Stoichiometry intro
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Transcript Stoichiometry intro
SCH3U
5.2 Introduction to Stoichiometry
What is Stoichiometry?
Stoichiometry is the study of the quantities
involved in chemical reactions.
The word stoichiometry means
“measuring the elements”
Stoichiometry allows a chemist to predict
how much of a reactant reacts or how
much of a product forms in a chemical
reaction.
Balanced Reaction Equations
and the Mole Ratio
Consider this balanced reaction equation:
4 Al(s) + 3 O2(g) → 2 Al2O3(s)
The coefficients of this reaction represent:
- The number of reacting PARTICLES or
- The number of reacting MOLES
These numbers are FIXED, the ratio of reacting
substances NEVER changes.
Balanced Reaction Equations
and the Mole Ratio
We can interpret this reaction in 2 ways:
4 Al(s) + 3 O2(g) → 2 Al2O3(s)
1) Every time 4 atoms of Al react with 3 molecules
of O2, 2 molecules of Al2O3 form.
OR
2) Every time 4 moles of Al atoms react with 3
moles of O2 molecules, 2 moles of Al2O3
molecules form.
Using the Mole Ratio
Remember that the coefficients from a balanced
reaction represent the ratio of the moles of
substances that react and form during a
chemical reaction.
These numbers are fixed - they do not change
We can use these ratios to predict the amounts
of substances that react and form in a reaction
when any amount of a substance is reacted.
The conversion factor we use to make these
predictions is called a MOLE RATIO.
Using the Mole Ratio
In this reaction:
4 Al(s) + 3 O2(g) → 2 Al2O3(s)
What is the minimum amount of O2 moles
that react with 1.20 moles of Al?
To solve this, create a mole ratio:
coefficient of unknown
moles of unknown = moles given
coefficient of given
Using the Mole Ratio
To solve, substitute in the values:
- moles given = 1.20 moles Al
- moles of unknown = ? moles O2
- coefficient of given = 4 Al
- coefficient of unknown = 3 O2
The mole ratio becomes:
3 O2
3.60 mol O2
moles O2 = 1.20 moles Al
=
= 0.900 mol O2
4 Al
4
We predict that 0.900 mol of O2 react with 1.20
mol of Al.
Using the Mole Ratio
We can also predict the “theoretical yield” - which is
defined as the amount of a product forms if all the
reactants are converted to products.
What it the theoretical yield of Al2O3 when 1.20 moles of
Al reacts? Using the mole ratio:
2 Al2O3
2.40 mol Al2O3
moles Al2O3 = 1.20 moles Al
=
= 0.600 mol Al2O3
4 Al
4
We predict that 0.600 mol of Al2O3 should form, if all the
reactants are converted into products (the theoretical
yield)
Terminology of Stoichiometry
Coefficients
- Numbers that represent the ratios of reacting moles in a
balanced chemical reaction.
Mole Ratio
- A conversion factor that allows you to determine how
much of one substance reacts or forms in a chemical
reaction.
Theoretical Yield
- The amount of a product that should form in a chemical
reaction, if all of the reactants are converted into
products.