How is using a coffee cup calorimeter an effective way to determine enthalpy?

Enthalpy is defined as the measure of energy in a thermodynamic system. The value of enthalpy is considered to be equal to the internal energy of the system and the product of pressure and volume. Calorimeter is usually used to obtain the value of enthalpy of a system. 
A coffee cup calorimeter is an effective way of measuring the enthalpy of a system because it is a constant pressure calorimeter. This means that, the heat that is measured in a coffee cup calorimeter is equivalent to the change in enthalpy; also its use result in little or no volume change to the system involved.

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