Developing understanding of reacting ratios: masses

Developing understanding is a series of resources that encourage learners to connect their thinking at the macroscopic, sub-microscopic and symbolic levels.

Learning objectives

1. Describe what a balanced chemical equation means in terms of the ratio of reactant and product atoms or molecules.
2. Explain why the mass of carbon dioxide formed is not equal to the mass of carbon that reacts.
3. Calcaulate the mass of a given number of moles of carbon, oxygen and carbon dioxide.
4. Calculate the mass of product formed from a given mass of reactant using a chemical equation with a 1:1 ratio.
5. Calculate the mass of product formed from a given mass of a reactant using a chemical equation with a 1:2 ratio.

How to use this resource

This resource aims to develop learners’ understanding of a balanced chemical equation and what this means at the sub-microscopic level in terms of the ratio of a very large number of number (measured in moles) of atoms or molecules reacting or being produced. The questions encourage learners to think about how the measured mass of reactants or products connects with the number of moles of atoms or molecules. The more secure mental models developed should help learners to understand calculations of the mass of product formed from a given mass of reactant using chemical equations with both a 1:1 and 1:2 ratio.

• When to use? Use after initial teaching or discussion of this topic to develop ideas further. You can also use as a revision activity.
• Group size? Suitable for independent work either in class or at home. Or use the questions for group or class discussions.
• How long? 15–30 mins

Johnstone’s triangle

Johnstone’s triangle is a model of the three different conceptual levels in chemistry: macroscopic, symbolic and sub-microscopic. You can use Johnstone’s triangle to build a secure understanding of chemical ideas for your learners.

Introduce learners to Johnstone’s triangle with our Tin from tin oxide Johnstone’s triangle worksheet which guides learners to consider the difference in appearance between tin and tin oxide; show that they understand how to represent the extraction of tin from the compound symbolically in a balanced sumbol equation before tackling calculations involving mass, relative formula mass and moles.

Johnstone’s triangle and this resource

The icons in the margin indicate which level of understanding each question is developing to help prompt learners in their thinking.

• Macroscopic: what we can see. Think about the properties that we can observe, measure and record.
• Sub-microscopic: smaller than we can see. Think about the particle or atomic level.
• Symbolic: representations. Think about how we represent chemical ideas including symbols and diagrams.

The levels are interrelated, for example, learners need visual representation of the sub-microscopic in order to develop mental models of the particle or atomic level. Our approach has been to apply icons to questions based on what the learners should be thinking about.

Questions may be marked with two or all three icons, indicating that learners will be thinking at more than one level. However, individual parts of the question may require learners to think about only one or two specific levels at a time.

Support

This worksheet is ramped so that the earlier questions are more accessible. The activity becomes more challenging in the later questions. You can give extra explanations for the more challenging questions. If completing as an in-class activity it is best to pause and check understanding at intervals, as often one question builds on the previous one.

It is useful for learners to observe macroscopic properties first-hand. You could circulate examples of one mole of a substance in the classroom, run a class practical of a chemical reaction or show a teacher demonstration of properties.

Give learners physical models to use and manipulate, such as a Molymod™ kit or counters.

Additional support may be needed for any learners still lacking in confidence in the required symbolic representation, for example by sharing and explaining a diagram or a simulation that can show movement of the particles.