A vapour phase catalytic reaction $(Q+R{\rightarrow}S)$ follows Rideal mechanism ($R$ and $S$ are not absorbed). Initially, the mixture contains only the reactants in equimolar ratio. The surface reaction step is rate controlling. With constant $a$ and $b$, the initial rate of reaction $(-r_{o})$ in terms of total pressure $(P_{T})$ is given by

1. $-r_{o}=\frac{aP_{T}}{1+bP_{T}}$
2. $-r_{o}=\frac{aP_{T}}{1+bP_{T}^{2}}$
3. $-r_{o}=\frac{aP_{T}^{2}}{1+bP_{T}}$
4. $-r_{o}=\frac{aP_{T}^{2}}{\left (1+bP_{T}\right)^{2}}$