Stoichiometry: Limiting reagent | Chemical reactions and stoichiometry | Chemistry | Khan Academy

Khan Academy
27 Aug 200915:04
EducationalLearning
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TLDRThe video script discusses a stoichiometry problem involving the reaction of ammonia gas with oxygen to produce nitrogen monoxide and water. The presenter explains the process of balancing the chemical equation, identifying the limiting reagent, and calculating the mass of nitrogen monoxide produced. The problem is solved step-by-step, highlighting the importance of understanding molar ratios and the concept of limiting reagents in chemical reactions.

Takeaways
  • 🧪 The script discusses a chemical reaction involving ammonia (NH3) and oxygen (O2) to produce nitrogen monoxide (NO) and water (H2O).
  • 📈 The problem presented is a stoichiometry problem, requiring a balanced chemical equation before proceeding.
  • 🔄 The initial equation provided was not balanced, with an incorrect number of hydrogen and oxygen atoms on each side.
  • 🎯 The balanced equation was achieved by multiplying the coefficients of the reactants and products by appropriate factors to account for the correct number of atoms.
  • 📊 Given 34 grams of ammonia and 32 grams of oxygen, the script calculates the number of moles of each reactant based on their molecular masses.
  • 🤔 The script identifies that oxygen is the limiting reagent in the reaction because there is not enough to fully react with the given ammonia.
  • 📐 With 1 mole of oxygen available, the reaction can only produce 0.8 moles of nitrogen monoxide.
  • 📌 The molecular mass of nitrogen monoxide (NO) is determined to be 30 atomic mass units.
  • 🧭 From the balanced equation, it is deduced that 0.8 moles of nitrogen monoxide will be produced, which corresponds to 24 grams.
  • 🚀 The script highlights the importance of understanding molar ratios and how they dictate the amount of product formed in a reaction.
  • 💡 The leftover ammonia, 1.2 moles, is noted, emphasizing that it does not participate in the reaction due to the limiting reagent scenario.
Q & A

  • What is the chemical formula for ammonia gas?

    -The chemical formula for ammonia gas is NH3.

  • What type of gas is molecular oxygen?

    -Molecular oxygen is a diatomic gas with the chemical formula O2.

  • What is the product of the reaction between ammonia and oxygen as mentioned in the script?

    -The reaction between ammonia (NH3) and oxygen (O2) produces nitrogen monoxide (NO), also known as nitric oxide, and water (H2O).

  • What is the difference between nitric oxide and nitrous oxide?

    -Nitric oxide (NO) is a pollutant gas with one oxygen atom, while nitrous oxide (N2O), also known as laughing gas, consists of two nitrogen atoms and one oxygen atom.

  • How much ammonia and oxygen are given in the stoichiometry problem?

    -In the stoichiometry problem, 34 grams of ammonia and 32 grams of oxygen are given.

  • What is the balanced chemical equation for the reaction between ammonia and oxygen?

    -The balanced chemical equation for the reaction is 4NH3 + 5O2 → 4NO + 6H2O.

  • How many moles of ammonia are present in 34 grams?

    -Since the molecular mass of ammonia (NH3) is 17 atomic mass units, 34 grams of ammonia is equivalent to 2 moles.

  • What is the role of oxygen in the given reaction?

    -Oxygen acts as the limiting reagent in the reaction because there is not enough oxygen to react with all the given ammonia, thus determining the amount of nitrogen monoxide produced.

  • How many grams of nitrogen monoxide will be produced with the given amounts of ammonia and oxygen?

    -With 1 mole of oxygen (the limiting reagent), 0.8 moles of nitrogen monoxide will be produced, which is equal to 24 grams.

  • What happens to the excess ammonia that does not react?

    -The excess ammonia, which is 1.2 moles in this case, will remain unreacted at the end of the reaction.

  • How can one determine the limiting reagent in a chemical reaction?

    -The limiting reagent is determined by comparing the mole ratios required for the reaction. In this case, since there is less oxygen (1 mole) than what is needed for the complete reaction with 2 moles of ammonia (which would require 2.5 moles of oxygen), oxygen is the limiting reagent.

