5.1 Define reaction rate - NSC Physical Sciences - Question 5 - 2017 - Paper 2

What is the relationship between the concentration of hydrochloric acid and the rate of hydrogen gas production in the reaction with zinc?

Curve Q represents the results of experiment I. This is because experiment I has a lower concentration of hydrochloric acid (0.25 mol.dm⁻³) compared to experiment II (0.40 mol.dm⁻³). Consequently, the reaction with the lower concentration is expected to produce hydrogen gas more slowly, resulting in a smaller gradient or less steep curve.

To find the mass of zinc used, we start with the average rate of hydrogen gas production from graph P, which is given as 15 cm³.s⁻¹. The total volume of hydrogen produced over a specific time period can be calculated. Using the molar volume, we find the amount of gas produced: $ext{Volume of } H_2 = 15 ext{ cm}^3.s^{-1} imes ext{time (in seconds)}$ To find the number of moles, we use the molar gas volume of 24 000 cm³: n(H_2) = rac{V(H_2)}{V_m} From the calculated moles of hydrogen, we can then determine the corresponding mass of zinc using the stoichiometric ratios from the reaction.

The heat of reaction for experiment II will be LESS THAN that of experiment III because the latter reaction occurs at a higher temperature of 35 °C, which generally increases the heat of reaction.

The activation energy for the reaction in experiment I will be MORE THAN that of the reaction in experiment III, as the higher temperature in experiment III typically reduces the activation energy required for the reaction.

This statement can be justified by referring to the collision theory, which posits that higher temperatures increase the kinetic energy of particles, leading to more effective collisions and, thus, a faster reaction rate in experiment III compared to experiment I.