Chapter 11: Measurements and data processing

Question 1

What are random uncertainties causes by?

Answer 1

Random uncertainties are caused by the limitations of the measuring apparatus and other uncontrollable variables that are inevitable in any experiment.

Question 2

What are several sources of random uncertainties?

Answer 2

Variations due to air currents in the room, the heating effects of the current in the circuits, friction between various mechanical parts etc

Question 3

How can the effect of random uncertainties be reduced?

Answer 3

Repeating the measurements more often

They can never be completely eliminated

Question 4

How to calculate the uncertainty of a measurement?

Answer 4

It is half the smallest division to which you take a reading.

for example,

on the 100cm3 measuring cylinder, it is divided apart by 1cm3 and so the uncertainty is + or - 0.5cm3

Question 5

On an electronic balance, it reads to two decimal places, the smallest division is 0.01g, what is the uncertainty?

Answer 5

±0.005 g

Question 6

What does precision relate to?

Answer 6

Precision relates to the reproducibility of results. If a series of readings is taken with high precision, it indicates that the repeated values are all very close together and close to the mean (average) value.

Question 7

What does accuracy refer to?

Answer 7

Accuracy refers to how close a measurement is to the actual value of a particular quantity.

Question 8

What causes systematic errors and can it be reduced?

Answer 8

A systematic error is an error introduced into an experiment by the apparatus or the procedure.

It cannot be reduced by repeating the readings?

Question 9

What are some example of systematic errors?

Answer 9

• the beaker is not that well insulated so heat will escape – the measured temperature rise will be less than the actual value
• the reaction does not occur instantaneously and the thermometer does not respond instantaneously and so the measured temperature rise will be less than the actual value
• the concentration of the sodium hydroxide is less than 0.10moldm−3 and so the measured temperature rise will be less than the actual value

Question 10

How to calculate percentage error?

Answer 10

Question 11

What is the difference between accuracy and precision?

Answer 11

Precision: the reproducibility of results

Accuracy: how close a value is to the true value of the measurement

Question 12

What does it mean when the percentage error is greater than the percentage random uncertainty?

Answer 12

this suggests that the experiment involves some systematic errors

Question 13

How can systematic errors be reduced?

Answer 13

By changing the way the experiment is carried out

Question 14

See this example, how do we find out the uncertainty of the temperature change?

Answer 14

Add 0.02 +0.02 together

Question 15

Wen quantities with uncertainties are added or subtracted, what is done to the absolute uncertainty?

Answer 15

They are added

Question 16

When multiplying or dividing quantities with uncertainties, what is done to the uncertainties?

Answer 16

The percentage uncertainties should be added

Question 17

What is done to the percentage uncertainty when multiplying or dividing a quantity with a pure number?

Answer 17

The absolute uncertainty is multiplied/divided by that number so that the percentage uncertainty stays the same

Question 18

How should the uncertainty be quoted?

Answer 18

The uncertainty is usually quoted to one sig fig and your measurement should be stated so that the uncertainty is in the last sig dig - no figures should be quoted after the uncertainty

Question 19

What does the gradient represent in a graph?

Answer 19

The rate of the reaction

Question 20

What is an infrared spectroscopy used for?

Answer 20

To determine the structure of an organic compound

Electromagnetic radiation in the range 400-4000cm01 is passed through a sample. The printout of the spectrum then shows which frequencies (wavenumber) are absorbed. Can be used to determine which bonds are present in a molecule because the precise wavenumber at which infrared radiation is absorbed by a particular functional group depends on the adjacent atoms.

Question 21

What happens to a sample of organic compound is injected into a mass spectrometer?

Answer 21

1. Bombarded with high energy electrons to produce positive ions.
   
   1. Pass on through the mass spectrometer where they are separated according to mass and detected

Question 22

What are the other peaks other than the molecular ion peak?

When do fragmentation pattern arise?

Answer 22

When a molecule can break apart into smaller fragments when it is bombarded by high energy electrons

Question 23

Only what type of ions give peaks in a mass spectrum?

Answer 23

Positive ions

Question 24

What elements nucleus has a property called spin?

