Download rules for predicting products of chemical reactions

RULES FOR PREDICTING PRODUCTS OF CHEMICAL REACTIONS
Here are a few important things to remember when predicting products:
- The compounds form must be neutral ionic compounds (which means you’ll be paying attention to their
charges)
- You do NOT carry subscripts from the reactants to the products.
- You always balance your equation LAST
PREDICTING REACTION PRODUCTS: COMBUSTION REACTIONS
- A hydrocarbon and oxygen (O2) indicate a combustion reaction
- If it is combustion, then just write H2O and CO2 as products
- Then, balance the equation (can be tricky)
Examples:
__CH4 + _2 O2  ___ CO2 + __2 H2O
_2 C4H10 + __13 O2  __8 CO2 + _10 H2O
PREDICTING REACTION PRODUCTS: REPLACEMENT REACTIONS
- A metal will not always replace a metal in a compound dissolved in water because of differing
reactivities
- An activity series can be used to predict if reactions will occur
- To replace a metal, the other metal must be MORE REACTIVE
- Same applies for halogens
PREDICTING REACTION PRODUCTS: SINGLE REPLACEMENT REACTIONS
- To predict the products: Replace the single element with the corresponding element in the
compound
- A metal will replace the cation in the compound
- A non-metal will replace the anion in the compound
- If the element is more reactive than the one it attempts to replace, then the reaction will be
spontaneous and exothermic
- Otherwise, it will not be spontaneous and will be endothermic (NO REACTION)
PREDICT IF THESE REACTIONS WILL OCCUR
3 Mg + 2 AlCl3  2 Al + 3 MgCl2
Can magnesium replace aluminum?
Activity Series- YES, magnesium is more reactive than aluminum.
Can aluminum replace magnesium?
Activity Series - NO, aluminum is less reactive than magnesium.
Therefore, no reaction will occur.
No reaction - MgCl2 + Al No reaction
The question we must ask is can the single element replace its counterpart? metal replaces metal or nonmetal
replaces nonmetal.
Order of reactants DOES NOT determine how they react.
PREDICTING REACTION PRODUCTS: DOUBLE REPLACEMENT REACTIONS
- If it is double replacement, then the cation and anions switch places
- It is helpful to separate each compound into their cation and anionic parts with their charges
- Then switch places, and criss cross the new compounds to make sure they are the correct neutral
compound formulas
- AB and CD are both ionic compounds
- Separate each compound & write their charges
- Switch places and re-write new compounds
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Example: Pb(NO3)2 + KI
Pb2+ NO3- K+ IPb2+ I- K+ NO3-
Pb(NO3)2 + 2KI  PbI2 + 2KNO3
Balance at the end!!
Not all double replacement reactions will occur
In order for a double replacement reaction to take place:
- Both of the reactants must be soluble in water
- If a compound contains at least one of the ions that is proven soluble, then the compound will be at
least moderately soluble
- One product must be soluble and one product must be insoluble
o The insoluble product is the precipitate that forms when the reaction takes place
o Usually a precipitate is a solid, but it doesn't have to be, could be an insoluble gas
Predict if a reaction will occur when you combine aqueous solutions of iron (II) chloride with aqueous
sodium carbonate solution.
- If the reaction does occur, write a balanced chemical equation showing it.
FeCl2(aq) + Na2CO3(aq)  2 NaCl(aq) + FeCO3(ppt)
Using a SOLUBILITY TABLE:
Both reactants are soluble
SO REACTION OCCURS!
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Using a Solubility table: NaCl is soluble and
FeCO3(ppt) is insoluble so reaction occurs!
Predicting Synthesis Products:
A + B  AB (Synthesis/Combination)
2 elements indicates synthesis
Write Charges for each then write compound - Na+ Cl- - NaCl - Balance Reaction
Metal reacts with oxygen to form metal oxide – 2Mg + O2  2MgO
Nonmetal reacts with oxygen to form nonmetallic oxide – C + O2 -CO2
Metal oxides react with water to make metallic hydroxides – MgO + H2O  Mg(OH)2
Nonmetallic oxide react with water to make an acid – CO2 + H2O  H2CO3
A metal react with non-metal to make a salt – 2Na + Cl2 -2 NaCl
A few non-metals combine with each other – 3 P+ 3 Cl2  2 PCl3
These two reactions must be remembered:
Predicting Decomposition Products: AB  A + B (Decomposition)
A single reactant indicates a decomposition reaction.
Separate to elements (remember diatomics!) or less complex compounds (we will be memorizing some
of these later!)
Examples:2MgO  Mg + O2
NH4OH  NH3 + H2
H2O2  H2O + O2
H2O  H2 + O2
CaCO3(s) --> CaO(s) + CO2(g)
2KClO3(s) --> 2KCl (s) + 3O2(g)
(COOH)2. 2H2O --> (COOH)2 + 2H2O
Some need to just be memorized or observed in a reaction.