In The Equation \[ Zn + 2HCl \rightarrow ZnCl_2 + H_2 \], If 2.1 Moles Of Zinc React With 6.0 Moles Of Hydrochloric Acid, What Is The Limiting Reactant?

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In chemical reactions, reactants are not always present in stoichiometric amounts, which are the exact proportions required for complete reaction. When one reactant is consumed completely before the others, it limits the amount of product that can be formed. This reactant is known as the limiting reactant. Identifying the limiting reactant is crucial for predicting the yield of a reaction and optimizing experimental procedures. This article will delve into the concept of limiting reactants using the specific example of the reaction between zinc (Zn) and hydrochloric acid (HCl). We will explore the steps involved in determining the limiting reactant and understanding its significance in chemical reactions. By examining this specific reaction, we can gain a broader understanding of how to apply the concept of limiting reactants to various chemical processes.

Understanding Limiting Reactants

In the realm of chemistry, the concept of the limiting reactant is pivotal for understanding and predicting the outcomes of chemical reactions. In essence, the limiting reactant is the reactant that is completely consumed in a chemical reaction, thereby dictating the maximum amount of product that can be formed. To put it simply, imagine you are baking cookies and have a limited amount of one ingredient, such as flour. Even if you have plenty of all other ingredients, the amount of flour you have will determine how many cookies you can make. The flour, in this case, is the limiting ingredient.

To understand this concept fully, it's essential to distinguish the limiting reactant from the excess reactant. The excess reactant is the reactant that remains after the limiting reactant has been completely used up. Using the cookie analogy, the other ingredients that you have in abundance would be the excess reactants. Determining the limiting reactant is a critical step in stoichiometry, which is the calculation of quantitative relationships in chemical reactions. By identifying the limiting reactant, chemists can accurately predict the yield of a reaction, optimize experimental conditions, and avoid wasting valuable resources. Furthermore, understanding limiting reactants is not just an academic exercise; it has practical applications in various industries, including pharmaceuticals, manufacturing, and environmental science.

The Reaction Between Zinc and Hydrochloric Acid

The reaction between zinc (Zn) and hydrochloric acid (HCl) is a classic example of a single displacement reaction, frequently used in chemistry demonstrations and industrial processes. The balanced chemical equation for this reaction is:

Zn+2HCl→ZnCl2+H2{ Zn + 2HCl \rightarrow ZnCl_2 + H_2 }

This equation tells us that one mole of solid zinc (Zn) reacts with two moles of hydrochloric acid (HCl) to produce one mole of zinc chloride (ZnCl2{ ZnCl_2 }) and one mole of hydrogen gas (H2{ H_2 }). This stoichiometric relationship is the foundation for determining the limiting reactant when given amounts of Zn and HCl are reacted. In this reaction, zinc acts as a reducing agent, donating electrons to hydrogen ions from hydrochloric acid. The hydrogen ions are reduced to hydrogen gas, while zinc is oxidized to zinc ions, which combine with chloride ions to form zinc chloride. The evolution of hydrogen gas is often observed as bubbling, providing a visual indication that the reaction is occurring. Understanding the balanced chemical equation is paramount because it provides the molar ratios necessary for stoichiometric calculations. These ratios allow us to compare the amounts of reactants and determine which one will be completely consumed first, thereby identifying the limiting reactant.

Steps to Determine the Limiting Reactant

Identifying the limiting reactant involves a systematic approach that ensures accuracy and clarity in stoichiometric calculations. This process typically involves several key steps, each contributing to a comprehensive understanding of the reaction's constraints. By meticulously following these steps, chemists can confidently predict reaction outcomes and optimize experimental designs.

  1. Convert Given Amounts to Moles: The first critical step is to convert the given amounts of each reactant from grams or other units into moles. The mole is the standard unit of amount in chemistry, and it allows for direct comparison of reactants based on their molar masses. To perform this conversion, you will need the molar mass of each reactant, which can be obtained from the periodic table. For example, if you are given the mass of zinc in grams, you would divide it by the molar mass of zinc (approximately 65.38 g/mol) to find the number of moles of zinc. Similarly, if you are given the concentration and volume of hydrochloric acid, you can calculate the number of moles using the formula: moles = concentration (mol/L) × volume (L). Accurate conversion to moles is essential because the stoichiometric coefficients in the balanced chemical equation relate the reactants on a molar basis, not on a mass basis. This initial conversion sets the stage for all subsequent calculations and is paramount for correctly identifying the limiting reactant.

