a shorter, easier way to show chemical reactions, using symbols, not words, for the reactants and the products

hydrogen molecules react with oxygen molecules to form water molecules

H2 + O2 H2O reactant + reactant “YIELDS” products

total mass of reactants must equal total mass of products matter is NOT destroyed or created in a chemical reaction =

ELEMENTS – represented by a one or two-letter

symbol

(letter) C Na H COMPOUNDS– represented by a

chemical formula

uses

subscripts

(word) which to show the ratio of elements in the compound H 2 O 2 CO 2 C 3 H 8 O

coefficient

HOW MANY? in front of the chemical formula tells “how many” atoms or molecules 3H 2 O 2 4CO 2 2C 3 H 8 O

• • • • •

Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H) (note: only time a subscript can & must be changed) Count the number of atoms that are present in the basic equation and record Pick an element that is not balanced on both sides of the equation ( H & O last) Add a coefficient in front of the chemical formula to adjust the count and record the new values Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation (change coefficient, then change counts)

H + O H

2

O

• • • • •

Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H) (note: only time a subscript can & must be changed) Mg + O Count the number of atoms that are present in the basic equation and record Pick an element that is not balanced on both sides of the equation ( H & O last) C + Cl Add a coefficient in front of the chemical formula to adjust the count and record the new values Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation (change coefficient, then change counts) MgO CCl 4

Mg + O

2

------> MgO C + Cl

2

------> CCl

4

Al + O

2

Al

2

O

3

Note: add like elements on the same side of the equation

SO

2

+ O

2

SO

3

• • • • •

Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H) (note: only time a subscript can & must be changed) Count the number of atoms that are present in the basic equation and record Pick an element that is not balanced on both sides of the equation ( H & O last) Add a coefficient in front of the chemical formula to adjust the count and record the new values Al + O Al 2 O 3 SO 2 + O SO 3 Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation (change coefficient, then change counts)

SYNTHESIS – two or more substances combine to make a more complex substance 2 SO 2

AB + C ABC (synthesize means to put together)

+ O 2 + 2 H 2 O 2 H 2 SO 4 Synthesis of Acid Rain DECOMPOSTION - a complex substance breaks down into simpler substances

ABC AB + C (decompose means to break down)

2 H 2 O 2 2 H 2 O + O 2 Decomposition of Hydrogen Peroxide REPLACEMENT – two elements in two different compounds trade places

AB + CD AC + BD (replace means to find a new partner)

2 CuO + C 2 Cu + CO 2 Isolating Copper by Replacement