React to Reactions

Alright after a long break between posts, lets review what we have covered thus far…

1. Reading and Writing Reactions – Voicethread

Example: N2 (g) + 3H2 (g) 2NH3(g)

To read or write out a reaction, we will first describe each part of the reaction. To do that you follow these rules:

  1. Each atom, molecule, or compound is described in moles. (If it is an atom, you describe it as “Elemental”; if it is a compound, you describe it using the proper name from our last unit)
  2. The Coefficient is equal to the number of moles of that substance
  3. To finish the statement, the state of matter is used.
  4. Each part of the reaction is connected to the others by a descriptor, which represents a symbol from the reaction (i.e. “Reacts with” & “To Form”)

So for the example listed above it would read as follows:
1 mole of elemental Nitrogen gas
Reacts with
3 moles of elemental Hydrogen gas
to form
2 moles of Ammonia* Gas (the * means it has a special name)

2. Balancing Reactions – Voicthread

To balance a reaction means to have equal parts, or each type of element, on either side of a reaction. The easiest way to do this process is by showing your work! Here’s how we do it:

Example: Ca3(PO4)2 +  C  → Ca3P2  + CO

Step 1:  list all the elements shown in the reaction below the reaction Arrow

  •                                  Ca3(PO4)2 +  C  → Ca3P2  + CO
  •                                                         ____  Ca ____
  •                                                          ____  P  ____
  •                                                          ____  O ____
  •                                                          ____  C ____

Step 2:  Write the number of each type of element, from that side of the reaction, on that side of the symbol.

  •                                                                                Ca3(PO4)2 +  C  → Ca3P2  + CO
  •                                                                                                          _3_  Ca _3__
  •                                                                                                         __2_  P  _2__
  •                                                                                                         __8_  O _1__
  •                                                                                                         __1_  C _1__

Step 3:  In order to make the numbers match on either side you can change the number of any type of element by changing the Coefficient of that element or molecule. You cannot ever change the subscripts. Changing the coefficient of a compound effects all of the elements in that compound.

  •                                                                                Ca3(PO4)2 + 8 C  → Ca3P2  + 8 CO
  •                                                                                                          _3_  Ca _3__
  •                                                                                                         __2_  P  _2__
  •                                                                                                         __8_  O _1_8
  •                                                                                                         8_1_  C _1_8

Important: If there is more than one of the same type of element on one side of the reaction arrow, you must add them together

3. Types of Reactions – Voicethread

There are 6 types of reactions. Knowing the type of reaction that will occur based on the reactants, allows us to predict the products. Here are the three types we have discussed and how to predict the products (Important: These reactions are not balanced. You must determine the products before you can determine the reactions)

Synthesis: Element + Element → 1 Ionic Compound

The important step when doing synthesis reactions is to determine the ionic charge of the reactants.

If the charges are the same → The subscripts of the elements in the compound are 1 to 1

Ca    +    S8     →    CaS
+2          -2

If the charges are different → Use the crossing rule (the charge of one, becomes the subscript of the other

Zn    +   N2  →    Zn3N2
+2         -3

Decomposition:  Compound  → Element + Element (+ Element …)

The important step when doing decomposition reactions is to determine Elemental State of each element. If it is Diatomic, it always has a subscript of 2, if it’s tetraatomic, it always has a subscript of 4, if it’s octoatomic it always has a subscript of 8.

Ca3(PO4)2  →   Ca +     P4 +     O2

Combustion: Hydrocarbon + O2  → CO2 + H2O

That’s it. As long as a compound that contains Carbon & Hydrogen reacts with Oxygen, the products will ALWAYS be Carbon Dioxide and Water. As easy as that sounds, these are the problems that are difficult to balance.

So that’s 2 weeks of work in a nutshell. We’ll continue on the other 3 types of reactions in the near future. AND, hopefully get into the lab sometime soon as well. Watch the voicethreads for more in-depth understanding of what we are covering! Good Luck!


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