Outlines
00:00
🌟 Introduction to the Chemical Reaction

The video script begins with an explanation of a chemical reaction involving ammonia (NH3) and oxygen (O2) gases to produce nitrogen monoxide (NO), also known as nitric oxide. The script emphasizes the importance of having a balanced chemical equation before proceeding with the stoichiometry problem. It points out the initial imbalance in the equation, with three hydrogen atoms on the left side and only two on the right side. The video then demonstrates how to balance the equation by multiplying the molecules to achieve the correct stoichiometric ratios, resulting in a balanced equation with 4 molecules of ammonia and 5 molecules of oxygen yielding 4 moles of nitrogen monoxide and 6 moles of water (H2O).

05:02
📈 Calculating Moles and Understanding the Reaction

This paragraph delves into the calculation of moles from given masses of reactants. It explains how to determine the molecular mass of ammonia (NH3) and oxygen (O2) and convert the given grams of these substances into moles. The script specifies that 34 grams of ammonia corresponds to 2 moles, and 32 grams of oxygen corresponds to 1 mole. It then addresses the discrepancy between the moles of ammonia and oxygen, pointing out that there is not enough oxygen to react with all the given ammonia. This identifies oxygen as the limiting reagent in the reaction, meaning that the amount of nitrogen monoxide produced will be determined by the amount of oxygen available.

10:03
🎓 Determining the Limiting Reagent and Reaction Outcome

The final paragraph focuses on the concept of the limiting reagent and how it affects the outcome of the chemical reaction. With oxygen identified as the limiting reagent, the script calculates the amount of ammonia that can react with the available oxygen, which is 0.8 moles. It then uses the molecular mass of nitrogen monoxide to determine that 0.8 moles of NO will have a mass of 24 grams. The script concludes by explaining that the remaining 1.2 moles of ammonia will not react due to the insufficiency of oxygen, and it encourages viewers to review the material if they found the concepts challenging, emphasizing the importance of understanding molar conversions and stoichiometric ratios.

Mindmap
Keywords
💡ammonia gas
Ammonia gas, denoted as NH3, is a compound consisting of one nitrogen atom bonded to three hydrogen atoms. In the context of the video, it is a reactant in the chemical reaction being discussed, which involves combining ammonia with oxygen to produce nitrogen monoxide and water. The video emphasizes the importance of understanding the stoichiometry of the reaction, including the molecular mass of ammonia, which is 17 atomic mass units, to solve the problem at hand.
💡molecular oxygen
Molecular oxygen, represented as O2, is a diatomic molecule consisting of two oxygen atoms. It is the form of oxygen that is involved in the chemical reaction with ammonia in the video. The script discusses the molecular mass of oxygen, which is 32 atomic mass units for the O2 molecule, and how it relates to the stoichiometry of the reaction. The video also clarifies the distinction between molecular oxygen and atomic oxygen, which is crucial for understanding the reaction and solving the problem.
💡nitrogen monoxide
Nitrogen monoxide, also referred to as nitric oxide and denoted as NO, is a compound consisting of one nitrogen atom and one oxygen atom. It is one of the products of the chemical reaction between ammonia and oxygen discussed in the video. The script highlights that NO is a pollutant and is also used in the body, indicating its dual role in both environmental and biological contexts. The calculation of the amount of NO produced from the given quantities of ammonia and oxygen is the central problem being addressed in the video.
💡stoichiometry
Stoichiometry is the mathematical calculation of the amounts of reactants and products involved in a chemical reaction based on their respective molecular masses and the balanced chemical equation. In the video, stoichiometry is used to determine the amounts of ammonia and oxygen that will react to form nitrogen monoxide and water. The script emphasizes the importance of balancing the chemical equation before performing these calculations and understanding the mole concept to solve the problem.
💡moles
A mole is a unit of measurement used in chemistry to represent the amount of a substance. It is defined as the amount of a substance that contains as many elementary entities (such as atoms or molecules) as there are atoms in 12 grams of carbon-12, which is approximately 6.02 x 10^23 entities. In the video, the concept of moles is crucial for calculating the quantities of reactants and products in the chemical reaction. The script explains how to convert grams to moles and use this information to determine the limiting reagent and the amounts of products formed.
💡limiting reagent
The limiting reagent in a chemical reaction is the reactant that will be completely consumed first and thus determines the maximum amount of product that can be formed. In the video, the script identifies oxygen as the limiting reagent because there is not enough oxygen to react with all the given ammonia. Understanding the limiting reagent is essential for calculating the quantities of products that can be formed in a reaction.
💡molecular mass
Molecular mass is the mass of a molecule, typically expressed in atomic mass units (amu). It is calculated by summing the atomic masses of all the atoms in the molecule. In the video, the molecular masses of ammonia (NH3), oxygen (O2), and nitrogen monoxide (NO) are crucial for performing stoichiometric calculations. The script explains how to determine the molecular mass of each compound and how it relates to the problem of calculating the amounts of reactants and products.
💡balanced chemical equation
A balanced chemical equation is a chemical equation in which the number and type of atoms on the reactant side are equal to the number and type of atoms on the product side. This ensures the law of conservation of mass is obeyed. In the video, the script emphasizes the importance of having a balanced chemical equation before performing stoichiometric calculations. The process of balancing the equation involving ammonia, oxygen, and nitrogen monoxide is demonstrated, highlighting how it is necessary to adjust coefficients to achieve balance.
💡atomic mass units
Atomic mass units (amu) are a unit of mass used to express the mass of an atom. The amu is defined such that one twelfth of the mass of a carbon-12 atom is exactly 1 amu. In the video, atomic mass units are used to calculate the molecular mass of compounds like ammonia, oxygen, and nitrogen monoxide. Understanding atomic mass units is fundamental to performing stoichiometry calculations and solving the problem presented in the video.
💡conversion between moles and grams
The conversion between moles and grams is a fundamental calculation in chemistry that allows the transformation of the mass of a substance (in grams) to the amount of substance (in moles) and vice versa. In the video, this conversion is essential for determining the moles of ammonia and oxygen given their masses, and subsequently calculating the amounts of products formed in the reaction. The script provides a clear example of how to perform this conversion using the molecular mass of the substances involved.
💡chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. In the video, the chemical reaction involves ammonia and oxygen as reactants and produces nitrogen monoxide and water as products. The script discusses the stoichiometry of this reaction, including the balanced chemical equation and the calculation of the amounts of reactants and products based on the given masses of ammonia and oxygen.
Highlights

