Determination of enthelpy of solution (solubility/temp stuff)

In summary, by using the van't hoff equation and plotting ln(solubility) against 1/Temp, we can determine the enthalpy change of dissolving benzoic acid in water at different temperatures and volumes. It is not necessary to convert solubility into mol/l because it is the difference in solubility that is important. The second question asks why solubility can be used instead of equilibrium constant in the equation, and the answer is that they are proportional to each other, with solubility being the concentration of the substance in solution at equilibrium. This is only true for weak acids like benzoic acid, where the majority of the substance is in the non-ionic form.
  • #1
_Greg_
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Just done an experiment where you dissolve benzoic acid in water at different temperatures and different volumes of water.

From the van't hoff equation you then plot a graph of ln(solubility) against 1/Temp which produces a straight line with gradient -ΔH/RT from which you get the enthalpy change.

(vant hoff equation): lnk = -ΔH/RT + constant

The first question was:

why isn't it necessary to express solubility in mol/l despite ΔH beight evaluated in J/mol?

I answered with:

It isn’t necessary to convert solubility into mol/l because it’s the difference in solubility that’s important. Therefore plotting solubility with either units will always produce a line of the same gradient but with different Y intercept (the intercept being insignificant)

Is that ok?

The second question I am stuck on is:

Why is it that solubility, S, may be used instead of the equilibrium constant, K, in the van't hoff equation?

I would guess they are proportional to each other or something?

would really appreciate any help :smile:
 
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  • #2
_Greg_ said:
(vant hoff equation): lnk = -ΔH/RT + constant

why isn't it necessary to express solubility in mol/l despite ΔH beight evaluated in J/mol?

I answered with:

It isn’t necessary to convert solubility into mol/l because it’s the difference in solubility that’s important. Therefore plotting solubility with either units will always produce a line of the same gradient but with different Y intercept (the intercept being insignificant)

Is that ok?
I believe you are right. As long as -H/RT is using the correct units, the slope should be the same. Remember that this is only true if the units being used have a direct linear correlation to the other units being used.
Why is it that solubility, S, may be used instead of the equilibrium constant, K, in the van't hoff equation?

I would guess they are proportional to each other or something?
Correct, but you should probably say why that is true. Wikipedia has an explanation of solubility equilibrium. Under nonionic it says this:

If the activity of the substance in solution is constant (i.e. not affected by any other solutes that may be present) it may be replaced by the concentration.

[tex]K_s = \left[\mathrm{{C}_{12}{H}_{22}{O}_{11}}(aq)\right]\[/tex]

The square brackets mean molar concentration in mol dm-3 (sometimes called molarity with symbol M).

This statement says that water at equilibrium with solid sugar contains a concentration equal to K. For table sugar (sucrose) at 25 °C, K = 1.971 mol/L.
Remember that this is only true because benzoic acid is a weak acid meaning the majority of it is COOH (which applies to the above quote). Only a tiny fraction of it is the ionic COO- form, to which Ka applies, but it's so small you can probably just ignore it.

edit: Firefox actually copies tex code from wikipedia. Fascinating.
 
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  • #3


Your answer is partially correct. The reason why solubility, S, can be used instead of the equilibrium constant, K, in the van't hoff equation is because they are directly related to each other. In fact, solubility is equal to the concentration of the solute in a saturated solution, which is also equal to the equilibrium constant. This means that as the solubility increases, the equilibrium constant also increases, and vice versa. Therefore, using solubility in the van't hoff equation allows us to directly measure the solubility at different temperatures and use it to calculate the enthalpy change of the solution. On the other hand, using the equilibrium constant would require additional calculations and assumptions about the reaction. So, using solubility simplifies the process and gives more accurate results.
 

1. What is the enthalpy of solution?

The enthalpy of solution is the energy change that occurs when a solute is dissolved in a solvent. It is a measure of the heat absorbed or released during the dissolution process.

2. How is the enthalpy of solution determined?

The enthalpy of solution can be determined experimentally by measuring the temperature change that occurs when a known amount of solute is dissolved in a known amount of solvent. This can be done using a calorimeter.

3. How does temperature affect the enthalpy of solution?

Temperature can affect the enthalpy of solution as it can influence the strength of the intermolecular forces between the solute and solvent particles. Generally, as temperature increases, the enthalpy of solution decreases, as more heat energy is needed to break the intermolecular bonds.

4. What factors can affect the solubility of a substance?

The solubility of a substance can be affected by factors such as temperature, pressure, and the nature of the solute and solvent. For example, polar solutes tend to be more soluble in polar solvents, while nonpolar solutes are more soluble in nonpolar solvents.

5. Why is the determination of the enthalpy of solution important?

The determination of the enthalpy of solution is important in understanding the thermodynamics of a solution and its ability to dissolve a solute. It can also provide valuable information for various industrial processes, such as in the production of pharmaceuticals or in the design of chemical reactions.

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