Analytical Techniques

Question 1

Where are analytic techniques such as spectroscopy used?

Answer 1

In a wide range of analytical laboratories including forensic, public health and industrial

Question 2

What does mass spectrometry provide information about?

Answer 2

Relative atomic mass and relative molecular mass

Question 3

What does high-resolution mass spectrometry enable?

Answer 3

Empirical formulae to be determined

Question 4

What do fragmentation patterns in mass spectrometry provide?

Answer 4

Clues about the bonds and functional groups present which can be used to work out the structure of a substance

Question 5

For simple quick and relatively inexpensive identification of functional groups what can be done?

Answer 5

Qualitative analysis using test tube reactions

Question 6

How do you test for alkenes?

Answer 6

Add dilute bromine solution which rapidly decoulorises when reacted with an alkene. This is because the bromine reacts with the c=c bond

Question 7

How do you test for halogenoalkanes?

Answer 7

• The carbon-halogen bond in a halogenoalkane I’d polar and susceptible to nucelophillic substitution. The hydroxide ion is a nucleophile. reaction with a halogenoalkane and a halide ion is one of the products formed
• the halide ion can be identified using acidified silver nitrate solution followed by ammonia solution

Question 8

Why are fluoroalkanes not detected when testing for a halogenoalkane?

Answer 8

Because they do not hydrolysed because the C-F bond is very strong and cannot be easily broken

Question 9

Describe the process of testing for a chloroalkane and what the results are

Answer 9

• warm with dilute sodium hydroxide solution
• add acidified silver nitrate solution: white precipitates of silver chloride form
• add dilute ammonia solution followed by a concentrated ammonia solution: white precipitate dissolves conform a colourless solution when dilute ammonia is added

Question 10

Describe the process of testing for a chloroalkane and what the results are

Answer 10

• warm with dilute sodium hydroxide solution
• add acidified silver nitrate solution: white precipitates of silver chloride form
• add dilute ammonia solution followed by a concentrated ammonia solution: white precipitate dissolves conform a colourless solution when dilute ammonia is added

Question 11

Describe what the process of testing for a bromoalkane is and what the procedure is

Answer 11

• warm with dilute sodium hydroxide solution
• add acidified silver nitrate solution and a cream precipitate of silver bromide forms
• add dilute ammonia solution, followed by concentrated ammonia solution and a cream precipitate dissolves to form a colourless solution only when concentrated ammonia is added

Question 12

Describe the process of testing for an iodoalkane and what the result is

Answer 12

• warm with dilute sodium hydroxide solution
• add acidified silver nitrate solution and a yellow precipitate of silver iodide forms
• add dilute ammonia solution followed by concentrated ammonia solution and the precipitate is insoluble in both dilute and concentrated ammonia solution

Question 13

Describe the process of testing for an iodoalkane and what the result is

Answer 13

• warm with dilute sodium hydroxide solution
• add acidified silver nitrate solution and a yellow precipitate of silver iodide forms
• add dilute ammonia solution followed by concentrated ammonia solution and the precipitate is insoluble in both dilute and concentrated ammonia solution

Question 14

What is the test you do to test whether something is a primary or secondary alcohol?

Answer 14

You add acidified potassium dichromate solution (VI). If it is a primary or secondary alcohol it will be oxidised to an aldehyde and the colour will change from orange to green. If it is a tertiary alcohol no reaction will take place

Question 15

What is the test you do to test whether something is a primary or secondary alcohol?

Answer 15

You add acidified potassium dichromate solution (VI). If it is a primary alcohol it will be oxidised to an aldehyde and if it is a secondary alcohol it will be oxidised to a ketone and the colour will change from orange to green. If it is a tertiary alcohol no reaction will take place

Question 16

What test do you do to distinguish between a primary and secondary alcohol?

Answer 16

• Warm with Fehlings solution after it has been oxidised: a brick red precipitate will form if it is a primary alcohol and has been oxidised to an aldehyde. It will stay blue if it is a secondary alcohol oxidised to a ketone
• warm with Tollens reagent:a silver mirror will form on the test tube if it is a primary alcohol and has been oxidised to an aldehyde. It will stay blue if it is a secondary alcohol oxidised to a ketone

Question 17

How do you test for a carboxylic acid?

Answer 17

Add sodium hydrogen carbonate solution and carbon dioxide will be given off. There will also be a low pH of the solution

Question 18

In qualitative tests what is it not necessary to do?

Answer 18

Measure volumes exactly

Question 19

Testing for an alkene involves adding bromine water. If the alkene is a liquid the two reagents can be added in equal volumes in a clean test tube. The mixture needs to be shaken. How do you do this?

Answer 19

Hold the test tube just below the lip and shake it with a jiggling motion

Question 20

How can you react an alkene gas with bromine water?

Answer 20

You can either bubble it through the bromine water or if it is collected in test tubes, bromine water can be added to the test tube and the test tube shaken

Question 21

How do you test the pH of a carboxylic acid?

