RI 2022 Chemical Energetics II v2.0

Raffles Institution Year 5 H2 Chemistry 2022 Lecture Notes Sb -Chemical Energetics 2 Content • Entropy • Free energy change, spontaneity of reactions Learning Outcomes Candidates should be able to: (a) explain and use the term entropy AG=AH-TAS (b) discuss the effects on the entropy of a chemical system by the following: (i) change in temperature (ii) change in phase (iii) change in the number of particles (especially for gaseous systems) (iv) mixing of particles [quantitative treatment is not required] (c) predict whether the entropy change for a given process or reaction is positive or negative (d) state and use the equation involving standard Gibbs free energy change of reaction, /iG6, tiG6 = 1i,;e -T tise [the calculation of standard entropy change, tise, for a reaction using standard entropies, se, is not required) (e) state whether a reaction or process will be spontaneous by using the sign of /iG6 (f) understand the limitations in the use of /J.Ge to predict the spontaneity of a reaction (g) predict the effect of temperature change on the spontaneity of a reaction, given standard enthalpy and entropy changes Lecture Outline 1 Direction of chemical change 2 Entropy and Entropy Change 3 Gibbs Free Energy Change 4 Effect of Temperature on the Spontaneity of Reaction -1-

1 Direction of Chemical Change • When a piece of hot sodium metal and chlorine gas come In contact, a violent reaction occurs and sodium chloride Is formed. 2Na(s) + C/i(g)--+ 2NaC/(s) Why do sodium and chlorine react readily to form sodium chloride? Why does sodium chloride not form sodium and chlorine on its own accord? • Chemists are always interested in the direction of change. By understanding the factors that make a reaction go in a particular direction, it is possible to predict the conditions which might allow a reluctant reaction to go the way that is wanted. 1.1 Spontaneous Change • What is a spontaneous process? A spontaneous process Is one that, once started, will continue without any external assistance. • Conversely, a non-spontaneous process will not occur unless some external assistance is continuously applied. • Examples of spontaneous processes: H2O(s) --+ H2O(I) at 298 Kand 1 atm NaOH(aq) + HCl(aq)--+ NaCl(aq) + H2O(I) CH4(g) + 202(9)--+ CO2(g) + 2H2O(I) • If a process is spontaneous, the reverse process is non-spontaneous. Both spontaneous and non-spontaneous processes are possible, but only spontaneous processes will occur without intervention. Non-spontaneous processes require the system to be acted on by an external agent. • Note: The term "spontaneous" signifies nothing about how fast a process occurs and has nothing to do with how long a process takes to occur. A spontaneous process can either be slow or fast. 1.2 Criteria for Spontaneity • In early days of physical chemistry, it was thought that systems reacted or changed so as to minimize their energy. In the 19th century, Marcellin Berthelot suggested that all exothermic reactions are spontaneous. • However, things are not quite simple since many endothermic processes do take place spontaneously. Examples include the following: o NH4NQ3(s) + aq.--+ NH4NQ3(aq) o H2O(s)--+ H2O(I) lif-16 = +26 kJ moI·1 lif-16 = +6.01 kJ moI·1 • Since some spontaneous reactions are exothermic and others are endothermic, enthalpy (H) alone cannot account for the direction of spontaneous change; a second factor must be involved. This second thermodynamic factor is termed entropy (S). • There are two natural tendencies behind spontaneous processes: o the tendency to achieve a lower energy state and o the tendency toward a state of greater entropy. -2- '

2 Entropy and Entropy Change 2.1 What Is Entropy? Candldate1 should able to: (a) explain and me the tenn entropy • Definition The entropy (symbol: S) of a system is a measure of the disorder of matter and energy in the system. The more ways matter in the system can be arranged, and the more ways energy in the system can be dispersed, the more disordered the system is and the larger is its entropy. Simplified version: The entropy (symbol: S) of a system is a measure of the disorder in the system. The more disordered the system is, the larger is its entropy. • The entropy of a system is related to the number of ways the particles (atoms, molecules or ions) in the system can be arranged or distributed and also the number of ways the energy in the system can be dispersed or spread out. • Example: For any substance, the entropy increases as the substance changes from solid to liquid to gas. The solid state • has the lowest entropy • is the least disordered • has a regular and ordered structure in which the particles are only able to vibrate about fixed positions in the lattice • has the least energy dispersed within it solid liquid entropy increases The liquid state • has a higher entropy than the solid state but lower than that of the gas • is more disordered than the solid state • the particles have sufficient kinetic energy to undergo translational motion to some extent • more ways to arrange the particles • the particles are more randomly arranged than those in the solid • more ways to disperse the greater amount of energy in the liquid -3- 0 0 0 0 0o 0o 0 O oc 0 0 0 00 0 0 0 0 0 o 0 0 0 ooo o 00 oo 0 0 gas The gaseous state • has the highest entropy • is the most disordered • the particles in the gas have greater freedom of motion than those in the liquid • more ways to arrange the particles • the particles in the gas are even more randomly arranged than those in the liquid • more ways to disperse the greater amount of energy in the gas

2.2 Some Entropv Values • Using a variety of experimental methods, it is possible to work out the absolute entropy values of elements and compounds under different conditions. The tables below list the standard entropies, S0, of some substances at 298 K. Element set J K·1 mol·1 5.6 2.4 158 41.6 155 131 205 192 203 223 152 116 set J K·1 mo1·1 186 229 270 127 198 214 48.0 70.0 189 72.1 26.9 91.7 H20(s) H20(I) LlSe = seH20(1) -seH20(s) = +70-48 = +22 J mo1-1 K-1 C(g) C(graphite) LlS8 = S6C(graphlte) -S6c(g) = +5.6-158 = -152.4 J mo1-1 K-1 • When comparing the same or similar substances, entropies of gases are much larger than those for liquids, and entropies of liquids are larger than those for solids. For example, the standard entropies of h(s), Br2(I) and C/2(g) are 116, 152 and 223 J K-1 moI·1 respectively. • For a series of related compounds, entropy increases with increasing molecular complexity. 2.3 Entropy Change • The entropy change (AS) for a reaction or a process is a measure of the change in disorder in a system. • This entropy change (iiS) depends only on the entropy of the initial state (S1n1t1a1) and entropy of the final state (Snna1) of the system (its composition, volume, pressure, temperature). • The entropy change (iiS) for a reaction can be worked out using tabulated standard entropy values but this is not required as stated under learning outcome (d) in the syllabus. • Note: entropy change is positive entropy change is negative (iiS > 0) (AS< 0) => final state is more disordered => final state is less disordered than initial state than initial state • A qualitative idea of the entropy change can be obtained by inspecting the equation for the reaction, and taking note of the various factors affecting entropy as discussed in the next section. -4-

2.4 Factors Affecting the Entropy of a Chemical System Candidates should be able to: (bl di5CUSS the effects on the entropy of a chemical system by the following: (I) change in temperature (Ii) change In phase (ill) change in the number of particles (especially for gaseous systems) (iv) mixing of particles [ quantitative treatment is not required] (c) predict whether the entropy change