$Zn(s) + HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$ A piece of zinc is dropped into $0.1 cm^3$ of $0.1 mol \cdot dm ^{-3} HCl$ and the following data were obtained at 4 second intervals. | Time (s) | Mass of zinc (g) | | -------- | ---------------- | | 0 | 0.016 | | 4 | 0.012 | | 8 | 0.010 | | 12 | 0.009 | | 16 | 0.008 | | 20 | 0.008 | 5.1 Calculate the average rate of the reaction for the first 12 s in $mol ^{-1}$. 5.2 Explain why the mass of the zinc remained constant after 16 s. 5.3 Explain how the rate of the reaction changes as time passes. Answer only INCREASES, DECREASES, or REMAINS THE SAME. 5.4 A graph that shows the amount of $H _2$ gas that was produced against time for this reaction is shown below:
Answer (2)
Calculate the change in mass of zinc: Δ m Z n = − 0.007 g .
Convert the change in mass to moles: Δ n Z n = − 0.000107066 m o l .
Calculate the average reaction rate: r a t e = 8.922 × 1 0 − 6 m o l / s .
The rate of the reaction decreases as time passes. 8.922 × 1 0 − 6 m o l / s
Explanation
Calculate the change in mass of zinc First, we need to calculate the change in mass of zinc during the first 12 seconds of the reaction. The initial mass of zinc is 0.016 g, and after 12 seconds, the mass is 0.009 g. Therefore, the change in mass is: Δ m Z n = 0.009 g − 0.016 g = − 0.007 g
Convert mass change to moles Next, we convert the change in mass of zinc to moles using the molar mass of zinc, which is approximately 65.38 g/mol: Δ n Z n = M Z n Δ m Z n = 65.38 g / m o l − 0.007 g = − 0.000107066 m o l
Calculate the average reaction rate Now, we calculate the average rate of the reaction for the first 12 seconds. The change in time is 12 seconds. The rate is the change in the number of moles of zinc divided by the change in time: r a t e = Δ t − Δ n Z n = 12 s − ( − 0.000107066 m o l ) = 8.922 × 1 0 − 6 m o l / s
Explain constant zinc mass The mass of zinc remains constant after 16 seconds because all of the hydrochloric acid ( H Cl ) has been consumed. Once the H Cl is used up, the reaction can no longer proceed, and the mass of the zinc will no longer decrease.
Explain rate change As the reaction progresses, the concentration of H Cl decreases. Since the rate of the reaction is dependent on the concentration of H Cl , the rate of the reaction DECREASES as time passes.
Summary of Results To summarize, the average rate of the reaction for the first 12 seconds is 8.922 × 1 0 − 6 m o l / s . The mass of zinc remains constant after 16 seconds because the H Cl is completely consumed. The rate of the reaction decreases as time passes.
Examples
Understanding reaction rates is crucial in many real-world applications. For example, in the pharmaceutical industry, knowing the rate at which a drug degrades helps determine its shelf life. Similarly, in environmental science, understanding the rate at which pollutants break down helps assess their impact on the environment. In manufacturing, controlling reaction rates can optimize production processes, making them more efficient and cost-effective. By studying the reaction between zinc and hydrochloric acid, we gain insights into how reaction rates change over time and how factors like reactant concentration affect these rates, which are principles applicable across various scientific and industrial fields.
The average rate of the reaction for the first 12 seconds is approximately 8.922 x 10^{-6} mol/s. The mass of zinc becomes constant after 16 seconds as all hydrochloric acid is consumed. The reaction rate decreases over time due to the reduction in HCl concentration.
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