# Thermodynamics: The molar enthalpy of a solution

• Chemistry

## 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.

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Chestermiller
Mentor
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
mjc123