Introduction to Chemical Reactions and Equations

Recognizing Chemical Reactions A CHEMICAL REACTION is a process in which one or more substances are changed into new* substances. “New” simply means substance that were not there before the reaction!

Chemical Equations A chemical reaction cannot be seen; it occurs on the SUBMICROSCOPIC LEVEL! Since reactions can’t be seen, we use a CHEMICAL EQUATION to represent a CHEMICAL REACTION.

All CHEMICAL EQUATIONS have THREE BASIC PARTS: A CHEMICAL EQUATION is a WRITTEN EXPRESSION that illustrates what happens during a chemical reaction. All CHEMICAL EQUATIONS have THREE BASIC PARTS: REACTANT(S) PRODUCT(S) One or more substances that enter the reaction, separated by PLUS SIGNS. The arrow is called a “YIELD SIGN”…it’s like an equal sign in a math equation. One or more new substances that are formed during the reaction, separated by PLUS SIGNS.

Type of Equations FORMULA EQUATIONS are written using the chemical symbols and formulas for substances. EXAMPLE Mg + S  MgS

WORD EQUATIONS are written using the names of the elements and compounds involved. EXAMPLE Potassium + Oxygen  Potassium Oxide

Equations MUST be BALANCED! Since a chemical equation illustrates what happens to atoms on the SUBMICROSCOPIC level, it must show exactly what happens to ALL ATOM INVOLVED! The LAW OF CONSERVATION OF MATTER, says MATTER CANNOT BE MADE OR DESTROYED.

Consider this equation: Na + Cl2  NaCl Because of this law, we must write BALANCED equations: equations that have the SAME NUMBER of EACH ATOM on BOTH SIDES of the equation. Consider this equation: Na + Cl2  NaCl Are there the SAME NUMBER of SODIUM and CHLORINE atoms on BOTH SIDES of the equation? NO! There are 1 sodium and 2 chlorine atoms on the reactant side, but only 1 sodium and 1 chlorine on the product side…this is NOT BALANCED!

Balancing an Equation Balancing an equation is easy. We can only add numbers BEFORE the formulas to adjust the number of atoms on each side….we CANNOT change subscripts! These “numbers” are called COEFFICIENTS Coefficients MULTIPLY the number of each kind of atom in a formula!

*****USE A PENCIL!!****** Let’s balance our equations now: Na + Cl2  NaCl 1. Count the number of EACH atom on both sides: There is 1 Na atom and 2 Cl atoms on the left side. There is 1 Na atoms and 1 Cl atom on the right side. Start with the first unbalanced atom and try adding a coefficient to balance it. 3. Continue adding coefficients as needed until all atoms are balanced in number on both sides. *****USE A PENCIL!!****** 2 2

Try balancing these equations: K + O2  K2O Al + S  Al2S3 C + Cl2  CCl4

4 K + O2  2 K2O 2 Al + 3 S  Al2S3 C + 2 Cl2  CCl4

How do you know when a chemical reaction has occurred? There are several factors that identify a chemical reaction: ENERGY is given off as HEAT or LIGHT, such as burning, or an explosion. A GAS is formed. A PRECIPITATE forms [a solid that settles to the bottom]. A change in COLOR or ODOR may indicate a reaction, but not always [can be physical].