Transcript Slide 1

Calculating Masses of
Reactants and Products
Review: Balance the following equations:
Ex. 1:
Cu(s) + AgNO3(aq) ® Ag(s) + Cu(NO3)2(aq)
Ex. 2:
NH3(g) +
O2(g) ® H2O(l) +
NO2(g)
Ex. 3:
Write a balanced equation for the reaction of
aqueous sodium phosphate and aqueous
calcium chloride to produce sodium chloride and
calcium phosphate
Mole Ratios in Balanced Chemical Equations
a balanced equation tells us the ratio of the amounts
of reactants and products participating in the reaction
this also applies to moles!
Ex. 1:
N2(g)
+
3H2(g)
2NH3(g)
1 molecule
3 molecules
2 molecules
1 dozen
molecules
3 dozen
molecules
2 dozen
molecules
1 mol of nitrogen
3 mol hydrogen
2 mol ammonia
1 (6.02 X 1023)
molecules
3 (6.02 X 1023)
molecules
2 (6.02 X 1023)
molecules
when moles are used to express
coefficients in a balanced equation, we
have mole ratios
Mole ratio: the ratio of the amount in
moles of reactants and products
Using Mole Ratios to Calculate the Mass
of Reactants and Products
we use this because in chemistry we need
to be able to calculate the amount of
reactant that will give us an amount of
product
this is just like when you are baking
cookies and put in enough
ingredients to make 24 cookies
To do this:
1) Start with a balanced equation
2) Convert your mass of reactant into moles
3) Use the mole ratios given by the
coefficients in the equations to get the
amount of product in moles
4) Convert from moles -> mass
mass of
reactant
(g)
n=m
M
moles of use mole
reactant
(mol)
ratios
mass of
product
(g)
m= n X M
moles of
product
(mol)
Ex. Aluminum oxide is decomposed to produce
aluminum metal and oxygen. What mass of
aluminum metal can be produced from 125g
aluminum oxide?
2Al2O3(s) -> 4Al(s)
+ 3O2(g)
Ex. How many grams of KClO3 must be
decomposed to give 0.96g of oxygen?
How many moles of KCl will be produced
during this same reaction?
KClO3(s) ->
KCl(g) +
O2(g)
Homework:
Pg. 212 # 2,3
Pg. 227 # 1-6
How do we choose which reactant to use to find the
mass of a product?
Does it matter which one we choose??
Limiting and Excess Reagents
in a chemical reaction, there can be a reactant
that is all used up and one that is left over
so the limiting reagent is the reagent that is
_________________, it also:
-limits the amount of product that is
produced
-stops the reactions
To get the proper mass of product that was
produced, we need to use the limiting reagent!
Limiting And Excess Reagents Activity
Making Nut 'n' Bolts :
# of nut 'n' bolts produced:
Making Nut 'n' Bolt 'n' Bolts :
# of nut 'n' bolt 'n' bolts produced:
What's left over?
Making Bolt 'n' Nut 'n' Nuts:
# of bolt 'n' nut 'n' nuts produced:
What's left over?
How can we find which reagent is the
limiting reagent?
Ex. 1 - If 14g of iron and 12g of sulfur react
to form iron(II) sulfide, which reagent will be
the limiting reagent?
Fe(s) + S(s)
FeS(s)
M (g/mol)
m(g)
*B
w
h
re
What we can do is :
1) Find the amount of moles that we have for
each reagent, Fe & S
2) Choose 1 reagent, Fe and determine how
many moles of S it needs to REACT FULLY
3) If this calculation shows that you have more
S than needed, then Fe is the limiting reagent
and S is the excess reagent.
Fe(s) + S(s)
M (g/mol)
m(g)
FeS(s)
Ex.2 - What is the maximum mass of
ammonia that can be produced by reacting
10g of hydrogen with 50g of nitrogen?
Yield of a Chemical Equation
Theoretical Yield : the maximum amount of
product that can be formed in a chemical
reaction
-measured in mass or moles
-determined using the mass of our limiting
reagent
Actual Yield: amount of product that is
actually obtained from a chemical reaction
-should always be less than the theoretical
Percentage Yield = Actual Yield X 100%
Theoretical Yield
Ex. If the theoretical yield is 10 kg and the
actual yield is 9 kg, what is the percentage
yield?
Homework:
Pg. 235 #7-10
Pg.239 #3 &4