9.1 information given by reactions
Equations
?What can an equation tell us?
?why must we always balance equations ?
?what is one mole = to?
9-2 mole-mole relationships
Problem: What number of moles of O2 will be produced from the decomposition of 5.8 moles of H20?
1.find your equation:
2H20 à 2H2 + O2
1.find relationship between moles
2H20 à2H2 + 02
?What is the ratio of water to oxygen?
2 to 1
2.make equivlance statement:
1 mol 02
2 mole H20
3.add known quantities for cancellation:
?What do we already know?
*remember to write down what you know(5.8 mol H20)
5.8 mol H20 x 1 mol 02 = 2.9mol 0
2 mol H20
9.3 mass calculations
Problem
What mass of oxygen will be required to react exactly with 44g of propane from the following reaction:
C3H8 + 5O2 à 3CO2 + 4H2O
Convert grams of propane to grams of oxygen:
*remember to write down what you know and get what you need on top
44gC3H8 x 1molC3H8 x 5molO2 x 32gO2 = 160gO2
44.09gC3H8 1molC3H8 1molO2
Problem
Convert .5moles of NH3 to moles NH4Cl by using the following chemical equation:
NH3 + HCl à NH4Cl
?Where did the equivalence statements come from?
The coefficients(mole ratio)
9.4 using scientific notation
Put the following into scientific notation:
495 =
3446 =
.4589
Review steps for calculating the masses of reactants
1.
2.
3.
4.
?What do we use to calculate between mass and moles?
EX: 1mole of H2O is 18g
Problem
What mass of CO2 can 1 x 103g of lithium hydroxide absorb?
1.2LiOH + CO2 à Li2CO3 + H2O
2.turn masses to moles
3.plug in mol ratio
4.turn moles back into mass
?What is the answer in scientific notation?
?What does this problem look like in a string conversion?
9.6
limited reactant
Excess reactant
Problem
2.5 x 104gN2 is mixed with 5x103gH2 forming ammonia. Calculate the limited reactant.
N2 + 3H2 à 2NH3
1.Calculate the moles available of both reactants using the periodic chart(turn g to moles)
2.determine which reactant is limited using coefficient ratio(calculate moles required)use moles from step one and make an equivalent statement with coefficients
3.compare moles available to moles required
8.92 x 102 mol N2 available 8.27 x 102 mol N2required
2.48 x 103mol H2 available 2.68x103mol H2required
Which reactant is limiting AND why?
Which is the excess reactant AND why?
We can now determine how much of the product(ammonia can be formed) as it is based on the limited reactant:
Try It:
You have just calculated the theoretical yield!
Problem
If 4moles of ZnS are combined with 3moles of O2, how many moles of ZnO can be produced?
2ZnS + 3O2 à 2ZnO + 2SO2
Step 1 moles available is given to you:
3moles ZnS and 4moles O2
Step 2 find the moles required
What do we use to find the moles required?
Step 3 compare available to required to find limiting reactant
3moles ZnS available 2.67 moles ZnS required
4moles O2 available 4.5moles O2 required
Which is the limited reactant AND why?
How many moles of the product ZnO can be produced?(convert moles of the limited reactant to moles of the product)
How many moles of the excess reactant will be left over after the reaction stops?
?How did you get this answer?
Problem: What mass of FeCl2 could be produced from 25g of Fe and Excess HCl if the balanced reaction is:
Fe + 2HCl à FeCl2 + H2
How would you solve this problem
Turn grams Fe to grams FeCl2 because if HCl is the excess reactant than Fe must be the limiting reactant and that is what we need to use
Problem:
When ammonia burns in pure oxygen, the reaction is:
4NH3 + 3O2 à 2N2 + 6H2O
What masses of nitrogen and water could be produced from 25g of ammonia?
How would you solve this problem?
9.8
Theoretical yield:
Actual yield-
Percent yield-
Percent yield = actual yield x 100
Theoretical yield
Limited reactant must be calculated to get theoretical yield to compare to actual yield to calculate percent yield
*limited reactants are important*
Problems
If 10g of magnesium reacts with excess oxygen and 20g of magnesium oxide are produced, what is the percent yield?
What does the 20g represent?
2Mg + O2 à 2MgO
Actual x 100
Theoretical
What do you need to calculate to find the percent yield to this problem?
Which is the limited reactant?
Try calculating it using a string conversion:
You just calculated the theoretical yield!
Try calculating the percent yield: