Kaplan MCAT General Chemistry Review

Because heat and work are measured independently, we can assess the transfer of energy in the form of heat through any process regardless of the work done (or not done). Processes in which the system absorbs heat are called endothermic (+Q), while those processes in which the system releases heat are called exothermic (-Q). The unit of heat is the unit of energy: joule (J) or calorie (c) for which 1 c = 4.184 J.

Key Concept

Endothermic: positive H

Exothermic: negative H

You know how people who live in really hot desert climates, when asked, “How can you stand the heat?” often respond by pointing out, “But it’s a dry heat!” Well, there’s some truth to that. One of the most important ways that the body works to prevent overheating is through the production of sweat—that exocrine secretion of water, electrolytes, and urea. However, it’s not the production of sweat, per se, that is the cooling mechanism. It’s the evaporation of the sweat that helps cool the body. Evaporation (vaporization) from the liquid to gas phase is an endothermic process: Energy must be absorbed for the particles of the liquid to gain enough kinetic energy to escape into the gas phase. So the sweat that is excreted onto the skin must absorb energy in order to evaporate. Where does that necessary energy come from? It comes from the body itself. Hot, arid desert air has lower partial pressure of water vapor than humid tropical air, so sweat vaporizes more readily in the dry air than it does in the humid air. Although it might be hard to believe that any temperature in excess of 100°F could ever be considered comfortable, it probably is true that most people will feel more comfortable in “dry heat” than in “tropical heat.”

When substances of different temperatures are brought into thermal contact with each other (that is, some physical arrangement that allows for the transfer of heat energy), energy—in the form of heat—will transfer from the warmer substance to the cooler substance. When a substance undergoes a chemical reaction that is exothermic or endothermic, heat energy will be exchanged between the system and the environment. The process of measuring transferred heat is called calorimetry. Two basic types of calorimetry that you should know and understand for Test Day include constant-pressure calorimetry and constant-volume calorimetry. The coffee-cup calorimeter, introduced at the beginning of this lesson, is a low-tech example of a constant-pressure calorimeter, while the bomb calorimeter is an example of a constant-volume calorimeter. Constant-pressure and constant-volume are terms used to describe the conditions under which the heat changes are measured.

The heat (q) absorbed or released in a given process is calculated from this equation:

q = mcT

where m is the mass, c is the specific heat of the substance, and T is the change in temperature (in either Celsius or Kelvin). Specific heat is the amount of energy required to raise the temperature of one gram or kilogram of a substance by one degree Celsius or one unit Kelvin. Specific heat values will be provided for you on Test Day, but one value that you need to remember is the specific heat of H₂O (l): one calorie per gram per Celsius degree (1 c/g°C). Speaking of easy to remember, you ought not have any problem remembering this equation, given that q = mcT looks like “q equals MCAT.”

Mnemonic

Ever notice how the equation for heat spells out MCAT? Call it the Calorimetry Test if it helps you remember the equation!

Constant-Volume Calorimetry