Limiting and excess reactants: quantitative | Developing understanding | 14–16 years | Resource

No comments

Use balanced chemical equations and the number of moles of each reactant to determine the limiting reactant and the number of moles of product that could be formed

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

Download this

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

Download All

View and download more Developing understanding worksheets

Learning objectives

Use observation to identify an excess reactant.
Determine the excess and limiting reactant using a balanced chemical equation and number of moles of each reactant at the start.
Determine the number of moles of product that can be produced from a given number of moles of two reactants for a chemical reaction with a 1:1 reactant ratio.
Determine the number of moles of product that can be produced from a given number of moles of two reactants for a chemical reaction with a 1:2 reactant ratio.

How to use this resource

This resource aims to develop learners’ quantitative understanding of limiting and excess reactants. The questions encourage learners to think about how information from a balanced chemical equation (symbolic understanding) and the number of moles of each reactant at the start (sub-microscopic understanding) can be used to determine the limiting reactant and the number of moles of product that could be formed. 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 introduce learner’s to the idea of limiting and excess reactions, including an analogy using ice creams and ice cream cones, try our Developing understanding of limiting and excess reactants (qualitative) 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.

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 three multi-part questions in the student worksheet. Answers can be found in the teacher notes.

The first question develops learners’ understanding of what they observe in a beaker at the end of a reaction (macroscopic understanding) and how this connects to the balanced chemical equation (symbolic understanding). The right-hand side of a chemical equation shows the products that are made and not necessarily what is found in the reaction vessel at the end of a reaction. Ideas about limiting and excess reactants at a qualitative level are explored qualitatively in the Developing understanding of limiting and excess reactants (qualitative) resource.

In question two learners develop understanding of how the number of moles of the product is limited by the number of moles of the limiting reactant (sub-microscopic understanding) and how the number of moles of product is determined by the ratio with this limiting reactant shown in the balanced chemical equation (symbolic understanding).

Question three develops learners’ understanding of how to determine the number of moles of product (sub-microscopic understanding) when the balanced chemical equation of the reaction has a ratio of reactants that is 1:2 (symbolic understanding). This question uses bar diagrams to support learners in recognising the limiting reactant which is a necessary step in determining the number of moles of the product. The last part of the question requires students to recognise that the number of moles of product relates to the mass of product formed (macroscopic understanding).