1. Chapter 9 – Introduction to Equations Writing and Balancing Equations Section 9.1

2. Chemical Reactions One or more substances are rearranged to form different substances One or more substances are rearranged to form different substances Example: C 8 H 18 + O 2  CO 2 + H 2 O + ENERGY

3. Indicators of Chemical Reactions Animation of Experiment (GIF Format, 321 k) Animation of Experiment (GIF Format, 321 k) Temperature change Color change Light is produced Gas is produced (bubbles) Formation of a solid - precipitate

4. Representing Chemical Reactions Statements called equations are used to describe what happens during a chemical reaction Reactants are the starting substances in a reaction. The arrow points away from reactants. Products are the substances formed during a reaction. The arrow points toward the products. SymbolMeaning + Separates two or more reactants or products  Separates reactants from products (s) Solid state (l) Liquid state (g) Gaseous state (aq) Aqueous/water solution (solid dissolved in water)

5. What is a Chemical Equation? A chemical equation is like a recipe in that you have the ingredients and you make your product. In a chemical equation we call the ingredients reactants and what we form are called the products. The reactants and products come in various physical states. For instance, you can add melted butter to a recipe or solid butter. You can add solid, liquid, or gas reactants and you can make solid, liquid, or gaseous products.

6. Can you match the physical state of the compounds on the left with the correct symbol it represents on the right? _____ solidA. (aq) _____ liquidB. (g) _____ gasC. (s) _____ aqueous solutionD. (l)

7. Can you list all of the physical states of each compound in the chemical reaction below? NH 3 (g) + H 2 O(l)  NH 4 + (aq) + OH - (aq) NH 3 (g) _______________ H 2 O(l) ________________ NH 4 + (aq) _____________ OH - (aq) ______________

8. Can you label the reactants and the products? NH 3 (g) + H 2 O(l)  NH 4 + (aq) + OH - (aq) These are the ______

9. Word Equations and Skeleton Equations Word Equations -Statements that indicate the reactants and products in a rxn -Statements that indicate the reactants and products in a rxn -Describe chemical reactions -Describe chemical reactions -Lack important information -Lack important information Iron + chlorine  iron (III) chloride Skeleton Equations -Use chemical formulas to identify reactants and products Fe (s) + Cl 2 (g)  FeCl 3 (s)

10. Practice Problems Write skeleton equations for the following word equations: 1. hydrogen (g) + bromine (g)  hydrogen bromide (g) H 2 (g) + Br 2 (g)  HBr (g) 2. carbon monoxide (g) + oxygen (g)  carbon dioxide (g) CO (g) + O 2 (g)  CO 2 (g) 3. potassium chlorate (s)  potassium chloride (s) + oxygen (g) KClO 3 (s)  KCl (s) + O 2 (g)

11. Balancing Equations Mass/matter cannot be created nor destroyed Mass/matter cannot be created nor destroyed Atoms you start with (reactants) must still be there when you end (products) Atoms you start with (reactants) must still be there when you end (products) This is called balancing the equation This is called balancing the equation Mass of Reactants = Mass of Products Mass of Reactants = Mass of Products Chemical Equations also must obey the Law of Conservation of Mass. The number of atoms that are used in the reactants must equal the number of atoms found in the products. Matter can neither be created nor destroyed. A chemical equation that follows the law of conservation of mass is called a balanced chemical equation A Chemical Equation H 2 + O 2 -> H 2 0 A Balanced Chemical Equation Coefficient… -> 2H 2 + O 2 -> 2H 2 O

12. What happens in every chemical reaction?

13. Balancing Equations Mg + 2HCl  MgCl 2 + H 2 Big numbers = Coefficients Big numbers = Coefficients - can change Small numbers = Subscripts Small numbers = Subscripts - DO NOT CHANGE, if you change you no longer have the same substance

14. Balancing Chemical Equations There is a procedure for balancing chemical equations: There is a procedure for balancing chemical equations: 1. Write formulas for all compounds (sometimes given) 1. Write formulas for all compounds (sometimes given) (remember your diatomic molecules) 2. Separate reactants and products by a line 3. Count how many atoms of each element are present on the reactant and the product side (element inventory) 4. Add or adjust the coefficients so that element inventory is balanced 4. Add or adjust the coefficients so that element inventory is balanced (subscripts cannot be changed) 5. Reduce coefficients to lowest possible terms (simplify) 6. Check your work Hydrogen + Chlorine Yields Hydrochloric Acid H 2 + Cl 2 -> HCl

15. Which of the compounds in the equation is one of the 7 diatomic molecules? ______ CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g) List the other diatomic molecules (remember they make a 7 on the periodic table) ___________________________________________ __________________________________________ What are the physical states of the compounds in the reaction? _________________ Notice those little numbers in front of the O 2 (g) and the H 2 O(g)? Those are called coefficients. Coefficients help to balance out the atoms on both sides of the chemical reaction.

16. Balancing Equations Practice 1. Al + O 2 -> Al 2 O 4Al + O 2 -> 2Al 2 O 3 2. C 3 H 8 + O 2 -> CO 2 + H 2 O C 3 H 8 + 5O 2 -> 3CO 2 + 4H 2 O 3. KNO 3 -> KNO 2 + O 2 2KNO 3 -> 2KNO 2 + O 2 4. O 2 + CS 2 -> CO 2 + SO 2 3O 2 + CS 2 -> CO 2 + 2SO 2