Pogil Limiting And Excess Reactants Answer Key

Pogil Limiting and Excess Reactants: A Comprehensive Guide with Answers

Stoichiometry, the section of chemistry dealing with quantitative relationships between reactants and products in chemical reactions, often involves determining the limiting reactant and calculating the amount of excess reactant remaining. This concept is crucial for understanding reaction yields and optimizing chemical processes. This comprehensive guide will delve into the concepts of limiting and excess reactants, provide detailed explanations, and offer solutions to common problems.

Understanding Limiting and Excess Reactants

Chemical reactions occur when reactants combine in specific ratios, as defined by the balanced chemical equation. The limiting reactant (or limiting reagent) is the reactant that is completely consumed first in a chemical reaction, thus limiting the amount of product that can be formed. Once the limiting reactant is used up, the reaction stops.

Conversely, the excess reactant is the reactant that remains after the limiting reactant has been completely consumed. Some amount of this reactant will be left over at the end of the reaction.

Identifying the limiting reactant is essential for predicting the theoretical yield of a reaction – the maximum amount of product that can be formed given the amounts of reactants. Understanding the concept of excess reactants helps in calculating the amount of unused reactant remaining after the reaction is complete.

Identifying the Limiting Reactant: A Step-by-Step Approach

Determining the limiting reactant involves several steps:

1. Balance the Chemical Equation: Ensure the chemical equation representing the reaction is correctly balanced. This provides the molar ratios between reactants and products.

2. Convert Quantities to Moles: Convert the given masses (or volumes for solutions) of reactants into moles using their respective molar masses (or molarities).

3. Determine Mole Ratios: Use the stoichiometric coefficients from the balanced equation to determine the mole ratios of reactants. This establishes the relationship between the number of moles of each reactant required for complete reaction.

4. Compare Mole Ratios to Actual Ratios: Compare the actual mole ratio of the reactants to the required mole ratio from the balanced equation. The reactant with the smaller ratio (meaning less available relative to the stoichiometric requirement) is the limiting reactant.

5. Calculate Excess Reactant: Once the limiting reactant is identified, calculate the amount of excess reactant remaining using the stoichiometry of the balanced equation.

Let's illustrate this with an example:

Example Problem 1: Burning Propane

Problem: Propane (C₃H₈) burns in oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O). If 10.0 grams of propane reacts with 10.0 grams of oxygen, which reactant is limiting, and how much CO₂ is produced?

Solution:

1. Balanced Equation: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

2. Moles of Reactants:

• Moles of C₃H₈ = (10.0 g) / (44.1 g/mol) = 0.227 mol
• Moles of O₂ = (10.0 g) / (32.0 g/mol) = 0.313 mol

3. Mole Ratios:

• From the balanced equation, 1 mole of C₃H₈ reacts with 5 moles of O₂.
• Mole ratio of O₂ to C₃H₈ required = 5:1

4. Comparing Ratios:

• Actual mole ratio of O₂ to C₃H₈ = 0.313 mol O₂ / 0.227 mol C₃H₈ ≈ 1.38
• Since 1.38 < 5, there is not enough oxygen to react with all the propane. Therefore, oxygen (O₂) is the limiting reactant.

5. Calculating CO₂ Produced: Use the limiting reactant (O₂) to calculate the moles of CO₂ produced:

• Moles of CO₂ = (0.313 mol O₂) × (3 mol CO₂ / 5 mol O₂) = 0.188 mol CO₂
• Mass of CO₂ = (0.188 mol) × (44.0 g/mol) = 8.27 g CO₂

Therefore, 8.27 grams of CO₂ are produced, and propane is the excess reactant.

Example Problem 2: Synthesis of Ammonia

Problem: Ammonia (NH₃) is synthesized from nitrogen (N₂) and hydrogen (H₂). If 2.00 moles of N₂ react with 6.00 moles of H₂, which reactant is limiting, and how much excess reactant remains?

Solution:

1. Balanced Equation: N₂ + 3H₂ → 2NH₃

2. Moles of Reactants: Already given: 2.00 moles N₂ and 6.00 moles H₂.

3. Mole Ratios:

• From the balanced equation, 1 mole of N₂ reacts with 3 moles of H₂.
• Mole ratio of H₂ to N₂ required = 3:1

4. Comparing Ratios:

• Actual mole ratio of H₂ to N₂ = 6.00 mol H₂ / 2.00 mol N₂ = 3.00
• Since the actual ratio (3.00) equals the required ratio (3:1), both reactants are used up completely in this specific scenario. There is no limiting reactant. Neither reactant is in excess.

5. Excess Reactant Calculation: In this case, there's no excess reactant because both are completely consumed.

Solving More Complex Limiting Reactant Problems

Many problems involve more than two reactants or require multiple steps to solve. Here's a strategy for tackling these more complex scenarios:

1. Identify all Reactants: Clearly list all reactants involved in the reaction.

2. Determine Limiting Reactant for Each Pair: If you have three or more reactants, consider each pair of reactants separately and determine which one is limiting within that pair.

3. Overall Limiting Reactant: The reactant that limits the reaction overall will be the one that runs out first when considering all possible pairs.

4. Sequential Reactions: If the reaction involves multiple steps, treat each step individually. The product of one step may be the reactant of the next. The limiting reactant in each step may influence the overall yield.

5. Percent Yield: After calculating the theoretical yield (based on the limiting reactant), you can calculate the percent yield, which compares the actual yield obtained in an experiment to the theoretical yield.

Common Mistakes to Avoid

• Ignoring the Balanced Equation: Always use the stoichiometric coefficients from the balanced equation when calculating mole ratios. Incorrect ratios will lead to incorrect identification of the limiting reactant.

• Incorrect Unit Conversions: Carefully convert all quantities into moles before performing calculations. Mixing units (e.g., grams and moles) will result in incorrect answers.

• Misinterpretation of Mole Ratios: Remember that mole ratios are based on the balanced chemical equation. The actual amounts of reactants present might not reflect these ratios perfectly.

• Neglecting Excess Reactant Calculation: The problem might ask for the amount of excess reactant remaining after the reaction. Don't forget to perform this calculation.

Conclusion

Mastering the concept of limiting and excess reactants is fundamental to stoichiometry. By systematically following the steps outlined in this guide, you can accurately identify the limiting reactant, calculate the amount of excess reactant remaining, and predict the theoretical yield of a chemical reaction. Remember to always double-check your work, pay close attention to detail, and practice regularly to improve your proficiency in solving these types of problems. This understanding is crucial for various applications in chemistry, including industrial chemical processes, environmental studies, and pharmaceuticals. The ability to predict reaction outcomes and optimize resource utilization is a key skill for any chemist or chemical engineer.