Demo – Elephant Toothpaste What do you think is going to happen? What happens to the reaction once you drop in the catalyst? Based on your abservations is this a chemical reaction. If so, explain.

Chemical Reactions

Chemical Reactions A chemical reaction produces new substances by changing the way in which atoms are arranged. Bonds between atoms are broken and new bonds form between different atoms The new arrangements of atoms form different substances

Reactants and Products Reactants are the substances present at the beginning of a chemical reaction. Products are the substances formed by a chemical reaction Reactant + Reactant Product The mass of the reactants is always equal to the mass of the products

Detecting a Chemical Reaction Color Change Formation of a Precipitate Formation of a Gas Temperature Change Evolution of Light

Color Change Substance often change color during a chemical reaction. Example: when gray iron rusts, the product that forms is brown.

Formation of a Precipitate Precipitate: A solid product formed when chemicals in two liquids react. Example: Seashells are formed when sea creatures release a liquid that reacts with seawater.

Formation of a Gas Chemical reactions may produce a gas, like that often formed when antacid pills are mixed with excess stomach acid.

Most chemical reactions involve temperature change Example: Concrete gets warmer as it hardens

Evolution of Light Energy can be released in the form of light during a chemical reaction Example—Glow Sticks

Classifying Chemical Reactions Synthesis: A new compound is formed by the combination of simpler reactants Decomposition: a reactant breaks down into simpler products, which could be elements or other compounds (reverse synthesis) Combustion: one reactant is always oxygen and another reactant often contains carbon and hydrogen. Example: the burning of methane is a combustion reaction

Endothermic and Exothermic Reactions Exothermic Reactions: If more energy is released when the products form than is needed to break the bonds in the reactants then energy is released during the reaction Energy is released Endothermic Reactions: If more energy is required to break the bonds in the reactants than is released when the product is formed then energy must be added to the reaction Energy is absorbed

Chemical Equations Draw a line under your Chemical Reaction Notes, then start the Chemical Equation Notes

Chemical Equation Shorthand form for writing what reactants are used and what products are formed in a chemical reaction Sometimes shows whether energy is produced or absorbed Example: C6H12O6 + 6O2  6CO2 + 6H2O + energy

Chemical Equations Reactant + reactant  product

Components of a Chemical Equation Formulas Chemical Formula  2H2 + O2 2H2O Subscript Coefficient Coefficient Subscripts (Yield) (Products) (Reactants) Sometimes you will see a “yields” sign that looks like this. What do you think it means?

Law of Conservation of Mass Proposed by Antoine Lavoisier In a chemical reaction, atoms are neither created nor destroyed All atoms present in the reactants are also present in the products Chemical equations must account for/show the conservation of mass  balancing equations In its present form, does this chemical equation show a conservation of mass? 2 H2 + O2  2 H2O Reactants Products How would you balance this equation to show the conservation of mass? H  2 O  2 4 2 H  2 O  1 4 2

Hints For Balancing Equations Count the atoms List the number of atoms of each element to see which elements must be balanced Use a coefficient to add atoms to one side of the equation Start with the reactant or product that has the greatest number of different elements Add a coefficient to another reactant or product Make sure that the coefficients in your balanced equation are the smallest whole numbers possible (they should have no common factor other than one)

Tutorial on Balancing Equations http://users.wfu.edu/ylwong/balanceeq/balance q.html