Chapter 11 : StoichiometrySection 1 : defining Stoichiometry

Objectives Describe the types of relationships indicated by a balanced chemical equation.

State the mole ratios from a balanced chemical equation

Rally coach:Balance the following chemical equation Partner A ;H2 + O2 = H2OPartner B :Mg +Cl2 = Mg Cl2

Stoichiometry Stoichiometry: The study of quantitative

relationships between the amounts of reactants used and amounts of products formed by a chemical reaction

Stoichiometry is based on the law of conservation of mass. Which states that states that matter is neither created nor destroyed in a chemical reaction

In any chemical reaction, the amount of matter present at the end of the reaction is the same as the amount of matter present at the beginning. Therefore, the mass of the reactants equals the mass of the products.

Amounts of reactants = Amounts of products

Relationships Derived From A Balanced Chemical EquationThe balanced chemical equation for the following

chemical reaction ….Fe(s) + …. O 2 (g) → …. F e 2 O 3 (s)

4Fe(s) + 3 O 2 (g) → 2F e 2 O 3 (s)the coefficients in an equation represent 1- numbers of individual particles 2- numbers of moles of particles.Therefore, you can also say that four moles of iron

react with three moles of oxygen to produce two moles of iron(III) oxide.

Converting mole to mass

However, by converting the known mole quantities to mass, the mass relationships become clear.

Remember : The chemical equation does not directly tell you anything about the masses of the reactants and products

moles are converted to mass by multiplying by the molar mass.

So the mass of the reactant is calculated as follows4Fe(s) + 3 O 2 (g) → 2F e 2 O 3 (s) we have two reactants 1- 4 mol Fe multiplying by the molar mass. ( from the periodic table ) 2- 3 mol O 2multiplying by the molar mass. ( from the periodic table )

Now, total mass of the reactant is given by The total mass of the reactants is: (………. + ………… ) = ………… g

Similarly, the mass of the product is calculated as follows:

2 mol F e 2 O 3Note that the mass of the reactants

equals the mass of the product.

Mole ratio A mole ratio: is a ratio between the numbers of moles of any two of the substances in a balanced chemical equation

So , for the following chemical equation

2K(s) + B r 2 (l) → 2KBr(s)

the mole ratios are