Thermodynamics: Basic TermsThermodynamics: The branch of science that deals with the study of different forms of energy and the quantitative relationships between them.System: Quantity of matter or a region of space which is under consideration in the analysis of a problem.Surroundings: Anything outside the thermodynamic system is called the surroundings. The system is separated from the surroundings by the boundary. The boundary may be either fixed or moving.Closed system: There is no mass transfer across the system boundary. Energy transfer may be there.Open system: There may be both matter and energy transfer across the boundary of the system.Isolated system: There is neither matter nor energy transfer across the boundary of the system.State of the system and state variable: The state of a system means the conditions of the system. It is described in terms of certain observable properties which are called the state variables, for example, temperature (t), pressure (p), and volume (v).State function: A physical quantity is a state function in the change in its value during the process depends only upon the initial state and final state of the system and does not depend on the path by which the change has been brought about.Macroscopic system and its properties: If as system contains a large number of chemical species such as atoms, ions, and molecules, it is called macroscopic system. Extensive properties: These properties depend upon the quantity of matter contained in the system. Examples are; mass, volume, heat capacity, internal energy, enthalpy, entropy, Gibb's free energy. Intensive properties: These properties depend only upon the amount of the substance present in the system, for example, temperature, refractive index, density, surface tension, specific heat, freezing point, and boiling point.Types of thermodynamic processes: We say that a thermodynamic process has occurred when the system changes from one state (initial) to another state (final).Isothermal process: When the temperature of a system remains constant during a process, we call it isothermal. Heat may flow in or out of the system during an isothermal process.Adiabatic process: No heat can flow from the system to the surroundings or vice versa.Isochoric process: It is a process during which the volume of the system is kept constant.Isobaric process: It is a process during which the pressure of the system is kept constant.Reversible processes: A process which is carried out infinitesimally slowly so that all changes occurring in the direct process can be exactly reversed and the system remains almost in a state of equilibrium with the surroundings at every stage of the process.Zeroth Law of Thermodynamics<<(Basic Terms) Previous | Next (First Law)>>When two bodies A and B are separately in thermal equilibrium with a third body C, they are in thermal equilibrium with each other.Internal energy: It is the sum of all forms of kinetic and potential energy of the system. It is the sum of the kinetic energy of motion of the molecules and the potential energy represented by the chemical bonds between the atoms and any other intermolecular forces.Examples:(i) In an ideal monoatomic gas, the internal energy is in the form of translational kinetic energy of the atoms.(ii) in a plyatomic gas, the internal energy is in the form of translational, rotational and vibrational kinetic energy of the molecules.(iii) In a liquid or solid, the internal energy is in the form of translational, rotational and vibrational kinetic energy of the molecules and potential energy associated with the intermolecular attractive forces.We cannot measure the absolute value of internal energy of a system. The best one can do is measure the changes in internal energy. Or we can measure it relative to some arbitrary reference state.We use the symbol U for internal energy.First Law of Thermodynamics<<(Zeroth Law) Previous | Next (Enthalpy)>>Let us consider one of the most fundamental principles of the physical world - The Law of Conservation of Energy - It states that energy cannot be created or destroyed.Now, let us apply this law to various processes in chemistry. It can be written as U = Q + WWhere U is the change in internal energy Q is heat W is workAccording to this, there are two kinds of processes that can lead to a change in the internal energy of the system - they are heat and work.Let us consider the sign convention:(i) Heat flowing into the system is positive. It increases the internal energy of the system.