Module 4: Causal Mechanistic Reasoning

What is in this module?

Overview

Ultimately, scientists seek to explain the natural world. Therefore, a goal of chemistry education should be that students use their chemistry knowledge to explain observable phenomena. Causal mechanistic reasoning explains both WHY and HOW something occurs at a level deeper than the level of the phenomenon. For chemistry, this generally means at the particulate level or subatomic level.

Instructional videos that support students’ abilities to provide causal mechanistic explanations must include explanations of concepts at a causal mechanistic level. Prior to watching videos on a given topic, we first need to identify what a causal mechanistic explanation needs to include.

For example, a causal mechanistic explanation of Le Chatelier’s principle would include: 

How: Collisions between molecules lead to forward and reverse reactions occurring. Changes in concentration or temp alter the relative rate of success of collisions to lead to the forward or reverse reaction. 

Why: Changes in concentration or temp alter the relative forward and reverse reaction rates. If rates are unequal, then the rate of one will increase and the other decrease (due to changing concentrations of reactants and products) until they become equal

Both parts must be present for a complete causal mechanistic explanation!

Watch the video clips below and answer the following questions: 

  1. Did the video use causal mechanistic reasoning to explain how and why the equilibrium system shifts?
  2. If the video did not use causal mechanistic reasoning, what was missing? What would need to be added to meet the criteria?

Video 1

What did you decide?

  1. Did the video use causal mechanistic reasoning to explain how and why the equilibrium system shifts?
    • No
  2. If the video did not use causal mechanistic reasoning, what was missing? What would need to be added to meet the criteria?
    • At 5:20 in the video, he talks about how there would be less ammonia to break down (if NH3 was removed) and that the rate of the formation of NH3 will exceed the rate of the formation of N2 and H2 so more NH3 will be formed until the equilibrium is reestablished. However, he was missing a connection to the changes in collisions as a result of changing the concentrations that would cause this to happen. So while it was a better explanation than many, it did not reach the level of Causal Mechanistic Reasoning.

Video 2

What did you decide?

  1. Did the video use causal mechanistic reasoning to explain how and why the equilibrium system shifts?

Yes, the video explains the casual understanding of how the mechanisms work and how the equilibrium shifts happens. At about 1 min she clearly articulates how an increase in the amount of reactant will affect collisions between the reactant particles and increase the rate of the forward reaction, thus forming products more quickly than the reverse reaction forms reactants. As the number of product particles increases, this will increase the rate of the reverse reaction until the forward and reverse rates are again the same. 

Video 3

What did you decide?

  1. Did the video use causal mechanistic reasoning to explain how and why the equilibrium system shifts?
    • No
  2. If the video did not use causal mechanistic reasoning, what would need to be added to meet the criteria?

    • In the second minute, there was a description of how concentrations will change the amount of materials, but they do not address the ideas of collisions or rates.

      In the 5th minute, they talk about the fact that the number of moles of gas on either side of the equation are equal, so that a change in pressure/volume will impact both reactions rates equally. This is close, but since this is a system that doesn’t change, it doesn’t actually describe an example of Le Chatelier’s Principle nor the reestablishing of equilibrium.