Stilbene isomerizes to isostilbene when dissolved in benzene with an appropriate catalyst. Isostilbene also isomerizes to stilbene under the same conditions. When both compounds are present in solution, both reactions proceed. However, the reaction rates are slow enough that the "opposite" reaction can be ignored during initial rate kinetics experiments starting with pure stilbene or pure isostilbene.
a. The table below provides initial rate data for the isomerization of stilbene (S) to isostilbene (I) at 25 degrees Celsius. Determine the order of the forward (S to I) isomerization reaction and find its rate constant kf (give units). Fill in the table.
Concentrations are for I
initial S&I at t=0 20 sec 1 min 2 min rate
S0=.5 mol/L I0=O .0075 .015
S0=1 mol/L I0=0 .005 .015
S0=3 mol/L I0=0 .015 .045
b. A second set of experiments on isomerization of purified isostilbene to stilbene indicates that the reverse reaction has the same order and a rate constant kr=2kf. When the concentration of stilbene is .8 mol/L and the concentration of isostilbene is .2 mol/L, find the net rate of production of isostilbene per liter of reaction mixture.
c. Find the equilibrium constant of this reaction. Hint: When equilibrium is attained, what is the NET rate of production/destruction of stilbene? Of isostilbene?
d. Find the standard free energy of reaction for conversion of stilbene to isostilbene. You may assume that the standard free energy of formation for these dissolved compounds is the same as that calculated for the compounds in their pure state. If you didn't get c, assume the equilibrium constant is .25.
e. When the reaction is conducted at 75 degrees Celsius, the equilibrium mixture contains stilbene and isostilbene in a 1 to 1 ratio. Given this data, and your results from parts c and d, is it possible to tell whether the reaction is exothermic, endothermic or has an enthalpy of reaction of 0? If it's impossible, state why (e.g., what data is missing); if it's possible, give the sign of the enthalpy of reaction.