Chapter 13 (Incomplete - dilutions, limiting reagents and excess reagents to be calculated)

Separation of compounds

In a reaction pathway, intermediate compounds and by-products are also formed

Separation of the compounds enables us to get the desired product from the mixture, disregarding the by-products

Intermediate compound

A substance that is identified between the change from a reactant to a product

Distillation

A way of separating two or more liquids based on their boiling point

The difference in boiling point between the mixtures must be at least 50 degrees for effective separation

How it works

The vessel with the mixture is placed at the bottom - a tube connects it to a round-bottom vessel at the top

The liquid with a lower boiling point travels up the tube and is collected at the top vessel (distillate)

Is condensed back into a liquid by a water-cooled condenser

Visual explanation (Insert here)

Distillate

The vapor which is condensed and returned as back into a liquid

Fractional Distillation

A process of distillation used when the boiling points of the reactants are very similar, and it may be harder to seperate/distinguish them

The distillate has a higher concentration of species that have a lower boiling point

The original mixture of liquids has a higher concentration of mixtures that have a higher boiling point

How it works

Mixtures with a higher boiling poinjt condense in the fractionating column, as the heat is not high enough for them to continue as a gas

Vapours with a lower boiling point continue and are seperated as the distillate

Note: Heat gets lower as the height increases

Visual Of Fractionating Column (Insert)

What is it commonly used for

To separate volatile liquids (liquids that easily vaporize at low temperatures)

Anti-bumping granules

Small and irregular shaped stones in fractionating column where condensation of higher boiling point species occurs

Textbook explanation

Ethyl Ethanoate Fractional Distillation

Explanation

Ethyl Ethanoate - Lowest boiling point - hence higher proportion is sent to distillate

Melting Point Determination

A measurement of when the solid first begins melting, to when the solid has completely melted

Utilises a melting point range

Melting point rangwe

The temperature difference from when the solid first shows signs of melting to when it has completely finished melting

If pure - 0.5-2 degrees (the range) - is called as narrow

If impure - the temperature difference is more than that - broadens the melting point range

Why impurities have a broader melting point range

Impurities are when multiple organic structures combine - is impure

• Impurities introduce different particles into the lattice.

• These particles:

  • Distort the lattice

  • Create regions with weaker intermolecular forces

  • Are unevenly distributed throughout the solid

Some parts are pure - melt quicker

Some parts are impure - melt slower

As a rnage - it has a broader melting point raneg

Why impurities have a lower melting point

If pure - lattice is structured and very compact - has strong bonding due to its crystalline nature

If Impure - branching and an irregular structure contributes to decreasing the overall melting poitn

Why pure structures have sharp/narrow melting point range

• A pure solid has particles arranged in a uniform, repeating lattice.

• This means all particles require about the same amount of energy to overcome the intermolecular forces → sharp melting.

If melting point of compound is higher

It is not the desired product

If the melting point of the compound is lower

It is either impure or not the desired product

Mixed Melting Point Determination

When a pure sample is mixed with an unknown sample

If this new mixture has the same melting point range and melting point of the known substance, then it is pure

If not it is impure

Identifying Prescence of functional groups (C=C bond)

Adding a liquid such as bromine (red or red-orange in color) to the alkene

As it is unsaturated, the alkene undergoes an addition reaction and takes in the Bromine atoms - rapidly decolourises the solution

The color of the solution thus turns colorless

Visual Representation

Testing Prescence for hydroxyl group

Is mixed with a carboxylic acid + sulfuric acid (as a catlyst)

If an ester is formed - then there is hydroxyl group

Testing if it’s a tertiary alcohol or primary and secondary alcohols (Review Chapter 11)

Primary and secondary alcohols oxidize easily but tertiary alcohols do not

So when adding acidified potassium dichromate or acidified potassium permanganate - if it changes color - then primary or secondary alcohol

If not - tertiary

If manganese

The permanganate turns into Mn⁺² which is colourless

Testing for Prescence of carboxyl group

As the carboxyl group is acidic, when it reacts with a carbonate (acid and base reaction), it produces CO₂ gas

