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 SymbolMeaning (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) + Cl 2(g) → FeCl 3(s) Step 1: Add more Cl 2 Fe(s) + 2Cl 2 (g) → FeCl 3 (s) Step 3: Add more Fe and Cl 2 2Fe(s) + 3Cl 2 (g) → 2FeCl 3 (s) Step 2: Add more FeCl 3 Fe(s) + 2Cl 2 (g) → 2FeCl 3 (s)

Advanced Counting 5O 2 = 10 Oxygen NH 4 = 1 Nitrogen, 4 Hydrogen 2NH 4 = 2 Nitrogen, 8 Hydrogen 3(MnO 4 ) 2 = 6 Manganese, 24 Oxygen

Flipping the Subscripts O 5  O 2 Use the subscripts as the 2O 5  5O 2 opposite one’s coefficient

Chunking Polyatomic Ions Hg(MnO 4 ) 2 + Al 2 (Cr 2 O 7 ) 3  HgCr 2 O 7 + Al(MnO 4 ) 3 Notice that there are the same polyatomic ions on both sides. Instead of breaking them up, balance them together. 3Hg(MnO 4 ) 2 + Al 2 (Cr 2 O 7 ) 3  HgCr 2 O 7 + 2Al(MnO 4 ) 3 Balanced MnO 4 by flipping the subscripts 3Hg(MnO 4 ) 2 + Al 2 (Cr 2 O 7 ) 3  3HgCr 2 O 7 + 2Al(MnO 4 ) 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 Double If you have an element in multiple places do it last C 4 H 8 S + O 2  4CO 2 + 4H 2 O + SO 3 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. 2C 4 H 8 S + 15O 2  8CO 2 + 8H 2 O + 2SO 3

Reaction Types Combustion – O 2 on the left, CO 2 & H 2 O on the right. Synthesis – X + Y  XY Decomposition – XY  X + Y Single Replacement – AX + B  A + BX Double Replacement – AX + BY  AY + BX Acid-Base – HX + YOH  XY + H 2 O