An exothermic reaction ( $Δ H = - 40 kJ mo l^{- 1}$ ) has $E_{a} = 100 kJ mo l^{- 1}$ for the forward step without a catalyst. A catalyst lowers the forward $E_{a}$ to $75 kJ mo l^{- 1}$ . The activation energy of the reverse reaction in the presence of the catalyst, in $kJ mo l^{- 1}$ , is

Answer: C

Δ H = E_{a, f} - E_{a, r}

holds with or without a catalyst, because

Δ H

is unchanged. With the catalyst

E_{a, r} = 75 - (- 40) = 115 kJ mo l^{- 1}

. The value 140 is the uncatalysed reverse barrier.