StoichiometryChapter 12

Stoichiometry • STOY-KEE-AHM-EH-TREE – Founded by Jeremias Richter, a German chemist– Greek orgin

• stoikheion – element & metron – measure

• The branch of chemistry that deals with the quantities of substances that enter into, and are produced by, chemical reactions.

• Shows the quantitative relationship between reactants and products in a chemical reaction. – How much product is formed from a given amount

of reactants.

Balanced Chemical Equation (BCE)• Coefficients are used to balance chemical

equations. • The BCE illustrates that chemical reactions

follow Law of Conservation of Mass– The number and types of atoms on the reactants

side is equal to the number and types of atoms on the products.

– Reactants (left)Products (right)

The coefficients are molar ratios of reactants and products.We can use the molar ratios to convert from one substance to another using Stoichiometry

Stoichiometry Problems• Each problem will be 3 steps

1. Convert to moles

2. Mole to mole ratio using balanced chemical equation (BCE)

3. Convert to required unit.

BCE

Example• What is the mass of Al2O3 that can be made

from 25 grams of aluminum?• Balanced Equation: 4 Al + 3 O2 2 Al2O3

BCE

1. You have to use units to understand the process2. Units include g, mol, L, particles AND chemical formulas3. Convert your given to moles4. You have to use units to understand the process5. Use BCE coefficients to convert from one substance to another6. You have to use units to understand the process7. Convert answer to required unit 8. You have to use units to understand the process9. Units include g, mol, L, particles AND chemical formulas10. It is not that hard

You Should Know About Stoichiometry

Percent Yield• Indicates the percent of theoretical yield that was obtained from the

final product in an experiment. • The closer to 100% the better • Percent Yield = actual mass x 100 theoretical mass Actual: what was formed in the lab experimentTheoretical: what should have been formed from the given

***Use stoichiometry***

Percent Error• Percent Error =(actual mass–theoretical mass)x100

theoretical mass

• The closer to zero the better % error

Limiting Reactants Video Clip• The limiting reactant is the reactant present in the

smallest stoichiometric amount– In other words, it’s the reactant you’ll run out of first

5N2 and 9H2 6NH3 and 2N2

Hydrogen is limiting

Hydrogen is limiting

How 2 Calculate the Limiting Reactant

• Convert each of the given reactants to a product• The reactant that produces the least product is the limiting

reactant.Example:• Nickel replaces silver from silver nitrate in solution according to the following equation:

2AgNO3 + Ni → 2Ag + Ni(NO3)2 If you have 22.9 g of Ni and 112 g of AgNO3 ,what mass of nickel(II) nitrate would be produced?

• 22.9g Ni x 1 mol Ni x 1 mol Ni(NO3)2 x 182.7g Ni(NO3)2 = 71.3 g Ni(NO3)2

58.7g Ni 1 mol Ni 1 mol Ni(NO3)2

• 112 g AgNO3 x 1 mol AgNO3 x 1 mol Ni(NO3)2 x 182.7g Ni(NO3)2 = 60.2 g Ni(NO3)2

169.9g AgNO3 2 mol AgNO3 1 mol Ni(NO3)2

• AgNO3 is limiting & 60.2 g Ni(NO3)2 can be produced

Limiting Reactant Challenge• Given 20 mL HCl (1.46g HCl) calculate the amount of

Mg to add to generate the maximum production of Hydrogen gas with Mg still limiting the reaction.

• Calculate the percent yield of Hydrogen gas