(ii) Heat flowing out of the system is negative. It decreases the internal energy of the system.(iii) Work done by the system is positive. It decreases the internal energy of the system.Note that: U is a state function. From a given value of ?U, one cannot make out whether this change has come about by adding heat to it or by doing work on it.System work: When we talk about work done by a thermodynamic system, we are usually talking about the work done by a gas in expanding.(i) Work done at constant pressureW = PVIt is given by the area under the PV graph(ii) Work done when the pressure is changingW = PdVNote: That work and heat are not state functions. They do depend on the path followed between the initial and final states.ExampleCalculate the internal energy change in each of the following cases:(i) A system absorbs 5kJ of heat and does 1kJ of work.(ii) 5kJ of work is done on the system and 1kJ of heat is given out by the system.(i) Q = +5kJ W = -1kJ U = Q + W = 5 + (-1) = 4kJ(ii) W = +5kJ Q = -1kJ U = Q + W = -1 + 5 = 4kJ In both the cases, the interval energy of the system increases.Enthalpy of Heat Content<<(First Law) Previous | Next (Second Law)>>Let us again consider first law of thermodynamicsU = Q + Wor Q = U - WIf Q is the heat absorbed by the system, U is the increase in internal energy and W is the work done by the system. If pressure is constant,W = - PUQ = U - (-PU) = U + PUIf internal energy increases from U1 to U2 and volume increases from V1 to V2 then,U = U2 - U1 and U = V2 - V1Substituting we getQ = (U2 - U1) + P(V2 - V1)= (U2 + PV2) - (U1 + PV1)U1, P and V are functions of state, thus the quantity U + PV must also be a state function.(U + PV) is called the heat content or enthalpy of the system. It is represented by symbol H.H = U + PVNow H2 = U2 + PV2and H1 = U1 + PV1Q = H2 + H1Q = VEnthalpy change of a system is equal to the heat absorbed or evolved by the system at constant pressure.Enthalpy of a substance or a system is the energy stored within the substance or the system that is available for conversion into heat.Hess's law of constant heat summation: The total amount of heat evolved or absorbed in a reaction is the same whether the reaction takes place in one step or in a number of steps.In other words, the total amount of heat exchange in a reaction depends only upon the nature of the initial reactants and the nature of the final products and is independent of the path or the manner by which the change is brought about.Thus, the thermo chemical equations can be treated as algebraic equations which can be added, subtracted, multiplied or divided.Example: calculate the enthalpy of formation of carbon monoxide (CO) from the following data:(i) C(s) + O2(g) ? CO2(g) ----- (1)H = - 393.3 kJ/mol(ii) CO(g) + ½O2(g) ? CO2(g) ----- (2)H = -282.8 kJ/molSolution:We have to obtainC(s) + ½O2(g) ? CO(g)Subtracting equation (2) from equation (1)C(s) + O2(g) - CO(g) - ½O2(g) - CO2(g)C(s) + ½O2(g) ? CO(g)H = -393.3 - (-282.8) = -110.5 kJ/molThus, heat of formation of CO is Hf = 110.5 kJ/mol Entropy and Second Law of ThermodynamicsFirst law gives us an insight into a new property called internal energy. Second law leads to a new property cal entropy.Entropy is a property which sets the direction of irreversible processes in a closed system. According to the entropy postulate - If an irreversible process occurs in a closed system, the entropy S of the system always increases, it never decreases.Consider a hot bowl of soup in a room. Heat will flow from the bowl to the room till the temperature is equalized. It is an irreversible process. Heat can never flow from the room back to the soup bowl and heat up the soup again! This flow takes place till equilibrium is attained. The entropy increases in this process.Change in entropy: The change in entropy Sf - Si of a system during a process that takes the system from an initial state i to a final state f as S = Sf - Si = dQ/Twhere:Q - Energy transferred as heat to or from the system during the process.T - Temperature of the system in Kelvin.Entropy as a state function:Entropy like pressure, energy and temperature is a property of the state of a system and is independent of how that state is reached.Second law of thermodynamics: S >= 0In words, if a process occurs in a closed system, the entropy of the system increases for irreversible processes and remains constant for reversible processes.Thus if entropy decreases in one part of the system, then it increases by an equal or larger amount in another part of the system. Thus, the total change in entropy is either zero or greater than zero.