Master the MCAT: Thermochemistry
Key Takeaways
System vs. Environment
Always define your boundaries. The "system" consists of the actual molecules and bonds reacting, while the "environment" (or surroundings) is everything else.
Temperature vs. Heat
Temperature is a state function that measures average kinetic energy, while heat ($Q$) is a process function representing the actual transfer of energy due to temperature differences.
The Laws of Thermodynamics
Energy cannot be created or destroyed, only transferred (First Law), and systems naturally tend toward increasing entropy or disorder (Second Law).
Enthalpy Directs Heat Flow
Endothermic reactions absorb heat from the environment (positive $\Delta H$), whereas exothermic reactions release heat into the environment (negative $\Delta H$).
Gibbs Free Energy Dictates Spontaneity
The MCAT often tests your ability to reason through the signs of $\Delta H$ and $\Delta S$ rather than requiring complex arithmetic. A negative $\Delta G$ guarantees a spontaneous (exergonic) reaction, while a positive $\Delta G$ means it is non-spontaneous (endergonic).
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Transcript
Master the MCAT: Thermochemistry
Welcome back, future doctors! If you're prepping for the Chem/Phys section of the MCAT, you know that Thermochemistry is a guaranteed high-yield General Chemistry topic. Today, we’re going to distill everything you need to know about heat, energy, and chemical thermodynamics into a quick, targeted review. Let's dive in!
The Laws of Thermodynamics
First, let's cover the foundational laws of thermodynamics.
When two objects are placed in contact with each other and experience no heat transfer, the two objects are said to be in thermal equilibrium. The Zeroth Law states that if system A is in thermal equilibrium with system B, and system B is in thermal equilibrium with system C, then systems A and C are in thermal equilibrium, too. This law introduces the concept of temperature—which is a measure of the average kinetic energy of the molecules in a system. You should also know that temperature is a state function, meaning that it is path independent
The First Law of Thermodynamics is the conservation of energy. Energy cannot be created or destroyed. For a closed system, the change in internal energy equals the heat added to the system, minus the work done by the system.
The Second Law of Thermodynamics introduces entropy, which you can think of as a measure of a system's disorder. This is a bit of a simplification but is perfectly adequate for the MCAT. The universe always leans toward increasing entropy. When comparing the entropy of different phases, you should keep in mind that gases have the most entropy, liquids are in the middle, and solids have the least.
Enthalpy and Gibbs Free Energy
Now, let's talk about chemical reactions, which involves breaking and forming chemical bonds. Remember, breaking bonds requires energy and forming bonds releases energy.This change in heat of a reaction at constant pressure is called Enthalpy change, or Delta H.
If a reaction absorbs heat from its environment, it’s endothermic, and Delta H is positive. If it releases heat, it’s exothermic, and Delta H is negative.
To predict if a reaction will happen on its own, we use the Gibbs Free Energy equation: Delta G equals Delta H minus T Delta S.
If Delta G is negative, the reaction is exergonic and spontaneous. If Delta G is positive, it’s endergonic and non-spontaneous. To determine when a reaction will be spontaneous, focus on signs of the enthalpy change and entropy change. If enthalpy change is negative and entropy change is positive, the reaction will be spontaneous at all temperatures. If enthalpy change is positive and entropy change is negative, the reaction will be nonspontaneous at all temperatures. If enthalpy change and entropy change are both positive, the reaction will be spontaneous at high temperatures. If enthalpy change and entropy change are both negative, the reaction will be spontaneous at low temperatures.
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