Thermodynamics Of Potassium Nitrate Dissolving In Water Essay

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The Thermodynamics of Potassium Nitrate Dissolving in Water


This experiment focuses on thermodynamics or the study of heat and molecular randomness. It helps illustrate how and why potassium nitrate (KNO3) dissolves in water. When it dissolves, it dissociates into potassium (K+) and nitrate (NO3-) ions. These ions can also recombine into solid potassium nitrate. This reaction can reach a state where concentrations of ions remain constant or Ksp; which stands for the equilibrium concentration for the solubility product. The solubility of a compound changes with temperature, so Ksp changes too. Thermodynamics focuses on three large areas; which are enthalpy, entropy, and free energy. Enthalpy change or ΔH, for KNO3 dissolving
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Set up a hot plate and place a 400-mL beaker filled half-way with tap water.
2. Use a balance to weigh out 10 g of KNO3 on weighing paper. Place the KNO3 from the weighing paper into the 25 * 200 mm test tube.
3. Use a graduated cylinder to add 7.5 mL distilled H2O to the test tube. Heat the test tube while it is suspended in the hot water bath by the utility clamp on the ring stand.
4. Stir the KNO3 in the distilled H2O until all the KNO3¬ dissolves.
5. Remove the test tube from the water bath and keep stirring the contents until potassium nitrate crystals start to appear.
6. Record the temperature this occurs at and continue stirring until all of it has crystallized again.
7. Add 2.5 mL distilled H2O to the test tube and start to heat it in the water bath. Dissolve all the potassium nitrate while continuously stirring.
8. Once dissolved, let the contents cool and once again record the temperature at which crystal form.
9. Repeat this process for a total of five trials.


Table 1: Results and Calculations from all Trials Relevant to the Dissolving and Crystallization of Potassium Nitrate
Figure 1: Graph of lnKsp (Natural Logarithm of Equilibrium Constant) vs. 1/T (Reciprocal of Kelvin Temperature) with linear regression equation to identify the
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Then a linear regression line is added and determined in a y = mx + b format, where m is the slope. ΔH is calculated by taking the negative constant 8.314 J/K*mol and multiplying it by the negative slope. The value for ΔH is positive meaning that heat must be added for potassium nitrate to dissolve. ΔS is calculated by taking ΔH and subtracting ΔG, then divide by the temperature in kelvin. This is calculated separately for each trial and then the separate values for each trial are averaged together. As expected, the value is positive because the two ions possess more disorder as products than one molecule of potassium nitrate as a

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