Chapter 12 - stoichiometry Section 12.1 - the Arithmetic of Equations

What does a balanced chemical equation tell you? ● A balanced chemical equation tells you what amounts of reactant to mix and

what amounts of products to expect● If you know the quantity of one substance then you can calculate the quantity

of any of the other substances consumed or created in the reaction● The calculation of quantities in chemical reactions is called stoichiometry ● Calculations involving balanced equations are called stoichiometric

calculations

What information does a balanced chemical equation give you? ● The number of atoms involved in the reaction● The number of molecules involved in a reaction● Number of moles: Often more practical than talking about atoms and

molecules; instead you can discuss the number of avogadro's numbers of representative particles of a substance

Anything else? ● All chemical reactions must abide by the law of conservation of mass: no

matter is created or destroyed ● Although the number of moles of reactants and products do not have to be

equal, the mass of the reactants and products do! ● If the reaction involves gasses; you can use the knowledge that 1 mole of a

gas at STP has a volume of 22.4 L to determine the volume of gaseous products and reactants

Example - balanced chemical equation for ammonia

Chemical CalculationsSection 12.2

What is a mole ratio?● A mole ratio is a conversion factor obtained by interpreting the coefficients of

a balanced chemical equation in terms of moles● A mole ratio can be used to convert between a given number of moles of one

substance to moles of a different substance

Mole - Mole calculations ● The mole ratio can be

used to calculate the moles of one substance if you know the moles of another substance

● You simply multiply the given number of moles by the mole ratio

Mass - Mass calculations● Just with moles, you can calculate the

mass of any substance in a given chemical equation from the mass of another substance

● You are able to convert the mass to moles using the the molar mass and our formula triangle

● You can then use the mole ratio to work out the number of moles of the product

● This can then be converted to mass using the molar mass and the formula triangle

Worked example - page 364

Other stoichiometric calculationsUsing the two formula triangles you can also convert easily between moles and mass & moles and volume

Using avogadro's number it is also possible to convert between moles and particles

Simply multiply the number of moles by 6.02 x 1023

Limiting reagent and percent yield

Section 12.2

How do you make the ideal s’more ?

How many s’mores can you make with the following ingredients?

7 Graham crackers

5 Marshmallows

3 squares of chocolate?

How do you identify a limiting reagent? ● Using a balanced chemical equation, and the amount of reactants given, it is

possible to determine which reactant is the limiting reagent● Simply convert both masses to numbers of moles, and use mole ratios from

the balanced chemical equation to determine which reactant is the limiting reagent

○ The limiting reagent determines the amount of the product that is formed ○ The reactants that is smaller in mass or volume is not always the limiting reagent - it all

depends on mole ratios in the balanced chemical equations

Theoretical Yields● A balanced chemical equation can be used to calculate the theoretical yield

for a reaction○ The maximum amount of product that can be formed from the given amount of reactants

● In reality, the amount produced may vary. The amount produced is called the actual yield

○ The actual yield is an experimental value that must be measured ○ The actual yield is usually less than the theoretical yield, although in rare cases due to an error

it could end up being more

● The actual yield and theoretical yield can be used to calculate the percent yield

Factors that affect percent yield ● Many factors can cause percent yield to be less than 100 %

○ Examples: Impure reactants or competing side reactions

● Laboratory procedures such as filtration or transferring materials between containers can also lead to a decrease in percent yield

○ Achieving a yield close to 100% also requires precise measuring

● Remember, the percent yield is a measure of the efficiency of a reaction carried out in a laboratory