2.8 - Instrumental analysis

Question 1

Where is A and Z on an element?

Answer 1

Z = atomic number
A = mass number

Question 2

Why is the mass number on a periodic table a decimal?

Answer 2

Decimal number due to mass of isotopes

Question 3

What is the definition of an isotope?

Answer 3

Atoms of the same element with different numbers of neutrons and different masses

Question 4

How does the reactivity of isotopes vary?

Answer 4

They react in the same way because they have the same electronic configuration

Question 5

What is the abundance?

Answer 5

Tells us how common each isotope is.

Question 6

How does a mass spectrometer work?

Answer 6

• element with all its isotopes placed in a sample chamber
• ionsation converts all atoms into positive ions
• positive ions are attracted to the neegatively charged plates, causing acceleration and increase in KE (ions with same charge have same KE)
• ions pass plate and stop accelerating, drifts towards the detector (ions drift at different velocities light - fast, heavy - slow)
• ions reach detector and gains electrons to make it neutral, causes a current flow, the time taken is used to determine mass and size of current produced is used to determine abundance

Question 7

Why is mass spectrometry done within a vacuum?

Answer 7

To stop the ions colliding with elements in the air

Question 8

What does a mass spectrum look like?

Answer 8

Relative aundance (%) on y axis
M/Z ratio or relative mass on x axis

Question 9

What is the definition of relative isotopic mass?

Answer 9

Mass of an atom of an isotope compared with 1/12th the mass of carbon 12.

Question 10

What are the 3 rules of relative isotopic mass?

Answer 10

• RIM has no unis
• RIM is a whole number
• 1 RIM per isotope of an element

Question 11

What is the definition of relative atomic mass?

Answer 11

Weighted mean mass of an atom of an element compared with 1/12th the mass of carbon-12

Question 12

What is the formula for relative atomic mass through mass spectroscopy?

Answer 12

Ar = (RIM of isotope 1 x abundance)+(RIM of isotope 2 x abundance)/100

Question 13

What is infrared spectroscopy?

Answer 13

Identifying the bonds in a molecule due to them absorbing different frequencies of infrared.
Used to identify functional groups

Question 14

What does a infrared spectroscopy look like?

Answer 14

Transmittance / wavenumber
Uses trophs to shows bonds present in a molecule

Question 15

What is the fingerprint region?

Answer 15

Area thats really messy between 500-1000, specific to the molecule

Question 16

What is nuclear magnetic resonance spectroscopy?

Answer 16

• using absorption of different frequency radiowaves to indentify different parts of a molecule
• similiar to IFS, but this is about the nucleons in a atom

Question 17

How does NMR work?

Answer 17

• protons and neutrons have spins too
• if a magnetic field is applied, their spins line up along or against it
• if no. of nucleons is even, they are balanced
• if no. of nucleons is odd, radio waves can be absorbed so that the spin can be flipped
• nucleons in different environments can absorb different frequencies

Question 18

What is NMR measured relative to?

Answer 18

Standard substance Si(CH3)4 or TMS for short
Used because it is unreactive and volatile, and only asorbs one frequency because its 1 environment

Question 19

What does 13C NMR do?

Answer 19

give the proportion of different carbon bonds in a sample

Question 20

What affects chemical shifts?

Answer 20

The distance from functional groups, can effect the number of peaks due to different environments (can be detected through symmetry)

Question 21

What is H NMR?

Answer 21

• aka proton NMR
• can give proportions of H in different environments
• use of ratios

Question 22

What do high resolution NMR’s show?

Answer 22

Peaks split due to neighboring carbons
no of splits = no, H’s on neighbouring atom + one

Question 23

What is the difference between a 13C NMR and 1H NMR?

Answer 23

13C NMR - gives you what carbons are bonded to
1H NMR - what hydrogens are bonded to, how many hydrogens in the same environment, how many hydrogens on neighbouring carbons (split peaks)