Answer 24

Hydrogen

Question 25

How many decimal places should the final answer have?

Answer 25

If a calculation involves just adding or subtracting numbers, the final answer should be quoted to the same number of decimal places as the piece of original data that has the fewest decimal places

e. g.
23. 57-8.4=15.17 but the final answer should be quoted as 15.2

Question 26

If a calculation involves multiplication/division, what is the general rule for the final answer compared to a calculation with addition/subtraction?

Answer 26

The final answer should be quoted to the number of significant figures of the piece of data with the fewest significant figures.

for addition and subtraction, it is the same number of decimal places rather than significant figures

Question 27

When should rounding values in calculations take place?

Answer 27

Only when an answer to a particular part of a question is required

Question 27

When should rounding values in calculations take place?

Answer 27

Only when an answer to a particular part of a question is required

Question 27

When should rounding values in calculations take place?

Answer 27

Only when an answer to a particular part of a question is required

Question 28

How to calculate percentage uncertainty?

Answer 28

Question 29

What values do you put in the formula to calculate percentage uncertainty?

Answer 29

Question 30

What unit is used for a percentage uncertainty?

Answer 30

A percentage uncertainty has no units

Question 31

What unit is used for a percentage uncertainty?

Answer 31

A percentage uncertainty has no units

Question 31

What unit is used for a percentage uncertainty?

Answer 31

A percentage uncertainty has no units

Question 32

How are uncertainties propagated in multiplication/division calculations?

Answer 32

The percentage uncertainties should be added

for addition and subtraction the absolute uncertainties are added

Question 33

What to do about the uncertainty when multiplying or dividing a quantity with uncertainty by a pure number?

Answer 33

the absolute uncertainty is multiplied/divided by that number so that the percentage uncertainty stays the same

Question 34

Does proportional and directly proportional mean the same thing?

Answer 34

Yes

Question 35

What is a proportional and inversely proportional relationship?

Answer 35

Question 36

How to draw a line of best-fit?

Answer 36

• Do not join the dots
• may be a straight line or a curve
• should represent the trend in the data
• points should be evenly distributed about the line
• is very much a matter of judgement and no two lines of best fit drawn by different people will be identical

Question 37

When should the line of best fit not be drawn?

Answer 37

When discrete data are plotted

• points may be joined or the data could be plotted as a bar chart

Question 38

What does the gradient tell us?

Answer 38

Gives us an idea how much one quantity (the dependent variable) is affected by another quantity (the independent variable)

Question 39

What does it mean when the gradient is big?

Answer 39

A small change in the IV has a large effect on the DV

Question 40

How to calculate the gradient?

Answer 40

Change in y or change in x

Question 41

How are the units of the gradient obtained?

Answer 41

by dividing the units of y by the units of x

Question 42

How to find the gradient of a curve?

Answer 42

• Draw a tangent

Gradient = (70-0)/(32-0) = 2.2cm3s-1

Question 43

How should a tangent be drawn?

Answer 43

As large as possible to minimise uncertainties caused by reading values off the graph

Question 44

How do we know if there is one double bond or a ring present?

Answer 44

for every two hydrogen atoms fewer than in the alkane with the same number of carbon atoms

Question 45

How many H will an alkane have with 4 carbons? (C₄H?)

Answer 45

C₄H₁₀

CnH2n+2 - alkane formula

Question 46

If the parent alkane is C₄H₁₀, what would the molecule that contain one double bond or one ring look like?

Answer 46

C₄H₈

Question 47

How many rings or double bonds or triple bonds would there be present in C₄H₆?

Answer 47

Has four hydrogens fewer than in the alkane and therefore has two double bond equivalents
* 2 double bonds
* one triple bond
* two rings (not possible with 4 carbon atoms)
* one right and one double bond

Question 48

What is IHD (degree of unsaturation or the index of hydrogen deficiency)?

Answer 48

The number of double bond equivalents

Question 49

How many double bond is a ring equivalent to?

Answer 49

1

Question 50

How many double bond is a triple bond equivalent to?

Answer 50

2

Question 51

WHat is the IHD of C₂H₆O?

