Unit 6: Thermochemistry and Thermodynamics
In this unit, we study thermodynamics and thermochemistry. Thermodynamics is the study of heat transfer. Thermochemistry is specifically the study of heat transfer in chemical reactions. We were introduced to thermodynamics in Unit 5 when we learned about the energy associated with phase changes. Thermodynamics and thermochemistry allow us to predict whether a reaction will produce heat, such as the burning of a candlewick, or if a reaction will require heat to proceed, such as the reaction that occurs inside an disposable cold pack. In this unit we also learn about Gibbs Free Energy, which tells us whether a reaction is spontaneous, meaning the reaction will occur without external "help".
Completing this unit should take you approximately 10 hours.
Upon successful completion of this unit, you will be able to:
- define temperature;
- define heat and state its units;
- define and perform enthalpy change, enthalpy of reaction, enthalpy of combustion, and enthalpy of formation calculations;
- define entropy;
- describe the driving force of a chemical reaction and relate it to Gibbs free energy;
- define Hess' law and state its functions; and
- use Hess' law to solve thermodynamic problems.
6.1: Energy
Before we begin our study of thermodynamics and thermochemistry, we need some definitions.
Thermodynamics incorporates many everyday words, such as energy, heat, work, system, and surroundings, which have precise meanings in this scientific context. In thermodynamics, the system refers to whatever you are interested in learning about. For example, the system could be a chemical reaction, or a substance undergoing a phase change. The surroundings refers to everything the system comes into contact with. For example, if you are doing a chemical reaction in a beaker, the system is the reaction and the surroundings is the beaker and air directly surrounding the beaker.
There are two overall types of energy: kinetic and potential. Kinetic energy is the energy of motion. Any object in motion has kinetic energy. Potential energy refers to stored energy that can be due to an object's position, or the energy stored in chemical bonds. Heat refers to the disordered flow of energy between a system and its surroundings when there is a temperature difference. Work refers to the ordered flow of energy between a system and its surroundings when there is not a temperature difference.
Read this page. Pay close attention to how the system and surroundings are defined; this will determine the direction of energy flow in thermodynamic systems.
Read this page. Make sure you can follow the concept map at the bottom of the page, which relates potential and kinetic chemical energy to the enthalpy of a system.
Watch these videos. The enthalpy of a system is the energy contained in its chemical bonds plus any pressure-volume work the system can do.
In this virtual lab, you will measure the enthalpy of a reaction. To see the instructions, click the name of the problem ("Camping 1") on the virtual lab screen.
In this virtual lab, you will determine change in the enthalpy of a reaction as the concentration of reactants are varied. To see the instructions, click the name of the problem ("Camping 2") on the virtual lab screen.
Read this page. Calorimetry is used to measure the change in enthalpy of reactions in a laboratory setting. Since enthalpy is a state function, Hess' law is applied to calculate the change in enthalpy from literature values.
Watch these videos to reinforce your understanding of Hess' Law.
This virtual lab will demonstrate Hess' Law using three reactions: the solubility of NaOH in water, the solubility of NaOH in HCl, and the reaction of a solution of HCl and a solution of NaOH. To see the instructions, click the name of the problem ("Heats of Reaction") on the virtual lab screen.
Read this page. Using bond energy to calculate the enthalpy of reaction is not particularly accurate because energies are affected by each molecule's unique surroundings, such as intermolecular forces. For this reason, energies are only an average across many different reactions.
6.2: Thermodynamics
Thermodynamics is the general study of heat and energy transfer. It is governed by the three laws we will examine in this section. Some chemists call the first law of thermodynamics the law of conservation of energy. It states that energy can be neither created nor destroyed, but it may change form. For example, imagine a campfire: the energy is stored in chemical bonds in the wood and is released as light and heat.
Read this page. The first law of thermodynamics is sometimes referred to as the law of conservation of energy. It states that energy can be neither created nor destroyed, but it may change form. For example, imagine a campfire: the energy is stored in chemical bonds in the wood and is released as light and heat.
Watch this lecture, which covers exothermic and endothermic reactions.
Read this page. The second law of thermodynamics deals with heat transfer and explains why a perpetual motion machine can never exist.
Read this page, which gives a detailed description of entropy and how it relates to the system and surroundings. The third law of thermodynamics deals with a theoretical absolute zero state, which is a physical impossibility.
Read this page. The Gibbs free energy equation predicts whether a reaction will happen spontaneously or not. Gibbs free energy applies to reactions of every magnitude, from water freezing to industrial chemical processes. It is important to note that while a reaction may be predicted to be spontaneous, its reaction rate may be very slow.
Watch these videos. If the computed change in Gibbs free energy is negative for a chemical reaction, then the reaction is spontaneous.
Unit 6 Assessment
Take this assessment to see how well you understood this unit.
- This assessment does not count towards your grade. It is just for practice!
- You will see the correct answers when you submit your answers. Use this to help you study for the final exam!
- You can take this assessment as many times as you want, whenever you want.