Colour by Design

Question 1

How do dyes attach themselves to fibres?

Answer 1

By intermolecular bonds, ionic bonds, or by covalent bonding

Question 2

Why do good dyes need to be colourfast?

Answer 2

So they don’t wash out too quickly or fade in light

Question 3

What does colourfastness depend on?

Answer 3

The strength of the bond between the dye and the fibre.

Question 4

If there’s a -NH2 group in a dye, how will it bond to fabrics?

Answer 4

The -NH2 group allows H-bonding to take place with fibre. This type of bonding works particularly well with cellulose fibres because there’s lots of -OH groups

Question 5

Are dyes that use H-bonding colourfast?

Answer 5

Not particularly because H-bonds aren’t that strong

Question 6

How do dyes with acidic groups bond to fibres?

Answer 6

If there’s an -NH2 group in the fibre, H+ ions from the dye move to the -NH group to make -(NH3)+. This allows ionic interactions to take place

Question 7

What are the two strategies attaching dyes to fibres by covalent bonds?

Answer 7

Mordanting or adding fibre-reactive groups to the dye

Question 8

Describe mordanting

Answer 8

This makes use of a metal ion to join the dye to the fabric: groups on the fabric and the dye form dative covalent bonds to the metal ion. This forms a chelate complex ion

Question 9

What is a chelate?

Answer 9

A complex ion in which the metal ion is bonded to two or more atoms on the same molecule

Question 10

Why is adding fibre-reactive groups to dyes used?

Answer 10

The reactive groups attached to the dye form a bridge between the dye and the fibre

Question 11

When do instantaneous dipole-induced dipole bonds form between dyes and fabrics?

Answer 11

If the fibre doesn’t have many polar groups, it can only bond using instantaneous dipole-induced dipole bonds. These dyes are known as disperse dyes

Question 12

How are disperse dyes added to fabrics?

Answer 12

The dyes are suspended in water. Then the mixture is added to the fibre and the small dye molecules are able to diffuse into the fabric. They are held there by instantaneous dipole-induced dipole bonds.

Question 13

What is a chromophore?

Answer 13

The part of a molecule, which is delocalised, that causes it to absorb light.

Question 14

What types of functional groups modify the chromophore?

Answer 14

Groups that affect the degree of delocalisation, like benzene rings

Question 15

Name a functional group that makes a dye more soluble.

Answer 15

Sulfonic acid groups on benzene rings

Question 16

How does the addition of a nitro group to a benzene ring modify the chromophore?

Answer 16

It changes its properties by changing the wavelength of light the chromophore absorbs, thereby changing the colour of the mixture.

Question 17

Which functional groups can affect a dye’s ability to bond to fibres?

Answer 17

The addition of -OH and -NH2 groups mean that more H bonds can form between the dye and the fibre.
The addition of ionic groups like SO3- mean that more ionic bonds can form between the dye and the fibre.

Question 18

Define the term ‘triglyceride’

Answer 18

An ester molecule formed by the esterification of glycerol by three fatty acids

Question 19

Why are triglycerides also known as triesters?

Answer 19

The systematic name of glycerol is propan-1,2,3-triol, and so can form three ester linkages to carboxylic acids.

Question 20

What are two examples of triglycerides?

Answer 20

Fats and oils

Question 21

What is the main difference between fats and oils?

Answer 21

Fats are solid at room temperature and oils are liquid at room temperature

Question 22

Describe the hydrolysis of triglycerides.

Answer 22

They are heated with concentrated sodium hydroxide to give glycerol and the sodium salts of the fatty acids. The fatty acids are then released from their salts by adding dilute HCl.

Question 23

Explain why fats and oils aren’t soluble in water (4 marks)

Answer 23

• Intermolecular bonds in fats/oils are instantaneous-induced dipole
• Intermolecular bonds in water are hydrogen bonds
• Only instantaneous-induced dipole/permanent-induced dipole are possible between water and fats/oils
• Energy released when fat/oil dissolved isn’t enough to break bonds in solute and solvent