Introduction to Chemical Reactions and Equations

What are the clues that a chemical reaction has occurred?

A gas bubbles off.

A new solid material is formed (a precipitate.)

Color change.

Heat and/or light are emitted.

The key to chemical reactions! The Law of Conservation of Mass In a chemical reaction, no atoms are created or destroyed. The atoms in the reacting molecules simply re-arrange to form new molecules. Since atoms aren’t created or destroyed, no mass is created or destroyed either. Mass is “conserved.”

Count the atoms, before & after 2H2 + O2 2H2O

In the reaction 2NaCl 2Na + Cl2 In the reaction N2 + 3H2 2NH3 If 14 grams of N2 reacted with 3 grams of H2, how many grams of ammonia would be produced? In the reaction 2NaCl 2Na + Cl2 If 58.5 grams of NaCl is decomposed, and 23 grams of Na is formed, how many grams of Cl2 must also be formed?

What else is conserved? In N2 + 3H2 2NH3 besides mass (grams), what else is being conserved? Atoms? Molecules? Moles?

Coefficients In a chemical reaction, if more than 1 molecule of a substance reacts or is produced, the number of molecules is shown with a coefficient. 6CO2 + 6H2O C6H12O6 + 6O2 Coefficients multiply subscripts through the whole molecule that follows, so 6CO2 shows that there are 6 Carbon atoms and 12 Oxygen atoms.

How many? How many of each atom are shown by 4Al2(SO4)3 3(NH4)3PO4

Terminology In a chemical reaction, the materials to the left of the arrow are the reactants. Reactants are consumed in the reaction. The arrow is like an equal sign in math. It can be read “react to produce” or “produces” or “forms”. The materials to the right of the arrow are the products. They form as the reaction proceeds. H2O + SO3 H2SO4

Phase Notation A chemical equation can also show the physical state of the materials in the reaction. This is done with phase notations: (s) solid (l) liquid (g) gas (aq) aqueous or dissolved in water (ppt) or ( ) means “a precipitate forms” ( ) means “a gas bubbles off” means “is heated” Phase notations are written to the lower right of each formula. Clues to phase notations are often given in equations written in words.

An example When pellets of sodium hydroxide are dropped into an aqueous solution of iron(III) chloride, iron(III) hydroxide precipitates and sodium chloride stays dissolved. 3NaOH(s) + FeCl3(aq) Fe(OH)3 (ppt) + 3NaCl(aq)

Now onward to “balancing chemical equations”