Experiment 5 Enthalpy of Chemical Reactions

Topics: Sodium hydroxide, PH, Titration Pages: 2 (800 words) Published: March 16, 2013
Experiment 5
In the first part of this lab, the objective was to get the molar concentration of sodium hydroxide by using the secondary standard, sodium hydroxide solution with the primary standard, potassium hydrogen phthalate. With this information, we could create a second derivative plot, which can be used to obtain the exact concentration of the secondary standard, sodium hydroxide. Also, the indicator can give us these results along with the end points and with the readings from the pH meter; we can get the equivalence points.

In part I of the lab, the primary standard was the potassium hydrogen phthalate and the secondary standard was the sodium hydroxide solution. The differences between the two standards is that the primary standard, potassium hydrogen phthalate is a weak acid that reacts with NaOH compared to the secondary standard, sodium hydroxide that is basic and hygroscopic, meaning it absorbs moisture from the air. It was necessary to standardize NaOH solution because the compound is never a 100% NaOH since it absorbs a lot of moisture from the air.

The processes used to determine the molar concentration of sodium hydroxide was by: 1) using the volume of NaOH at the end point and 2) by using the volume of NaOH at the equivalence point. The pH meter was used for the end point and the second derivative and titration curve allowed us to get the volume of the equivalence point. The key difference between the end point and the equivalence point is that the end point is just an indicator of when the color changes and titration stops. The equivalence point is when the H+ ions are neutralized by OH- ions, which occurs when both are used up in the reaction causing the pH to be 7.

The molarity of NaOH from the indicator was exactly .500 M, but the molarity for the equivalence point from the second derivative was .688 M. This resulted in a range of roughly 27.3%, which is way over the 15% error mark of each other. This may have occurred due to the...
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