As the name suggests thermodynamics is the study of transfer of energy/heat. Why do we need to study it? What’s the role it plays in chemistry? We all know energy is involved whenever a reaction takes place. So it is as important as money for any business. When someone starts a business, he calculates the possible profit and loss, and he starts it only after ensuring that it is profitable. Similarly a reaction happens only when it is profitable. In terms of chemistry what does it mean and how can we calculate it?
Internal energy
The profit in a reaction can be calculated in terms of its internal energy. Internal energy is the sum of all types of energies of the molecules which is very difficult to calculate. But there is an easy way to calculate the difference in internal energy of a system using thermodynamics.
If we want to calculate internal energy of a reaction, we have to focus on the reaction and where it is taking place. If the reaction is taking place in a reaction vessel such as a beaker, flask or container then we consider the vessel as a system. And we consider everything other than the vessel as surrounding.
Types of System
In order to calculate internal energy we have to observe properties of a system. First it is necessary to understand the system. System is the place where the reaction takes place and any place in this universe other than system is the surrounding. System and the surrounding are separated by a boundary. You can understand it by taking an example of your room. Suppose you are playing with a ball in your room. Playing with ball is a reaction and it is taking place in your room so your room is the system and the walls of the room are the boundary. All the other places in the house are surrounding. You and ball may or may not go out of the room; it depends on the design of your room. If you keep all of the windows and doors open, you or the ball can go outside but if you close all the windows and doors, no one can go out of the room, only the sound of the ball hitting the walls can be heard from outside. If your room is closed and sound proof too, then neither sound nor the ball or you can go outside. Similarly a system can be of three types.
- Open system where energy and matter can be exchanged with the surrounding.
- Closed system where energy can be exchanged with the surrounding but there is no exchange of matter.
- Isolated system is the one which doesn’t allow any exchange of matter or energy.
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| Types of Thermodynamic System |
State Variables
How can we define the state of the system? We can define it by measurable quantities like pressure, temperature, volume and amount. If you have to define the state of the room in above example, you will define it by its present pressure, temperature, volume, you and ball. How this room gets that temperature or pressure doesn’t affect the state of room. Such variables are called as state variables; those values depend on the state of system not on the path how it is reached. Internal energy “U” is also a state variable.
Possibility of any reaction happening depends on the benefit of internal energy it gets. If a reaction gets benefit of internal energy then it happens. That means to check the feasibility of any reaction we have to calculate the benefit of U or ΔU. Let’s see how internal energy of the system changes. If we add or remove any kind of energy to/from the system we can increase/ decrease the internal energy of the system. Energy deference can be created as a result of some work just like when you workout you lose energy or when someone gives you massage your body gets relaxed and you feel energetic again. If you add some heat to the system it also increases the internal energy of the system and vice versa. Removal or addition of matter also affects the internal energy of the system. Now we will discuss different ways to create difference in internal energy of the system.
Internal Energy of a Closed System
Let’s take hot water in a beaker and cover it with a lid. It’s an example of closed system. Now define the state of the system at time t1 by pressure, volume and temperature. Note the state of the system after time t2. You will find only difference in temperature of initial state and final state of the system. So the change in internal energy of the system can be given by the temperature difference. The energy which is a result of temperature difference is called heat q.
ΔU = Tfinal - Tinitial
ΔU = q
In above example Tfinal is lesser than Tinitial because heat is transferred from the system to the surrounding and you will get q negative. Similarly when you give heat to the system, q will get a positive sign.
Internal Energy of a Adiabatic System
The other way to bring change in the internal energy of the system is work. Let’s see how you can change the U by the work done on the system or by the system. To observe the energy change by doing work we have to keep other factors constant, that means there should be no heat transfer between system and surrounding. To make this sure we can take adiabatic system. Take some water in a flask with thermally insulated walls. Make some arrangement to add paddle and thermometer to it. Note the initial temperature of the water. Now stir up water for some time with the help of paddles and note the temperature at the final state. This change in temperature will give us change in internal energy.
ΔU = Tfinal - Tinitial
Here you will find Tfinal is greater than Tinitial so the difference in internal energy will be positive. Here mechanical work is done on the system which adds some more energy to the system and you get positive change in U. Difference in internal energy is equal to the work done in an adiabatic system.
ΔU = w
When work is done on the system it will get positive sign and when it is done by the system it will get the negative sign.
First law of thermodynamics
We have studied two types of systems one is closed system which allows heat transfer and the other is adiabatic system which doesn’t allow heat transfer. In the first system, change in internal energy is carried out by heat and in the latter it is done by doing work on the system. What happens when we put both in order to change the internal energy?
In that case internal energy difference is calculated by the equation:
ΔU = q + w
This equation is the mathematical expression of “First law of thermodynamics” it states that “The energy of an isolated system is constant.”
Internal energy is a state function; its value depends only on initial and final state. It will be independent of the way the change is carried out. In the above example you did mechanical work on the system but if you replace paddle with an emersion rod and do electrical work on the system you will get the same results. In the next post we will study its applications.
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