Describing Chemical Reactions The process by which the atoms of one or more substances are rearranged to form different substances is called a chemical reaction.

Indications of a Chemical Reaction 1. Change in Temperature -Exothermic- Heat Released Ex: Freezing, ice formation Phase change from liquid to solid -Endothermic- Heat absorbed Ex: Melting of ice, Phase change from liquid to gas

2. Gas production Bubbles, fizzing 3. Precipitate formed- A solid appears, “falls down” after two solutions are mixed together.

Writing Chemical Equations In writing chemical equations, the LAW OF CONSERVATION OF MASS must be satisfied. The same number of atoms of each element must appear on each side of the the equation.

How do you equalize the numbers of atoms on each side of the equation? You add a COEFFICIENT!

What is a coefficient? A coefficient is a small whole number that appears in FRONT of a formula in a chemical equation. Placing a coefficient in front of a formula specifies the relative number of MOLES of the substance: if no coefficient is written the coefficient is assumed to be 1.

WHAT IS A WORD EQUATION? A WORD EQUATION is an equation in which the reactants and products in a chemical reaction are represented by words. methane + oxygen → carbon dioxide + water The arrow → is read as react to yield, or yield, forms, produces, makes, etc.

Methane + oxygen → carbon dioxide + water CH 4 + O 2 → CO 2 + H 2 O (not balanced) CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O (balanced)

STEPS FOR BALANCING EQUATIONS Step 1 Write the skeleton equation for the reaction. H 2(g) + Cl 2(g) → HCl g 2 hydrogen atoms + 2 chlorine atoms → 1 hydrogen atom 1 chlorine atom