# Thermodynamics: The molar enthelpy of a solution

• Chemistry
• Traced
In summary, the enthalpy of solution at a given temperature is defined as the amount of heat needed to add to the solution to hold the temperature constant. Since the temperature in the test has risen, heat would need to be removed to keep the temperature constant, resulting in a negative enthalpy of solution. This should not be confused with the enthalpy of the solution, which refers to the overall system of solid and solvent releasing energy. The correct answer is -400 kJ/mol.
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Homework Statement
When 2.35 g of Mg(OH)2 is added to 250.0 mL of water, the temperature of the water raises from 20.5 degrees Celsius to 36.0 degrees Celsius. Calculate the molar enthalpy of the solution.
Relevant Equations
Q=mc ΔT
My answer is -0.40 kJ/mol. I'm having trouble understanding why the molar enthalpy of the solution is a negative number. Wouldn't this mean that the solution is losing energy? If the temperature of the water heats up from 20 degrees Celsius to 36 degrees Celsius shouldn't the final answer be positive? If the solution gains energy, that would heat up the water right?

I understand why the enthalpy of the system is negative, since it's being released from the system and into the solution, but why the solution is also a negative number confuses me.

The enthalpy of solution at a given temperature is defined as the amount of heat you need to add to the solution to hold the temperature constant. Since the temperature in the test has risen, you would have had to remove heat in order to hold the temperature unchanged from its original value. So the heat of solution at 20 C is negative.

Last edited:
BvU
I don't know what the exact wording of your question was, but I think you are looking for the "enthalpy of solution" (that is, of the process of dissolving), not the "enthalpy of the solution". The "system" is solid plus solvent (or solution once it's dissolved), not one or the other. The system releases energy; this would be released into the surroundings as heat if thermal transfer were perfectly efficient; as it isn't, the heat goes to raise the temperature of the solution.

Oh, and your answer is a factor of 1000 out. It should be -400 kJ/mol.

## 1. What is the molar enthalpy of a solution?

The molar enthalpy of a solution is the amount of energy required to dissolve one mole of a solute in a specific amount of solvent at a constant pressure and temperature.

## 2. How is the molar enthalpy of a solution calculated?

The molar enthalpy of a solution can be calculated by subtracting the enthalpy of the solvent from the enthalpy of the solution.

## 3. What factors affect the molar enthalpy of a solution?

The molar enthalpy of a solution is affected by the nature of the solute and solvent, temperature, pressure, and concentration of the solution.

## 4. Why is the molar enthalpy of a solution important?

The molar enthalpy of a solution is important because it helps determine the energy changes that occur during a chemical reaction or physical process involving a solution. It also provides insight into the stability and strength of the bonds between solute and solvent molecules.

## 5. How is the molar enthalpy of a solution used in practical applications?

The molar enthalpy of a solution is used in various industries, such as pharmaceuticals, food and beverage, and chemical manufacturing, to optimize processes, design new products, and ensure product quality. It is also used in environmental studies to understand the impact of solutions on ecosystems.

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