R2.2 How fast? The rate of chemical change

The question asks for the mean rate between 30 s and 90 s. At $t=30$ s, mass $=84.76$ g; at $t=90$ s, mass $=84.46$ g.

$\Delta m=84.76-84.46=0.30\text{ g}$ The mass has decreased by 0.30 g over the interval.

$\Delta t=90-30=60\text{ s}$

$\text{Mean rate}=\frac{\Delta m}{\Delta t}=\frac{0.30}{60}=0.0050{\text{ g s}}^{-1}$

The mean rate of mass loss between 30 s and 90 s is 0.0050 g s${}^{-1}$.

We need $\frac{\Delta m}{\Delta t}$ between $t=30$ s and $t=90$ s, where $\Delta m=0.30$ g and $\Delta t=60$ s.

0.0025 g s${}^{-1}$ corresponds to $\frac{0.30}{120}$, which uses an incorrect time interval of 120 s instead of 60 s. Eliminated.

0.0033 g s${}^{-1}$ would correspond to $\frac{0.30}{90}$, incorrectly using the endpoint time (90 s) rather than the interval length (60 s). Eliminated.

0.0040 g s${}^{-1}$ does not match any correct calculation of $\Delta m/\Delta t$ using the given data. Eliminated.

Only 0.0050 g s${}^{-1}$ equals $0.30÷60$, the correct calculation. This is the answer.

Halving the volume at constant temperature doubles the pressure and therefore doubles the concentration of all gaseous species.

According to collision theory, more particles occupying the same volume means they are closer together and collide more frequently per unit time.

Temperature is constant, so average kinetic energy does not change. Activation energy is an intrinsic property of the reaction and is also unchanged.

The correct explanation is: concentration of reactant molecules increases, resulting in a higher frequency of collisions. This is the mechanism by which pressure increases the rate.

Halving the volume compresses the gases, increasing pressure and concentration at constant temperature.

'The activation energy is reduced...' is incorrect — pressure changes do not alter $E_a$. Only a catalyst can lower $E_a$. Eliminated.

'The average kinetic energy of the gas molecules increases...' is incorrect because temperature is held constant, so average kinetic energy is unchanged. Eliminated.

'A greater proportion of molecules now possess energy ≥ $E_a$...' is incorrect — this describes the effect of a temperature increase, not a volume decrease at constant temperature. Eliminated.

The remaining option — concentration of reactant molecules increases, resulting in higher collision frequency — correctly describes the effect of reduced volume at constant temperature.