The rate constant for a reaction was determined to be 4.25 * 10 ^ -3 L/mol*s at 22.5 C. The rate constant was then measured at 48.0 C and found to be 1.95 * 10 ^ -2 L/mol*s. Calculate the value of the activation energy. Is the reaction first of second order?
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According to Arrhenius Equation , K = A e -Ea / RT
Where
K = rate constant
T = temperature
R = gas constant = 8.314 J/mol-K
Ea = activation energy
A = Frequency factor (constant)
Rate constant, K = A e - Ea / RT
log K = log A - ( Ea / 2.303RT ) ---(1)
If we take rate constants at two different temperatures, then
log K = log A - ( Ea / 2.303RT ) --- (2)
& log K' = log A - (Ea / 2.303RT’) ---- (3)
Eq (3 ) - Eq ( 2 ) gives
log ( K' / K ) = ( Ea / 2.303 R ) x [ ( 1/ T ) - ( 1 / T' ) ]
Ea = [(2.303R x T x T’) / (T’ - T)] x log (K’ / K)
T= 22.5oC=22.5+273= 295.5 K
T'= 48oC= 48+273=321 K
K= 4.25*10^-3 L/mol-s
K'= 1.95*10^-2 L/ mol*s
Plug the values we get Ea= 47.125*10^3 J= 47.125 kJ
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