Topic: Principle of mass conservation

Target group

Elementary school student (grades 7. and 8.)

Core curriculum:

Elementary school. Chemistry.

III. Chemical reactions. Student:

7) applies the constancy law and mass conservation law to calculations (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 the mother tongue;

• communication in foreign languages;

• mathematical competence and basic competences in science and technology;

• digital competence;

• learning to learn.

Criteria for success
The student will learn:

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

• 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;

• notebook and crayons/felt‑tip pens;

• interactive whiteboard, tablets/computers;

• methodician or green, yellow and red cards.

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 „Does the mass of substrates change during a chemical reaction?” They explain what the principle of mass conservation is and who formulated it, the teacher completes the information explaining cases in which this principle is not preserved. Next, the teacher explains that in modern physics, according to the special theory of relativity, the law of preserving the total resting mass of the isolated system takes energy into account.

2. The teacher announces a movie titled “How to control mass of substances involved in the reaction of baking soda with vinegar”. He instructs his pupils to write a research question and a hypothesis in the form provided in the abstract. Then he plays the video and the students note their observations and conclusions. Referring to observations and conclusions, the teacher encourages young people to discuss.

3. The teacher explains how can the principle of mass conservation be used in chemical calculations. He presents and discusses examples from the abstract (the reaction of the synthesis of magnesium and oxygen in magnesium oxide, the reaction of exchanging copper oxide and carbon in copper and carbon dioxide).

4. The students, working in pairs, solve the tasks: a) Heating 30 g of copper with oxygen, 38 g of copper(II) oxide were obtained. Calculate how much oxygen was used in this reaction; b) The iron is obtained from iron ore. 14 g of iron(III) oxide were used for the reaction with 6.4 g of carbon monoxide and 9.2 g of iron was obtained. Calculate the volume of the separated carbon dioxide, if its density is equal to 1.96 g / dmIndeks górny 3³; c) 6.4 grams of oxygen were consumed when magnesium was burned, yielding 16 grams of magnesium oxide. Calculate the mass of magnesium used in the chemical reaction.

5. The teacher plays the abstract recording for all students. Participants listen carefully and give feedback on the difficulty of the text being heard using the traffic light method. Students are provided with green, yellow and red cards. While listening to the recording, they display the appropriate color for self‑assessment and to inform the teacher: green - I'm fine, I understand everything; yellow - I have some doubts; red - I do not understand anything, please help. The teacher responds depending on the needs of the students, deciding to repeat the recording, listen to the recording while following the text or translate the text.

Summary

1. The teacher asks the students to finish the following sentences:

   • Today I learned ...

   • I understood that …

   • It surprised me …

   • I found out ...

   The teacher can use the interactive whiteboard in the abstract or instruct students to work with it

Homework

1. Carry out interactive exercise.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

Principle of mass conservation

While observing the course of chemical reactions, we can describe effects that occur during it, for example changes in colour, sounds, light emission. Sometimes one can also get impression that the quantity of substances involved in the reaction is decreasing or increasing.

Masses of substrates and products were compared already in the 18th century. Due to these studies, conducted independently by two chemists, Mikhail Lomonosov from Russia (1756) and Antoine Lavoisier from France (1785), a general law of nature was formulated. It was called principle of mass conservation. In line with this law, total mass of substrates is equal to total mass of products in an isolated system (in which reaction products and energy does not leave this system). This means that the same mass of substrates produces the same mass of products; that is mass of substances involved in chemical reaction does not change.
The need to balance chemical equations results in fact from the principle of mass conservation. If the total mass of substrates is to be equal to the total mass of products, numbers of atoms of the same type on both sides of the equation have to be identical.

Nowadays the law of mass conservation is extended by the energy of ingredients. Reagents are characterized by their own internal energy called the resting mass. However, due to the fact that the resting mass of the system or chemical reaction contributes not only to the rest masses of the components, but also all forms of internal energy related to the movement of elemental atoms in space and their mutual interactions, the rest mass of the system is equal to the sum of masses its components and their energy.

During chemical reactions, the structure of the resting mass of the system may change, eg by reducing the sum of the rest masses of its components, and increasing the sum of their energy.

For closed systems but not insulated, the right to maintain the rest mass is not satisfied, because there is a flow of energy between the system and the environment, which results in a change in the rest mass of the system.

However, during chemical reactions, the amounts of energy exchanged are so small that the mass change of the system is not detectable by standard methods, hence the stability of the mass of the reaction system is assumed. In chemical reaction, the sum of the masses of products and substrates are the same.

The principle of mass conservation helps determine mass of one substance if we know masses of other substrates and products. If you know this principle, you can calculate for example the quantity of products resulting from a given mass of substrates. For example, if we know that 2.4 g of magnesium and 1.6 of oxygen were involved in the reaction, we can easily determine that of magnesium oxide was produced in this chemical reaction:

In case of another reaction – exchange reaction of copper(II) oxide with carbon – we can determine mass of copper if we know masses of substrates and mass of the second product:

In line with the principle of mass conservation, total mass of substrates is to be equal to total mass of products:

After rearranging the equation and making calculations we will know the mass of copper:

$\text{x = 15.9 g + 1.2 g - 4.4 g = 12.7 g}$

Using the principle of mass conservation, we can conclude that 12.7 g of copper will be produced in a reaction of 15.9 g of copper(II) oxide and 1.2 g of carbon.

Calculate mass of hydrogen produced in this reaction and determine the number of molecules of this gas.

• According to the principle of mass conservation it is assumed that in each reaction total mass of substrates is equal to total mass of resulting products.

• Mass of each substrate or products can be calculated based on the principle of mass conservation, if you know masses of the other ones.

• According to the law of definite proportion, mass ratio of elements in a chemical compound is always constant and does not depend on its source and method of preparation (each chemical compound always contains its component elements in fixed qualitative and quantitative ratio).

• If you know mass ratio of chemical elements in a compound, you can calculate mass of chemical elements in given amount of this compound.

• Molecular formula of given compound may be determined based on mass ratio of its component elements.