  2. Use the Stoichiometric Ratio: Once the amounts of reactants are expressed in moles, the next step is to use the stoichiometric ratios from the balanced chemical equation to determine the mole ratio in which the reactants combine. The stoichiometric coefficients in the balanced equation provide the exact molar ratios required for the reaction to proceed completely. For the reaction between zinc and hydrochloric acid (Zn+2HCl→ZnCl2+H2{ Zn + 2HCl \rightarrow ZnCl_2 + H_2 }), the stoichiometric ratio is 1 mole of Zn to 2 moles of HCl. This ratio is crucial because it allows us to compare the available moles of each reactant to the required molar ratio. For instance, if you have 2 moles of Zn, you would need 4 moles of HCl to react completely with it, according to the stoichiometric ratio. By comparing the actual molar amounts to the stoichiometric ratio, you can assess which reactant is present in a lesser amount relative to its requirement, which is a key indicator of the limiting reactant. This step bridges the gap between the experimentally provided amounts and the theoretically required amounts, setting the foundation for the final determination.

  3. Determine the Limiting Reactant: After converting to moles and applying the stoichiometric ratio, the limiting reactant can be identified. This is the reactant that would be completely consumed first if the reaction were to proceed to completion. There are several methods to determine the limiting reactant. One common method is to calculate how much of one reactant is needed to react completely with the given amount of the other reactant. For example, you can calculate how many moles of HCl are required to react completely with the given moles of Zn. If the amount of HCl you have is less than the calculated amount, then HCl is the limiting reactant. Alternatively, you can calculate how many moles of Zn are required to react completely with the given moles of HCl. If the amount of Zn you have is less than the calculated amount, then Zn is the limiting reactant. Another approach is to divide the moles of each reactant by its stoichiometric coefficient in the balanced equation. The reactant with the smallest result is the limiting reactant. This method provides a direct comparison of the reactants relative to their stoichiometric requirements. Correctly identifying the limiting reactant is crucial because it dictates the maximum amount of product that can be formed in the reaction. Understanding this limitation is essential for both theoretical predictions and practical applications.

Applying the Steps to the Given Reaction

Now, let's apply the steps outlined above to the specific reaction in question: 2.1 moles of zinc (Zn) react with 6.0 moles of hydrochloric acid (HCl) in the equation:

Zn+2HCl→ZnCl2+H2{ Zn + 2HCl \rightarrow ZnCl_2 + H_2 }

  1. Moles Calculation: In this scenario, the amounts of reactants are already given in moles, so we can skip the conversion step. We have 2.1 moles of Zn and 6.0 moles of HCl.

  2. Stoichiometric Ratio: From the balanced equation, the stoichiometric ratio of Zn to HCl is 1:2. This means that for every 1 mole of Zn, 2 moles of HCl are required for complete reaction.

  3. Limiting Reactant Determination: To determine the limiting reactant, we can compare the available moles to the required ratio. If we start with 2.1 moles of Zn, we would need 2.1 moles Zn * (2 moles HCl / 1 mole Zn) = 4.2 moles of HCl for complete reaction. Since we have 6.0 moles of HCl, which is more than the 4.2 moles required, Zn is the limiting reactant. Alternatively, we can calculate how much Zn is needed to react with 6.0 moles of HCl. We would need 6.0 moles HCl * (1 mole Zn / 2 moles HCl) = 3.0 moles of Zn. Since we only have 2.1 moles of Zn, which is less than the 3.0 moles required, Zn is again identified as the limiting reactant. Another approach is to divide the moles of each reactant by its stoichiometric coefficient: For Zn: 2.1 moles / 1 = 2.1 For HCl: 6.0 moles / 2 = 3.0 Since 2.1 is smaller than 3.0, Zn is the limiting reactant. By using these different methods, we consistently arrive at the same conclusion: Zinc (Zn) is the limiting reactant in this reaction. This determination is crucial for predicting the maximum amount of products that can be formed.

Conclusion: The Limiting Reactant

In conclusion, after analyzing the reaction between 2.1 moles of zinc (Zn) and 6.0 moles of hydrochloric acid (HCl), we have definitively determined that zinc (Zn) is the limiting reactant. This determination was achieved by comparing the given molar quantities of the reactants to the stoichiometric ratio derived from the balanced chemical equation:

Zn+2HCl→ZnCl2+H2{ Zn + 2HCl \rightarrow ZnCl_2 + H_2 }

The stoichiometric ratio of 1 mole of Zn to 2 moles of HCl indicates that for every mole of Zn, two moles of HCl are required for complete reaction. By calculating the amount of HCl needed to react fully with 2.1 moles of Zn (4.2 moles) and comparing it to the available amount of HCl (6.0 moles), we found that HCl is present in excess. Conversely, when considering the amount of Zn needed to react completely with 6.0 moles of HCl (3.0 moles), we observed that the available amount of Zn (2.1 moles) is insufficient. This clearly identifies Zn as the limiting reactant. Therefore, the correct answer to the question of which reactant is limiting is A. Zn. Understanding the concept of limiting reactants is essential in chemistry, as it allows for accurate prediction of product yields and efficient use of resources in chemical reactions. This principle is not only vital in academic settings but also has significant implications in industrial applications, where optimizing reactions can lead to cost savings and increased efficiency.