Topic: The law of constant composition of a chemical compound

Target group

Elementary school student (grades 7. and 8.)

Core curriculum:

Elementary school. Chemistry.

III. Chemical reactions. Pupil:

7. Applies to the calculation of constancy law and mass conservation law (performs calculations related to the stoichiometry of the chemical formula and the chemical reaction equation).

General aim of education

The student interprets the law of mass preservation and solves tasks using the law of mass preservation.

Key competences

• communication in foreign languages;

• communication in the mother tongue;

• digital competence;

• mathematical competence and basic competences in science and technology;

• learning to learn.

Criteria for success
The student will learn:

• to knowledge of the content of the law of mass conservation and its interpretation;

• to solve tasks using the law of mass conservation.

Methods/techniques

• expository

  • talk.

• exposing

  • film.

• programmed

  • with computer;

  • with e‑textbook.

• practical

  • exercices concerned.

Forms of work

• individual activity;

• activity in pairs;

• collective activity.

Teaching aids

• e‑textbook;

• interactive whiteboard, tablets/computers;

• notebook and crayons/felt‑tip pens.

Lesson plan overview

Introduction

1. The teacher hands out Methodology Guide or green, yellow and red sheets of paper to the students to be used during the work based on a traffic light technique. He presents the aims of the lesson in the student's language on a multimedia presentation and discusses the criteria of success (aims of the lesson and success criteria can be send to students via e‑mail or posted on Facebook, so that students will be able to manage their portfolio).

2. The teacher together with the students determines the topic – based on the previously presented lesson aims – and then writes it on the interactive whiteboard/blackboard. Students write the topic in the notebook.

Realization

1. Students read the fragment titled „Do chemical compounds have the same composition?” They explain what the law of constant composition is and who formulated it.

2. Students get acquainted with the photo gallery in the abstract. They discuss it together with the teacher. The teacher mentions bertolides - compounds that do not have a fixed quantitative composition.

3. The teacher plays the presentation „Chemical calculations using the law of constant composition of a chemical compound” and explains how the law of constant composition can be used in chemical calculations. The teacher also plays the presentation showing how to determine the mass of aluminum in a sample of its oxide.

4. Students, working in pairs, solve the tasks: a) Calculate how many grams of oxygen will react with 7.2 g of magnesium, since it is known that magnesium is combined with oxygen in a mass ratio of 3: 2; b) During combustion of 10 g of copper in sulfur vapors, 12.50 g of copper sulphide were formed. Determine what sulphide was: copper(I) or copper(II). Volunteers present solutions, the teacher corrects mistakes.

5. Students perform exercise number 1. The teacher discusses the right solution with them.

Summary

1. The teacher briefly presents the most important issues discussed in class. He answers the additional questions of the proteges and explains all their doubts.

2. The teacher plays the recording of the abstract. Every now and then he stops it, asking the students to tell in their own words what they have just heard. In this way, students consolidate information learned during the lesson and practice listening comprehension.

Homework

1. Carry out exercise number 2.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

The law of constant composition of a chemical compound

At the end of the 18th century, the French chemist Joseph Louis Proust formulated a law referring to the composition of chemical compounds, which was called the law of constant composition.

Today, this law seems obvious to us and not very revealing. But we must remember that it was formulated at a time when nothing was known about the structure of matter, concepts such as the notion of an atom or a molecule were unknown and no one used the chemical formulas of compounds. Currently based on a chemical formula, for example water (${\text{H}}_{\text{2}}\text{O}$),and data contained in the periodic table, we can determine the mass ratio of hydrogen to oxygen in water ($\text{2}\text{: 16}$ then $1\mathrm{}:8$), less than 150 years ago, the conclusion of such an application required many tedious experimental works.

The formulation of this law was of fundamental importance for the further development of chemistry. It has become the foundation for further research leading to the creation of the atomic theory of matter.

Knowledge of the law of mass conservation and the law of constant composition is the basis of chemical calculations. Thanks to them, it is possible to determine the proportions in which the reactants reacted with each other to form specific products, or to estimate the amount of products produced based on the mass of the substrates used.

The mass ratio of hydrogen to oxygen in the water molecule, ${\text{H}}_{\text{2}}\text{O}$, is $1:8$, which means that in water for 1 part of the mass of hydrogen there are 8 parts of oxygen mass. Regardless of which units of mass we will use, these relationships will always be the same. For example, if there is 1 g of hydrogen in a given water sample, then oxygen will be 8 g and the water sample will have a mass of: $\text{1 g + 8 g = 9 g}$. The same mass ratio will exist both in one molecule of water with a molecular weight of 18 u and in samples of water with a mass of 18 g, 200 kg or 1 ton.
The mass ratio of the individual elements in the compound is always constant – independent of the mass of the compound sample as well as the way the compound is obtained.

• According to the conservation law, the mass states in each chemical reaction the total mass of substrates is equal to the sum of masses of products obtained as a result.

• The mass of one of the substrates or products can be calculated based on the mass conservation law if the residual masses are known.

• The law of constancy of composition says that the mass ratio of elements in a chemical compound is always constant and independent of the method and place of its receipt (each chemical compound has an unchanging qualitative and quantitative composition).

• Knowledge of the mass ratio of chemical elements in a compound allows to calculate the mass of chemical elements in a given mass of a compound.

• On the basis of the mass ratio of elements in a chemical compound, its total formula can be determined.