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Chemical Reactions – The Basics

A chemical reaction is what occurs when elements combine into compounds, separate into elements, or rearrange from one compound into another.

Chemical reactions are represented by chemical equations. The reactants (starting chemicals) are shown the left and products (ending chemicals) are shown on the right.

Other Symbols

Symbol Meaning

(s) Solid chemical

(l) Liquid chemical

(g) Gas chemical

(aq) Chemical is dissolved in water (aquated)

Balancing a Chemical Equation

Basically a guess and check process.

The only thing you can change is the coefficient!

Coefficients are the numbers in front of a compound and get multiplied to everything in that compound.

Fe(s) + Cl2(g) → FeCl3(s)

Step 1: Add more Cl2

Fe(s) + 2Cl2(g) → FeCl3(s)

Step 3: Add more Fe and Cl2

2Fe(s) + 3Cl2(g) → 2FeCl3(s)

Step 2: Add more FeCl3

Fe(s) + 2Cl2(g) → 2FeCl3(s)

Advanced CountingAdvanced Counting

• 5O2 = 10 Oxygen

• NH4 = 1 Nitrogen, 4 Hydrogen

• 2NH4 = 2 Nitrogen, 8 Hydrogen

• 3(MnO4)2 = 6 Manganese, 24 Oxygen

Flipping the SubscriptsFlipping the Subscripts

• O5 O2 Use the subscripts as the 2O5 5O2 opposite one’s coefficient

Chunking Polyatomic IonsChunking Polyatomic IonsHg(MnO4)2 + Al2(Cr2O7)3 HgCr2O7 + Al(MnO4)3

• Notice that there are the same polyatomic ions on both sides. Instead of breaking them up, balance them together.

3Hg(MnO4)2 + Al2(Cr2O7)3 HgCr2O7 + 2Al(MnO4)3

Balanced MnO4 by flipping the subscripts

3Hg(MnO4)2 + Al2(Cr2O7)3 3HgCr2O7 + 2Al(MnO4)3

Changing the number at the beginning increases the number of mercury, so by adding a 3 to the Mercury from the other side balances the whole thing.

Even Odd DoubleEven Odd Double

• If you have an element in multiple places do it last

• C4H8S + O2 4CO2 + 4H2O + SO3

• If you end up with an odd number of something on one side and an even number on the other… double everything then start counting from where you were.

• 2C4H8S + 15O2 8CO2 + 8H2O + 2SO3

All tricks togetherAll tricks together• C2H5N + O2 CO + H2O + NO2

– Balance the Carbon

• C2H5N + O2 2CO + H2O + NO2

– Flip the 2 and 5 for Hydrogen, then fix the Carbon

• 2C2H5N + O2 4CO + 5H2O + NO2

– Balance the Nitrogen

• 2C2H5N + O2 4CO + 5H2O + 2NO2

– Do Oxygen last. Oxygen is even on the left and odd on the right, so double everything.

• 4C2H5N + 2O2 8CO + 10H2O + 4NO2

– Recount the number of Oxygen

• 4C2H5N + 13O2 8CO + 10H2O + 4NO2

Reaction TypesReaction Types

• Combustion – O2 on the left, CO2 & H2O on the right.

• Synthesis – X + Y = XY• Decomposition – XY = X + Y• Single Replacement – AX + B = A + BX• Double Replacement – AX + BY = AY + BX