Question

72. Hydrogen peroxide is a very common oxidizing agent. It has been used as an oxidizer for rocket engines, it is a readily available antiseptic for home use and we have used it several times in the general chemistry lab. Below is some thermodynamic data for hydrogen peroxide xide at 25 C H (kJ/mol AG (k.J/mol Chemical Thermodvnamics of hydrogen 188 266 H20:1) H2(g) 02(g) H:0:(1) 20H(g H2O2(g) → 20H(g) H202(g) HO:(g) +H(g) H2O2(l) H2O(l)+%O2(g) 120 188 174 332 365 -98 H2O2(aq)H2(g)O2 134 a. From this data, determine the standard Gibb's Energy change of each of the following reactions or phase changes at 298 K. Which are spontaneous? H20) H2O2(l) H20(aq) → H2O2(g) (a) The solvation of hydrogen peroxide: (b) The vaporization of hydrogen peroxide (c) AH'rof the hydroxyl radical (d) Aqueous decomposition O (g)+H2(g)OH(g) 2H20(aq) >2H20) +O2(g) b. Based on the data in the table and your results in problem (a), would an c. If hydrogen peroxide is unstable, then offer an explanation as to why it can be d. Would liquid hydrogen peroxide be stable or would aqueous hydrogen peroxide solution be stable or subject to decomposition? found so readily in aqueous solutions. decomposition; what would the products be? be subject to

Answer 1

Before solving the problem need to remember the whenever the reaction given above is used in reverse direction the free energy values changes its sign that is positive values become negative and vice versa.

a)

This cah be achieved as fol lbel Ading above reachions we gel- 120 C-13)l

H202 U)-→ H202 (9) ing above equathons e ge XS = 188 +(-174)

→ 0H (9) 아, Adding abeve eguetions we gel auaons we ge uou mole

Go = 268灯/mole Adding above egwathens eget- 21 2 (a*) → 2H20(1)ナ02(3) ー-206灯[mole

b) From the data obtained from (a) free energy change for aqueous decomposition has high negetive value (-206 kJ/mole) means thermodynamically decomposition reaction is very much favoured. So hydrogen peroxide is thermodynamically unstable in aqueous solution.

c) Eventhough hydrogenperoxide is unstable thermodynamically we can find so readily in aqueous solutions. The reason behind this is the rate of decompostion is very slow at room temperature that means kinetically not favoured. Hydrogenperoxide decompostion follows first order kinetics ( rate proportional to concentration of reactant). So high concentration aqueous solutions are less stable than the diluted solutions. More over rate of decompostion can be increased using catalyst for example light (this is reason for the advice to store in black coloured bottles).

d)Since free energy change for decompostion of liquid H2O2 to give water(l) and oxygen gas is very negative, liquid hydrogen peroxide is unstable and will decompose to give water and oxygen gas readily.