Chemical Reactions

What You’ll Learn… How to write chemical equations to describe reactions How to classify and identify chemical reactions How to write ionic equations for reactions that occur in aqueous solutions

Reactions and Equations How to recognize evidence of chemical change How to represent chemical reactions with equations

Chemical Reaction: A process by which atoms of one or more substances are rearranged to form different substances A.K.A. chemical change

Evidence of Chemical Reactions Change of: Temperature: Exothermic – energy released (heat…hot and light) Endothermic- absorb energy (heat…cold) Color Production of: Odor Gas Solid (precipitate)

What are the clues that a chemical reaction has occurred?

Production of a gas http://www.youtube.com/watch?v=Mx5JJWI2aaw&safety_mode=true&persist_safety_mode=1

Production of a solid (a precipitate.) http://www.youtube.com/watch?v=r8zukkc1xCY&safety_mode=true&persist_safety_mode=1 Production of a solid (a precipitate.)

http://www. youtube. com/watch http://www.youtube.com/watch?v=kKlXe2mrnHQ&safety_mode=true&persist_safety_mode=1 Change of color

Heat and/or light are emitted.

The key to chemical reactions! The Law of Conservation of Mass In a chemical reaction, no atoms are created or destroyed. The atoms in the reacting molecules simply re-arrange to form new molecules. Since atoms aren’t created or destroyed, no mass is created or destroyed either. Mass is “conserved.”

Count the atoms, before & after 2H2 + O2 2H2O

Representing Chemical Reactions Reactants: Starting substances, usually on the left Products Substances formed, usually on the right

Terminology In a chemical reaction, the materials to the left of the arrow are the reactants. Reactants are consumed in the reaction. The arrow is like an equal sign in math. It can be read “yields” or “produces” or “forms”. The materials to the right of the arrow are the products. They form as the reaction proceeds. H2O + SO3 H2SO4

Representing Chemical Reactions Symbols Used in Equations Symbol Meaning + Separates two or more reactants or products  “yields” or “react to produce” Separates reactants and products (s) Solid state (l) Liquid state (g) Gas state (aq) “dissolved in water” Aqueous solution Representing Chemical Reactions reactant 1 + reactant 2  product 1 + product 2 Physical state can provide clues about how the reaction occurs

Phase Notation A chemical equation can also show the physical state of the materials in the reaction. This is done with phase notations: Phase notations are written to the lower right of each formula. Clues to phase notations are often given in equations written in words.

Other Phase Notations (ppt) or ( ) means “a precipitate forms” ( ) means “a gas bubbles off” means “is heated”

Representing Chemical Reactions Word Equations Ex. Iron(s) + chlorine(g)  iron(III) chloride(s) Chemical Equations  BALANCED!!! 2Fe(s) + 3Cl2(g)  2FeCl3(s)

Chemical Equation: A statement that uses chemical formulas to show identities and relative amounts of the substances in a chemical reaction MUST show that the law of conservation of mass is obeyed # atoms of each reactant and each product is equal on BOTH sides of the 

Balancing Chemical Equations Steps Step 1 Write the skeleton equation for the reaction Step 2 Count the atoms of the elements in the reactants Step 3 Count the atoms of the elements in the products Step 4 Change the coefficients to make the # atoms of each element equal on both sides Step 5 Write the coefficients in their lowest possible ratio Step 6 Check your work Balancing Chemical Equations Coefficient: # written in front of reactant or product Whole numbers Value of 1 not usually written

REMEMBER!! ALL chemical reactions obey the law that matter is neither created nor destroyed!

What else is conserved? In N2 + 3H2 2NH3 what besides mass (grams) is being conserved? Atoms? Molecules? Moles?

Coefficients In a chemical reaction, if more than 1 molecule of a substance reacts or is produced, the number of molecules is shown with a coefficient. 6CO2 + 6H2O C6H12O6 + 6O2 Coefficients multiply subscripts through the whole molecule that follows, so 6CO2 shows that there are 6 Carbon atoms and 12 Oxygen atoms.

How many? How many of each atom are shown by 4Al2(SO4)3 3(NH4)3PO4