Analysis & Stoichiometry

Adv Higher Unit 2 Topic 1

Gordon Watson

Chemistry Department, Kelso High School

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 methanemolecule

Two oxygenmolecules

One carbon dioxidemolecule

Two watermolecules

CH4

2 O2 CO

22 H

2O

The MoleMolar relationships , in turn, allow us to establish measurable relationships between reacting substances.

1 mole 2 moles

1 mole 2 moles

16g

25 l

32g

50 l

44g

25 l

36g

Volumetric Analysis

This method of chemical analysis involves accurately measured volumes.

Instruments such as pipettes and burettes are used to measure volumes accuratelySolutions of unknown concentration are titrated against a solution of known concentration - a primary standard solution or standard solution

Gravimetric AnalysisThis 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 solubleSuitable substances include:

Potassium hydrogen pthalate (acid) and sodium carbonate (base)

Standard Solution

Stoichiometry

C = n / V

Stoichiometry

n = mass / gfm

Standard SolutionA 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

TitrationsAcid-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) → Mn2+(aq) + 4H2O(l)

2I-(aq) → I2(aq) + 2e-

Complexometric Titrations - based on ligand reactions, requiring an indicator that can be replaced

[Ni(In)](aq) + EDTA4-(aq) → [Ni(EDTA)]2-

(aq) +

In

Acid-Base Titrations

Equivalence pointThe 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.

IndicatorsIndicators 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 TitrationThe 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.

End of Topic 1

Analysis & Stoichiometry