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Introduction
The heat of solution represents the energy transfer when dissolving a solute in water.
Dissolution Process Summary
ΔH is measured in KJ/mol.
ΔH₁ - Breaking Solute-Solute Interactions
To dissolve a solute, you would need to break up the forces holding its particles together.
e.g. electrostatic attraction between ions, intermolecular forces between molecules.
This step is endothermic (ΔH > 0) because energy is required to overcome theses bonds or interactions.
ΔH₂ - Breaking Solvent-Solvent Interactions
Next, the forces holding the solvent together must be broken make way for the solute particles.
This step is endothermic (ΔH > 0) because energy is required to overcome these interactions.
ΔH₃ - Forming Solute-Solvent Interactions
To effectively form a solution, solvent particles surround solute particles.
This step is exothermic (ΔH < 0) because it establishes new interactions between the particles, which is more stable than the state that they were in before.
Heat of Solution Formula
The heat of solution (ΔHsoln) is determined by adding the three values representing the enthalpy changes:
ΔH₁ + ΔH₂ + ΔH₃ = ΔHsoln
If the sum is negative (< 0), this means that the process is exothermic because ΔH₃ released more energy than how much ΔH₁ + ΔH₂ absorbed, resulting in energy being released into the surrounding.
If the sum is positive (> 0), this means that the process is endothermic because ΔH₃ released less energy than how much ΔH₁ + ΔH₂ absorbed, resulting in a loss of temperature surrounding the system.
Using Calorimetry to Solve for ΔHsoln
To learn more about calorimetry, check out this tutorial.
Suppose you were to set up a lab to explore the energy transfer of dissolving KCl into water. Here is the data collected:
Mass of Water: 210.46g
Initial Temperature of Water: 18°C
Mass of KCl: 3.82g
Final Temperature of Water (KCl dissolved): 17°C
1) Determine the energy change of the water (using the formula q = mcΔT).
Add together the masses of KCl and water for the value of m.
q = (214.28)(4.184)(-1)
q = -896.55 J
2) Does KCl dissolve endothermically or exothermically? Justify your answer.
KCl dissolves endothermically. During the dissolution process, the salt absorbed heat from the surrounding, leading to a decrease in water temperature.
3) Calculate the ΔHsoln of KCl in KJ/mol.
Step I. Use the q value of water: Since water transferred 896.55 J to the KCl, that means that the q value of KCl should be +896.55 J, assuming perfect thermodynamics.
Step II. Convert J to KJ: 896.55/1000 = 0.897 KJ
Step III. Convert grams of KCl to moles: 3.82/74.55 = 0.051 mol KCl
Step IV. Divide KJ value by mole value to find ΔHsoln: 0.897/0.051 = +17.59 KJ/mol
4) The theoretical value of ΔHsoln is +17.58 KJ/mol. What is the percent error?
|(17.59-17.58)/17.58| × 100 = 0.057%
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