Chemical thermodynamics

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1. Chemical thermodynamics

2. From this chapter 12 marks will come and 5 marks numerical sure

3. General introduction: The branch of science which deals with the study of different form of energy and the quantitative relationship between them is known as thermodynamics. When we confine our study to chemical changes and chemical substances only, then that branch of thermodynamics is known as chemical thermodynamics.

4. Some basic terms and concepts: System : A system means that part of Universe which is under study. For example: a certain amount of solid, liquid or gas contain in a system. Surrounding: T he rest of the universe or environment around the system is called s urrounding. So, the surrounding means all other things which can interact with the system. Boundary : Anything that separates the system from the surrounding is called boundary. Thus, universe = system + surrounding

5. Depending upon how a system interacts with the surrounding it is classified into 3 categories. Closed system : A closed system is one which can’t exchange matter with surrounding. For example, a close container in a closed system, because no matter can enter or escape from it. However, the closed system can exchange heat or work with surrounding. Figure………….. Open system : An open system is one which can exchange both matter and energy( heat or work) with the surrounding. Hot water contained in an open beaker is an example of open system. Figure……………….

6. 3. Isolated system: An isolated system is one which can exchange neither matter nor energy with the surrounding. If the boundary is closed and insulated, no interaction is possible with the surrounding. Hence hot or cold water contained in a thermos is an isolated system. 4. State of a system and state variable: The state of a system means the condition of the system which is described in terms of certain measurable properties such as temperature (T), pressure(P), volume(V), etc. of the system. If any of these properties of the system changes, the system is said to be in different state i.e. the state of the system changes. That is why these properties of the system are called state variable . The properties is said to occur when the state of the system changes. The first and last state of the system are initial sate and final state respectively.

7. 5. State function : A physical quantity is said to be state function if its value depends only upon the state of the system and doesn’t depend upon the path by which this state has been attained. For example: potential energy, internal energy, enthalpy, free energy etc. Explanation: let us consider two points at the peak and a base of a hill. The climbers can move from base to the peak of the hill through different path. Work done depend upon the distance travelled by the climber (may be longer and a shorter route). So, it is not a state function . However a climbers standing on the peak of hill has a fixed value of PE, irrespective of the fact that whether he reached by stairs or lift. Thus PE of the person is a state function

8. 6. Thermodynamic properties: Intensive properties: T he properties of the system which do not depend upon the amount of the substance present is called intensive properties. For example: temperature, pressure, viscosity, density, surface tension, etc. The boiling point of water is 100 o c at one atmosphere whatever its mass. Extensive properties : The properties of the system which depend upon the amount of substance present is called extensive properties. For example, mass, volume, energy, work, entropy, etc. let us consider a glass of water, if we double the mass of water, the volume, no. of moles, internal energy etc. also get d oubled so they are extensive properties.

9. Types of thermodynamic process: Isothermal and Adiabatic process: A process is said to be isothermal , if the temperature of the system remains constant during each stage of the process. Such type of process may be achieved by placing the system in thermostat. Here , dT = 0, keeping temperature constant. A process is said to be adiabatic , if no heat enters or leaves the system during any step of the process i.e. the system is completely insulated from the surrounding. Here , dQ = 0, keeping heat constant.

10. Isochoric and isobaric process:  A process is said to be isochoric , if the volume of the system remains constant during each step of the process. Here , dv = 0. A process is said to be isobaric if the pressure of the system remains constant during each step of the process. Here , dp = 0

11. Exchange of energy between system and surrounding: As work : If the system and the surrounding are at two different pressure, then the energy is exchange between the system and surrounding in the form of work. Suppose there is a gaseous system enclosed in a cylinder fitted with a piston. If the pressure of the gas is higher than that of the surrounding, the gas will expand, push the piston and does a work against the surrounding. Here energy is transformed from the system to surrounding in the form of work. If the surrounding is at higher pressure, the gas contracts and work is done on the system by the surrounding. In this case energy is transferred from the surrounding to the system in the form of work. In both the case of expansion and contraction, the mechanical work is done by/on the system is given by W= P Work done by the system, W= -P Work done on the system, W= +P  

12. b. As heat : If the system and the surrounding are at two different temperature, energy is transformed or exchanged between the system and surrounding in the form of heat. For example: When ice is kept at room temperature, flow of heat energy takes place from the surrounding to the system (ice) unless both of them will have same temperature. Similarly if hot water in a vessel is kept at room temperature, flow of heat energy takes place from hot water( system) to surrounding unless both of them will have the same temperature.

13. Internal energy (E) : The energy of the thermodynamic system is called internal energy. It includes all the possible forms of energy of the system. The sum of different forms of energy associated with the molecule is called internal energy. Internal energy = Translational energy + rotational energy + vibrational energy + electrical energy + nuclear energy + bonding energy…….. The internal energy of the system depends upon the state of the system but not upon the path on which the system attains another state. So, internal energy is a state function. Therefore, the absolute value internal energy cannot be determined. However, the change in internal energy can be calculated as: ΔE = E 2  – E 1

14. In a chemical reaction,  ΔE = E product  – E reactant If E product  > E reactant , heat is absorbed by the system and hence reaction is endothermic. If E product  < E reactant , heat is released by the system and hence reaction is exothermic.

15. Sign convention: E = -ve, then energy is evolved. E = +ve, then energy is absorbed  

16. First law of thermodynamics: It states that, “ Energy can be neither be created nor destroyed although it may be converted from one form to another” OR The total amount of energy in the Universe remains constant although it may undergo transformation from one form to another.

17. Mathematical formulation of first law of thermodynamics:( relation between internal energy, work and heat) The internal energy of the system can be increased in 2 ways. By supplying heat to the system By doing work on the system in board…………….

18. Sign convention: Heat absorbed by the system: q is +ve Heat evolved by the system: q is –ve Work done on the system: W = +ve Work done by the system: W = -ve