Combining ammonia (NH3) and oxygen (O2) produces nitrogen monoxide (NO), also known as nitric oxide.

Nitric oxide is a pollutant and can be found in cigarette smoke, and it also has a role in the body.

The given problem involves a stoichiometry calculation with 34 grams of ammonia and 32 grams of oxygen.

The balanced chemical equation is crucial before proceeding with stoichiometry calculations.

The initial equation provided was not balanced, requiring adjustments to ensure equal numbers of each element.

By multiplying the equation by appropriate factors, the equation can be balanced, resulting in 4 moles of ammonia reacting with 5 moles of oxygen to produce 4 moles of nitrogen monoxide and 6 moles of water.

The molecular mass of ammonia (NH3) is 17 atomic mass units.

The molecular mass of oxygen (O2) is 32 atomic mass units.

Given 34 grams of ammonia, we have 2 moles of ammonia available for the reaction.

Given 32 grams of oxygen, we have 1 mole of oxygen available for the reaction.

The balanced reaction requires 2.5 moles of oxygen for every 2 moles of ammonia, indicating that oxygen is the limiting reagent in this scenario.

With 1 mole of oxygen, only 0.8 moles of ammonia can react, resulting in 0.8 moles of nitrogen monoxide.

The molecular mass of nitrogen monoxide (NO) is 30 atomic mass units.

From the reaction, 24 grams of nitrogen monoxide will be produced, as 0.8 moles of NO equate to 0.8 times 30 grams per mole.

There will be 1.2 moles of ammonia left unreacted due to the limiting reagent scenario.

Understanding the concept of limiting reagents is crucial for predicting the outcome of chemical reactions.

The practical application of stoichiometry is evident in predicting the quantities of reactants and products in chemical processes.

The problem-solving approach in chemistry involves step-by-step analysis of the chemical equation, balancing, and calculating moles to determine reaction outcomes.

The importance of accurate mole and mass conversions is emphasized in stoichiometry problems to ensure correct calculations.

Transcripts

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