Introduction to Chemical Introduction to Chemical ReactionsReactions and Equations and Equations

Recognizing Chemical ReactionsRecognizing Chemical Reactions

A A CHEMICAL REACTION CHEMICAL REACTION is is a a process in process in which one or more substances are which one or more substances are changed into new* substanceschanged into new* substances..

““NewNew”” simply means substance simply means substance that were not there before the reactionthat were not there before the reaction!!

Chemical EquationsChemical Equations

A chemical reaction cannot be seen; it A chemical reaction cannot be seen; it occurs on the occurs on the SUBMICROSCOPIC SUBMICROSCOPIC LEVELLEVEL!!

Since reactions canSince reactions can’’t be seen, we use a t be seen, we use a CHEMICAL EQUATIONCHEMICAL EQUATION to represent a to represent a CHEMICAL REACTION.CHEMICAL REACTION.

A A CHEMICAL EQUATIONCHEMICAL EQUATION is a is a WRITTEN WRITTEN EXPRESSIONEXPRESSION that illustrates what that illustrates what happens during a chemical reaction.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 EquationsType of Equations

FORMULA EQUATIONS FORMULA EQUATIONS are written using are written using the the chemical symbols and formulas chemical symbols and formulas for for substances.substances.

EXAMPLE EXAMPLE Mg + S Mg + S MgS MgS

WORD EQUATIONS WORD EQUATIONS are written are written using the using the names of the elements names of the elements and compoundsand compounds involved. involved.

EXAMPLE EXAMPLE

Potassium + Oxygen Potassium + Oxygen Potassium Oxide Potassium Oxide

Equations Equations MUST be BALANCEDMUST be BALANCED!!

Since a chemical equation illustrates what Since a chemical equation illustrates what happens to atoms on the happens to atoms on the SUBMICROSCOPICSUBMICROSCOPIC level, it must show exactly what happens to level, it must show exactly what happens to ALL ALL ATOM INVOLVEDATOM INVOLVED!!

The LAW OF CONSERVATION OF MATTER, says MATTER CANNOT BE MADE OR DESTROYED.

• Because of this law, we must write Because of this law, we must write BALANCEDBALANCED equations: equations that equations: equations that have the have the SAME NUMBER of EACH SAME NUMBER of EACH ATOM on BOTH SIDESATOM on BOTH SIDES of the equation. 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 EquationBalancing an Equation

Balancing an equation is easy. Balancing an equation is easy. We We can only add numbers BEFORE the can only add numbers BEFORE the formulasformulas to adjust the number of atoms on to adjust the number of atoms on each side….we each side….we CANNOT change CANNOT change subscriptssubscripts!!

These These ““numbersnumbers”” are called are called COEFFICIENTSCOEFFICIENTS

Coefficients Coefficients MULTIPLYMULTIPLY the number of the number of each kind of atom in a formula!each kind of atom in a formula!

• LetLet’’s balance our equations now:s balance our equations now:

Na + ClNa + Cl22 NaCl NaCl

1. Count the number of EACH atom on both sides:1. Count the number of EACH atom on both sides:

There is There is 11 Na atom and Na atom and 22 Cl atoms on the left side. Cl atoms on the left side.

There is There is 11 Na atoms and Na atoms and 11 Cl atom on the right side. Cl atom on the right side.

2.2. Start with the Start with the first unbalanced atom first unbalanced atom and try adding a and try adding a coefficient to balance it.coefficient to balance it.

3. Continue adding coefficients as needed until all atoms 3. Continue adding coefficients as needed until all atoms are balanced in number on both sides.are balanced in number on both sides.

******USE A PENCIL!!**********USE A PENCIL!!******

22

Try balancing these equations:Try balancing these equations:

K + OK + O22 K K22OO

Al + S Al + S Al Al22SS33

C + ClC + Cl22 CCl CCl44

44 K + O K + O22 22 K K22OO

22 Al + Al + 33 S S Al Al22SS33

C + C + 22 Cl Cl22 CCl CCl44

How do you know when a chemical How do you know when a chemical reaction has occurred?reaction has occurred?

There are several factors that identify a There are several factors that identify a chemical reaction:chemical reaction:

•ENERGYENERGY is given off as is given off as HEAT or LIGHTHEAT or LIGHT, such as , such as burning, or an explosion.burning, or an explosion.

•A A GASGAS is formed. is formed.

• A A PRECIPITATEPRECIPITATE forms forms [a solid that settles to the bottom].[a solid that settles to the bottom].

•A change in A change in COLOR or ODOR COLOR or ODOR may indicatemay indicate a reaction, but not always [can be physical].a reaction, but not always [can be physical].