Limiting and excess reactants (qualitative) | Developing understanding | 14–16 years | Resource

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Use observations, state symbols and the number of reacting atoms and molecules to determine the limiting and excess reactants

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

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A ramped worksheet to help learners develop their qualitative mental models of limiting and excess reactants. With icons to indicate the conceptual level/s of each question.

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Learning objectives

Recognise in an everyday situation why not having enough of one ingredient can limit the quantity of the final roduct that can be made.
Identify a reactant that is in excess.
Identify the excess and limiting reactants of different chemical reactions using evidence from observations and state symbols.
Use particle diagrams to determine the limiting reactant.
Suggest why copper carbonate is added in excess when making the salt copper sulfate.

How to use this resource

This resource aims to develop learners’ understanding of limiting and excess reactants. The questions encourage learners to think about how observations (macroscopic understanding), state symbols in a chemical equation (symbolic understanding) and the number of reacting atoms and molecules (sub-microscopic understanding) can be used to determine the limiting reactant and excess reactants. As a result, learners should develop more secure mental models to support their thinking about this topic.

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

To develop learner’s quantitative understanding of limiting and excess reactions, including calculation using moles, try our Developing understanding of limiting and excess reactants (quantitative) worksheet.

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 Excess copper oxide Johnstone’s triangle worksheet which introduces the idea of limiting and excess reactants in the context of observing excess copper oxide reacting with sulfuric acid to produce copper sulfate solution.

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 substances in the classroom, run a class practical of a chemical reaction or show a teacher demonstration of properties.

For question four consider giving learners a physical model e.g. 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.

Answers and guidance

There are five multi-part questions in the student worksheet. Answers can be found in the teacher notes.

The first question develops learners’ understanding of the basic concept of limiting and excess ingredients when making an overall product. The question uses the everyday example of scoops of ice cream and cones rather than chemical reactions to make the idea as accessible as possible.

In question two learners develop understanding of how observation of a chemical reaction (macroscopic understanding) can be used to determine the excess and limiting reactant. The question also builds leaners’ understanding of how an indicator (macroscopic understanding) may be used to show that an acid is in excess.

Question three develops learners’ understanding of observation of a chemical reaction (macroscopic understanding) and state symbols from a balanced chemical equation (symbolic understanding) can help determine the limiting and excess reactants. The question also builds learners’ understanding of the use of an indicator (macroscopic understanding) to distinguish whether an acid or alkali is in excess.

In question four learners develop understanding of how the ratio shown in a balanced chemical equation (symbolic understanding) and particle diagrams (sub-microscopic diagram) can be used to explain which reactant is the limiting reactant and which is in excess. Please note that to determine this from given masses of reactants quantitative methods are required. These are explored in the Developing understanding of limiting and excess reactants (quantitative) resource.

In the final question develops learners’ understanding of why limiting and excess reactants are important to consider when making a salt. It develops understanding of how a chemical reaction only shows the products of a chemical reaction and not an excess reactant (symbolic understanding). It builds understanding of why a solid excess reactant is used and how this can be removed to leave the desired product (macroscopic understanding). If learners have not yet covered methods of making salts, then this question could be omitted. Note: in part c it is also important to add excess copper carbonate from a safety perspective so that all the sulfuric acid has reacted before heating.