Molar Heat of Dissolution Calculator

An expert tool to calculate the theoretical molar heat of dissolution for endothermic processes based on calorimetric data.

Calculation Results

— kJ/mol

Enter your experimental data above to see the calculated molar heat of dissolution.

Heat Absorbed by Solution (q)

— J

Moles of Solute (n)

— mol

Temperature Change (ΔT)

— °C

Visual representation of the Molar Heat of Dissolution (kJ/mol).

What is Molar Heat of Dissolution?

The molar heat of dissolution, also known as the molar enthalpy of solution (ΔH_sol), is the total energy change that occurs when one mole of a substance (the solute) dissolves in a solvent. When this process absorbs heat from the surroundings, the reaction is classified as endothermic, and the ΔH_sol value is positive. This calculator is specifically designed to help you calculate the theoretical molar heat of dissolution using table endothermic process data derived from a calorimetry experiment.

The process of dissolution involves two main energy steps:

1. Lattice Energy: The energy required to break the bonds holding the solute particles together in their crystal lattice. This is an endothermic step (requires energy).
2. Hydration Energy: The energy released when the individual solute particles are surrounded and stabilized by solvent molecules. This is an exothermic step (releases energy).

For an endothermic dissolution, the energy required to break the solute’s lattice is greater than the energy released during hydration. As a result, the system draws heat from its surroundings, causing the temperature of the solution to drop. A common example is the cold pack used for sports injuries, which often uses the endothermic dissolution of ammonium nitrate in water.

The Formula for Molar Heat of Dissolution

To calculate the molar heat of dissolution from experimental data, we use the principles of calorimetry. The key idea is that any heat absorbed by the dissolving process is lost by the surrounding solution, causing its temperature to decrease.

Step 1: Calculate the heat absorbed by the solution (q).
The formula is:
q = m × c × ΔT

Step 2: Calculate the moles of solute (n).
n = mass of solute / molar mass of solute

Step 3: Calculate the molar heat of dissolution (ΔH_sol).
Because the heat absorbed by the reaction is equal to the heat lost by the solution, we use a negative sign to represent the change from the solution’s perspective.
ΔHsol = -q / n
The result is typically converted from J/mol to kJ/mol by dividing by 1000.

Formula Variables

Variable	Meaning	Unit (Auto-Inferred)	Typical Range
q	Heat absorbed by the solution	Joules (J)	-10,000 to +10,000 J
m	Total mass of the solution (solute + solvent)	grams (g)	10 – 1000 g
c	Specific heat capacity of the solution	J/g·°C	~4.184 for water
ΔT	Change in temperature (Tfinal – Tinitial)	°C	-20 to +20 °C
n	Moles of solute	mol	0.01 – 5 mol
ΔHsol	Molar heat of dissolution	kJ/mol	-100 to +100 kJ/mol

Practical Examples

Example 1: Dissolving Ammonium Nitrate

You dissolve 8.0g of ammonium nitrate (NH₄NO₃, molar mass 80.04 g/mol) in 100g of water. The temperature drops from 22.0°C to 16.1°C. The specific heat of the solution is 4.18 J/g·°C.

• Inputs: massSolute=8.0, molarMassSolute=80.04, massSolvent=100, specificHeat=4.18, initialTemp=22, finalTemp=16.1
• Calculations:
  • Total Mass (m) = 8.0g + 100g = 108g
  • ΔT = 16.1°C – 22.0°C = -5.9°C
  • q = 108g × 4.18 J/g·°C × -5.9°C = -2663 J
  • Moles (n) = 8.0g / 80.04 g/mol = 0.10 mol
  • ΔH_sol = -(-2663 J) / 0.10 mol = +26630 J/mol
• Result: The molar heat of dissolution is +26.6 kJ/mol.

Example 2: Dissolving Potassium Chloride

You dissolve 15.0g of potassium chloride (KCl, molar mass 74.55 g/mol) in 200g of water. The temperature drops from 25.0°C to 20.2°C. Assume specific heat is 4.10 J/g·°C.

• Inputs: massSolute=15.0, molarMassSolute=74.55, massSolvent=200, specificHeat=4.10, initialTemp=25, finalTemp=20.2
• Calculations:
  • Total Mass (m) = 15.0g + 200g = 215g
  • ΔT = 20.2°C – 25.0°C = -4.8°C
  • q = 215g × 4.10 J/g·°C × -4.8°C = -4231 J
  • Moles (n) = 15.0g / 74.55 g/mol = 0.201 mol
  • ΔH_sol = -(-4231 J) / 0.201 mol = +21050 J/mol
• Result: The molar heat of dissolution is +21.1 kJ/mol. This is a topic our experts explain in more detail in our exothermic vs endothermic reactions guide.

