Enthalpy change of water [closed]








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A given amount of (liquid) water in an open vessel is heated from 20 degree celsius to 50 degree celsius. Suppose it absorbs x joule in the process. What is the enthalpy change in the process? Is it = x J ? But many authors define enthalpy change as heat absorbed or emitted at constant temperature and pressure. And here temperature is increasing.
What exactly is meant by constant T and P in enthalpy definition ? (in context of system and surroundings)




Enthalpy change is not ***defined*** as the heat added at constant temperature and pressure. Please cite a reference where you think you saw that.


For a pure species, enthalpy is a function both of temperature and pressure, and also varies with change of phase at constant temperature and pressure. It is defined as $$H=U+PV$$ where U is internal energy, P is pressure, and V is volume. So the change in enthalpy for any process of a pure substance is given by $$\Delta H=\Delta U+\Delta (PV)$$ This applies to all processes including the one you described. If the first law of thermodynamics is applied to the process you described at constant pressure, you have:
$$\Delta U=Q-P\Delta V$$From this, if follows that $$\Delta H=\Delta U+P\Delta V=Q$$So, for the particular process you described, the change in enthalpy turns out to be equal to the amount of heat added.


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