Chemical Reactions 2

CR2 Represent chemical reactions symbolically using models, word equations, and balanced chemical equations

Suggested time: 6-8 hours

Nomenclature, the systematic naming of chemical compounds, provides chemists with distinct names for every chemical compound. Naming systems also provide information about the chemical composition and structure of compounds. Students should learn to represent chemical reactions using models, word equations, and chemical equations. Students should investigate the concept of conservation of mass in chemical reactions and use the understanding of that concept to write balanced chemical equations for a variety of chemical reactions. Throughout the unit, students should continue to extend their understanding of the structure, patterns, and predictive nature of the periodic table.

Many students find the concept and process of balancing chemical equations to be difficult. This difficulty can be minimized by providing the students with multiple opportunities to manipulate physical models of elements and compounds, enabling students to physically rearrange (but not add or subtract) atoms for each chemical reaction studied.

The concepts of reaction type (synthesis, decomposition, single replacement, and double replacement) and activity series are addressed in Chemistry and need not be introduced at this grade.

Learning Objectives

Represent common chemical compounds using models.
Name and write formulas for common ionic compounds using the periodic table and a list of ions. (COM)
Name and write formulas for common molecular compounds using the periodic table and a list of numerical prefixes.
Describe the usefulness of scientific nomenclature systems such as the International Union of Pure and Applied Chemistry (IUPAC) naming conventions.
Reflect upon how knowledge develops and changes in science. (CCT)
Value the contributions made by women and men from many societies and cultural backgrounds in the development of international standards in chemistry. (PSD)
Represent chemical reactions using word equations.
Design an experiment to investigate the Law of Conservation of Mass, identifying and controlling major variables. (CCT)
Represent chemical reactions and conservation of mass using models.
Represent chemical reactions and conservation of mass using balanced chemical equations. (NUM)

Enrichment Learning Objectives

Investigate and identify properties of ionic and covalent compounds.
Conduct tests to determine the presence of ions in solutions.
Indicate substance state symbols ( s, l, g, aq ) in chemical equations.

Key Questions

Why have scientists worldwide adopted common naming systems for chemicals?
How and why do scientists represent chemical reactions symbolically?
What is the Law of Conservation of Mass and why is it a foundation of chemistry?
How does the concept of conservation of mass relate to the concept and process of balancing chemical equations?
What does it mean to balance a chemical equation?

Key Concepts

An ion is an atom that has become charged by gaining or losing one or more electrons.
An anion is a negatively charged ion, the result of gaining one or more electrons.
A cation is a positively charged ion, the result of losing one or more electrons.
A covalent bond is the bond formed by two or more atoms sharing one or more pairs of electrons.
A molecular compound is a neutral compound composed of two or more non-metallic elements held together by covalent bonds.
A polyatomic ion is composed of two or more non-metallic atoms bonded together covalently.
An ionic bond is the bond formed by the transfer of electrons from one atom (usually a metal) to another (usually a non-metal).
An ionic compound is a neutral compound that consists of positive and negative ions held together by an ionic bond.
A word equation identifies the reactants and products in a chemical reaction using only the names of the elements and compounds.
A chemical equation represents the reactants and products in a chemical reaction using their symbols or formulas.
The Law of Conservation of Mass states that in a chemical reaction the total mass of reactants is equal to the total mass of the products.
A balanced chemical equation represents the identities and relative amounts of reactants and products in a chemical reaction. The total number of each type of atom remains the same.
Designing experiments involves planning a series of data-gathering operations that will provide a basis for testing a hypothesis or answering a question.
A scientific law is a statement that summarizes an observed pattern in nature.

Pre-Instructional Questions

Are students familiar with models of atomic structure that include protons, electrons, and neutrons?
Are students aware of the basic structural features of the periodic table (i.e., atomic number, atomic mass, families, and groups)?
Are students able to identify which elements are metals and which are non-metals using a periodic table?
Do students know the difference between an atom and an ion?
Are students familiar with numerical prefixes up to 10 (deca)?
Do students know the difference between a subscript, a superscript, and a coefficient?
Do students know the eight elements that occur in nature as diatomic molecules (H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂, and At₂)?
Are students able to identify the products and reactants in word equations?
Are students able to represent elements and compounds using models?
Do students understand the concept of conservation of mass?

