Rate of reaction - A double meaning

 

Featured article

Cakmakci, G. (2009). Emerging issues from textbook analysis in the area of chemical kinetics. Australian Journal of Education in Chemistry, 70, 31–37

Sometimes when reading a research paper with my “misconception mining” lens something quite simple jumps out at me.

This paper by Cakmakci includes the findings of an analysis of textbooks used at both school and university. Although this analysis was carried out in the 2000’s, and textbooks in different countries vary, it is easy to recognise the same issues in textbooks today as well as in online and even generative AI content.

The textbook analysis revealed some imprecise use of language that authors suggest could have an impact on student learning. One example was in how catalysts may be defined in a way that implies that they do not interact in any way with the reactants or products ( "a catalyst does not enter into the reaction in any way”). As the author points out, this may promote misconceptions about the mechanism of catalysis (which forms part of more advanced study). 

The other area of imprecision that may have a more immediate impact on student understanding is the use of term “reaction rate”. The author gives two example questions that are familiar to anyone who has taught the rates of reaction topic to 14 to16 year olds which shows how this imprecise language use is replicated in the teaching language of the classroom.

"How would a rise in temperature affect the reaction rate?"

"How would the reaction rate change during the reaction?"

The simple point, that is so easily overlooked, is that the reaction rate referred to in each question is not the same.

In the first question the reaction rate is the average (mean) rate of reaction. In the second question it is the instantaneous rate of reaction (the rate of reaction at any given point in time). The concept of instantaneous rate receives greater focus once students are required to draw the tangent to find the gradient and hence rate of reaction shown on a curved graph. However, the idea that the rate of reaction can change during a chemical reaction is needed much sooner.

Average (mean) reaction rate has one value for a chemical reaction whereas the instantaneous rate of reaction changes during the course of the reaction. This idea of a changing reaction rate is a central idea to the rate of reaction topic. 

Why does this matter in the classroom?

The paper presents a diagram to show the importance of understanding and connecting three different perspectives (very similar to Johnstone’s Triangle): 

• the macroscopic observable event
• a particulate model of what is happening 
• mathematical modelling (equivalent to the symbolic representation). 

At age 14-16 this would correspond to:

• observation of the rate of formation of hydrogen decreasing with time when magnesium reacts with acid
• explanation for an increase in the average (mean) rate of reaction if the concentration of acid is increased in terms of collision theory
• the curve graph line obtained if plotting volume of time vs the volume of hydrogen produced 

If students develop the mental model of reaction rate being equivalent to a single average (mean) rate how can they fully interpret experimental observations such a decrease in the rate of formation hydrogen when magnesium reacts with an acid?

If students have not developed the concept that reaction rate changes during a chemical reaction how can they interpret the curve of a time vs volume of gaseous product graph effectively?

Reflective questions

How could the double meaning of the term "reaction rate" be addressed in the classroom? Is the more complex term “instantaneous” essential to introduce or would including the term average/mean reaction rate be more accessible and have sufficient impact?

What are the implications for curriculum sequencing when planning the rate of reaction topic?

Could there be benefits in building connections with physics learning about speed and acceleration (a rate and a change in rate)? How could this be achieved?

BEST Diagnostic question

Some students are discussing whether a chemical reaction has only one rate of reaction.

Who do agree with, and why?

Who do you disagree with, and why?

Oscar “We only put one rate of reaction in the results table so a chemical reaction can only have one rate.”

Pritam “The rate of a reaction changes during a chemical reaction.”

Naomi “A chemical reaction can have a different rate if the reaction takes place at a higher temperature.”

Catherine “A chemical reaction has a different rate if you change the concentration of a reactant.”

Oscar, Naomi and Catherine are correct if they are referring to the average (mean) rate of reaction.

Answer: The instantaneous rate of reaction changes during a chemical reaction so Pritam is also correct.

A student who thinks that Pritam is incorrect may only think of rate of reaction in terms of the average (mean) rate. The student may be unaware of the idea of an instantaneous rate of reaction.

BEST Response activity

Chemistry textbooks and websites often use the term “rate of reaction” but which rate of reaction do they mean?

Fill in the gaps to complete these sentences about rate of reaction. You should only use the words average and instantaneous.

1.     At a higher temperature, the _________ rate of reaction is faster.

The ___________ rate of reaction is fastest at the start of the reaction.

During a reaction, the ____________ rate of reaction gradually decreases.

There is one ____________ rate for a chemical reaction.

Answers: average, instantaneous, instantaneous, average

Useful links

Best Topic 6 Key Concept 1 Instantaneous rate

Diagnostic questions to check for student misconceptions about rates of reaction as part of a five-part progression (and including response activities)

University of York Science Education Group