Chapter 6 : Reactions - LCHS Physical Science LCHS Main SiteSD271 Main siteUof I New Century Classroom  
Objectives: The learner will...
...identify types of chemical reactions
...balance the main types of chemical equations
Vocabulary:
decomposition : complex substance breaks down into 2 or more simpler substances
synthesis : 2 or more simpler substances combine to form a more complex substance
single replacement : uncombined element takes the place of a similar element in another compound
double replacement : similar atoms in separate compounds trade places with each other to form 2 new compounds
combustion : requires oxygen as a reactant and carbon dioxide and water are the 2 products
 


REACTION TYPES

Elements, compounds and molecules can recombine during reactions to form new products. Often these reactions occur because the new products tend to be more stable and better suited in their new combinations. 6 typical types of reactions are listed below with examples.

Reaction Type Example
Decomposition
AB => A + B		 2Al2O3 =>4Al + 3O2
H2O => H+ + OH-
Synthesis
A + B => AB		 4Al + 3O2 => 2Al2O3
H+ + OH- => H2O
Single Replacement
A + CB => AB + C
8Al + 3Fe3O4 => 9Fe + 4Al2O3
Mg + 2HCl => H2 +MgCl2
Double Replacement
AD + CB => AB + CD
MgCl2 + 2AgNO3 => 2AgCl + Mg(NO3)2
3H2S + 2AuCl3 => Au2S3
Combustion
CxHy + O2 => CO2 + H2O		 2C6H6 +15O2 => 12 CO2 + 6H2O
Acid/Base H+ + [OH]- => H2O
HxB + A[OH]y => AB + H2O		 3H2[SO4] + 2Al[OH]3 => Al2[SO4]3 + 6H2O

Decomposition
In a decomposition reaction a complex substance breaks down into 2 or more simpler substances. Decomposition is the reverse of the synthesis reaction.

Synthesis
In a synthesis reaction 2 or more simpler substances combine to form a more complex substance. Synthesis is the reverse of a decomposition reaction.

Single Replacement
An uncombined element takes the place of a similar element in another compound in a single replacement reaction.

Double Replacement
In a double replacement reaction similar atoms in separate compounds trade places with each other to form 2 new compounds.

Combustion
All combustion reactions require oxygen (O2) on the reactant side of the equation (left side). Carbon dioxide (CO2) and water (H2O) are the 2 products (right side) of a the combustion reaction.

Acid/Base
In an acid/base reaction an acid (compound always starts with H) and a base (compound always ends with [OH]) combine in a double replacement fashion to form a salt and water. If every H is neutralized with an [OH] then the pH of the final solution is 7 (neutral). Excess acid will have a pH less than 7 and excess base will have a pH greater than 7.


BALANCING REACTIONS

  1. Check the subscripts (e.g. Al2O3) on each molecule/compound for correctness first
  2. Once the subscripts are checked and are correct, DO NOT ALTER THEM AGAIN
  3. To balance go back and forth repeatedly on both sides of the equation and adjust the COEEFICIENTS ONLY
    (e.g. 4Al + 3O2 => 2Al2O3 ) until there are the same number of each atom on both sides of the equation.

You might go too far in your balancing efforts yet the chemical equation will still balance (e.g. 8Al + 6O2 => 4Al2O3 ). Just go back and check the coefficients to see if you can divide out a common factor (i.e. 2 in the previous example). Combustion reactions can be tricky at times to balance. Always balance the carbon atoms first, then the hydrogen, saving the oxygen for last. If a fractional coefficient is required to balance oxygen (e.g. 3.5 O2 ) you should go back and double the values of all the coefficients.

Practice balancing equations with this awesome Javascript!

 

 
Practice:
PB1
What type of Reaction is show below?
(1=Decomp 2=Syn 3=SingleRplc 4=DblReplc 5=Combust 6=Acid/Base)
H3[PO4] + Al[OH]3 -> Al[PO4] + 3 H2O
PB2
What type of Reaction is show below?
(1=Decomp 2=Syn 3=SingleRplc 4=DblReplc 5=Combust 6=Acid/Base)
BaF2 + Na2[SO4] -> Ba[SO4] + 2 NaF
PB3
What type of Reaction is show below?
(1=Decomp 2=Syn 3=SingleRplc 4=DblReplc 5=Combust 6=Acid/Base)
2 PbO2 -> 2 PbO + O2
PB4
Enter the missing subscripts (no spaces e.g. Ba1Cl2 is 12 )
Mg_F_ + Al 2 O 3
PB5
Enter the missing subscripts (no spaces e.g. Ba1Cl2 + C1H4 is 1214 )
K_Br_ + Ca_As_
PB6
Enter the missing COEFFICIENT(S)
2 Li 3 P 1 + 1 B 2 O 3 -> 3 Li 2 O 1 + _ B 1 P 1
PB7
Enter the missing COEFFICIENT(S)
1 Mg 3 P 2 + 1 Ga 2 S 3 -> _ Mg 1 S 1 + _ Ga 1 P 1
PB8
Enter the missing COEFFICIENT(S)
_ K 1 F 1 + _ Mg 3 As 2 -> _ K 3 As 1 + _ Mg 1 F 2
PB9
Enter the missing COEFFICIENT(S)
_ Li 1 Cl 1 + _ B 2 O 3 -> _ Li 2 O 1 + _ B 1 Cl 3
PB10
Enter the missing COEFFICIENT(S)
_ Na 2 Se 1 + _ Ga 1 Cl 3 -> _ Na 1 Cl 1 + _ Ga 2 Se 3
answer bank:
6
4
1
3261 		12
1132
2		 32
6123
6132

Scientist Spotlight:
Alfred Nobel 1833-1896 Swedish inventor who worked on manufacturing nitroglycerine.

Resources (Study Links/Study Tips/Reading Lists)
Practice balancing equations with this awesome Javascript!

Chapter 6 : Reactions - LCHS Physical Science LCHS Main SiteSD271 Main siteUof I New Century Classroom  
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