Acid + carbonate (or bicarbonate) —→ Salt+ Water + CO₂

This can be confirmed by doing the limewater test (tube connecting the system to the limewater - if cloudy/milky, then CO₂ was produced)

Visual representation (Insert here)

Summary for testing all functional groups

Example (Must do)

Solutions

Example 2

Are tertiary alcohols soluble

Yes

if short chains - very soluble because OH bond is polar (proportion is equally matched)

if longer chain - not much - the polar bond is outweighed by the chain’s non-polar characteristic

Example 2 Answers

A - But-2-ene

B - Propan-1-ol

E - Ethanoic Acid

D - Pentane

C - 2-Methylpropan-2-ol

Instead of Bromine, why is Iodine more preferable to use in checking for C=C bonds?

More stable

Less dangerous - Aqueous bromine releases harmful fumes of bromine - creates respiratory hazard

How is Iodine prepared to make it a suitable alternative to Bromine (unclear)

Making it standardised using a sodium thiosulfate solution

How Iodine number is calculated

As one mole of I₂ reacts with one mol of C=C bonds, the nymber of moles of I₂ = the no. of moles of C=C bonds in the fatty acid molecule

Must find how much I₂ reacts with 100 g of that molecule

Example

Worked Example (Insert Solution)

Standard Solution

A solution of accurately known concentration

Primary Standrad

A very pure substance, such that its moles can be calculated accurately from its mass

Conditions of a primary standard

How standard solutions are prepared

Dissolving accurately measured mass of primary standard in water to make an accurately measured volume of solution

Using titration with another standard solution to find its concentration

If a a standard solution is to be made from something that is not a primary standard (already pure)

• Primary standard → can make a solution directly (accurate from weighing).

• Impure/unstable chemical → must be titrated first → then you get a standardised solution.

Steps In Tiration

Equivalence Point (Key wording)

When the reactants have when the amount of titrant added is exactly enough to completely react with the analyte.

The reactants are in stoichiometric proportion as indicated in the equation

Endpoint

Observable change in the titration’s colour - signals that the equivalence point is reached - shows that reaction is very close to or is completed

Steps of titration

How To Rinse Glassware

Steps Of Volumetric Analysis

How far should concordant titres be away from each other

by 0.1ml (maximum gap)

Errors In Volumetric Analysis (Refer to chapter 1 errors)

Depends on the calibration and the accuracy of the materials used

Quantitative analysis - aim to make as accurate as possible

typical errors—→

Redox titration

Reaction of an oxidising agent with a reducing agent (one solution is pipetted and another one is dispensed into the flask)

Adding indicator to redox titrations

Some molecules (e.g. those having MnO₄^-) don’t need an indicator - it will change color by itself

For other molecules in redox, an indicator (e.g. starch solution) is needed to represent the equivalence point

Purpose

To check the composition of the substances

To check when they reach equivalence point

Examples

(e.g. by mixing iodine in fruit juice, we can test for the presence of Vitamin C)

Malic Acid

An acid that gives many fruits their “sour” taste

Is a dicarboxylic acid and an alpha-hydroxy acid (AHA).

It plays a vital role in the Krebs cycle (or Citric Acid Cycle), which is the process your body uses to turn food into energy (ATP).

Alcohols undergoing oxidation

Are weak reducing agents (can’t oxidise as much)

They are already partially oxidized with the C-O bond; thus are weaker reducing agents

When oxidized, the OH group turns into a COOH group (carboxylic acid)

Why Alcohols are weak reducing agents

• Ethanol is partially oxidised

• Strong C–H and C–O bonds

• Electron density pulled towards oxygen

This makes oxidation:

• kinetically slow

• less favourable

Criteria for oxidising weak reducing agents such as ethanol

Must have very strong oxidising agent (can pull electrons towards itself)

Must have very harsh conditions:

Heat (reflux)

Acidic conditions

Time)

Steps on titration (unclear)

Example (has longer working out)

Worked example (shorter working) - Insert solution here

Criteria of indicators for redox titrations

Must be weak oxidizing or reducing agents (so that they don’t react too soon, causing a color change that doesn’t reflect the equivalence point accurately)

Must change color drastically (big change) in their reduced or oxidized form - to indicate equivalence point

ChatGPT explanation