Need help preparing for the General Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach Part 1 about Enthalpy of Chemical Thermodynamics. Watch this video to get all the mcat study tips you need to do well on this section of the exam!
Chemical thermodynamics is the study of the heat, work, and energy involved with chemical reactions. Chemical reactions describe a thermodynamic process within a given system, where chemical bonds are broken and formed. A change in enthalpy (ΔH) is a change in a system’s heat content. Knowing the heat of formation of a chemical bond (the heat energy required to form a bond), scientists can predict the relative enthalpy of a reaction system before and after a reaction takes place. Note that the energy content of a system can additionally change without chemical reactions occurring - for instance, in the compression of a container of gas.
Enthalpy
Enthalpy is a state function, which means that the value of enthalpy in a chemical system is independent of the path of the system. All you need to consider about enthalpy is that a system’s starting reactants will have some amount of enthalpy, and its products will have some amount of final enthalpy. A chemical system has a bit of a circular definition: it is whatever we define to be the system we are interested in. The change in enthalpy of that system is the sum of its change in internal energy plus work done by the system related to changes in the system’s pressure and volume term. That’s a mouthful, so here’s the equation:
ΔH = ΔU + PΔV
Don’t focus too much on the definition of enthalpy - the MCAT primarily tests your ability to reason about changes in the enthalpy of a system. A question might test your understanding of how enthalpy changes when a chemical reaction releases energy into your system, increasing the internal energy (U) term. If the enthalpy of a reaction system increases, ΔH will be positive, and if the enthalpy of a reaction system decreases, ΔH will be negative.
Positive values of enthalpy change mean that heat is absorbed by the system. When heat is absorbed, we say that we're dealing with what is called an endothermic process. If the enthalpy change is negative, then we say that heat is released, which means that we are working with an exothermic process. Considering chemical reactions, changes in enthalpy will result from breaking and forming chemical bonds. Specifically, breaking chemical bonds requires discrete amounts of energy. Thus, breaking bonds is an endothermic process. Conversely, forming chemical bonds releases energy, meaning that the formation of chemical bonds is an exothermic process.
Heat of formation
The enthalpy change during a reaction can also be called the heat of reaction. The reaction occurring under standard state conditions (298 K, 1 atm, 1 molar concentrations) implies a standard heat of reaction, denoted by ΔH° reaction. Heat of formation is defined as the enthalpy change of forming one mole of a compound from its constituent elements in their standard states. The standard state of a compound is its most stable state (lowest in energy) under standard conditions. An element’s standard state - the state of an atom when it exists either as a monoatomic atom (e.g. carbon) or, more rarely, a diatomic pair (e.g. fluorine and other diatomic atoms) - requires zero energy to form.
So, how can the heat of formation be used to calculate the enthalpy change of a reaction? The enthalpy change of a reaction under standard conditions is equal to the sum of the number of moles, times the heat of formation of all the products, minus the sum of the number of moles of reactants, times their heats of formation. In short, you’re just adding up the heats of formation of all the products, and subtracting the heats of formation of all the reactants. Note that for the MCAT, aside from the knowledge that elements in their standard state have a heat of formation of zero, you will be provided with all necessary heats of formation in a table.
MEDSCHOOLCOACH
To watch more MCAT video tutorials like this and have access to study scheduling, progress tracking, flashcard and question bank, download MCAT Prep by MedSchoolCoach
IOS Link: play.google.com/store/apps/de...
Apple Link: apps.apple.com/us/app/mcat-pr...
#medschoolcoach #MCATprep #MCATstudytools