Unit 9: Stoichiometry

Stoichiometry

Using coefficients and compounds/elements to solve for different amounts of moles

1 mole =

6.02 x 10^23 molecules

"ratio of molecules" and "ratio of moles"

The coefficients give us "ratio of molecules" the that react. Since the moles are just a large grouping of molecules, the coefficients also give us the "ratio of moles" that react.

Moles to moles conversions

Moles A ---> Moles B

Use mole ratio from the chemical equation that is given or made

Moles to grams conversions

Moles A --> Moles B ---> Grams B

To get from Moles A to B use mole ratio from the coefficients of balanced equation

To get from Moles B to Grams B multiply by the molar mass

Grams to Moles conversions

Grams A --> Moles A ---> Moles B

To get from Gram A to Moles A divide by the molar mass

To get from Moles A ---> Moles B use the mole ratio

Grams to Grams conversions

Grams A ==> Moles A ===> Moles B ===> Grams B

To get from Grams A to Mole A divide by the molar mass

To get from Moles A to Moles B use the mole ratio

To get from Moles B to Grams B multiply by the molar mass

Limiting and Excess Reactant

- Limiting reactant is the thing that is stopping from making more products

- excess reactant - after the reaction there will be left over

Limiting reactant in problems

A reactant that is totally used up and limits the amount of product formed.

- The reactant that produced the smaller amount of product

Excess reactant in problems

- a reactant that has extra left over after the reaction is complete

- The reactant that produced the larger amount of product

Theoretical yield in problems

When calculating how many of that compound will form, the theoretical yield is the smaller value of that compound

Calculating yield

theoretical yield - calculated value

actual yield - experimental value

Percent yield = actual/theoretical x 100 (all the same unit) (efficient reactions have a higher % yield)

Bond Energy

What makes a reaction endothermic or exothermic

For chemical reactions you have to break bonds in the reactants before you can put hte atoms back into a new arrangement to form the product

both of these processes involve changes in energy.

the "heat of reaction" or"the change in enthalpy"

The net energy change for a reaction

Endothermic

when there is a "positive energy change", meaning energy was added or absorbed to allow the reaction to occcur

Exothermic

energy was released when the reaction occured

Bond energies

The bond energy for a particular type of bond can vary from one moleculte to another because the atomic environment of a bond can influence the amount of energy needed to break the bond

- Bond energy can be useful for calculating the net enery change in a reaction