Chemistry Made Easy: An Illustrated Study Guide For Students To Easily Learn Chemistry by NEDU

Author:NEDU [NEDU]
Language: eng
Format: epub
Publisher: NEDU LLC
Published: 2021-04-21T00:00:00+00:00

Enthalpy and Entropy

We should take this opportunity to look more closely at enthalpy and entropy. Enthalpy is often equated with heat, which is inaccurate. In the same way, heat and temperature are not the same things. Heat is a concept, while temperature is a way of quantifying heat, so it makes sense to us.

The term enthalpy means, in Greek, to put heat into something. In specific terms, enthalpy is related to heat, energy, and pressure. It is related to energy but really isn't the same thing at all. One way to describe enthalpy is to take a system under unchanging pressure and change the substances in some way. If you do this, you will add or subtract heat, AND you will add or subtract work to change the substance. This combination of heat plus work is equal to enthalpy .

You can also apply enthalpy to a gaseous system. If you increase the heat, you will increase the energy of the system without changing its pressure by trapping the gas. You might also add work to the system to expand the gas. This expansion of gas is how many engines and piston systems work.

There is a calculation used to describe enthalpy. It is described as the internal energy of a system plus pressure multiplied by volume. It looks like this:

H = E = Pv

Enthalpy applies to chemical systems as well as physical systems. In chemistry, it is not uncommon for the end products in a reaction to be colder or warmer than they were before the reaction. Heat will flow out of the system until it equilibrates with its surroundings. If you can keep this from happening and also keep the pressure the same, the temperature change will say a lot about what happened to the enthalpy of the situation (because it relates to the heat change directly).

For example, if you have a reaction that burns up the substrates or gives off a great deal of heat, you can not only say that the enthalpy of the substance is the same, but you can also say that the enthalpy of the substance you have burned is now less. The lost enthalpy went into the surrounding air as heat (that heated the air around it). In addition, if you can quantitate the heat given off (usually in kilojoules or joules), you can actually say how many kilojoules of enthalpy were in the original substance.

What this means is that a reaction that gives off heat is said to lose enthalpy, and you will report it as a number like this: ∆H = –100 kJ. If you have a reaction that actually needs heat or one where the end products are colder after the reaction is over, you now have a positive enthalpy situation because work has been put into the reaction. This work, as you know, counts as part of enthalpy. You could write the reaction like this: ∆H = +100 kJ.

Chemical reactions that give off heat are called exothermic reactions. Those that become cooler after the reaction are called endothermic reactions.

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