Our system is simply our chemical reaction. Anything outside of that is considered our surroundings.
Concept #1: Thermodynamics vs. Thermochemistry
Thermodynamics deals with the conversion of energy from one form to another. Thermochemistry is the branch of chemistry dealing with thermal (heat) energy.
Concept #2: System vs. Surroundings
Example #1: Whether our system (the chemical reaction) releases or absorbs heat or energy will determine if it is exothermic or endothermic.
Example #2: 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.
Concept #3: 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.
Concept #4: Heat vs. Work
The signs of heat (q) and work (w) of the system can be either negative or positive depending on the key words stated.
Example #3: 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 = -
Work is one key variable to find the internal energy of the system. It’s equals to – PΔV.
Concept #5: Calculating work
Once we’ve calculated work we can calculate the internal energy of the system once we also calculate the heat released or absorbed.
Concept #6: 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 #7: 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.
Practice: 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?