How to Use This Molar Heat of Dissolution Calculator

Follow these steps to accurately find the enthalpy of solution:

1. Enter Solute Mass: Input the mass of the solid you dissolved in grams.
2. Enter Molar Mass: Input the molar mass of your solute in g/mol. If you don’t know it, you may need a molar mass calculator.
3. Enter Solvent Mass: Input the mass of the water (or other solvent) you used in grams.
4. Set Specific Heat: The value for water (4.184 J/g·°C) is the default. Adjust this if you are using a different solvent or have a more precise value for your solution.
5. Enter Temperatures: Record the initial temperature of the solvent before mixing and the final, stable temperature after the solute has fully dissolved.
6. Calculate: Click the “Calculate” button to see the results. The calculator will show the primary result (ΔH_sol in kJ/mol), along with intermediate values for heat change (q), moles (n), and temperature change (ΔT).

Table of Common Endothermic Dissolutions

Standard Molar Enthalpies of Solution (ΔH°_sol) at 25°C

Compound	Formula	ΔH°sol (kJ/mol)
Ammonium Nitrate	NH₄NO₃	+25.7
Ammonium Chloride	NH₄Cl	+14.8
Potassium Chloride	KCl	+17.2
Potassium Iodide	KI	+20.3
Sodium Chloride	NaCl	+3.9
Sodium Thiosulfate Pentahydrate	Na₂S₂O₃·5H₂O	+47.4

This table can be used to compare your experimental results with theoretical values. Check out our specific heat database for more material properties.

Key Factors That Affect Molar Heat of Dissolution

• Lattice Energy: Stronger ionic bonds in the solute’s crystal lattice require more energy to break, leading to a more positive (or less negative) ΔH_sol.
• Hydration Energy: Stronger attraction between the ions and solvent molecules releases more energy, leading to a more negative (or less positive) ΔH_sol. This is a key part of the enthalpy of solution formula.
• Solute Concentration: The ideal ΔH_sol is measured at “infinite dilution.” At higher concentrations, ion-ion interactions in the solution become more significant, slightly altering the overall energy change.
• Temperature: The heat capacities of the solute and solvent can change slightly with temperature, which can affect the final calculation, though this effect is often minor in classroom settings.
• Type of Solvent: A solvent’s polarity and ability to form bonds (like hydrogen bonds) dramatically affect the hydration energy. Water is an excellent solvent for ionic compounds due to its high polarity.
• Pressure: For solid and liquid solutes, pressure has a negligible effect on the heat of dissolution. However, for gaseous solutes, it has a significant impact.

Frequently Asked Questions (FAQ)

1. Why is the result positive for an endothermic reaction?

A positive sign for ΔH_sol indicates that the system (the dissolving substance) absorbed energy from its surroundings (the solvent). This energy absorption causes the temperature of the surroundings to decrease. It’s a core concept in calorimetry calculation.

2. What does a negative result mean?

A negative ΔH_sol signifies an exothermic reaction, where dissolving the solute releases heat into the solvent, causing the temperature to rise.

3. Why is my experimental result different from the table value?

Discrepancies are common and can be caused by heat loss to the environment (imperfect insulation), inaccurate temperature or mass measurements, or impurities in the solute or solvent.

4. Can I use this calculator for exothermic reactions?

Yes. If you enter a final temperature that is higher than the initial temperature, the calculator will correctly compute a negative ΔH_sol, indicating an exothermic process.

5. What is the difference between heat of solution and lattice energy?

Heat of solution is the net energy change of the entire dissolving process. Lattice energy is just one component—the energy needed to break apart the solute’s crystal structure. The full picture also includes hydration energy.

6. Why do we use the total mass of the solution (solute + solvent) in the q=mcΔT equation?

Because the thermometer is measuring the temperature change of the entire mixture. The heat is being transferred throughout the whole solution, so we must account for the mass of everything that is changing temperature.

7. What if my substance doesn’t dissolve completely?

This calculator assumes the entire mass of the solute you enter dissolves. If it doesn’t, your calculation will be inaccurate because the moles of dissolved solute will be less than what you calculated. Ensure you are working below the solubility limit of the substance.

8. Is it important to follow safety procedures?

Absolutely. Some dissolution processes can be vigorously exothermic, and some chemicals are hazardous. Always follow proper lab safety procedures when performing experiments.