Chemical Reactions

Chapter 2

Section 1 & 2

Physical Background• Physical Property: Characteristics of matter

that can be seen through direct observations• Density• Melting point• Boiling Point

• Physical Change: Change in which the identity of the substance does not change

Chemical Background• Chemical Property: characteristics of

matter that can only be observed when one substance changes into a different substance– Iron to Rust

• Chemical Change: New Substance is formed

A Change of Properties• The properties of these substances

are very different from the starting substances

• In a chemical reaction, bonds are broken. Atoms are then rearranged to form new substances

Signs of a Chemical Reaction

• Gas

• Solid

• Energy– Endothermic– Exothermic

• Color change

Chemical or Physical• Glass Breaking

• Hammering wood together

• Burning Leaves

• Melting Ice

• Frying an egg

• Melting butter

• Bleaching your hair

• Fireworks

• Mixing salt with water

Chemical Formula

• A shorthand way to use chemical symbols and numbers to represent a substance

• Shows how many of each atom is present in a molecule

• 3H2O• Subscript 2 – (applies to symbol before it)• Coefficient 3 – (applies to entire

molecule)

Be Accurate Symbols can be mistaken!!

• Use chemical symbols and formulas as a shortcut to describe a chemical reaction

• Reactants = starting material

• Products = substances formed from a reaction

C + O2 CO2

reactants before arrow products after arrow

Chemical Equations

Law of conservation of mass

• During a reaction, mass is neither created nor destroyed

• Equations must be balanced because atoms are neither created nor destroyed

Mass of Reactants = Mass of Products

How to balance an equation

Step 1

Count the number of atoms on each side

Step 2

You can only add or change coefficients to balance the number of atoms on each side.

Chemical Equation

• Shows the reaction between reactants and products

• Reactants-starting material

• Products-what is formed

• C + O2 CO2

• C=1 C=1

• O=2 O=2 http://www.explorelearning.com/index.cfm?method=cResource.dspView&ResourceID=408