Embedded Files
Unit 2 Learning Objectives
Unit 2 Learning Objectives
Introduction
This unit bundles Student Expectations addressing chemical formulas and chemical reactions. Chemical formulas reveal the composition of a substance. Properties of matter change when substances are chemically combined, separated, or rearranged. All of the atoms in a chemical reaction can be accounted for before and after the reaction.
Prior to this Unit
Grade 6
6.5A – Know that an element is a pure substance represented by a chemical symbol and that a compound is a pure substance represented by a chemical formula.
6.5C – Identify the formation of a new substance by using the evidence of a possible chemical change such as production of a gas, change in temperature, production of a precipitate, or color change.
Grade 7
7.6A – Distinguish between physical and chemical changes in matter.
Grade 8
8.5A – Describe the structure of atoms, including the masses, electrical charges, and locations, of protons and neutrons in the nucleus and electrons in the electron cloud.
8.5B – Identify that protons determine an element's identity and valence electrons determine its chemical properties, including reactivity.
8.5C – Interpret the arrangement of the Periodic Table, including groups and periods, to explain how properties are used to classify elements.
During this Unit
Students learn how formulas are used to reveal the composition of substances and indicate the number of atoms of each element in a substance. They analyze chemical formulas to determine the numbers of atoms of each element present in a compound. Students use scientific practices and a variety of tools to investigate how evidence of chemical reactions indicate that new substances with different properties are formed and the relationship between chemical reactions and the law of conservation of mass. Students construct models of chemical formulas and chemical reactions and identify the advantages and limitations of models. Additionally, students communicate and discuss their observations and record and organize data in their notebooks. Students continue to demonstrate safe practices as outlined in the Texas Education Agency-approved safety standards, and consider environmentally appropriate and ethical practices with resources during investigations.
Note:
There are three perspectives that can be used to demonstrate an understanding of chemical reactions and their relationship with the law of conservation of mass, the macro level, the micro level, and scientific conventions.
Macro level investigations of chemical reactions and the conservation of mass will be the most familiar with students since they are directly related to the experiences students can have in the classroom. Students can determine the total mass in grams of the reactants, observe evidence of chemical reactions, and determine the mass in grams of the products. Students can verify that the total mass of reactants and products are equal in reactions occurring in a closed system to demonstrate the law of conservation of mass. Students can account for lost or gained mass in reactions occurring in an open system. For example, if a gas is produced and released into the atmosphere, a student can mass the remaining solid or liquid product and determine the mass of the released gas.
An understanding of the micro level perspective of chemical reactions and the conservation of mass comes from conceptual instruction of the phenomena. Students use models of chemicals and demonstrate changing bonds between atoms. Students can be expected to understand that the numbers of atoms in a chemical reaction will not change and only the arrangement of the atoms and bonds change in the chemical reaction to adhere to the law of conservation of mass.
An understanding of the scientific conventions used to illustrate chemical reactions and the law of conservation of mass is only partially expected in the streamlined TEKS. Students are no longer expected to recognize balanced chemical equations. However, an understanding of chemical formulas as a convention for representing chemicals can still be expected. Students can still be expected to recognize that a chemical equation is a representation of a chemical reaction and can be used to assess student’s conceptual micro level understanding or macro level calculations to demonstrate conservation of mass. For example, a student could be shown a balanced chemical equation with the mass of each reactant indicated and the mass of one product indicated. The student could be expected to find the unknown mass of a second product to demonstrate the relationship between chemical reactions and the law of conservation of mass (e.g., Grade 8 Science STAAR Spring 2016 item # 30).
After this Unit
In high school, students will study chemical formulas, chemical reactions, chemical equations, and the law of conservation of mass in more detail.
Additional Notes
STAAR Note
The Grade 8 Science STAAR will directly assess Student Expectations in the following Reporting Categories:
Reporting Category 1: Matter and Energy
Research
“By the end of 8th grade, students should know that:
Atoms may link together in well-defined molecules, or may be packed together in crystal patterns. Different arrangements of atoms into groups compose all substances and determine the characteristic properties of substances. 4D/M1cd*
An important kind of reaction between substances involves the combination of oxygen with something else—as in burning or rusting. 4D/M6b*
No matter how substances within a closed system interact with one another, or how they combine or break apart, the total mass of the system remains the same. 4D/M7a*
The idea of atoms explains the conservation of matter: If the number of atoms stays the same no matter how the same atoms are rearranged, then their total mass stays the same. 4D/M7b
Substances react chemically in characteristic ways with other substances to form new substances with different characteristic properties. 4D/M11** (NSES)
If samples of both the original substances and the final substances involved in a chemical reaction are broken down, they are found to be made up of the same set of elements. 4D/M12**
The idea of atoms explains chemical reactions: When substances interact to form new substances, the atoms that make up the molecules of the original substances combine in new ways. 4D/M13**”
American Association for the Advancement of Science. (2009). Benchmarks on-line. Retrieved from http://www.project2061.org/publications/bsl/online/index.php?chapter=4#D3.
What Are Chemical Formulas?
What Are Chemical Formulas?
A chemical formula is a notation used by scientists to show the number and type of atoms present in a molecule, using the atomic symbols and numerical subscripts. A chemical formula is a simple representation, in writing, of a three dimensional molecule that exists. A chemical formula describes a substance, down to the exact atoms which make it up.
Scientists use chemical formulas to express chemical equations, which are the method by which chemical reactions are expressed as the chemicals used (reactants) the yield (as the reaction occurs) and the new chemical substances made after the reactions (products).
In order for a chemical reaction to take place, the initial chemicals used must be bonded into larger groups called compounds. These compounds act like the ingredients used in cooking a recipe. Once you have all the ingredients needed, in the correct quantities, the reaction takes place. After the reaction is completed, one or more new substances are created.
The Law of Conservation of Mass
The Law of Conservation of Mass
The Law of Conservation of Mass is the universal standard which states that matter can neither be created, nor destroyed, only rearranged or transformed into energy. In chemical reactions, all atoms of all atoms MUST be equally represented on both side of an chemical equation. The elements and amount of atoms used will rearrange into new compounds or molecules.
What Exactly Happens During a Chemical Reaction?
What Exactly Happens During a Chemical Reaction?
All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. In terms of energy, there are two types of chemical reactions: endothermic reactions and exothermic reactions
In exothermic reactions, more energy is released when bonds form in products than is used to break bonds in reactants. These reactions release energy to the environment, often in the form of heat or light. In endothermic reactions, more energy is used to break bonds in reactants than is released when bonds form in products. These reactions absorb energy from the environment. The burning of wood is an example of an exothermic reaction. You can tell by the heat and light energy given off by a wood fire.
Page updated
Report abuse