Objectives:Describe how chemical reactions

produce new substances with different properties. Identify four signs that a chemical reaction might be taking place. Explain

what happens to chemical bonds during a chemical

reaction.

•Chemical Reactionprocess of a substance completely changing to a different substance with different properties.

•Clues of a chemical reaction

•1. color change •2. gas formation •3. solid formation called a precipitate

• 4. energy given off as light, thermal, or electrical energy.

•Chemical reactions happen when molecules bump into each other causing the starting bonds to break apart, the atoms rearrange, and new bonds are formed

•Example:

CH4 + O2 → CO2 + H2O

•Objectives:• Interpret and write simple

chemical formulas.• Write and balance simple

chemical equations. • Explain how a balanced

equation shows the law of conservation of mass

•Chemical SymbolShorter way to write an element’s name.

•Chemical Formulacombination of chemical symbols and numbers to represent a substance. It an easier way to write a molecules and compounds.

•A chemical formula shows the number of atoms of each element in a molecule

•Examples:H2OSO4

Ca(OH)2

•Chemical formulas use subscripts and coefficients to show the number of atoms and molecules.

•Subscript the small # to the right of the element that tells the # of atoms:

•Examples:H2O SO4

Ca(OH)2

•Coefficients large # to the left of the element. Applies to the whole molecule!

EXAMPLES

•2H2O 4SO4

3Ca(OH)2

Writing Formulas for Covalent Compounds

You can use the name of a covalent compound to write its chemical formula. The names of covalent compounds use prefixes. Each prefix represents a number, as shown below.

mono- 1 hexa- 6

di- 2 hepta-

7

tri- 3 octa- 8

tetra- 4 nona- 9

penta- 5 deca- 10

How to Write Formulas for Covalent Compounds

1. name the first element

2. add a prefix if it has a subscript

3. name the second element

4. Add a prefix

5. Change ending of second element to “ ide ”

Let’s try some practice

1. CO2

2. CO3. SO2

4. SO3

5. N2O6. N2O3

7. NO8. CCl4

9. SiO2

1. Carbon dioxide2. Carbon monoxide3. Sulfur dioxide4. Sulfur trioxide5. Dinitrogen monoxide6. Dinitrogen trioxide7. Nitrogen monoxide8. Carbon tetrachloride9. Silicon dioxide

Exceptions

• Some important exceptions to this naming scheme occur because the compounds were originally named before the methodical naming scheme above became widespread.  Now a days, these names are so common that they're officially recognized:

• H2O is "water" PO4 is “phosphate”• NH3 is "ammonia" CLO3 is “chlorate”• CH4 is "methane“ NO3 is “nitrate”• SO4 is “Sulfate”• CO3 is “carbonate”• OH is “hydroxide”

Ionic Compounds

• To write the formula for an ionic compound, make sure the compound’s charge is 0.

• No prefixes…ends in ide

•Chemical Equationan easier and shorter way to write a chemical reaction using chemical symbols and formulas as a shortcut to describe a chemical reaction

•Example:

CH4 + O2 → CO2 + H2O

•Reactantswhat you start with

Productswhat you end with

•CH4 + O2 → CO2 + H2O

Reactants Products

•Law of conservation of Massstates that mass can not be created or destroyed

•An equation is balanced when the # of atoms in the reactants are = to the # of atoms in the products

•You balance an equation by changing the coefficients, not the subscript!

• Steps to balancing a chemical equation:1. Write the unbalanced equation on your paper.2. Draw a small line in front of each molecule.3. Draw boxes around all the chemical formulas.

• 4. Make an element inventory. 5. Write numbers in front of each of the boxes until the inventory for each element is the same both before and after the reaction

Example

•__ Fe + __S → __FeSElement Before After

Fe = ____ _____S = ____ _____

•__H2 + __O2 → __H2OElement Before After

H = ____ _____O = ____ _____

•__Mg + __O2 → __MgOElement Before After

Mg = ____ _____O = ____ _____

•_Ca + _H2O → _Ca(OH)2 + _H2

Element Before After

H = ____ _____O = ____ _____ Ca = ____ _____