Suggested Teaching Strategies and Activities

Students should construct models of ionic and molecular compounds using materials such as Styrofoam balls, paper clips, marbles, marshmallows, Smarties, or commercial molecular model kits. Students could label the model to show the compound name, element(s), chemical formula, and common uses. Their models should demonstrate that the chemical formula indicates the number and type of each atom present in the compound (e.g., Al₂O₃ consists of 2 aluminum atoms and 3 oxygen atoms).
Students should write word equations for common chemical reactions based on written descriptions of the reaction (e.g., aluminum metal reacts with hydrochloric acid to form aluminum chloride and hydrogen gas can be written as aluminum + hydrochloric acid => aluminum chloride + hydrogen). Students should be able to identify the reactants and products of the chemical reaction from the word equation. (CCT)
Students should name and write formulas for common molecular compounds (e.g., CCl₄ - carbon tetrachloride, N₂O₃ - dinitrogen trioxide, etc.) using prefixes and a periodic table. Students should be able to memorize and use prefixes from 1 (mono) to 10 (deca). Students should be able to describe why the use of prefixes is essential to the naming of molecular compounds.
Students should name and write formulas for common ionic compounds using a periodic table and an ion chart. The ion chart should contain names of common simple ions (e.g., Al³⁺, Cl^-, etc.), polyatomic ions (e.g., NH₄⁺, CO₃^2- , NO₃^- , etc.), and ions that have multiple oxidation numbers (e.g., Fe²⁺, Fe³⁺, etc.). Students should be able to use the Stock system (e.g., iron (II), iron (III), etc.) for naming ionic compounds that are composed of substances that have multiple oxidation numbers. Students are not expected to use the classical system for naming ionic compounds (e.g., ferrous, ferric, etc.) or memorize lists of ions and charges. Students should be able to describe why prefixes are not required for naming ionic compounds although prefixes are required for naming molecular compounds.
Students should identify the name and chemical formula of chemical compounds that have common rather than systematic names (e.g., water, ammonia, sugar, baking soda, chalk, limewater, muriatic acid, potash, salt, bleach, battery acid, vinegar, Vitamin C, and pencil lead). Students should discuss why these substances have the names that they do and the disadvantages of using common names rather than systematic names. The term "common names" generally refers to compounds whose names were adopted before the development of formal nomenclature systems such as those developed by IUPAC. Some resources refer to these as "trivial names".
Students could research the historical development of naming systems in chemistry and identify the key contributors to the international naming systems. Students should explain how these systems have developed and why they are important as a communication tool. Students should note that there are still discrepancies among countries regarding the naming of new elements.
Students should design an experiment to determine whether mass is conserved in chemical reactions in both closed and open systems. Students should generate hypotheses, choose variables, collect appropriate data, and then conduct the experiment safely. Students should be able to explain how their data supports or refutes the Law of Conservation of Mass. Students could describe implications of the law in practical situations (e.g., wood burning, bread baking, swallowing an Alka-Seltzer, industrial processes). (NUM, TL, CCT)
Students should represent chemical reactions using word equations, chemical equations, and balanced chemical equations. Students should be able to convert a word equation into a chemical equation, convert a chemical equation into a word equation, and balance a chemical equation. The use of physical models (e.g., Styrofoam balls, paper clips, marbles, marshmallows, Smarties, or commercial molecular model kits) to represent individual atoms in chemical reactions enables students to readily see that the numbers of atoms on each side of a balanced chemical equation must be equal. Students should practise balancing equations until students clearly demonstrate understanding that mass is conserved in chemical reactions and that atoms are neither created nor destroyed in chemical reactions.
Enrichment: Students could represent chemical reactions in ways that require students to be able to predict the products from the reactants. This will help increase students' understanding of a balanced chemical reaction and increase their ability to write balanced chemical equations.