CBSE Class 11 Chemistry Thermodynamics Worksheet Set B

Short Answer

Question. What are the necessary conventions regarding thermochemical equations? 
Answer. The necessary conventions regarding thermochemical equations are:
1. The coefficients in a balanced thermochemical equation refer to the number of moles of reactants and products involved in the reaction.
2. The numerical value of Δr HƟ refers to the number of moles of substances specified by an equation.

Question. Distinguish between ionization energy and electron affinity? 
Answer. 1. Ionization energy is the amount of energy needed by a gaseous atom in order to remove an electron from its outermost orbital whereas electron affinity is the amount of energy released when a neutral atom or molecule gains an electron from outside.
2. Ionization energy is used to describe electron removing whereas electron affinity is used to describe electron gaining.
3. Ionization energy describes the absorption of energy from outside whereas electron affinity describes the release of energy to the surrounding.

Question. What do you mean by the term standard enthalpy of formation? 
Answer. The standard enthalpy of formation of a compound is the change of enthalpy during the formation of one mole of the substance from its constituent elements, with all substances in their standard states.

Question. Why Gibbs energy is free at equilibrium? 
Answer. Gibbs free energy is a measure of how much potential a reaction has left to do a net something. So if the free energy is zero, then the reaction is at equilibrium, and no more work can be done. It may be easier to see this using an alternative form of the Gibbs free energy, such as ΔG=−TΔS.

Question. What do you mean by the term Born-Haber cycle?
Answer. The Born Haber cycle is an approach to analyse reaction energies. Born Haber cycle is mainly used to calculate the lattice energy. It also involves several steps or processes such as electron affinity, ionization energy, sublimation energy, the heat of formation and dissociation energy.

Long Answer

Question. Explain second and third law of thermodynamics? 
Answer. Second law thermodynamics: The Second Law of Thermodynamics states that processes that involve the transfer or conversion of heat energy are irreversible. It is very important because it talks about entropy and as we have discussed, entropy dictates whether or not a process or a reaction is going to be spontaneous. The change in entropy delta S is equal to the heat transfer delta Q divided by the temperature T, for example: Foods like carbohydrates and fats, and liquid fuels like gasoline petrol for some of us, have highly concentrated potential energy stored in their chemical bonds. They have a lot of energy in a fairly small space and it is efficient for us to convert that concentrated energy into useful energy to keep our bodies and our machines going. Third law of thermodynamics: The third law of thermodynamics states that as the temperature approaches absolute zero in a system, the absolute entropy of the system approaches a constant value. It helps in the calculation of the absolute entropy of a substance at any temperature T. These determinations are based on the heat capacity measurements of the substance. For any solid, let S0 be the entropy at 0 K and S be the entropy at T K, for example: turning on a light would seem to produce energy; however, it is electrical energy that is converted.

Question. Difference between enthalpy and entropy? 
Answer. 1. Enthalpy is the thermodynamic quantity equivalent to the total heat content of a system whereas entropy represents the unavailability of a system’s thermal energy for conversion into mechanical work.
2. Enthalpy relates to the first law of thermodynamics whereas entropy relates to the second law of thermodynamics.
3. In enthalpy it gives the heat transfer takes place in constant pressure whereas in entropy it gives the idea of the randomness of the system.
4. Enthalpy can use to measure the change in the energy of the system after the reaction whereas entropy can use to measure the degree of the disorder of the system after the reaction.

Question. Explain Hess’s law of constant heat summation? 
Answer. Hess's law of constant heat summation states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes. This law is a manifestation that enthalpy is a state function. Reaction enthalpy changes can be determined by calorimetry for many reactions. The values are usually stated for processes with the same initial and final temperatures and pressures, although the conditions can vary during the reaction. Hess' law can be used to determine the overall energy required for a chemical reaction, when it can be divided into synthetic steps that are individually easier to characterize. This affords the compilation of standard enthalpies of formation that may be used as a basis to design complex syntheses. Hess' Law of Constant Heat Summation is useful in the determination of enthalpies of the following:
1. Heats of formation of unstable intermediates like CO (g) and NO (g).
2. Heat changes in phase transitions and allotropic transitions.
3. Lattice energies of ionic substances by constructing Born-Haber cycles if the electron affinity to form the anion is known.
4. Electron affinities using a Born-Haber cycle with a theoretical lattice energy.

Question. Write short note on spontaneity in thermodynamics? 
Answer. A spontaneous process is the time-evolution of a system in which it releases free energy and it moves to a lower, more thermodynamically stable energy state, for cases involving an isolated system where no energy is exchanged with the surroundings, spontaneous processes are characterized by an increase in entropy. Spontaneous reactions are accompanied by an increase in overall entropy or disorder. Criteria for spontaneity in terms of free energy change: (i) If ΔG is negative, the process is spontaneous. (ii) If ΔG is positive, the direct process is non-spontaneous. (iii) If ΔG is zero, the process is in equilibrium. Entropy is a universal criterion of spontaneity. Most of the chemical process takes place at constant temperature and pressure. During course of every spontaneous process, Gibb's function decreases. A spontaneous process is an irreversible process but we can actually reverse it by the application of some external agents. The entropy of any system is the amount of randomness in it. Total entropy change is the essential parameter which defines the spontaneity of any process. Since most of the chemical reactions fall under the category of a closed system and open system; we can say there is a change in enthalpy too along with the change in entropy. So, change in enthalpy also increases or decreases the randomness by affecting the molecular motions, entropy change alone cannot account for the spontaneity of such a process.

Question. What are the different types of reactions for enthalpies explain?
Answer. The different types of reactions for enthalpies are:
1. Standard enthalpy of combustion: The standard enthalpy change of combustion of a compound is the enthalpy change which occurs when one mole of the compound is burned completely in oxygen under the standard conditions and with everything in its standard state. Enthalpies of combustion can be used to compare which fuels or substances release the most energy when they are burned. They can be calculated using a bomb calorimeter.
2. Enthalpy of atomization: The enthalpy of atomization is the enthalpy change that accompanies the total separation of all atoms in a chemical substance.Enthalpy of atomisation (ΔatH) is enthalpy change involved when one mole of gaseous atoms is formed from a substance in elemental state. Enthalpy is the energy required to break one mole of bond to give separated atoms in gaseous state.
3. Bond enthalpy: Bond enthalpy is the enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form two free radicals, averaged over a range of compounds.
4. Lattice enthalpy: Lattice enthalpy is simply the change in enthalpy associated with the formation of one mole of an ionic compound from its gaseous ions under standard conditions. The lattice energy is exothermic, i.e., the value of ΔHlattice is negative because it corresponds to the coalescing of infinitely separated gaseous ions in vacuum to form the ionic lattice. The lattice enthalpy is reported as a positive value.
5. Enthalpy of solution: The enthalpy of solution is the enthalpy change associated with the dissolution of a substance in a solvent at constant pressure resulting in infinite dilution. The enthalpy of solution is most often expressed in kJ/mol at constant temperature.