ENERGETICS II- Definitions

Define the term “Lattice Energy”

Lattice energy refers to the enthalpy change when one mole of an ionic solid is formed from its freee gaseous ions.

Define the term “Enthalpy change of atomisation”

The standar molar enthalpy change of atomisation is defined as the enthalpy change when 1 mol of an **element in its standard state** is atomised to produce **1 mol of gaseous atoms**.

Define the term “Electron affinity”

Electron affinity is defined as the change in energy (in kJ/mole) of a neutral gaseous atom when an electron is added to the atom to form a negative ion.

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(in simpler terms, how easy it is for a neutral gaseous atom to accept an electron)

What determines the polarising power of a cation?

Charge density is the main factor affecting a cation’s polarising power.

Hence, smaller ions with a greater charge are more able to distort the electron cloud (higher charge density)

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remember: the more distorted the e- cloud, the more covalent character which makes the lattice energy more exothermic.

What determines the polarisability of an anion?

The larger the ionic radius and greater negative charge increase the polarisability of an anion

Define the term “Enthalpy change of solution”

Refers to the enthalpy change when one mole of an ionic compound dissolves in water to produce an *infinitely dilute solution*.

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Define the term “Enthalpy change of hydration”

Refers to the enthalpy change when one mole of gaseous ion are hydrated by water producing an *infinitely dilute solution*.

Why is the solution defined as *infinitely dilute*?

In an infinitely dilute solution no electrostatic attraction is interfering

Trends in hydration energy- across a period

Across a period the enthalpy change of hydration becomes more negative (more exothermic), this is due to an increase in charge density, hence, the electrostatic attraction with the water molecules is stronger and more energy is released.

Trends in hydration energy- down a group

As you go down the group on anions the enthalpy change of hydration becomes more positive (less exothermic) this is due to the ionic radius increasing, hence, the electrostatic forces of attraction to the water molecules are weaker so less energy is released.

Why does MgCl2 have a more exothermic *enthalpy change of hydration* compared to BaCl2 ?

MODEL ANSWER- apply to other similar Qs

* Mg 2+ has a smaller ionic radius, therefore, higher charge density.
* Hence, electrostatic attraction between Mg 2+ ion and Cl- ion
* Mg 2+ is a small cation so it could introduce covalent character in MgCl2 making it more exothermic

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Define the term “Entropy”

entropy is the measure of the amount of disorder. Change occurs in the direction in which the total entropy increases.

Predict the sign of ΔS *sys* by observing the following equation*:*

NH4NO3 *(s)* → N2O *(g)* + 2H2O (*g)*

MODEL ANSWER- apply to other similar Qs

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The equation shows that one mole of the crystalline solid, ammonium nitrate, forms one mole of gaseous nitrous oxide and two moles of steam with a total of 3 gaseous moles. The more the moles the higher the entropy and the particles are more disorderly when gaseous hence the value of ΔS *sys* is positive.

Explain why a reaction might not occur although calculations eggiest it is feasible in terms of thermodynamic stability and kinetic inertness

The reactants are **thermodynamically unstable** relative to the products (*hence, feasible*), however, the reaction does not occur due to a high activation energy that has not been met as the reactants are **kinetically inert** compared to the products.