To answer the question regarding the variation of the rate of evolution of gas from a given length of magnesium ribbon with an increase in the concentration of the added acid, we need to understand the relationship between concentration and reaction rate, particularly in the context of magnesium reacting with an acid (like hydrochloric acid). ### Correct Option: A ### Explanation of Why Option A is Correct 1. **Understanding the Reaction**: When magnesium ribbon reacts with an acid, it produces magnesium ions and hydrogen gas. The reaction can be represented as: \[ \text{Mg (s)} + 2\text{HCl (aq)} \rightarrow \text{MgCl}_2 (aq) + \text{H}_2 (g) \] The rate of hydrogen gas evolution is directly related to the concentration of the acid. Higher concentrations of acid provide more H⁺ ions, which increases the rate of reaction. 2. **Rate of Reaction and Concentration**: According to the principles of chemical kinetics, the rate of a reaction generally increases with an increase in the concentration of reactants. This is because more reactant molecules are available to collide and react, leading to a higher frequency of successful collisions. 3. **Graphical Representation**: In a graph where the y-axis represents the rate of gas evolution (e.g., volume of hydrogen gas produced over time) and the x-axis represents the concentration of the acid, we would expect to see a positive correlation. As the concentration increases, the rate of gas evolution should also increase, typically resulting in a curve that rises steeply at first and may level off at higher concentrations due to factors like saturation or limited surface area of the magnesium. 4. **Interpreting the Graph**: Option A likely represents a graph that shows this positive correlation effectively. It may depict a steep initial increase in the rate of gas evolution with increasing concentration, which is characteristic of many reactions where concentration is a key factor. ### Why Other Options Are Incorrect or Weaker - **Option B (II)**: This option might show a linear relationship or a decrease in the rate of gas evolution with increasing concentration, which contradicts the expected behavior of the reaction. A linear relationship would suggest that the reaction rate does not depend significantly on concentration, which is not true for this reaction. - **Option C (III)**: This option could depict a plateau or a decrease in the rate of gas evolution as concentration increases, which would imply that the reaction has reached a maximum rate and is no longer affected by concentration. While reactions can reach a maximum rate, this is typically at very high concentrations, and the initial behavior should still show an increase. - **Option D (IV)**: This option might illustrate a scenario where the rate of gas evolution decreases with increasing concentration, which is not consistent with the principles of reaction kinetics for this type of reaction. ### Common Pitfalls - **Misunderstanding Reaction Rates**: Students often confuse the concepts of reaction rate and equilibrium. While the rate may level off at high concentrations, the initial increase is crucial for understanding how concentration affects reaction rates. - **Graph Interpretation**: It’s important to carefully analyze what the axes represent. A common mistake is to misinterpret the axes or the shape of the graph, leading to incorrect conclusions about the relationship between variables. ### Revision Summary - The rate of gas evolution from magnesium reacting with acid increases with higher acid concentration. - The correct graph (Option A) shows a positive correlation between concentration and rate of reaction. - Other options depict incorrect relationships, such as linear or decreasing rates with increasing concentration. - Understanding the principles of chemical kinetics is essential for interpreting reaction behavior correctly. By grasping these concepts, students can better prepare for questions related to reaction rates and concentration effects in their chemistry exams.