Unit 6 Chemical Reactions

Objectives Write chemical equations to describe chemical reactions Balance chemical equations Classify and identify chemical reactions Predict products from chemical reactions

I. Reactions and equations Chemical reactions: A chemical change has occurred Evidence of a chemical reaction Temperature change (energy change) Change in color

Evidence of chemical change Gas production (bubbles, fizzing, odor) Formation of a solid (precipitate)

YouTube - ammonium dichromate volcano YouTube - Nitrogen Triiodide Detonation YouTube - Sodium and Water YouTube - Reaction of Sodium & Chlorine (with subtitles)

Representing chemical reactions: Reactants: starting substances (written on the left side) Products: substances that result from the reaction (written on the right side) An arrow is written between reactants and products and is read as “yields”

Symbols used in chemical equations (p 203) Meaning  Yield (placed between reactants and products) Separates reactants and products; indicates reversible reaction + Separate two or more reactants or products (s) Substance is in a solid state (l) Substance is in a liquid state (g) Substance is in a gaseous state (aq) Substance is in an aqueous (water) solution  Heat is applied A substance written above an arrow indicates it is a catalyst needed for the reaction Pt

Word Equations Skeleton equations Statements used to indicate reactants and products Lack important information Ex. Hydrogen (g) + oxygen (g)  water (l) Skeleton equations Chemical formulas are used to represent reactants and products Ex. H2 (g) + O2 (g)  H2O (l)

Learning Check: Write the skeleton equations for the following: Hydrogen (g) + Bromine (l)  hydrogen bromide (g) Carbon monoxide (g) + oxygen (g)  carbon dioxide (g)

Practice: Write the skeleton equations for the following: Hydrogen (g) + Bromine (l)  hydrogen bromide (g) H2 (g) + Br2 (l)  HBr (g) Carbon monoxide (g) + oxygen (g)  carbon dioxide (g) CO (g) + O2 (g)  CO2 (g) CW p 284 # 3, p 980 9.1 #1,2

II. Balancing chemical equations Must obey “law of conservation of matter” Must show that the number of atoms of each substance is the same before and after the reaction. A chemical equation has to be BALANCED To balance chemical equations, COEFFICIENTS (whole number)are written in front of a reactant or product. If coefficient is 1, it is not written

Steps for balancing equations Write the skeleton equation for the reaction. Ex. Hydrogen gas reacts with chlorine gas and yields gaseous hydrogen chloride. H2 (g) + Cl2 (g)  HCl(g) Count the atoms of each substance for the reactants and products. REACTANTS PRODUCTS H : H : Cl : Cl :

Place coefficients (ONLY IN FRONT OF A REACTANT OR PRODUCT) to make number of atoms of each element equal on both sides of the equation. (#atoms= coefficient x subscript) H2 (g) + Cl2 (g)  HCl(g) REACTANTS PRODUCTS H : H : Cl : Cl :

Write coefficients in lowest ratio possible H2 (g) + Cl2 (g)  2 HCl(g) Check your answer. REACTANTS PRODUCTS H : 2 H : 2 Cl : 2 Cl : 2

Practice 1. Write a balanced chemical equation if sulfur trioxide when heated yields sulfur dioxide and oxygen.

Write a balanced chemical equation for: Iron (III) oxide iron (II) oxide + oxygen

Balance the following reaction CaO + H2O  Ca(OH)2

Reactions for gummy bear sacrifice KClO3  KCl + O2

CW p balancing equations handout