Question 1

What is the standard enthalpy of formation?

Accepted Answer

It is the enthalpy change when one mole of a compound is formed from its constituent elements in their most stable physical states at standard pressure. The standard enthalpy of formation of every element in its own standard state is defined as zero.

Question 2

Why are the standard states of chlorine, bromine and iodine different?

Accepted Answer

Their most stable physical states at 298 K are Cl₂(g), Br₂(l), and I₂(s), respectively. Therefore, the formation equations must label these states correctly and include any required phase-change enthalpy when gaseous atoms are produced.

Question 3

How is lattice enthalpy calculated using a thermodynamic cycle?

Accepted Answer

Use Hess’s law to calculate the route from elements to one mole of gaseous ions and then to the ionic solid. The solid’s standard enthalpy of formation supplies the missing term. Under the NCERT convention, lattice enthalpy is the positive energy required to separate one mole of solid into gaseous ions.

Question 4

Why is lattice formation exothermic?

Accepted Answer

Oppositely charged gaseous ions attract each other and form a stable ionic lattice. Energy is released, so lattice-formation enthalpy is negative. Separating the solid into gaseous ions requires the same magnitude of energy, so the lattice enthalpy for dissociation is positive.

Question 5

What is the difference between the expanded route and a closed cycle?

Accepted Answer

The expanded route from elements to the solid gives the standard enthalpy of formation, so its component terms do not sum to zero. Adding the reverse of the direct formation step makes a complete closed cycle whose algebraic sum is zero. A separate lattice-dissociation step is not substituted into the unchanged formation route.

Born–Haber Cycle & Standard Enthalpy of Formation 3D Lab

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