Answer 51

Has the same number of hydrogen as ethene (C₂H₆) and contains no double bonds

IHD = 0

Question 52

What is the IHD of C₄H₈O?

Answer 52

Has two hydrogens fewer than butane and contains either one double bond or one ring

IHD = 1

Question 53

How to calculate the IHD of a compound with a halogen atom?

Answer 53

we count this as a hydrogen atom and work out the number of double bond equivalents

Question 54

What is the IHD of C₄H₈Cl₂?

Answer 54

We replce the Cl atoms with hydrogen atoms which has the same number of hydrogen atoms to make C₄H₁₀ = same number of hydrogen atoms as butane and therefore there are no double bond equivalents

IHD = 0

Question 55

How to calculate the IHD when a compound contains nitrogen atoms?

Answer 55

subtract one hydrogen for every nitrogen atom and then calculate as usual

Question 56

What is benzene’s formula?

Answer 56

C₆H₆

Question 57

What is benzenes IHD?
C₆H₆

Answer 57

4 because it contains one ring and the equivalent of three double bonds

Question 58

What is infrared spectroscopy used for?

Answer 58

when determining the structure of an organic compound

Question 59

What does an infrared spectroscopy printout?

Answer 59

printout of the spectrum then shows which frequencies (wavenumbers) are absorbed

Question 60

What does the baseline at the top in an infrared spectra show?

Answer 60

100% transmittance/zero absorbance of infrared radiation

Question 61

What do the trouphs represent?

Answer 61

usually called bands or peaks

represents wavenumbers at which radiation is absorbed

Question 62

What can an infrared spectrum be used to determine?

Answer 62

which bonds are present in a molecule

Question 63

What is the ginerprint region?

Answer 63

the segment of the spectrum below 1500cm-¹

is characteristic of the molecule as a whole

can be used to distubguish compounds with similar bands in the region above 1500cm-¹ (i.e. butanone and propanone, both have the same functional group C=O and can be distinguished using their fingerprint regions

Question 64

What does the precise wavenumber at which infrared radiatio is absorbed by a particular functional group depend on?

Answer 64

on the adjacent atoms

so a C=O bond in a ketone absorbs at a slightly different frequency to a C=O bond in an ester

Question 65

What can be not always distinguish using infrared spectra?

Answer 65

can identify the bonds present

but not always distinguish between funcitonal groups

i.e. we could identify the presence of C=O in a molecule but would not be able to distinguish between an aldehyde and a ketone

Question 66

What is the boradness of the O-H band due to?

Answer 66

due to hydrogen bonding between molecules

Question 67

If the infrared spectrum of a carboxylic acid is in gas phase, what would the O-H absorption band look like?

Answer 67

much narrower

Question 68

Where do we look at the infrared spectrum to identify the bonds present?

Answer 68

the region above 1500cm-¹
not the fingerprint region

Question 69

What is a characteristic of carboxylic acids in the 2400 and 3400cm-¹ region?

Answer 69

of carboxylic acid due to the O-H bond

Question 70

How do we make sure that our hypothesis that this is showing a carboxylic acid?

Answer 70

• we have to check the fingerprint region to look for the functional group of a carboxylic acid which has a C-O bond
• there is no band in this region (1000-1300cm-¹) so this may not be a carboxylic acid

Question 71

Why dos the region below 1500cm-¹ contain many absorptions?

Answer 71

due to C-C bonds and C-H bonds and is difficult to interpret

Question 72

When doe we usually only look at the fingerprint region?

Answer 72

to confirm the presence of a particular vibration once we have a good idea of the structure of the molecule

i.e. a band in the 1000-1300cm-¹ region does not confirm th presence of a C-O bond in a molecule - but the absence of a band in this region means that a C-O bond is not present

Question 73

What does a mass spectrometer do to a sample of organic compound?

Answer 73

sorts molecules according to their mass;charge (m/z or m/e) ratio

we assume here that only 1+ ions are formed so that the mass:charge ratio is equal to the mass

Question 74

How does a mass spectrometer work?

