STOICHIOMETRY:

calculation of quantities in chemical reactions.

FIRST, WE NEED TO UNDERSTAND “THE MOLE” Describes by Amedeo Avagadro. 1 mole = 6.022x1023 molecules, atoms,

formula units or ions. 1 mole = 22.4L of gas

A balanced chemical equation can be interpreted in terms of different quantities:numbers of atomsNumbers of moleculesMolesMassVolume

NUMBER OF ATOMS

a balanced equation indicates that the number and type of each atom that makes up reactants & products

Both the number and types of atoms are not changed in a reaction

Law of Conservation of Matter!

NUMBER OF MOLECULES Ammonia reaction: one molecule of nitrogen

reacts with three molecules of hydrogen. N2(g) + 3H2 (g) → 2 NH3 (g) Nitrogen and Hydrogen react in a 1:3:2 ratio of

molecules. So 10 molecules of nitrogen react with 30

molecules of Hydrogen to make 20 molecules of Ammonia.

take Avogadro’s number of Nitrogen molecules and make them react with three times Avogadro’s number of Hydrogen molecules.

This would be the same 1:3 ratio of molecules of reactants.

The reaction would form two times Avogadro’s number of ammonia molecules.

MOLES: take Avogadro’s number of Nitrogen

molecules and make them react with three times Avogadro’s number of Hydrogen molecules.

This would be the same 1:3 ratio of molecules of reactants.

The reaction would form two times Avogadro’s number of ammonia molecules.

So 6.02x1023 molecules of N react with 18.06x1023 molecules of H2 to make 12.04x 1023 molecules of NH3

SO… A balanced chemical equation also tells you the

number of moles of reactants and products. The coefficients indicate the relative numbers

of moles of reactants and products NOW you can calculate the amounts of

reactants and products. For ammonia, one mole of N2 reacts with three

moles of H2 to form two moles of Ammonia molecules.

N2(g) + 3H2 (g) → 2 NH3 (g) total number of moles of reactants does not

equal the total number of moles of product.

MASS number and type of atoms does not

change in a rxn. total mass of the atoms does not

change The mass of 1 mol of N2 (28.0 g) plus

the mass of 3 mol of H2 (6.0 g) equals the mass of 2 mol of NH3 (34 g).

the total number of grams of reactants does equal the total number of grams of product

VOLUME

Assume STP (unless stated otherwise) Remember 1 mol of any gas at STP

occupies a volume of 22.4 L. N2(g) + 3H2 (g) → 2 NH3 (g) The equation indicates that 22.4 L of N2

reacts with 67.2 L (3 × 22.4 L) of H2. This reaction forms 44.8 L (2 × 22.4 L)

of NH3.

VOLUME STP, we’ve seen it before Stand for “Standard Temperature and

Pressure” Not commonly used on the Regents

anymore. Usually says the numbers, but if they

don’t, where do you find them?

MASS CONSERVATION IN CHEMICAL REACTIONS Mass and atoms are conserved in

every chemical reaction. molecules, formula units, moles, and

volumes are not necessarily conserved—although they may be.