Thermodynamics and Thermochemistry
23.0 Important Facts
23.0 Important Facts
(1) The capability of doing work is called energy.
(2) Reaction in which heat is released on completion of the reaction, are called exothermic reactions.
(3) Reaction in which heat is absorbed on completion of the reaction, are called endothermic reactions.
(4) Old bonds break in chemical reactions and new bonds are formed in which energy is released. Thus there is a change in energy of a system after the reaction.
(5) The energy content of a substance is due to its molecular structure, temperature, pressure, mass and volume. This is called internal energy.
(6) Total of the product of internal energy, pressure and volume of a substance is called enthalpy.
(7) The difference of energy of the final and initial state is called internal energy change.
(8) The thermal chemical equation is represented by showing a change in the heat with chemical change.
(9) A measure of disorder or randomness of a system is called entropy.
(10) Heat released on neutralisation of one gram equivalent of an acid and one gram equivalent of a base is called heat of neutralisation.
(11) When the same changes are performed by a different method, the heat change in all of them will be same.
(12) $?G = ?H – T?S$ shows the relationship between enthalpy and entropy. When the value of free energy change is negative, the process is spontaneous and when it is positive, the process is nonspontaneous.
(13) The energy produced by the combustion of petrol in an automobile engine is in the form of heat and mechanical work.
(14) An open system involves the exchange of both matter and energy with the surroundings a closed system involves an only exchange of energy whereas an isolated system involves neither exchange of matter nor energy.
(15) Internal energy is an extensive property i.e. depends upon the amount of the substance. It is a state function i.e. its value depends only on the state. Its absolute value cannot be determined.
(16) $\Delta E = {q_V}$ i.e. internal energy change = heat evolved or absorbed at constant volume.
(17) Enthalpy $H = E + PV$
(18) Enthalpy change, $?H = ?E + P?V$
(19) $\Delta H = {q_P}$ i.e. enthalpy change $=$ heat evolved or absorbed at constant pressure.
(20) The pollutants of the atmosphere are $C{O_2}, CO,$ oxides of nitrogen and sulphur and unburnt hydrocarbons.
(21) An endothermic reaction which may be non-spontaneous at low temperature may become spontaneous at high temperature whereas an exothermic reaction which may be non-spontaneous at high temperature may become spontaneous at low temperature.
(22) For hydrated salts like $CuS{O_4}.5{H_2}O,CaC{l_2}.6{H_2}O$ etc. or for salts which do not form hydrates (like $NaCl, KCl$ etc). the process of dissolution is endothermic.
(23) $\Delta {H_{reaction}} = $ Sum bond energies of reactants $–$ Sum bond energies of products.
(24) According to the first law of thermodynamics (law of conservation of energy), $?E = q + W$ or $q = ?E – W.$
(25) $?H$ is negative for exothermic reaction while it is positive for endothermic reactions.
(26) The heat of formation of ${H_2}O\left( \ell \right)\left( {\Delta H{^\circ _f} = - 285kJ\;mo{l^{ - 1}}} \right)$ is greater than that of ${H_2}O\left( g \right)$ $\left( {\Delta H{^\circ _f} = - 248.8kJ\;mo{l^{ - 1}}} \right)$ because the former includes heat of condensation of water vapour.
(27) ${q_P} = {q_V} + P\Delta V = {q_V} + \Delta {N_g}$ $RT$ where $\Delta {N_g} = {\left( {{N_P} - {N_r}} \right)_{gaseous}}$. Thus ${q_P} = {q_V}$ when
(i) The reaction is carried out in a closed vessel.
(ii) The reaction does not involve any gaseous reactant or product
(iii) $\Delta {N_g} = i.e.\;{N_P}\left( g \right) = {N_r}\left( g \right)$
(28) Standard enthalpy changes are measured at $298 K$ and 1 atmospheric pressure.
(29) $\Delta H{^\circ _{reaction}} = \sum {\Delta H{^\circ _f}\left( {product} \right) - \sum {\Delta H{^\circ _f}\left( {reac\tan ts} \right)} } $
