Transcript Stoichiometry PPT - Chemistry Teaching Resources
Analysis & Stoichiometry
Gordon Watson Chemistry Department, Kelso High School
Adv Higher Unit 2 Topic 1
Introduction
This topic explores various aspects of
Chemical Analysis
, leading to an appreciation of the importance of
Stoichiometry
in chemical reactions.
Stoichiometry
Stoichiometry
involves the understanding of
numerical
relationships between reacting substances.
One methane molecule CH 4 Two oxygen molecules 2 O 2 One carbon dioxide molecule CO 2 Two water molecules 2 H 2 O
The Mole
Molar relationships
, in turn, allow us to establish
measurable
relationships between reacting substances.
1 mole 2 moles
16g 32g 25 l 50 l
1 mole 2 moles
44g 36g 25 l
Volumetric Analysis
This method of chemical analysis involves accurately measured volumes.
Instruments such as
pipettes
and
burettes
are used to measure volumes accurately Solutions of
unknown concentration
are titrated against a solution of
known concentration
- a
primary
standard solution
or
standard solution
Gravimetric Analysis
This method of analysis involves accurate weighing.
Access to an
Analytical Balance
, capable of reading to 2 decimal places at least, is essential.
The analysis will usually involve the production of a suitable
precipitate
:-
very low solubility high molecular mass
Stoichiometry 1 mole = gfm
Primary standard
A
Primary standard
is a substance that has the following characteristics: • a high purity (> 99.9%) • is stable in air and in solution • a reasonably high formula mass • is reasonably soluble Suitable substances include: Potassium hydrogen pthalate (
acid
) and sodium carbonate (
base
)
Standard Solution
Stoichiometry n = mass / gfm Stoichiometry C = n / V
Standard Solution
A
Standard Solution
is one whose concentration has been established by titrating against a
Primary Standard
…..
or against another
Standard Solution
whose concentration had been established by titrating against a Primary Standard …..
Stoichiometry C1V1 p1 = C2V2 p2
Dilutions
Once prepared, standard solutions can be used a
stock solutions
and further diluted solutions can be made.
Stoichiometry C1V1 = C2V2
Titrations
Acid-Base Titrations
- neutralisation reactions requiring an indicator to detect the end-point
NaOH(aq) + CH3COOH(aq)
NaCH3COO(aq) + H2O(l)
Redox Titrations
- based on redox reactions, often self-indicating due to strong colours, e.g. KMnO4
MnO4 (aq) + 8H + (aq) 2I-(aq)
Mn2+(aq) + 4H2O(l) I2(aq) + 2e
Complexometric Titrations
- based on ligand reactions, requiring an indicator that can be replaced
[Ni(In)](aq) + EDTA 4 (aq)
[Ni(EDTA)]2-(aq) + In
Acid-Base Titrations
Equivalence point
The
equivalence point
is when the reaction is just completed For a titration between a
strong acid
(e.g HCl) and a
strong base
(e.g NaOH) the
equivalence point
will be when pH = 7.
However, not all indicators will complete their colour change at this point so
end point observed
may be different.
Indicators
Indicators
change colour
over a
pH range
.
End point
In this case both indicators would change just before or just after the
equivalence point
In this case one indicator would change just after the
equivalence point
, but the other would be no good.
Redox Titrations
An
excess
of MnO4 - must be added to detect the
end-point
.
Fortunately MnO4 - is so strongly coloured that
end-point
is very close to
equivalence point
.
Complexometric Titration
Murexide indicator forms a yellow green complex with Ni2+ ions.
EDTA is added and starts to complex with any free Ni2+ ions first.
Finally, EDTA will replace the murexide molecules and the colour of free murexide - purple - will be produced. Any decision about the end-point relies on there being enough free murexide to produce a distinct colour change. What about the equivalence point?
Difficult Titrations
The weaker the acid, the smaller the region of rapid pH change which includes the
equivalence point
.
For very weak acids, it is impossible to detect an
end point
close to the
equivalence point
Back Titration
The solution to this problem is a technique known as a back titration.
A carefully measured volume of base would be added to ensure complete reaction of the weak acid.
A strong acid would then be used to determine the excess base left over.
The amount of base which reacted with the weak acid can now be calculated and, hence, the amount of weak acid present originally.
Analysis & Stoichiometry
End of Topic 1