Dissolving KNO3 in water; getting weird enthelpy results?

In summary, a dissolution reaction of potassium nitrate and water was performed at different temperatures, and the solubility was measured. A graph of ln K (y) vs 1/T (x) was created, and the slope was found to be -ΔH/R, resulting in a ΔH value of about 34700 J and a ΔS value of about 137. However, it was noted that the reaction is exothermic, yet both the individual and the lab instructions had positive enthalpy values. This could be due to the fact that the dissolution of KNO3 is actually endothermic, as mentioned in a Yahoo Answers post.
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Sean1218
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I did a simple dissolution reaction (potassium nitrate + water), and measured the solubility at different temperatures.

I did a graph of ln K (y) vs 1/T (x), where K is the rate constant, and T is absolute temperature. Following the lab experiment instructions, where it says the slope is -ΔH/R, I get about 34700 J for ΔH (about the same as the values they give), and very similar ΔS (about 137 for most) values as well.

However, it's an exothermic reaction, so why do both of us have positive enthalpy values? They even say that ΔH is negative "as expected", but they list a positive value. And they wouldn't have gotten the ΔS values they listed (which they say is also as expected) if they hadn't used the positive ΔH value in the ΔG = ΔH - TΔS calculations.

What's going on?
 
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FAQ: Dissolving KNO3 in water; getting weird enthelpy results?

1. Why is the enthalpy of KNO3 different when dissolved in water compared to other solvents?

The enthalpy of a substance when dissolved in a solvent is dependent on the strength of the intermolecular forces between the solute and solvent molecules. KNO3 has strong ionic bonds, and when dissolved in water, the polar water molecules surround and interact with these ions, resulting in a different enthalpy compared to other solvents.

2. Can temperature affect the enthalpy of KNO3 when dissolved in water?

Yes, temperature can affect the enthalpy of KNO3 when dissolved in water. As temperature increases, the kinetic energy of the particles also increases, leading to more collisions between the solute and solvent molecules. This can result in a greater dissolution of KNO3 and a higher enthalpy value.

3. How does the concentration of KNO3 solution impact the enthalpy results?

The concentration of KNO3 solution can impact the enthalpy results, as a more concentrated solution means there are more solute particles present. This can lead to stronger interactions with the solvent molecules, resulting in a different enthalpy compared to a more dilute solution.

4. Is there a difference in enthalpy values when dissolving KNO3 in hot water compared to cold water?

Yes, there can be a difference in enthalpy values when dissolving KNO3 in hot water compared to cold water. As mentioned before, temperature can affect the solubility and enthalpy of the solute. In general, dissolving a solute in hot water requires more energy and results in a higher enthalpy compared to dissolving in cold water.

5. How can experimental errors affect the enthalpy results when dissolving KNO3 in water?

Experimental errors such as improper measurement of the amount of KNO3 or water, temperature fluctuations, and incomplete dissolution of KNO3 can all affect the enthalpy results when dissolving KNO3 in water. It is important to carefully follow procedures and minimize these errors to obtain accurate and reliable enthalpy values.

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