Master the MCAT: Calorimetry

Welcome back, future doctors! In our last segment, we covered the laws of thermodynamics and the concepts of enthalpy and entropy. Today, we are going to look at how we actually measure these energy changes in a lab setting. This brings us to calorimetry.

Calorimetry is a technique used to calculate the heat transfer associated with a chemical or physical reaction. We do this using an insulated container called a calorimeter.

Types of Calorimetry

There are two main types of calorimetry you need to know for the MCAT. The first is constant-pressure calorimetry. Just like the name sounds, the pressure remains constant throughout the experiment. We call this an isobaric process. Think of a coffee-cup calorimeter—the reaction happens in a cup open to the atmosphere, so the pressure doesn’t change.The second type is constant-volume calorimetry. Here, the volume is kept constant, making it an isochoric or isovolumetric process.

The classic example is a bomb calorimeter.In bomb calorimetry, the reaction is conducted inside a completely sealed, rigid steel container—the "bomb"—which is submerged in water. Because the environment is perfectly sealed, any heat generated by the reaction can only be exchanged with the water inside the calorimeter. Because of the First Law of Thermodynamics—the conservation of energy—we know that the heat released by the reaction must be exactly equal to the heat gained by the water.

The Heat Transfer Equation

So, how do we calculate that heat? Whether you are dealing with a calorimeter or just a pot of water on the stove, the equation for heat transfer is the same:q = mcDelta T, where q is the heat absorbed or released, measured in Joules or calories, m is the mass of the substance, usually in kilograms or grams, and c is the specific heat. This is a crucial term. Specific heat is the amount of energy required to raise the temperature of one gram of a substance by exactly one degree. Its units are typically Joules per kilogram-Kelvin. DeltaT is the change in temperature.

For temperature, the MCAT will often use Celsius and Kelvin interchangeably when talking about changes in temperature. That’s because one degree Celsius is exactly the same size as one Kelvin. If a temperature increases by 10 degrees Celsius, it also increased by 10 Kelvin. If you ever need to convert an absolute temperature from Celsius to Kelvin, just add 273.15.

Forms of Heat Transfer