Topic 7: Modern Analytical Techniques I

Question 1

What type of laboratory method is used to identify functional groups in organic compounds?

Answer 1

Test-tube reactions

Reagents produce characteristic colour changes, precipitates or gas evolution.

Question 2

What reagent is used to test for the presence of a carbon–carbon double bond?

Answer 2

Bromine water

Alkenes rapidly decolourise orange bromine water.

Question 3

Fill in the blank:

When an alkene reacts with bromine water, the solution changes from orange to _______.

Answer 3

colourless

The bromine adds across the C=C bond.

Question 4

True or False:

Carboxylic acids produce effervescence when reacted with carbonates.

Answer 4

True

CO₂ gas is released during the reaction.

Question 5

What gas is produced when a carboxylic acid reacts with sodium carbonate?

Answer 5

Carbon dioxide

The reaction forms CO₂, water and a salt.

Question 6

True or False:

Alcohols produce a colour change with acidified potassium dichromate when oxidised.

Answer 6

True

The solution changes from orange to green.

Question 7

Fill in the blank:

The reagent commonly used to oxidise alcohols in identification tests is acidified potassium _______.

Answer 7

dichromate

K₂Cr₂O₇ turns from orange to green during oxidation.

Question 8

What reagent forms a silver mirror when reacting with aldehydes?

Answer 8

Tollens’ reagent

Aldehydes reduce Ag⁺ ions to metallic silver.

Question 9

True or False:

Ketones give a positive result with Tollens’ reagent.

Answer 9

False

Ketones are not readily oxidised under these conditions.

Question 10

Fill in the blank:

Fehling’s solution gives a _______ precipitate when reacting with aldehydes.

Answer 10

brick red

Cu²⁺ ions are reduced to Cu₂O.

Question 11

What functional group produces carbon dioxide when reacting with sodium carbonate?

Answer 11

Carboxylic acid

The –COOH group reacts with carbonate ions releasing CO₂.

Question 12

Why are test-tube reactions useful in organic chemistry analysis?

Answer 12

Functional group identification

Observations help distinguish between different types of organic compounds.

Question 13

What analytical technique is used to determine the molecular mass of a compound?

Answer 13

Mass spectrometry

It measures the mass-to-charge ratio of ions produced from molecules.

Question 14

What property of ions is measured in a mass spectrometer?

Answer 14

Mass-to-charge ratio

This is often written as m/z.

Question 15

Fill in the blank:

In mass spectrometry, the horizontal axis of the spectrum represents _______.

Answer 15

m/z

It shows the mass-to-charge ratio of detected ions.

Question 16

True or False:

Mass spectrometry can be used to determine the molecular formula of a compound.

Answer 16

True

Precise molecular mass helps identify the exact combination of atoms.

Question 17

What peak in a mass spectrum usually represents the molecular ion?

Answer 17

Molecular ion peak

This corresponds to the mass of the intact molecule.

Question 18

True or False:

The molecular ion peak normally has the highest intensity in the spectrum.

Answer 18

False

The base peak has the highest intensity.

Question 19

Fill in the blank:

The most intense peak in a mass spectrum is called the _______ peak.

Answer 19

base

It is assigned a relative abundance of 100%.

Question 20

What type of masses are used when calculating molecular formula using mass spectrometry data?

Answer 20

Precise atomic masses

These allow accurate determination of molecular composition.

Question 21

True or False:

The molecular ion in mass spectrometry is formed by removing an electron from the molecule.

Answer 21

True

This produces a positively charged ion (M⁺).

Question 22

Fill in the blank:

The molecular ion formed in mass spectrometry is usually written as _______.

Answer 22

M⁺

It represents the intact molecule with a positive charge.

Question 23

Why are precise molecular masses important in determining molecular formulas?

Answer 23

Distinguish formulas

Different formulas can have similar relative molecular masses.

Question 24

What information can be determined by combining atomic masses with the molecular ion mass?

Answer 24

Molecular formula

The exact composition of atoms in the molecule can be deduced.

Question 25

What **analytical technique** is used to identify **functional groups by measuring absorption of infrared radiation**?

Answer 25

Infrared spectroscopy ## Footnote Different bonds absorb infrared radiation at characteristic frequencies.

Question 26

What **property of molecular bonds** allows them to absorb **infrared radiation**?

Answer 26

Bond vibrations ## Footnote Bonds stretch and bend when absorbing infrared energy.

Question 27

# Fill in the blank: In **infrared spectroscopy**, absorption is usually measured using _\_\_\_\_\_\_ numbers.

Answer 27

wavenumbers ## Footnote Wavenumber is measured in cm^-1.

Question 28

# True or False: Different **chemical bonds** absorb **infrared radiation at characteristic wavenumbers**.

Answer 28

True ## Footnote This allows identification of specific functional groups.

Question 29

What **region of an infrared spectrum** is unique for each molecule and used for identification?

Answer 29

Fingerprint region ## Footnote This region provides a distinctive pattern for each compound.

Question 30

# True or False: The **fingerprint region** is identical for most organic compounds.

Answer 30

False ## Footnote It is highly specific and differs between molecules.

Question 31

# Fill in the blank: **Infrared spectroscopy** helps identify _\_\_\_\_\_\_ groups within organic molecules.

Answer 31

functional ## Footnote Each functional group absorbs IR radiation at characteristic values.

Question 32

What additional information can **infrared spectra** provide besides identifying functional groups?

Answer 32

Impurities ## Footnote Unexpected peaks may indicate the presence of impurities.

Question 33

# True or False: **Infrared spectra** can be compared with **reference spectra** to identify unknown substances.

Answer 33

True ## Footnote This comparison process is known as fingerprinting.

Question 34

# Fill in the blank: **Greenhouse gases** absorb infrared radiation and contribute to _\_\_\_\_\_\_ warming.

Answer 34

global ## Footnote Molecules like CO₂, CH₄ and H₂O trap heat in the atmosphere.

Question 35

Which **atmospheric gas** absorbs infrared radiation and contributes significantly to the **greenhouse effect**?

Answer 35

Carbon dioxide ## Footnote Its molecular vibrations absorb infrared radiation.

Question 36

Why do molecules such as **CO₂**, methane and water vapour contribute to **global warming**?

Answer 36

Infrared absorption ## Footnote They absorb and re-emit infrared radiation, trapping heat in the atmosphere.