately came out of the solution. Each drop of HCl that touched the solution created a little whirlwind of granules of NaCl.
NaCl(s) + H2O = Na+ (aq) + CL- (aq)
When the HCL was added, it became a source of CL- ions, which caused the reaction to drive to the left. The production of more NaCl solid was evidenced by the observed formation of NaCl precipitate.
In pure water, the BTB gave the solution a light, translucent, blue color. Each drop of acid that his the liquid in the test tube sent a swirl of yellow swishing into the solution. By the time five or so drops were in the test tube, the liquid was completely yellow, indicating a low pH, and a high H+ concentration. The yellow was pale, almost pastel, but noticeably different from the previous blue color. The addition of sodium hydroxide neutralized the high H+ concentration and increased the pH. The liquid in solution changed back to blue. As the reaction was taking place, and drops of sodium hydroxide were put into the liquid, swirls of blue and gold occurred, similar to the way the crowd looked at Heinz Field for the 2011 WPIAL football championship.
Le Chateliers principle applied here because each time a new substance was added, the stress on the system forced the equilibrium to change. The reaction taking place basically consistjs of two different forms of aqueous BTB changing back and forth at equilibrium. There is blue colored BTB and yellow colored BTB. When the acid was added, it increased the concentration of the H+ ions, and drove the reaction to the side that has the blue colored version of BTB. When the base was added, the H+ ions were neutralized. This drove the reaction to the side that contained yellow colored BTB. The perceived color of the solution depended on whether there was more blue BTB or more yellow BTB.
When ferric nitrate was added to a solution of potassium thiocyanate, it turned a brilliant blood red.
Fe(NO3)3 + KSCN = [Fe(NCS)(H2O)5]2+ + K+ + 3NO3-
Adding a crystal of KSCN drove the reaction to the right, causing more [Fe(NCS)(H2O)5]2+ to form, causing the liquid to become more red. This was observed experimentally. Adding a couple drops of ferric nitrate drove the reaction further to the right, causing the liquid to become even more red.
The addition of Na2HPO4 which is phosphoric acid that has been neutralized by salt introduced other ions to the solution. These ions caused the large [Fe(NCS)(H2O)5]2+ to break apart. With out that molecule, the solution lost its red color and became a pale almost entirely clear yellow. This last part is not necessarily an application of le Chatliers