6.0 Unit 6 Overview: Thermochemistry and Reaction Thermodynamics

Welcome to Unit 6 of AP Chemistry! This unit focuses on the energy⚡ involved in reactions, specifically the energy of heat, also known as enthalpy 🔥 We will be diving deep into how energy flows when a reaction occurs, culminating in calculations involving how heat enters or leaves a system. You will learn how to measure the heat released or absorbed from a reaction using a calorimeter and understand how these heat changes connect to this through heat capacities.

According to the College Board, these are the key points to unit 6 to focus on: 

As we mentioned, this unit is all about energy and heat flow within reactions. We’ll look at everything from what energy is, how heat works in the form of enthalpy, and do plenty of calculations along the way!

Energy is crucial to chemical reactions. Like we learned in unit 5, all reactions need energy to happen in the form of an activation energy. Unit 6 takes energy a step further, describing how reactions can release energy or absorb it. We’ll introduce basic concepts in thermodynamics that help us understand how energy flows and the different types of energy.

The first few sections in unit 6 introduce the basics of thermodynamics. In these sections, you’ll learn what heat is, how we represent them in pictorial forms, and how different processes use heat. You’ll learn about exothermic and endothermic processes and how these either absorb energy or expel energy. We will also take a look at the laws of thermodynamics, specifically focusing on the first law, the law of Conservation of Energy, which states that energy cannot be created or destroyed, but only transferred or changed into a different form. Heat, as will be covered in this unit, is just a form of energy. Diagrams can show the flow of heat as a process occurs, with the system either “losing” energy or “gaining” energy depending on whether the process was exothermic or endothermic. Heat can also transfer from one body to another. Unit 6.3 will cover how heat transfers work and a concept known as thermal equilibrium, in which heat flows between two bodies at the same rate leading to a net-zero change in heat. These three sections form the foundation for your study of thermochemistry as you progress through applying thermodynamics to chemistry.

Sections 6.4 and 6.5 are your first tastes of thermodynamics truly applied to chemical reactions. Calorimetry uses mathematical study to determine how much energy is released or absorbed by a process using a calorimeter. A calorimeter (usually a coffee-cup calorimeter) records changes in temperatures and mass and heat capacity to calculate a certain amount of energy transferred. We calculate these changes using the equation q=mcΔT, where q is the amount of energy either in J or kJ, m is the mass, c is the heat capacity, and ΔT is the change in temperature. Heat capacity is the amount of energy it takes per unit mass to change the temperature one degree, notated using (the unit’s energy)/(mass * temperature). Most commonly, J, g, and C are used, so J/gC is the most common unit of heat capacity. 

Finally, you'll learn about applying heat energy, also called enthalpy, to chemical reactions. By studying the change in heat caused by a reaction (notated as ΔH°), whether a reaction is exothermic or endothermic can also be seen. If ΔH° is positive, the system gains energy, and therefore the reaction is endothermic. Conversely, if ΔH° is negative, the system loses energy, and therefore the reaction is exothermic. We can calculate enthalpy in many ways, such as by using bond enthalpies, the amount of energy needed to break a chemical bond. Because reactions are just reorganizations of bonds, the net change in heat from breaking and reforming bonds is equivalent to the enthalpy of reaction.

Similarly, every compound has an enthalpy of formation, which is the amount of heat that it takes to form that compound. By using enthalpies of formation, we can calculate the enthalpy of reaction. The final section of unit 6 will discuss Hess's Law. Hess's Law states that enthalpy is a state function, meaning it is pathway independent. Using this, you'll be able to calculate the enthalpy of reaction using OTHER reactions by adding and multiplying them in specific ways to form the reaction you need.

In conclusion, unit 6 is an absolute whirlwind of topics covering everything from heat to how reactions handle energy. This unit is incredibly applicable to chemistry and the AP Exam, and the CollegeBoard loves asking for both calculations of enthalpy and calorimetry and conceptual knowledge that requires you to truly understand how heat works.

You got this! Good luck!