Answer 21

• Dip the end of a glass rod into the liquid, withdraw it with a drop of liquid clinging to it and touch this onto a piece of pH paper.
• Use a pH probe

Question 22

How do you test for carbon dioxide given off after a carboxylic acid has reacted with a carbonate or hydrogen carbonate?

Answer 22

Use lime water

Question 23

When oxidising the alcohol what is the technique you carry out?

Answer 23

Add 1cm3 acidified potassium dichromate (VI) solution to the substance being tested, shake the test tube and then place it in a hot water bath for a few minutes

Question 24

How do you add fehlings solution to a sample being tested?

Answer 24

Add about 1cm3 of Fehling’s A and B solutions to a test tube. Add a few drops of the sample being tested. Fehlings solution is unstable so needs to be mixed right before. Then place the tube in a hot water bath for a few minutes

Question 25

What technique do you carry out when testing with tollens reagent?

Answer 25

Place 2cm3 of the sample being tested in a test tube and add about 5cm3 of tollens reagent. Leave at room temperature for a few minutes

Question 26

What can mass spectrometry be used for?

Answer 26

* to determine the mass numbers of an elements isotopes and their relative abundance * to determine the relative molecular mass and the molecular formula of a compound

Question 27

A sample of ethanol injected into a time of flight mass spectrometer vaporises. In the ionisation area the gas is bombarded with a stream of electrons. What are the two possibilities?

Answer 27

• electrons are knocked off the molecules and positive ions form. Most molecules lose one electron. Ethanol molecules lose an electron to become [C2H5OH]+: C2H5OH -> [C2H5OH]+ • some of the molecular ions break into smaller fragments. The charged fragments will also live through the spectrometer and produce lines in the spectrum.

Question 28

[C2H5OH]+ ions are the heaviest ions that can be produced from a sample of ethanol. What is the peak they produce on a mass spectrum called?

Answer 28

The molecular ion peak or parent ion because other molecular ions are made from it when it fragments in the mass spectrometer.

Question 29

What is the m/z value of the molecular ion peak?

Answer 29

The molecular mass of the compound

Question 30

Why cant you suggest the molecular formula of a molecule from the molecular ion peak using a low- resolution mass spectrometer?

Answer 30

Because there are several possible macular formulae for an particular mass both propane and ethanol have a relative molecular mass of 44

Question 31

Why can a high resolution mass spectrometer provide the information to work out the molecular formular of a compound?

Answer 31

It gives the relative isotopic masses to 6 significant figures so it allows an accurate identification of the molecular formula from the molecular ion peak

Question 32

What happens if infrared radiation is passed through a compound?

Answer 32

Some of the radiation is absorbed and some is transmitted

Question 33

What does infrared spectroscopy measure?

Answer 33

The wavelengths of infrared radiation that has been absorbed

Question 34

What is the infrared spectrum?

Answer 34

The chart that comes from the wavelength of infrared radiation. Wavelength is plotted along the x axis and the amount of radiation transmitted on the y axis

Question 35

What can infrared radiation be used to do?

Answer 35

Identify bonds in a compound

Question 36

In infrared spectroscopy you use wavenumber yo describe radiation. What is this and how can it be calculated?

Answer 36

* it is the number of wave cycles per cm | * wavenumber= 1/ wavelength/cm cm-1

Question 37

What can the absorbances of the infrared spectrum be used to do?

Answer 37

Identify bonds and hence the functional groups in a molecule. Each bond will absorb a different wavenumber of radiation

Question 38

What is the fingerprint region.

Answer 38

The region of the infrared spectrum between 500 and 1500 cm-1

Question 39

What happens in the fingerprint region?

Answer 39

The wave numbers in this range are also absorbed by vibrations of the whole molecule in addition to the wave numbers absorbed by the C-C and C-O bonds thus means that this region of the infrared spectrum ie usually complicated with many absorptions

Question 40

What is the radiation transmitted from the sun to earth and what is the radiation emitted from the earth's surface and what does it do?

Answer 40

* sun to earth: UV | * earth: infrared. It warms earth as the greenhouse gases absorb it

Question 41

What means that greenhouse gases absorb infrared radiation?

Answer 41

* The O-H bonds in water molecules strongly absorb infrared radiation at 3000-3800 cm -1 * The C-H bonds in methane absorb infrared at about 3000 cm-1 (characteristic of all alkanes) * The C=O bonds in carbon dioxide absorb strongly at 2350 cm-1

Question 42

What happens as the concentration of greenhouse gases in the atmosphere increases?

Answer 42

* infrared radiation absorbed by greenhouse gas molecules does not escape the atmosphere * as the concentration of greenhouse gases in the atmosphere increases the amount of infrared radiation being absorbed increases * this results in an increase in the kinetic energy of the molecules in the atmosphere which is detected as a temperature increase and referred to as global warming