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

Digestion - Children's understanding of what happens to the food that they eat

 Featured article

Teixeira, F. (2000). What happens to the food we eat? Children’s conceptions of the structure and function of the digestive system. International Journal of Science Education, 22 (5), pp.507–520

The children (aged 4 to 10) taking part in this study were each given a bar of chocolate and an outline, on paper, of a human body. They were asked to eat the chocolate and draw on the paper the parts of the body this food will pass through. The children were then asked to name any organs that they had drawn and to describe what happened to the food as it passed through each organ. They were also asked what the food would look like at each stage.

[AI generated impression of a child's drawing]

The study took place a private primary school in Brazil and the paper specifies that none of the children had had formal education about the digestive system. Clearly the outcomes may be different for children who have received scientific teaching about this during their primary years. However, the approach taken by the researcher and the outcomes still provoke some interesting questions about what children understand about what happens inside their bodies and how they build these ideas based on daily experiences.

The researcher analysed children’s drawings and explanations in terms of understanding of both structure and function. Clearly biologically these are linked as the structures inside the body all have a function. However, this distinction was designed to find out how much the children understood about the internal structure of their own bodies and how much awareness they had about the function of these parts.

The researchers categorised the responses based on three different broad ways of thinking about the path of food through the body:

1.  All food that has been ingested (eaten) remains in the body.

2. All the food that has been ingested eventually leaves the body.

3. Part of what has been ingested stays in the body but the rest leaves.

Broadly speaking number 1 was the predominant way of thinking of the four-year-olds in the study, number 2 was the more common thought process of the 6- to 8-year-olds and only for the oldest children (age 10) was number 3 the majority view.

At age four the most commonly used body part name was “tummy”. For children aged 4, 11% of children left the abdominal area in the drawing as empty space. The authors later comment that for a child that thinks all their food remains in the body this is not unexpected. There is no need for any other structures or compartments because the body is perceived as being simply a container. Interestingly for some of these children the only way food could leave their tummy was through moving down into their legs (or their arms if they bended). It was even suggested that the accumulation of food results in a stretching of the body. The children were aware (probably due to experience of chewing) that the food was broken into smaller pieces.

The children who thought that all the food that has been ingested leaves the body appeared to have some concept of physical change to the food but did not give any evidence of awareness that the identity of the food changed.

The oldest children showed some awareness of the limitations of the body realising that the body cannot absorb everything that is ingested. They also appeared to have some awareness of the idea that parts of food that are useful to the body are kept inside but other parts, which are not useful, leave the body.

Only six children (all older than 8) indicated that there maybe substances in the body that could modify others. This enabled them to conceive of the possibility that ingested food could be changed into something different to what it was. The idea of chemical transformation (change) was noted to be a complex one but difficulty with the idea was acknowledged to limit children’s explanations of the function of the digestive system.

Reflective questions

Primary (age 5-11)

The research suggests that even young children form ideas about what happens to the food they eat. What everyday discussion about food and eating may influence children’s reasoning?

e.g. An adult asking, “Is your tummy full?”

In what ways could the researcher’s approach of asking children to draw and then discuss what they think is inside their bodies be used in the classroom? How could the idea be extended to other topics?

What explanation of what happens to food when you eat do you think is acceptable for children aged 10 to 11 and what should be left for later learning?

Secondary (age 11-16)

In what ways could the researcher’s approach of asking the children to draw and discuss what they think is inside of them be used in a secondary context?

What chemistry learning is needed to support the development of biological understanding about digestion?

To what extent/ how do you build connections between the two subjects in your teaching?

 

 

The particle model

Featured article

Johnson, P. (1998). Progression in children’s understanding of a ‘basic’ particle theory: A longitudinal study. International Journal of Science Education, 20 (4), pp.393–412.

Very often students will get asked to label a particle diagram or to draw the particle diagram for a given state, however this paper suggests that even if answering ‘correctly’ students may hold a range of misconceptions.

The particle model under discussion in this paper is described as a model with sufficient detail to account for the characteristic properties of the three states of matter. It is not, the author specifies, a model that distinguishes between the types of particle (atoms, molecules and ions). 

The author summarises the common findings of the existing literature about student understanding of the particle model, identifying five areas of difficulty:

1. The relative spacing between particles in the three states.

Showing the spacing between particles in the liquid state as intermediate between the solid and gas states.Typical diagrams for the gas state underrepresent the relative spacing of particles.

2. The intrinsic motion of particles

Many students showed little appreciation of the intrinsic motion of particles.

3. Ideas of forces of attraction between particles

Very few students used the idea of forces of attraction between particles.

4. The ‘space’ between particles

The idea that there is ‘nothing’ between particles seemed to cause a lot of difficulties for students. Some preferred to think that something must be there (often referring to this as ‘air’).

5. The nature of the particles themselves

Many students were found to give particles the same properties as the bulk material. For example, a copper atom was thought to have the same properties as copper metal.

The paper reports on a three-year longitudinal study following a cohort of pupils in a non-selective English secondary school as they moved from year 7-9 (ages 11 to14). Students were periodically interviewed following teaching of four planned unit designed to develop the idea of a chemical substance.

The responses were used to identify four distinct particle models held by the students.

Model X: Substances are continuous (and not made of particles).

Model A: Particles are found in the continuous substance.

Model B: Particles are the substances, but with the macroscopic character of the bulk substance.

Model C: Particles are the substance and the properties of the substance in a given state are a collective property of those particles.

The interview questions enabled the author to identify the model of thinking held by students at the time of the interview. Whilst there were some students were inconsistent and applied different models in different circumstances, the majority had complete models of either X, A, B or C. This method of categorisation enabled the author to explore how student thinking about the particle model changed over time.

In general, many students were found to progress in the model that they were using. The author identified two different ‘dimensions’ of this progression: a continuous to particulate dimension and a macroscopic to collective properties dimension.

BEST Diagnostic question

Imagine you could see the particles in a jar of methane gas.

Which diagram best matches what you would see?

 

The correct answer is C.

Reflective questions

When teaching the particle model, do you teach what is between the particles? How do/could you explain the concept of ‘nothing’ to students?

It is not uncommon to refer to the ‘particles in a solid’. To what extend could this language reinforce existing misconceptions and how could the language be changed to avoid this?

An atom is often defined as ‘the smallest particle of an element’. To what extent could that reinforce earlier misconceptions about the particle model? How could this be mitigated?

Useful links

BEST Topic 1 Key concept 1: Particle model for the solid, liquid and gas states

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

University of York Science Education Group

Developing understanding: States of matter

A ramped student worksheet that aims to help students to deepen their understanding of the particle model and to strengthen their mental models.

Royal Society of Chemistry

Acknowledgements

The example BEST diagnostic question was developed by Helen Harden (UYSEG), from an idea by Andrew Hunt selected from a collection of ASK items devised for research by Philip Johnson (Durham University).

The particle model image is from the RSC Johnstone's Triangle Resource: States of Matter