To prepare a sample of soap by base hydrolysis/saponification

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30 Terms

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Theory

• Fats and oils found in animals and plants are naturally occurring esters.

• They can be reacted and refluxed with a base to produce a soap and alcohol

• Known as base hydrolysis/saponification

Example: Glyceryl tristearate, is an ester typically found in animal fats - if it is reacted and refluxed with the base sodium hydroxide, the soap sodium stearate and the alcohol glycerol are produced

<p>• Fats and oils found in animals and plants are naturally occurring esters.</p><p>• They can be reacted and refluxed with a base to produce a soap and alcohol</p><p>• Known as base hydrolysis/saponification</p><p>Example: <strong>Glyceryl tristearate</strong>, is an ester typically found in animal fats - if it is reacted and refluxed with the base <strong>sodium hydroxide</strong>, the soap <strong>sodium stearate</strong> and the alcohol <strong>glycerol</strong> are produced</p>
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Procedure: step 1!

Lard/plant oil is placed into the reaction flask, pellets of sodium hydroxide, ethanol and some anti-bumping chips are added

➢ The reactants are heated in the reaction flask using a hot plate and water bath and refluxed using a Liebig condenser for ̴30 minutes

<p>➢ <strong>Lard/plant oil</strong> is placed into the reaction <strong>flask</strong>, pellets of <strong>sodium hydroxide, ethanol </strong>and some <strong>anti-bumping chips are added</strong></p><p>➢ The <strong>reactants</strong> are heated in the reaction flask using a hot plate and water bath and <strong>refluxed using a Liebig condenser for ̴30 minutes</strong></p>
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Procedure: step 2/

➢ The apparatus is rearranged to a distillation and the flask is heated using a hot plate and water bath to 78˚C to allow most of the ethanol to be distilled off

<p>➢ The <strong>apparatus</strong> is <strong>rearranged to a distillation</strong> and the flask is <strong>heated</strong> using a hot plate and water bath to <strong>78˚C </strong>to allow most of the <strong>ethanol to be distilled off</strong></p>
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Procedure: step 3?

➢ The reaction mixture is poured from the hot reaction flask into a beaker of brine solution, rinsing the reaction flask with the minimum amount of boiling water to remove any residue remaining

➢ A solid material (soap) is observed precipitating out of solution when the mixture is added to the brine with the glycerol, ethanol and excess sodium hydroxide remaining dissolved in the brine

<p>➢ The reaction mixture is <strong>poured</strong> from the hot reaction flask <strong>into</strong> a beaker of <strong>brine</strong> solution, <strong>rinsing</strong> the reaction <strong>flask with</strong> the <strong>minimum amount of boiling water</strong> to <strong>remove</strong> any <strong>residue</strong> remaining</p><p>➢ A solid material (<strong>soap</strong>) is <strong>observed precipitating</strong> out of solution when the mixture is added to the brine with the <strong>glycerol, ethanol and excess sodium hydroxide remaining dissolved in the brine</strong></p>
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Procedure: step 4%

➢ The soap is filtered using vacuum/suction filtration using filter paper, a Buchner funnel and a Buncher flask – the brine solution containing glycerol, ethanol and excess sodium hydroxide will pass through as the filtrate

➢ The soap is washed with fresh brine followed by ice cold water to remove any sodium hydroxide that may be still present

➢ The soap is left to air dry on the filter paper in a warm place or placed in a desiccator

<p>➢ The <strong>soap</strong> is <strong>filtered using vacuum/suction filtration</strong> using filter paper, a Buchner funnel and a Buncher flask – the <strong>brine solution containing glycerol, ethanol and excess sodium hydroxide</strong> will pass through as the <strong>filtrate</strong></p><p>➢ The soap is <strong>washed with fresh brine followed by ice cold water</strong> to <strong>remove any sodium hydroxide</strong> that may be still present</p><p>➢ The soap is <strong>left to air dry on the filter paper </strong>in a warm place or placed in a desiccator</p>
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Why is oil or lard used in this experiment?!

Oils or lard contain naturally occurring estershydrolysing these esters with a base will produce a soap

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Why is sodium hydroxide added to the reaction flask?!

Hydrolysing an ester with a base will produce a soapsodium hydroxide is the base used

Note: Potassium hydroxide could be used as the base

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What is the difference between soap made with sodium hydroxide and soap made with potassium hydroxide?!

Soaps made with potassium hydroxide are milder than soaps made with sodium hydroxide

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Why is excess base (sodium hydroxide) used?!

The base is used in excess to ensure ALL of the ester (lard/oil) reacts and is converted to soapmaximises the yield of soap obtained – the ester is the limiting reagent

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Why is ethanol added to the reaction flask?!

