Unit I: CHEMICAL REACTIONS I.5. Balancing Chemical Equations

Chemical Equations  Remember:  Formulas must BE CORRECT  Ionic – use charges   Angela goes fishing Chris uses the crisscross method   Others look at net overall charge Covalent [molecular] – use prefixes

Phases  Sometimes a chemical reaction will also show the phases in which the reactants and products exist     (s)

– solid

(l)

– liquid

(g)

– gas

(aq)

– aqueous (water solution)

Typical Reaction 2

Mg

(

s

) 

O

2 (

g

)  2

MgO

(

s

)

PRODUCTS(S) REACTANT(S) REACTS TO FORM

Typical Reaction

Phases

2

Mg

(

s

) 

O

2 (

g

)  2

MgO

(

s

)

Subscripts Coefficients

General Rules 1.

2.

3.

4.

5.

6.

Always check your formulas Count the number of each kind of atom. Treat polyatomics like atoms.

Put in coefficients to equalize atoms.

If an atom appears in more than one place on the same side of the equation, do it last.

Do ‘singletons’ LAST…

Balancing Equations Even-Odd Trick  An odd number x 2 = an even number

K + O

2 

K

2

O

Balancing Equations Even-Odd Trick

Ca + O

2 

CaO

Balancing Equations Two’s and Three’s

N

2

+ H

2 

NH

3

Balancing Equations Two’s and Three’s

Al + Cl

2 

AlCl

3

Balancing Equations PAI’s    Trick is to look for a PAI on both sides of the equation A PAI can be thought of as a single “thing” Start with the most complex compound

Balancing Equations PAI’s Ca(NO 3 ) 2 + Na 2 S  CaS + NaNO 3

Balancing Equations PAI’s Ca(NO 3 ) 2 + Na 2 SO 4  CaSO 4 + NaNO 3

Balancing Equations PAI’s AlBr 3 + SrCO 3  Al 2 (CO 3 ) 3 + SrBr 2

Balancing Equations PAI’s Fe(NO 3 ) 3 + (NH 4 ) 2 CO 3  Fe 2 (CO 3 ) 3 + NH 4 NO 3

Balancing Equations PAI’s AgNO 3 + K 2 CrO 4  Ag 2 CrO 4 + KNO 3

Hebden  Do Exercises 11, 13, 14, 16, 17, 18, 20, 21, 23, 25 on page 110-111

Balancing Equations C, H, O  Balance in order  C then H then O 

C

an’t

H

ave O rder

Balancing Equations C, H, O CH 4 + O 2  CO 2 + H 2 O

Balancing Equations C, H, O C 2 H 6 + O 2  CO 2 + H 2 O C

To get whole number coefficients = Must get rid of the fraction so you times all coefficients by 2

2 H 6 + O 2  CO 2 + H 2 O

Balancing Equations C, H, O C 4 H 10 + O 2  CO 2 + H 2 O C 4 H 10 + O 2  CO 2 + H 2 O

Balancing Equations C, H, O C 24 H 50 + O 2  CO 2 + H 2 O C 24 H 50 + O 2  CO 2 + H 2 O

Tricky Question #18 Ca(OH) 2 + NH 4 Cl  NH 3 + CaCl 2 + H 2 O

Balancing Equations C, H, O- alcohols CH 3 OH + O 2  CO 2 + H 2 O

Balancing Equations C, H, O- alcohols CH 3 OH + O 2  CO 2 + H 2 O

Balancing Equations C, H, O- alcohols CH 3 OH + O 2  CO 2 + H 2 O

Balance These C 14 H 29 OH + O 2  C 6 H 12 O 6 + O 2  CO 2 + H 2 O CO 2 + H 2 O C 9 H 50 O 4 S 2 + O 2  CO 2 + SO 2 + H 2 O

Worksheet  Do Problem Set #1 on page 4 in your Student Workbook