Let S Try This in Another Example the Thermite Reaction

Introduction to Stoichiometry

Today we are going to begin the topic of stoichiometry. What is the definition of that word? The study and calculation of quantitative (measurable) relationships of the reactants and products in chemical reactions.

Let’s watch this video clip now about the new Arizona Cardinals football stadium. When it’s over I’d like you to tell me how stoichiometry is involved.

The thermite reaction. Used in rockets, underwater welding and incendiary bombs.

Fe2O3 (s)+Al(s)  Fe(l) + Al2O3(s) (Unbalanced)

(review names iron(III) oxide and Aluminum oxide (why no (III)?)

If I want to generate a definite amount of Fe to fill in the gap in the rails, I need to know how much Al and how much Fe2O3 is needed to start.

Every problem in stoichiometry is based on a chemical equation and its interpretation of moles. Here’s the thermite reaction:

Fe2O3 (s) +2Al(s)  2 Fe (l) + Al2O3(s) - may be interpreted as saying that 1 mole of iron (III) oxide combines with 2 moles of aluminum to form 2 moles of iron and 1 mole of aluminum oxide. The coefficients in the equation express the relative reacting quantities in terms of moles. From this it follows that if the reaction quantity of aluminum is 4 moles (twice as much as is indicated by the equation) the reaction quantities of everything else would double also: 2Fe2O3 (s) +4Al(s)  4 Fe (l) + 2Al2O3(s)

Example 1:

Methane burns in oxygen to produce carbon dioxide and water. If 8.2 moles of methane are burned in the reaction, how many moles of water will be produced?

Solution Step 1 Write the balanced equation for the reaction and the mole ratio for the given and required substances.

CH4 + 2O2 ------> CO2 + 2H2O 1 mol 2 mol

Step 2 Write the given and the unknown beneath the appropriate substance.

CH4 + 2O2 ------> CO2 + 2H2O 1 mol 2 mol 8.2 mol X mol

Step 3 Set up a proportion:

1 mol = 2 mol 8.2 mol X mol Step 4 Solve for "X". Use cross multiplication. 1 mol x X mol = 8.2 mol x 2 mol X = 16.4 mol

Therefore the number of moles of H2O formed from 8.2 mol of methane is 16.4 mol

Example 2: You try this one!

Propane burns in oxygen to produce carbon dioxide and water. If 0.2 moles of propane are burned in the reaction, how many moles of oxygen are required?

Solution Step 1 Write the balanced equation for the reaction and the mole ratio of the given and required substances.

C H O2 --> CO2 H O 3 3 + + 2

mol mol Step 2 Write the given and the unknown beneath the appropriate substance.

C H O2 --> CO2 H O 3 3 + + 2

mol mol mol mol Step 3 Set up a proportion: 1 mol = 5 mol 0.2 mol X mol

Step 4 Solve for "X". Use cross multiplication. 1 mol x X mol = 0.2 mol x 5 mol X = 1 mol Therefore, 1 mole of oxygen is required when 0.2 mol of propane are burned.