Ethanol acts as a solvent for the oil or lard

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Why are anti-bumping chips added to the reaction flask?!

Anti-bumping granules are added to allow boiling to take place more smoothly

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Describe how the ester and base are reacted in this experiment!

The ester and base (oil/lard + sodium hydroxide) and the ethanol solvent are heated under reflux for 30 minutes - heated until they vapourised and are then cooled back to a liquid in the Liebig condenser and fall back into the reaction flask so they can completely react

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Why is the reaction mixture refluxed?!*

1. Allows the reactants time to react to completion

2. Ensures no loss of the ethanol solvent as it is continuously condensed again

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What type of reaction occurs during the reflux process?!

Saponification/Base hydrolysis

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As the reaction proceeds, a coating is observed on the walls of the reaction flask. What is this coating?!

1. Soap that has formed

2. Lard/oil that has not been hydrolysed

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Why is the reaction flask swirled from time to time under reflux?!

Swirling the reaction flask washes the coating from the walls of the flask back into the reaction mixture

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Give a bi-product of the reflux reaction and give a use for this bi-product!

Bi-product: Glycerol

Use: Manufacture the explosive nitro-glycerine

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Give the IUPAC systematic name for this bi-product and draw its structural formula!

Propane -1,2,3 triol

<p>Propane -1,2,3 triol</p>
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Why is the ethanol then distilled off after reflux?/

Distilling the ethanol off makes it easier to isolate the soap and maximises its yield – some soap would remain dissolved in the ethanol and it would not be precipitated

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What is a suitable heating method for first refluxing the reaction mixture and then distilling off the ethanol. Justify the suitability of this heating method/

• A hot plate and a water bath are used for the reflux and distillation procedures

• Allows for gentle heating where the temperature can be easily controlled

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What is brine? Why is the reaction mixture added to a saturated brine solution??

• Brine is a saturated solution of sodium chloride (salt) in water.

• The soap does not dissolve in salt solution and is precipitated out of solution while other substances (glycerol, sodium hydroxide, ethanol) remain dissolved

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Why is a minimum amount of hot water used to remove residue from the flask??

Using a minimum amount of hot water maximises the amount of soap that will be precipitated.

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Why is the reaction mixture then filtered?% *

• The reaction mixture is filtered to isolate the insoluble soap in the filter paper

• The glycerol, ethanol, sodium hydroxide will pass through dissolved in the brine solution into the Buchner flask in the filtrate

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Why is vacuum filtration performed in preference to gravity filtration?%

a) To speed up the filtration process – faster than using regular filter paper

b) To help dry the soap

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What two substances is the soap washed with after the filtration and why?%

i. Fresh salt solution (brine) - dissolves and removes any sodium hydroxide that may be present with the soap – sodium hydroxide causes burns

ii. Ice-cold water – To wash off the brine. The water is ice-cold to avoid any soap dissolving maximising its yield

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How is the soap left to dry overnight?%

The soap is placed on filter paper and left to air dry in a warm place

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Outline a test that is performed on the prepared soap%

The soap is added to a test tube of de-ionised water (soft water) and shaken

Result: The presence of a lather proves that the substance is soap

Note: If the soap is added to a hard water sample, it will be more difficult to form a lather

Example: river water

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Outline two safety precautions taken in this experiment%

1) Wear googles and glovessodium hydroxide causes burns

2) Take care when dealing with hot flasks during the reflux and distillation stages – keep them in a water bath to prevent overheating

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Explain with reference to its structure, how a soap such as sodium stearate can dissolve both oils and salts in sweat from the skin/Explain why one end of a soap molecule is described as water loving (hydrophilic) and one end is described as water hating (hydrophobic)%

• Soap consists of a non-polar hydrocarbon portion (C17H35) that dissolves non-polar substances such as oils and grease – water hating part

• Soap also consists of an ionic portion ( - COO – Na+) that dissolves salts from sweats and causes the soap to dissolve in water – water loving part

Example: Sodium stearate

<p>• Soap <strong>consists</strong> of a <strong>non-polar hydrocarbon portion (C17H35)</strong> that <strong>dissolves non-polar substances</strong> such as oils and grease – water hating part</p><p style="text-align: start">• Soap also <strong>consists of an ionic portion ( - COO – Na+)</strong> that <strong>dissolves salts </strong>from sweats and <strong>causes</strong> the <strong>soap to dissolve in water</strong> – water loving part</p><p style="text-align: start">Example: Sodium stearate</p>
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Soap can be produced using vegetable and animal fats. What is the principal chemical

difference between vegetable and animal fats?%

Vegetable fats are unsaturated (contain double bonds between some of the carbons)

Animal fats are saturated (contain only single bonds between carbons)