C 100 K 373 K mol J 1088 mol J 40626 S H T on vaporizati on vaporizati the

# C 100 k 373 k mol j 1088 mol j 40626 s h t on

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C 100 K 373 K mol J 108.8 mol J 40626 S H T 0 on vaporizati on vaporizati , the boiling point of H 2 O *note the cancellation of units
18-12 e.g Consider the decomposition of… CaCO 3 (s) CaO (s) + CO 2 (g) H 0 system = +178 kJ - endothermic S 0 system = +161 J/K - positive… gas is formed. Assuming that the reaction entropy and the enthalpy are independent of temperature, calculate the temperature at which CaCO 3 starts decomposing. Check the signs of the enthalpy and the entropy: Solution G = H system T S system G should be negative to start decomposition. The minimum temperature is when G = 0. Therefore… H system = T S system C 37 8 K 110 1 K J 61 1 kJ J 10 kJ 78 1 S H T 0 3 ion decomposit ion decomposit Temperature must be greater than 837 0 C, (T > 837 0 C), to start decomposition.
18-13 Standard Free Energy Change, G 0 G is a state function. G 0 is the standard free energy change when reactants and products are in their standard states. G 0 f is the standard free energy of formation of a compound. Many of these are tabulated at 25 0 C. G 0 f of elements = 0 G 0 reaction = n G 0 f (products) n G 0 f (reactants) e.g. Calculate G 0 for the reaction… 2 NO (g) + O 2 (g) 2NO 2 (g) G 0 f (kJ/mol) 86.7 0 51.8 G 0 reaction = n G 0 f (products) n G 0 f (reactants) = [(2 mol)(51.8 kJ/mol)] [(2 mol)(86.7 kJ/mol)] = 69.8 kJ (negative the reaction is spontaneous) G 0 reaction can also be calculated from H 0 and S 0 of the reaction. e.g. Calculate G 0 reaction : 2NO(g) + O 2 (g) 2NO 2 (g) Given: H 0 = -114.1kJ and S 0 = -146.2 JK -1
18-14 e.g. Given the following: C (s, diamond) + O 2 (g) CO 2 (g) G 0 reaction = 397 kJ C (s, graphite) + O 2 (g) CO 2 (g) G 0 reaction = 394 kJ Calculate the G 0 reaction for… C (s, diamond) C (s, graphite) C (s, diamond) + O 2 (g) CO 2 (g) G 0 reaction = 397 kJ CO 2 (g) C (s, graphite) + O 2 (g) G 0 reaction = +394 kJ Add: C (s, diamond) C (s, graphite) G 0 reaction = 3 kJ G 0 reaction is negative… Therefore, diamond should spontaneously change to graphite. In other words, the reaction is feasible, but it is very slow. e.g. Is it feasible to make ethyl alcohol by the following reaction? C 2 H 4 (g) + H 2 O (l) C 2 H 5 OH ethylene water alcohol Given: G 0 f (kJ/mol) 68 237 125 G 0 reaction = n G 0 f (products) n G 0 f (reactants) = ( 175 kJ) (68 kJ 237 kJ) = 6 kJ Thus… negative, thus the process is spontaneous at 25 0 C. e.g. Hydrogen peroxide decomposes according to the reaction… 2 H 2 O 2 (l) 2 H 2 O (l) + O 2 (g) G 0 reaction = 234 kJ Since G 0 reaction is negative, one can conclude that H 2 O 2 (l) has the tendency to decompose into H 2 O (l) and O 2 (g). Indeed, prolonged storage of H 2 O 2 had shown that decomposition does occur within a certain noticeable amount of time. Beware, when H 2 O 2 (l) goes on sale in the supermarket. It might have lost some of its potency.
18-15 Kinetics and Thermodynamics: The Sign of G 0 f 1) When G 0 f is negative, it means that, under standard conditions, the compound can spontaneously form from its constituent elements.

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• Summer '09
• Marky