Thermodynamics:
Heat and Enthalpy

It turns out that the amount of energy needed to do these processes depends on the circumstances. As an illustration, suppose that 1.00 g of rubbing alcohol (2-propyl alcohol, (CH3)2CHOH) at 82.4 ºC (its boiling point) is placed in a plastic bag. The heat of vaporization of rubbing alcohol is 10,063.5 J/g. How much heat is necessary to change the rubbing alcohol to a gas? Step 1: Define the system and surroundings. Write your answer in the space below.
Step 2: Identify and assign signs to all the kinds of energy and work that enter or leave the system. Write your answer in the space below.
Step 3: Predict the units your answer should have. Write your answer in the space below.
Step 4: Predict the approximate size of your answer. Write your answer in the space below.
How much heat is needed? Enter your answer in the space below. If you would like to review your answer, click on the Review Answers button. Otherwise, click on the Check Answers button.
Correct! 10,100 J of heat are required to vaporize the alcohol. Remember to include the units with your answer. While numerically correct, your answer has the wrong number of significant digits. Try again. 10,100 J of heat are required to vaporize the alcohol. That is incorrect. Please try again.

Now imagine that the same amount of alcohol is placed in large flask with all the air pumped out of it. The energy required to vaporize the alcohol is 49.2 J less. Why is more energy needed when the alcohol vaporizes in the plastic bag?

Think about what happened when the alcohol vaporized in the flask. Since there was no other gas there, the alcohol was free to expand to occupy all the volume inside the flask. On the other hand, when the alcohol in the bag vaporized, it had to push up the bag. It had to make room for itself by pushing back the atmosphere. In other words, it had to do work. Thus, of the 10.1 kJ that were put into the bag, most went into changing the alcohol from a liquid to a gas, but 49.2 J went into pushing back the atmosphere and changing the volume.

The name scientists use to describe this is the enthalpy . Enthalpy is the energy put into a system that does not do work to change the volume of the system. Mathematically, we can write this as:

DE = DH + w

where H is enthalpy and DH represents the change in enthalpy. This is another way of stating the First Law of Thermodynamics (See the Energy Module): all the change in energy a system undergoes during any process is completely accounted for by enthalpy and work entering or leaving the system.

In general, if there is no change in volume in the system, DE and DH are the same. If there is an increase in volume, however, DH will be smaller than DE because of the work involved in pushing back the atmosphere. If there is a decrease in volume, DH will be larger than DE because the atmosphere does work on the system.

Since the value of the enthalpy change for a reaction depends on the conditions of the air around the system, which in turn depends on the temperature and pressure at the time, scientists have settled on standard values to be reported. When a reaction takes place at 1 atm of pressure, the conditions are said to be standard conditions, and the enthalpy change is said to be the standard enthalpy change (DHº) (the same applies for the other thermodynamic functions that will be discussed in later modules). Any time you see the symbol for a thermodynamic quantity followed by the degree sign (º), you should automatically think of standard conditions: 1 atm of pressure. Temperature usually also has an effect on the thermodynamic values, so tables of these values should also list what temperature the values apply at. The most common temperature used is 25 ºC.

In most of chemistry, except when gases are involved, the volume change is small. Thus, DH and DE are similar and can be used interchangeably, to a good first approximation. Thus, in the rest of this tutorial, enthalpy will be used instead of energy.