Question

State with reason which phase of the following substances is more stable at 298 K and 1 bar:

i)      Ethanol (liquid / gas).                                                                                                               

ii) Solid calcium carbonate.                                                                                                            

iii) Water (liquid / gas).                                                                                                                   

iv) HNO3 (liquid / aqueous).                                                                                                          

v) Ammonia (liquid / aqueous).

Answer 1

Stability in chemistry means, the system is in lowest energy state.

Decision rule: to compare the stability at different phases, we need to understand the ability of the molecule to interact with the surroundings. These interactions may be in the form of some bonds with other molecules or with itself etc.

1. Ethanol

               In liquid form, ethanol forms the inter molecular hydrogen bond between oxygen of one ethanol and Hydrogen of other ethanol. This inter molecular hydrogen bond strengthen the molecule and helps to stay as a liquid in room temperature.

In gaseous form, there are no or weak connections between two molecules of ethanol which makes it less stable than liquid form.

Hence, the liquid form of ethanol is more stable than its gaseous form.

2. Solid calcium carbonate.

Calcium carbonate is an ionic solid, this compound has high lattice enthalpy and hydration enthalpy, this makes this form more stable and sparingly soluble in water.

    Solid form is more stable

3. Water

Water is more in liquid form as it can form hydrogen bonds with other water molecules. This hydrogen bonding is responsible for the anomalous properties of water.

As the water is heated, the hydrogen bonding has broken and stability is been reduced.

4. HNO3

This is little complicated to judge. HNO3 is highly resonance stabilized which is possible both in liquid and aqueous conditions.

Studies showing that, aqueous solutions of HNO3 can form highly polarized hydrogen bonding. And HNO3 molecule is stored in aqueous condition.

Hence HNO3 is more stable in aqueous condition.

5. Ammonia

    Ammonia can form strong hydrogen bonding with water which sustains the molecule in aqueous conditions.

In ammonia aqueous conditions is more stable

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