Chemistry
November 21st, 2011
Brainteaser 11-21-11
Using your knowledge of chemical reactions, predict
what type of reaction the following equations are,
looking solely at the reactants:
H2 + O 2
HgO
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Zn + H2SO4
AgNO3 + NaCl
CH4 + O2
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Answers
H2 + O 2
HgO
Synthesis
Decomposition
Zn + H2SO4
AgNO3 + NaCl
CH4 + O2
Single Replacement
Double Replacement
Combustion
Predicting Products
You have to use the knowledge of what type of
reaction you have in order to predict your products.
There are 5 types of reactions I’ve taught you so far:
Synthesis
Decomposition
Single and Double Replacement
Combustion
Synthesis reactions
Start with what type of reactants you have. They will simple in form, and
usually elements.
Look at the base element and it’s charge.
Crisscross to get product.
Example:
Reactants: H2 + O2
Break into base elements and their charges:
(H+) + (O-2)
Last step is to criss-cross the charges to get subscripts for final
product.
Product: H2O
Balance the equation.
Decomposition
Starts with a single compound that decomposes into two
simple compounds/elements.
Look for the cation and the anion in compound.
Example
Reactant: HgO
Hg is cation (metal), O is anion (nonmetal).
Write your product by breaking apart the anion and
the cation. Don’t forget the special 7!!
Products: Hg + O2
Balance the equation.
Single Replacement
Starts with an element and compound, then switches and creates new element and
compound.
Cations will switch with cation, if single element is cation. Anion will switch with
anion if single element is anion.
Break compound into respective elements/ions and their charges, and switch
elements/ions for product.
Example:
Reactants: Zn + H2SO4
Break into elements/ions:
(Zn2+) + (H+)(SO42-)
Cations/anions switch! Crisscross for final compound. Don’t forget
special 7 for single element!
Products: H2 + ZnSO4
Balance the equation.
Double Replacement
Starts with two compound reactants, who switch partners and create two
new compound products.
Break compounds up into their elements/ions.
Anions will switch with each other, and cations will switch with each other.
DON’T FORGET TO CRISSCROSS IF CHARGES ARE DIFFERENT FOR
PRODUCTS!!
Example:
Reactants: AgNO3 + NaCl
Break into single elements/ions and charges and crisscross if needed
for final products:
(Ag+)(NO3-) + (Na+)(Cl-)
Products: NaNO3 + AgCl
Balance the equation.
Combustion
Will always start with carbon and hydrogen
containing (CxHy) compound, that reacts with
oxygen (O2).
Products formed are always carbon dioxide (CO2)
and water (H2O)
Example:
Reactants: CH4 + O2
Products: CO2 + H2O
Balance.
Let’s Practice
Predict the reactants for the following
equations:
MgBr2 + Cl2
Al + Fe2O3
NiCl2 + (NH4)2S
Let’s Practice
MgBr2 + Cl2
Answer: SINGLE REPLACEMENT
Element/Ion: (Mg2+)(Br-) + (Cl-)
Crisscross for product compound
Products: ____ + _____
Balanced Equation:
__MgBr2 + __Cl2 --> __MgCl2 + __Br2
Al + Fe2O3
Answer: SINGLE REPLACEMENT
Element/Ion: (___) + (___)(___)
Crisscross for product compound
Products: _____ + ___
Balance:
__Al + __Fe2O3 --> __Al2O3 + __Fe
NiCl2 + (NH4)2S
Answer: __________ REPLACEMENT
Element/Ion: (___) + (___)(___)
Crisscross for product compounds
Products: _____ + _____
Balance:
__ ____ + __ ____ --> __ ____ + __ ____