ASRJC H2 Chem 5b. Thermodynamics

2024 JC1 H2 CHEMICAL ENERGETICS II (THERMODYNAMICS) 2024/ASRJC/Chemistry 1 Anderson Serangoon Junior College H2 Chemistry CHEMICAL ENERGETICS II (THERMODYNAMICS) Contents/Outline 1 Introducing spontaneous process 2 Defining entropy and explaining the factors affecting entropy of a chemical system 3 Predicting the spontaneity of a reaction using Gibbs Free Energy Change 4 Determining the effect of temperature on the spontaneity of a reaction Learning Outcomes Students should be able to: (a) explain and use the term entropy. (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) define standard Gibbs free energy change of reaction by means of the equation ∆G = ∆H − T∆S (e) calculate ∆G for a reaction using the equation ∆G = ∆H − T∆S [the calculation of standard entropy change, ∆S, for a reaction using standard entropies, S, is not required] (f) state whether a reaction or process will be spontaneous by using the sign of ∆G (g) predict the effect of temperature change on the spontaneity of a reaction, given standard enthalpy and entropy changes References 1. Chemistry for Advanced Level, Cann and Hughes, Murray 2. Understanding Advanced Physical Inorganic Chemisry, Jeanne Tan and Chan Kim Seng, WS education 3. Chemistry, The Molecular Nature of Matter and Change (Fourth Edition), Silberberg, McGraw Hill 4. Chemistry & Chemical Reactivity (Sixth Edition), Kotz, Treichel and Weaver, Thomson 5. Chemistry The Central Science (Ninth Edition), Brown, LeMay, Bursten, Prentice Hall

2024 JC1 H2 CHEMICAL ENERGETICS II (THERMODYNAMICS) 2024/ASRJC/Chemistry 2 1 SPONTANEOUS PROCESS • What is a spontaneous process? • Can an endothermic reaction be spontaneous? The main objective of studying chemical thermodynamics is to be able to predict whether a reaction is spontaneous. A spontaneous process is one that takes place without any outside interference i.e. takes place by itself without an ongoing input of energy from outside the system. It occurs in a definite direction. Spontaneous reactions can be fast or slow. Slow spontaneous reactions can include ripening, rusting and aging. A spontaneous change is irreversible as it cannot be brought back to its original state unless some outside influence is introduced. Some examples of spontaneous reactions: • A lump of sugar dissolves spontaneously in a cup of coffee but dissolved sugar does not spontaneously reappear in the original form. • Iron exposed to water and oxygen forms rust but rust does not spontaneously change back to iron. • The expansion of a gas in an evacuated bulb is a spontaneous process. The reverse, i.e. the gathering of all the gas molecules into one bulb, is not. Generally, when enthalpy H < 0, the tendency of a spontaneous reaction is higher. However, the signs of H alone cannot be used to predict the spontaneity of a reaction. There are other factors that affect the spontaneity of a reaction. Some examples of spontaneous reactions that are endothermic: • The melting of ice H2O(s) H2O(l) H = +6.01 kJ mol−1 Ice melts spontaneously above 0 oC even though the process is endothermic. • The dissolving of ammonium nitrate in water NH4NO3(s) NH4+(aq) + NO3−(aq) H = +25 kJ mol−1 The temperature of water decreases as ammonium nitrate dissolves in water. Hence, the dissolution is spontaneous even though it is endothermic. Hence, it is possible for endothermic reactions to be spontaneous and possible for exothermic reactions to be non−spontaneous. Thus, consideration of energy changes (H) is not enough to predict the spontaneity of a process. In this part of the Chemical Energetics, the concept of entropy and free energy will be introduced to provide an explanation why some reactions are spontaneous while others are not spontaneous.

2024 JC1 H2 CHEMICAL ENERGETICS II (THERMODYNAMICS) 2024/ASRJC/Chemistry 3 2 ENTROPY, S • What is entropy? • What are some factors that affect the entropy of a chemical system? 2.1 Definition Entropy (S) measures the degree of disorder/ randomness of matter. It gives a measure of the extent to which particles (atoms, molecules or ions) and their energies (vibrational, translational and rotational) are distributed in a system. The greater the number of ways to distribute particles and their energies within a system, the greater the degree of disorder in the system and hence the larger the entropy of the system. Hence the more spontaneous the reaction will be. It has the unit of J mol-1 K-1. Entropy, like enthalpy, is a state function because its value depends solely on external conditions of temperature, pressure and concentration/ amount/ composition. At absolute zero (i.e. 0 K or –273 °C), all matter is in a crystalline solid state. In such a state, all particles attain a perfectly ordered arrangement with no lattice vibration. In this state, the matter is considered to possess zero entropy (S) . Entropy starts to increase when temperature increases from 0 K. Thus, the change in entropy of the system ( Ssys) depends only on the difference between its final and initial values, and is independent of how the change is brought about: Ssys = Sfinal – Sinitial (Recall Hess’ Law: H is independent of reaction pathway used) Example 2.1.1 At temperatures below 13 oC, shiny, ductile metallic tin, known as ‘white tin’, changes slowly into a grey powder which is brittle. Data for each form of tin are given in the table. S / J K−1 mol−1 white grey 51.4 44.1 What is the value of ∆S for the formation of grey tin from white tin at 12 oC? Solution: white tin → grey tin ∆S = 44.1 − 51.4 = −7.3 J K−1 mol−1

2024 JC1 H2 CHEMICAL ENERGETICS II (THERMODYNAMICS) 2024/ASRJC/Chemistry 4 2.2 Factors affecting the Entropy of a Chemical System (a) Temperature Maxwell–Boltzmann distributions of kinetic energy of an ideal gas at 2 different temperatures • Based on the diagram, the energy distribution curve broadened at a higher temperature. • When temperature increases, the average kinetic energy of the particles increases. • Particles move more randomly and vigorously. • The kinetic energies will be distributed over a wider range at a higher temperature. • There are more ways to distribute the particles and their energies in the system, resulting in greater disorder in the system. • Hence, entropy of the system increases when temperature increases.

2024 JC1 H2 CHEMICAL ENERGETICS II (THERMODYNAMICS) 2024/ASRJC/Chemistry 5 (b) Change in phase • For a fixed amount (in moles) of a substance in a system, entropy increases as it changes from a solid to a liquid to a gas (Ssolid < Sliquid << Sgas). • From solid to liquid (melting) ➢ When a solid melts, the order in the solid is destroyed but the volume change is negligible. ➢ Particles in liquid can now move about more freely and there are more ways to distribute the particles and their energies, hence resulting in greater disorder in the system. ➢ There is an increase in entropy (∆Sfus > 0). • From liquid to gas (boiling) ➢ When a liquid boils, the gas particles can now move with great disorder in a larger volume of space as compared to particles in the liquid. ➢ There are even more ways to distribute the particles and their energies hence resulting in the greatest disorder in the system among the three states. ➢ There is an even greater increase in en