Question

The enthalpies of the elements in their standard states are assumed to be:[A] Zero at 298K[B] Unity at 298K[C] Zero at all temperatures[D] Zero at 273K

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Hint: In simple words, we can say that enthalpy gives us the measure of total energy of the system. Under standard condition, if a reaction takes place and the products obtained is also 1 mole i.e. follows standard condition, there is no change in energy of the system.

Enthalpy is a property of a thermodynamic system and it is given as the sum of the total energy of the system and the product of pressure and volume.

We can write it as-
$H=U+pV$
Here, $H,U and V$ is the enthalpy, internal energy and the volume respectively and p is the pressure of the system.
We know that the standard condition is 1atm pressure and 25 degree Celsius temperature.
At these standard conditions we can define the standard enthalpy of formation as the change of enthalpy of during the formation of 1 mole of the substance where all the substances are in their standard states.
At standard conditions, we denote the change in enthalpy as $\Delta {{H}^{\circ }}$.
At standard states, the change in enthalpy of elements is zero as the enthalpy of formation is zero. It means that, if we are making one mole of water from one mole of hydrogen gas and half mole of oxygen gas, there is no change in enthalpy as the product formed is 1 mole i.e. under standard conditions.
Therefore, the enthalpy is zero at standard conditions which is 25 degree Celsius or 298 kelvin and 1atm pressure.
Therefore, the correct answer is [A] Zero at 298K
So, the correct answer is “Option A”.

Note: We should not confuse enthalpy with entropy. Enthalpy is a measure of change in work done or heat released or required during the reaction whereas entropy is the generally randomness or the degree of disorder of the system. Enthalpy is zero at standard states whereas entropy is not zero under standard conditions. Entropy is zero at absolute zero or 0 kelvin temperature, which cannot be reached.