Internal Energy, Heat & Work

Our system is simply our chemical reaction. Anything outside of that is considered our surroundings

Thermodynamics & Thermochemistry

Thermodynamics deals with the conversion of energy from one form to another. Thermochemistry is the branch of chemistry dealing with thermal (heat) energy.

Endothermic & Exothermic Reactions

Example: Whether our system (the chemical reaction) releases or absorbs heat or energy will determine if it is exothermic or endothermic


Example: Classify each of the following process as either exothermic or endothermic:

a)  Fusion of Ice.

b)  Sublimation of CO2.

c)  Vaporization of aqueous water.

d)  Deposition of chlorine gas.

e)  Condensation of water vapor.


Internal Energy of the System

Concept: Understanding the internal energy of the system


The internal energy (ΔE or ΔU)  of the system can be calculated from the heat and work of the system.

The signs of heat (q) and work (w) of the system can be either negative or positive depending on the key words stated. 

Example: Which of the following signs on q and w represent a system that is doing work on the surroundings, as well as losing heat to the surroundings?

q = - , w = -                   q = +, w = +          

q = -, w = +                   q = +, w =  -



Internal Energy Calculations

Work is one key variable to find the internal energy of the system. It’s equals to – PΔV.

Once we’ve calculated work we can calculate the internal energy of the system once we also calculate the heat released or absorbed. 

Concept: Calculating heat and the internal energy of the system


Under certain conditions either q (heat) or w (work) can be equal to zero. This makes it easier to calculate the internal energy of the system. 

Concept: Calculating the internal energy of the system in a vacuum


When work is done against a vacuum the pressure is equal to 0 atm. Since ΔE = q + w, the equation becomes only ΔE = q

Problem: The reaction of nitrogen with hydrogen to make ammonia has an enthalpy, ?H = - 92.2 kJ: N2 (g) + 3 H2 (g) ----> 2 NH3 (g) What is in the internal energy of the system if the reaction is done at a constant pressure of 20.0 atm and the volume compresses from 10 L to 5 L?