Word Problems: Predicting Products in Synthesis, Decomposition and Oxidation/Combustion reactions

Given the beginning of a chemical reaction, how can you figure out what will be made, and if the reaction will actually happen?

Steps to follow when writing your own equations Determine the type of reaction (Synthesis, Decomposition, Single

Replacement, Double Replacement or combustion)

Use the reaction type information to predict the likely products Synthesis

A + B AB N2 + 3H2 2NH3

Elements combine to make a compound

Decomposition AB A + B 6FeS 6Fe + S6

A compound fall apart back into its ELEMENTS We will always decompose compounds all the way back to the elements.

Oxidation/Combustion AB + O2 AxOy + BxOy

4FeS + 7O2 2Fe2O3 + 4SO2

When the reactants involve carbon and hydrogen, CO2 and H2O will be your products

Steps to follow when writing your own equations – Continued

Always check to make sure that you properly write each product making sure that the charges are balanced. Every compound must have zero net charge.

For Synthesis, Decomposition and combustion reactions you may assume that the reaction proceeds as written.

Use coefficients to balance the equation.

Example Problem 1

Sodium carbonate is decomposed by heating…

Na2CO3 The problem tells you that it is a decomposition reactionABC A + B + CUse the elements as your products

____Na2CO3 ____Na + ____C + ____O2

Now, in this case oxygen is a very active product. It will attack both the sodium and carbon to produce a second reaction

Na + C + O2 Na2O + CO2

So the real reaction in this case is…

____Na2CO3 ____Na2O + ____CO2

Example Problem 2

Aluminum strips and oxygen gas are allowed to react…

Al + O2

Looks like a synthesis (two elements react)A + B AB

It could also be called a combustion reaction too!A + O2 AxOy

Al +O2 AlORemember that Al becomes an Al+3 ion and the oxygen ion is O-2

Be sure that the charges in the product cancel out.

_____Al +______ O2 ______Al2O3

Example Problem 3

Cellulose, C6H10O5, is burned in air…

“Burned in air” is code for combustion or oxidationAB + O2 AO + BO C6H10O5 + O2 When C and H are reactants, CO2 and H2O are the products

_____ C6H10O5 + ______ O2 ______ CO2 + _____ H2O

Try these two problems on your own

Practice Problem A

Isopropanol, C3H8O combusts in air…

Practice Problem B

A chunk of lithium metal is left on a lab table exposed to the air…

Pause the video and try these two problems on your own