Thermodynamics is the study of the transformation of energy in a system to work energy - the dynamics of thermo.
Energy is the basis of life. Our body transforms the energy in to pump our heart, to keep our heart functioning. Our everyday living needs energy; for cooking, for moving from place to place, for reading, for our computer, etc. So the science of thermodynamics is as universal as you can get. So do not be surprised if you have to move out from chemistry into physics, engineering, and most of all philosophy.
Let me say from the very beginning that we do not know what energy is, although we have devised way of measuring it. For example we do not know what heat is. We only know that on a hot day the temperature is higher, and when there is a temperature difference heat will flow from the higher temperature to the lower temperature. Temperature is not heat, it is a measure of heat.
Secondly we want to state aloud that thermodynamics is about philosophy. We will make statements that are so obvious that we do not think proof is necessary, and we will give it high sounding titles. For example we know that heat energy flows because there is a difference in temperature. To make sure that everyone understand what we are talking about (no ifs or buts), we stated: if a system A has the same temperature as system B, and if system B has the same temperature as system C, then system A has the same temperature as system C. We then called this concept the Zeroth Law of Thermodynamics.
Since the subject is about the universe we have to define the particular space we are studying as the system and every other space as the surrounding or environment. Then study how the energy is transferred from the system into the surrounding. For example the reactor you are using can be defined as the system and any energy that goes into the reactor or out of the reactor, will be energy transferred from or into the surrounding. If the system is sealed so that the matter inside cannot escape out to the surrounding, only the energy can escape, it is known as a closed system. Otherwise it is an opened system.
Tutorial 1
What is the difference in emphasis between chemical kinetics and thermodynamics? Answer
Thermodynamics was started in the days of the industrial revolution and so many terms are culturally based on the steam engine. What a system do is known as the process. The energy of the system to do work is energy, and work is mechanical work, defined as a moving against a force for a certain distance. When the energy is transferred between the system and the surrounding because of a temperature difference, we say the energy is transformed in the form of heat.
When heat is transferred from the system to the surrounding we say the process is exothermic. If heat is transferred into the system from the surrounding it is endothermic. So the system is the reference, heat going out is going to the exterior (exo), and heat going in is endo. Exo heat is positive in value, and endo heat is negative in value.
The total amount of heat in a system is known as the internal energy of the system, U. This would include the sum total of energies of all the particles (atoms, molecules, their interactions, their motions, etc). We are not able to measure U since it is so complex. The best we can do is to measure the change in the internal energy of the system when a process has taken place. The parameters that define the value of U will be pressure (p), volume (V), temperature (T) and the mass (m). Since (p,V,T,m) are interrelated we will only need three of the parameters to define U.
Let us consider 100 ml of water as our system. Common sense will tell us that this 100 ml of pure water in Asia at a specific condition (T1, p1) will have the same amount of internal energy (U1) as another 100 ml of pure water in the America at (T1, p1). Similarly if the condition of both the 100 ml of water is changed to (T2, p2) the two systems at the new condition will still have equal amount of internal energy U2. The group in America can carry the change in whatever manner it sees fit and so can the group in Asia. As long as the final system is 100 ml of water is at (T2, p2) it should have an internal energy of U2. I warned you, this would get philosophical.
Any property of a system that is dependent only on the (p,V,T,m) is known as a state function. State functions are dependent only on the state they are in - an exact value, a unique value. In mathematics we say they are exact functions and you can integrate its function to compute its value. We will see later that not all properties of a system are state functions.
WORK and HEAT
Let us stay focus. Thermodynamics is the study of the transformation of energies to do work to the surrounding. Let us stay focus on our steam engine; heat and work.
Since we define input of heat energy as positive values we would want to do the same for work, to be consistent in the nomenclature. That is if we put into a closed system q amount of heat energy and do w (energy) amount of work on the closed system, the change in the internal energy would be;
Note. An open system means the particles can escape and the internal energy will be a different ball game.
FIRST LAW OF THERMODYNAMICS
Let us change the internal energy by doing work on the system, for example a 100 ml of water. To determine how this work we will make sure that no energy leave on enter the system. So beside a closed system we need an adiabatic system. We do this by insulating the system so that no heat can enter or leave the system. Then we do work on the system, like stirring the water with an electrical motor, until the internal energy is change from U1 to U2.
Applying the Law of the Conservation of Energy;
| ΔU = q + w q = 0 | ΔU = w |
Logic will tell us that the amount of work needed to change the system from U1 to U2 will always be the same whenever, whatever the process used and whoever conduct the process.
Actually what we saying is that;
| The internal energy is a state function | Or | ΔU = q + w |
Any of the three statements have been used as a statement for the First Law of Thermodynamics. However we will use ΔU = q + w most.
