7-1 Notes: Arranging the Elements

9-2 Notes: Chemical Formulas and Equations

All substances are formed from about 100 elements. Each element has its own chemical symbol, which is found in the periodic table. A chemical formula is a shorthand way to use chemical symbols and numbers to represent a substance. A chemical formula shows how many atoms of each kind of element are present in a molecule.

The chemical formula for water is H2O. This formula tells you that one water molecule is made of two hydrogen atoms and one oxygen atom. The small 2 in the formula is a subscript. A subscript is a number written below and to the right of a chemical symbol in a formula. If there is no subscript, only one atom of that element is present.

Simple covalent compounds are usually composed of two nonmetals. If the name of a compound contains the name of a metal and the name of a nonmetal, the compound is ionic.

A chemical equation uses chemical symbols and formulas as a short way to describe a chemical reaction. The starting materials in a chemical reaction are reactants. The substances formed from a reaction are products. The formulas of the reactants are written before the arrow and the formulas of the products are written after the arrow.

Atoms are never lost or gained in a chemical reaction. They are just rearranged. When writing a chemical equation, make sure that the total number of atoms of each element in the reactants equals the total number of atoms of that element in the products. This process is called balancing the equation.

A French chemist, Antoine Lavoisier, found that the total mass of the reactants was always equal to the total mass of the products. The law of conservation of mass states that mass is neither created or destroyed in chemical and physical changes. So, a chemical equation must show the same numbers and kinds of atoms on both sides of the arrow even though the atoms are rearranged.

You must use coefficients to balance an equation. A coefficient is a number that is placed in front of a chemical symbol or formula. For example, 2CO represents two carbon monoxide molecules.

For an equation to be balanced, all atoms must be counted. So, you must multiply the subscript of each element in a formula by the formula’s coefficient. Only coefficients – not subscripts – are changed when balancing equations.

Unit 4 (Ch. 9) – Reactions/Chem. Living Systems Bach 11/03/09 Section 9-2 Review 1. What is the difference between a chemical formula and a chemical equation? A chemical formula is a combination of chemical symbols and numbers to represent a substance. A chemical equation is a representation of a chemical reaction that uses symbols to show the relationship between the reactants and the products.

2. In the following chemical equation, put a circle around all subscripts and a box around all coefficients:

2SO2 + O2  2SO3

3. For the following chemical formulas, identify the elements in each formula and write how many atoms of each element are present in each molecule: a. NaCl Sodium – 1 atom; Chlorine – 1 atom

b. NH3 Nitrogen – 1 atom; Hydrogen – 3 atoms

c. MgSO4 Magnesium – 1 atoms; Sulfur – 1 atoms; Oxygen – 4 atoms

4. What is the difference between a reactant and a product? A reactant is a substance or molecule that participates in a chemical reaction. A product is a substance that forms in a chemical reaction.

5. What is the law of conservation of mass? The law that states that mass cannot be created or destroyed in ordinary chemical and physical changes.

6. Balance the following chemical equations by adding coefficients:

a. HCl + NaOH  H2O + NaCl

2HCl + 2NaOH  2H2O + 2NaCl

b. Fe2O3 + C  Fe + CO2

2Fe2O3 +3C  4Fe + 3CO2

Unit 4 (Ch. 9) – Reactions/Chem. Living Systems Bach 11/03/09 9-2 Notes: Chemical Formulas and Equations

All substances are formed from about 100 ______. Each element has its own chemical ______, which is found in the periodic table. A ______is a shorthand way to use chemical symbols and numbers to represent a substance. A chemical formula shows how many ______of each kind of element are present in a ______. The chemical formula for water is ______. This formula tells you that one water ______is made of two hydrogen atoms and one oxygen atom. The small 2 in the formula is a ______. A subscript is a ______written below and to the right of a chemical symbol in a formula. If there is no subscript, only ______atom of that element is present. Simple covalent compounds are usually composed of two ______. If the name of a compound contains the name of a ______and the name of a nonmetal, the compound is ______. A chemical ______uses chemical symbols and formulas as a short way to describe a chemical ______. The starting materials in a chemical reaction are ______. The substances formed from a reaction are ______. The formulas of the reactants are written before the ______and the formulas of the products are written after the arrow. ______are never lost or gained in a chemical reaction. They are just ______. When writing a chemical equation, make sure that the total number of atoms of each element in the ______equals the total number of atoms of that element in the ______. This process is called ______the equation. A French chemist, Antoine Lavoisier, found that the total ______of the reactants was always equal to the total ______of the products. The law of ______of ______states that mass is neither created or destroyed in chemical and physical changes. So, a chemical equation must show the same numbers and kinds of ______on both sides of the arrow even though the ______are rearranged.

You must use ______to balance an equation. A coefficient is a ______that is placed in front of a chemical symbol or formula. For example, 2CO represents two carbon monoxide ______. For an equation to be ______, all atoms must be counted. So, you must multiply the ______of each element in a formula by the formula’s coefficient. Only ______– not subscripts – are changed when balancing equations.

Unit 4 (Ch. 9) – Reactions/Chem. Living Systems Bach 11/03/09 Section 9-2 Review 1. What is the difference between a chemical formula and a chemical equation?

2. In the following chemical equation, put a circle around all subscripts and a box around all coefficients:

2SO2 + O2  2SO3

3. For the following chemical formulas, identify the elements in each formula and write how many atoms of each element are present in each molecule: a. NaCl

b. NH3

c. MgSO4

4. What is the difference between a reactant and a product?

5. What is the law of conservation of mass?

6. Balance the following chemical equations by adding coefficients:

a. HCl + NaOH  H2O + NaCl

b. Fe2O3 + C  Fe + CO2

Unit 4 (Ch. 9) – Reactions/Chem. Living Systems Bach 11/03/09