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