A 0.35 g sample of Li(s) is placed in an Erlenmeyer flask containing 100 mL of water at 25°C. A ball — Kinetics Chemistry Question

STEPS:

1. Identify the reaction type and phase of the reactants: The reaction describes solid lithium, $\text{Li}(s)$, reacting with liquid water, $\text{H}_2\text{O}(l)$. This is a heterogeneous reaction occurring specifically at the boundary (interface) where the liquid water meets the solid metal surface.
2. Understand how surface area affects heterogeneous reaction rates: For any reaction involving a solid phase, the reaction can only take place at the exposed surface of the solid where the other reactant molecules can physically collide with it. Consequently, the rate of the reaction is directly limited by and proportional to the surface area of the solid reactant.
3. Analyze the physical change of cutting the solid: Taking a $0.35\text{ g}$ sample of $\text{Li}(s)$ and cutting it into smaller pieces does not change its total mass, chemical identity, or the temperature. However, it dramatically increases the total exposed surface area of the metal.
4. Relate surface area to collision frequency: A larger surface area allows many more lithium atoms to be in direct physical contact with water molecules simultaneously. According to collision theory, this increase in contact area increases the frequency of reactant collisions per unit of time, which directly increases the rate of the reaction.
5. Select the correct option: Cutting the sample into small pieces (Option C) is the only change that increases collision frequency and successfully increases the rate of the reaction.

*

WHY_OTHERS_WRONG:

• Option A is incorrect: Increasing the volume of water from $100\text{ mL}$ to $125\text{ mL}$ does not alter the concentration of liquid water (since it is a pure liquid) or the surface area of contact on the lithium metal. Therefore, it will not affect the rate of the reaction.
• Option B is incorrect: Using a smaller mass of lithium ($0.25\text{ g}$ instead of $0.35\text{ g}$) decreases the total amount of reactant available. A smaller solid sample has less total surface area, which would decrease or maintain a lower overall reaction rate rather than increase it.
• Option D is incorrect: Decreasing the water temperature decreases the average kinetic energy of the water molecules. According to collision theory, lower temperatures lead to less frequent collisions and, more importantly, a much smaller fraction of colliding particles possessing sufficient kinetic energy to overcome the activation energy barrier. This would decrease the reaction rate.