We
need to know the relative amounts of substances to ensure that a reaction will
work properly. Really, it’s just like a
cooking recipe – if you bake a cake and need a ratio of 3 cups of flour to 1
cup of sugar, you know that messing around with that ratio will wreck the cake.
Interpreting Balanced Equations
1C3H8(g) +
5O2(g) → 3CO2(g) + 4H2O(g)
In Terms of Molecules
1
molecule of propane reacts with 5 molecules of oxygen to make 3 molecules of
carbon dioxide and 4 molecules of water.
But,
this is impractical – we need to be able to measure out chemicals and we can’t measure out molecules. It is better to look at the ratios in terms of moles, because we can physically mass out moles of chemicals. This is much more practical.
In Terms of Moles
1
mol of propane reacts with 5 mol of oxygen to make 3 mol of carbon dioxide and
4 mol of water - this is the mole ratio
of reactants and products.
Regardless
of how much propane is used, the propane and oxygen still react in a ratio of
1:5 to create products in a ratio of 3:4.
When we set up mole ratios mathematically (which we will do below), we will continue to use "n" as the variable to represent number of moles. However, we will add a chemical formula subscript to each "n" to indicate which substance with which we are working. For instance, "number of moles of water will be represented by “nH2O”.
When we set up mole ratios mathematically (which we will do below), we will continue to use "n" as the variable to represent number of moles. However, we will add a chemical formula subscript to each "n" to indicate which substance with which we are working. For instance, "number of moles of water will be represented by “nH2O”.
Mole Ratios
Knowing
the number of moles of any one substance in a balanced reaction equation, we can find the number of moles of any
other.
So, let's give this a try, shall we? Fair warning, be prepared for some cross-multiplication during the solution of these questions.
So, let's give this a try, shall we? Fair warning, be prepared for some cross-multiplication during the solution of these questions.
ex.
Given the equation for the combustion of methanol,
2CH3OH(l)
+ 3O2(g) →
2CO2(g) + 4H2O(g)
if 3.50 mol of methanol (CH3OH)
is burned completely in oxygen,
(a) how many moles of oxygen are used?
nCH3OH
nO2
-----------
= ---------
2 3
3.50
mol nO2
------------
= ----------
2 3
nO2
= 3/2 x 3.50 mol
nO2
= 5.25 mol
(b) how many moles of water are formed?
nCH3OH
nH2O
-----------
= ---------
2 4
3.50
mol nH2O
------------
= ----------
2 4
nH2O
= 4/2 x 3.50 mol
nO2
= 7.00 mol
(c) how many moles of carbon dioxide are formed? Try this one on your own.*
General Equation
For
the general reaction equation, aA + bB → dD
+ eE (where the lower case letters represent coefficients and the upper case numbers represent chemical substances in a balanced equation),
nA
nB nD
nE
-----
= ----- = ----- = -----
a b
d e
What the Equation Doesn’t Tell Us
- the rate of the reaction (we will study this in 4U chem, in the Chemical Kinetics unit)
- exothermic or endothermic (we will study this in 4U chem, in the Thermochemistry unit)
- description
of how the reactants actually react with each other to form products ((we will
study this in 4U chem, in the Chemical Kinetics unit * the answer is 3.50 molHomework: p.300 #11-18
Answers:
