Food for Brain - Enthalpy and Entropy

As a student of Thermodynamics, I had a lot of trouble understanding enthalpy. So during my first year of master's, I undertook this task of finding a very simplified understanding of this vast concept.

Some of the common definitions indicated that enthalpy is the energy required to change from one phase to the other. I applied this definition to the first law of thermodynamics: H (Enthalpy) = U (Internal Energy) + PV (work).

Imagine an ice cube taken out of the refrigerator directly put on a heating pan. The process can be described in three simple steps:

1. Initially its temperature remains zero until all of the ice is converted into water (Latent heat of fusion)

2. Next, the temperature of water rises to 100 degC (sensible heat). Here heat provided to the system corresponds to the rise in temperature

 3. Temperature remains constant at 100 degC until all the water is converted to vapor (Latent heat of vaporization)

Enthalpy would therefore describe the total heat provided to the system (1+2+3).

The concept of Entropy on the other hand is a little tricky. 

Imagine a perfect vacuum cylinder. Suppose a very small amount of gas inserted in one corner of this cylinder. The gas molecules (initially concentrated in one corner) will spread apart occupying the whole cylinder in some time. It is the natural tendency (read entropy) of any physical matter to spread and occupy as much space as possible. Entropy in this case is positive.

Now, if we were to force the molecules in one corner and create vacuum in the rest of the cylinder, we will be required to act against this natural tendency and provide external energy in some form. Entropy for doing this will be negative (?).

Lets take an example of acid + base --> salt + water. When acid comes in contact with a base, the reaction is going to take place naturally. Entropy is positive. 

For a reversible reaction, the value of entropy will depend on the direction of the reaction (?).

However, if the entropy is negative for a backward reaction, the reaction simply won't take place.

Gas Hydrates - An Overview

When molecules of one species are enclosed in a lattice formed by another, the resulting compounds are known as Clathrates. Gas hydrates, also known as Clathrate hydrates, or simply 'Clathrates' physically resemble ice and are formed when gas molecules (hosts) get trapped in a rigid ice lattice (guests).

Presence of hydrate forming gas, high pressure, low temperatures and abundant water are pre-requisites for the formation of gas hydrates. Thus, hydrates can be observed in abundance in the permafrost regions and ocean depths.

Due to their capacity to block high pressure pipelines, hydrates present a major challenge in subsea oil production.

Nevertheless, a lot of research is being carried out to harness this source of energy, as most of the natural gas in the depths is stored in the form of hydrates. Besides, gas hydrates promise a possible application for storage of gases or for that matter trapping of greenhouse gases - methane and carbon dioxide.

Fischer Tropsch Synthesis

The process is named after Franz Fischer and Hanz Tropsch, who reported the formation of an oily liquid due to hydrogenation of carbon monoxide on alkalized-iron turnings. It was extensively used during the Second World War for producing oil and gasoline from coal. Although the process has evolved over the years and a lot of data for the reaction between hydrogen and carbon monoxide made available, research on catalyst development and kinetics still prevails. The process, also known as ‘Hydrocol Process’ or ‘Synthine Process’ is primarily used to obtain wax and liquid organic compounds - straight and branched paraffins and α-olefins, methanol and higher alcohols.

The Fischer-Tropsch catalysts are based on iron (Fe), cobalt (Co) or ruthenium (Ru) as the active metal. The costs for Fe-based catalysts are low, but they have problems like low wax selectivity, deactivation, and inhibition of the productivity by water at large syngas conversions. Ru based catalysts have high activity but their utilization is limited to scientific studies because of its high price. However cobalt based catalysts are stable and allow high syngas conversions, promoting the formation of heavy wax. Cobalt is superior in terms of catalyst life and does not favor the water gas shift reaction.

The F-T synthesis converts syn-gas into a variety of product mixture consisting of linear and branched hydrocarbons and oxygenated products. The main products are linear. The intrinsic rate equations for the hydrocarbon forming for the cobalt and iron based catalysts are based on Langmuir-Hinshelwood-Hougen-Watson (L-H-H-W).