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 su