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Animation Activity: Limiting Reactant Mark as Favorite (2 Favorites)

ACTIVITY in Conservation of Mass, Conservation of Matter, Balancing Equations, Combustion, Stoichiometry, Conservation of Mass, Limiting Reactant, Conservation of Matter. Last updated December 12, 2023.


In this activity, students will view an animation that explores what happens in a limiting reactant problem on the particulate level. Assembling a bike is used as an analogy to introduce the concept of limiting reactant, and then the balanced equation of the combustion of methane is used in four quantitative examples to show what it means for a chemical to be a limiting reactant. The concept of the conservation of mass is also demonstrated by calculating masses from the mole quantities of the reactants and products.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

  • HS-PS1-7: Use mathematical representation to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
  • Scientific and Engineering Practices:
    • Using Mathematics and Computational Thinking
    • Developing and Using Models


By the end of this activity, students should be able to:

  • Explain the concept of limiting and excess reactants.
  • Identify limiting and excess reactants given a balanced equation and molar amounts of reactants.
  • Explain the principle of conservation of mass.

Chemistry Topics

This activity supports students’ understanding of:

  • Limiting reactants
  • Stoichiometry
  • Conservation of mass
  • Combustion reactions


Teacher Preparation: minimal
Lesson: 10-30 minutes



  • No specific safety precautions need to be observed for this activity.

Teacher Notes

  • All of the animations that make up the AACT Animation collection are designed for teachers to incorporate into their classroom lessons. Intentionally, these animations do not have any spoken explanations so that a teacher can speak while the animation is playing and stop the animation as needed to instruct.
  • If you assign this to students outside of class time, you can create a Student Pass that will allow students to view the animation (or any other video or ChemMatters article on the AACT website).
  • We suggest that a teacher pause this animation at several points or watch it more than once to give students the opportunity to make notes, ask questions, and test their understanding of the concepts presented. The animation is a little over a minute and a half long and moves quickly, so students will likely require pausing or multiple viewings to successfully complete the student activity sheet if you choose to use it. Here are some of the points at which you might want to pause the video to allow students to answer the questions on the activity sheet:
  • Question 1 – 0:11
  • Question 2 – 0:17
  • Question 3 – 0:20
  • Question 4 – 0:25
  • Question 5 – 0:35
  • Question 6 – 0:42
  • Question 7 – 1:03
  • Question 8 – 1:22
  • This animation depicts limiting and excess reactants in both real-world examples (building a bicycle from parts) and chemistry-specific examples (the combustion of methane). The chemistry examples also provide corresponding masses, which allows teachers to address the conservation of mass as well. Students should be familiar with the concepts of moles, atoms and molecules, and balanced chemical equations to get the most out of this animation. It can be used as an introduction before formal instruction on limiting reactants, or as a review at any point.
  • At any point in this animation, the images of molecules of each substance could be viewed as representing either a molecule of that substance or a mole of that substance. For example, at about 0:20, 1 molecule of CH4 and 2 molecules of O2 appear on the screen. These could be interpreted literally as 1 molecule CH4 and 2 molecules O2, or they could be used to represent an entire mole of those substances (1 mole CH4 and 2 moles of O2).
  • Please note that specific reaction mechanisms are not represented in this animation, so the atoms in reactant molecules separate and then reform into products, which does not accurately represent the way these reactions would occur in the real world. The focus is on representing limiting and excess reactants and the conservation of mass.
  • Related classroom resources from the AACT Library that may be used to further teach this topic:

For the Student

Limiting Reactant

As you view the animation, answer the questions below.

  1. What is the “equation” for building a bicycle?
  1. In the animation, several bicycles are built from a pile of parts. Which part of the bicycle do you run out of first? (This part is the “limiting reactant.”)
  1. What is the equation for the combustion of methane?
  1. What happens to the CH4 and O2 molecules in the combustion reaction? What do you end up with after the reaction?
  1. When you start with 5 CH4 molecules and 10 O2 molecules, do all the molecules get used up or are there some left over? Use the balanced equation from question 3 to explain why this is.
  1. When you start with 10 CH4 molecules and 10 O2 molecules:
    1. Does one type of reactant molecule get used up completely (the limiting reactant)? If so, which one?
    2. Are there any “unused” reactant molecules left over? If so, what kind and how many? (This is called the “excess reactant.”)
  1. When you start with 4 moles of CH4 and 2 moles of O2:
    1. What is the total mass of your reactants?
    2. What molecules do you end up with after the reaction is complete?
    3. Is there a limiting and an excess reactant? If so, which is which?
    4. What is the total mass of the substances after the reaction? How does it compare with the mass of the starting materials?
  1. When you start with 1 mole of CH4 and 3 moles of O2:
    1. What is the total mass of your reactants?
    2. What molecules do you end up with after the reaction is complete?
    3. Is there a limiting and an excess reactant? If so, which is which?
    4. What is the total mass of the substances after the reaction? How does it compare with the mass of the starting materials?


  1. Summarize in your own words how you can identify the limiting and excess reactants in a chemical reaction.
  2. The law of conservation of mass states that mass cannot be created or destroyed in a chemical reaction. Based on what you saw in this animation, does this law still apply when there is an excess reactant left over? Explain using evidence from the animation.
  3. Provide a real-world example, similar to the beginning of the animation, to explain the concept of limiting and excess reactants to a fourth grader. Write and provide the solution to a practice problem using your example where the fourth grader would have to identify the limiting and excess reactants.