**Correct Option: C** ### Explanation of the Correct Answer The question revolves around the rate of reaction between calcium carbonate (marble) and dilute hydrochloric acid. The rate of reaction is influenced by several factors, including the surface area of the reactants, concentration of the reactants, temperature, and presence of catalysts. 1. **Understanding Reaction Rates**: - The rate of a chemical reaction is defined as the change in concentration of reactants or products over time. In this case, we are looking at how quickly calcium carbonate reacts with hydrochloric acid to produce carbon dioxide, water, and calcium chloride. - The reaction can be represented by the equation: \[ \text{CaCO}_3 (s) + 2 \text{HCl} (aq) \rightarrow \text{CaCl}_2 (aq) + \text{H}_2\text{O} (l) + \text{CO}_2 (g) \] 2. **Surface Area and Reaction Rate**: - The surface area of a solid reactant plays a crucial role in determining the rate of reaction. When the surface area is larger, more particles of the solid are exposed to the acid, allowing more collisions between reactant molecules. - Small pieces of marble (calcium carbonate) have a larger total surface area compared to larger pieces. This means that when small pieces are used, there are more sites available for the acid to react with the marble, leading to a higher rate of reaction. 3. **Graph Interpretation**: - The graph mentioned in the question likely shows the rate of reaction on the y-axis and time on the x-axis. The steepness of the curve indicates how fast the reaction is occurring. - The initial slope of the curve is steepest when the reaction starts, which corresponds to the maximum rate of reaction. This occurs when small pieces of marble are used because they provide a larger surface area for the acid to react with. ### Why Other Options Are Incorrect - **Option A**: "the initial slope of the curve is steepest and this occurs when small pieces of marble are used and is due to a small surface area of reactant." - This option incorrectly states that the steep slope is due to a small surface area. In fact, the steep slope is due to a **large** surface area when small pieces are used. Therefore, this option is incorrect. - **Option B**: "the occur when large pieces of marble are used and is due to a small surface area of reactant." - This option is incorrect because it suggests that the steepest slope occurs with large pieces of marble. Large pieces have a smaller surface area, which would result in a slower reaction rate, not a steep slope. - **Option D**: "the initial slope of the curve is steepest and this occurs when large surface area of reactant." - While this option correctly identifies that a large surface area leads to a steep slope, it incorrectly states that this occurs with large pieces of marble. It is actually small pieces that provide the large surface area. ### Summary of Key Points - The rate of reaction increases with a larger surface area of solid reactants. - Small pieces of marble provide a larger surface area compared to large pieces, leading to a faster reaction rate. - The steepest initial slope on the graph indicates the highest rate of reaction, which occurs with small pieces of marble. - Understanding the relationship between surface area and reaction rate is crucial for predicting how changes in reactant size affect chemical reactions. This thorough understanding of the factors affecting reaction rates will help you in your studies and exams.