Answer 74

1. a sample of the organic compound is injected into the mass spectrometer
2. it is bombarded with high-energy electeons to produce positive ions
3. these pass on through the mass spectrometer where they are separated according to mass (actually the mass:charge ratio) and detected

Question 75

What happens when a sample of propane is introduced into a mass spectrometer? (C₃H₈)

Answer 75

C₃H₈+ positive ions are produced

Question 76

What is molecular ion, M+?

Answer 76

the ion produced when just one electron is removed from a molecule

the molecular ion peak is not necessarily the biggest peak in terms of abundance, but it has the highest m/z value

Question 77

What does the peak in the spectrum at the highest mass (m/z value correspond to?

Answer 77

corresponds to the molecular ion and indicates the relative molecular mass of the molecule

so the relative molecular mass of the compound being investigated is 44

Question 78

What are the other peaks besides the molecular ion peak called?

Answer 78

fragmentation pattern

Question 79

Why does the fragmentation pattern arise?

Answer 79

because a molecule can break apart into smaller fragments when it is bombarded by high-energy electrons

Question 80

Only which ions resulting from fragmentation will produce a peak inthe mass spectrum?

Answer 80

only the positive ions because only they can pass thorugh a mass spectrometer

Question 81

Which would show as a peak on the mass spectrum

Answer 81

only positive ions can pass through a mass spectrometer, so the CH₃ radical produced in this proces does not give rise to a peak

however there is a peak due to CH₃+ at m/z 15, which is produced in a different fragmentation process

Question 82

Why may a mass spectrum also show a peak with a mass one unit higher than the molecular ion - an (M+1)+ peak?

Answer 82

because this is caused by the presence of an artom of ¹³C in some molecules. It is an isotope of carbon - its natrual abundance is low at 1.1%

Question 83

What is ¹³C and its natural abundance?

Answer 83

it is an isotope of carbon
its natrual abundance is low at 1.1%

Question 84

What are 2 ways in which we can look at fragmentation patterns?

Answer 84

• the ion formed when a molecular ion breaks apart
• the group lost from a molecular ion

Question 85

Where does the molecular ion peak at?

Answer 85

m/z 74

Question 86

What is the relative molecular mass of this?

Answer 86

74
where the molecular ion peaks

Question 87

How do we know that at m/z 57, it correspond to the loss of OH?

Answer 87

because the difference from the relative molecular mass is -17 and according to this table, it suggests that it is an OH

Question 88

What is lost from C₂H₅COOH+ and what is formed at the peak at m/z 45?

Answer 88

the peak at m/z 45 corresponds to the loss of 29 mass units from the molecular ions. C₂H₅ is a group with mass 29 and so we can deduce that C₂H₅ is lost from C₂H₅COOH+ to form the COOH+ ion

Question 89

What is the relative molecular mass of chloroethane? Why?

Answer 89

64.5 because because chlorine has two isotopes ³⁵Cl and ³⁷Cl with an abundance ratio of 3:1

so some chloroethane molecules contain a ³⁵Cl atom and other have a ³⁷Cl atom - therefore we get molecular ion peaks at m/z 64 (C₂H₅³⁵Cl+) and m/z 66 (C₂H₅³⁷Cl+)

Question 90

What property does a hydrogen nucleus have?

Answer 90

a property called spin

Question 91

What does a spinning nucleus acts as?

Answer 91

like a tiny bar magnet

Question 92

What can the bar magnet (spinning nucleus) either align itself with?

Answer 92

align itself with (lower energy) or against (higher energy) an externally applied magnetic field

Question 93

What are the requirements of the nuclei so that they have the property of spin?

Answer 93

only nuclei with an odd value for the mass number and/or atomic number have the property of spin

Question 94

What can be used to cause a hydrogen nucleus to change its orientation relative to the applied magnetic field?

Answer 94

Energy in the radio frequency range of the electromagnetic spectrum can be used to cause a hydrogen nucleus to change its orientation relative to the applied magnetic field

it is these changes in energy state that occur in nuclear magnetic rosonance (NMR) spectra

Question 95

What are we looking at in an NMR spectra?

Answer 95

absorptions due to ¹H nuclei (usually just called protons)

Question 96

What information does NMR spectra provide?

Answer 96

information about the environments of protons in molecules

Question 97

What do the peaks in an NMR spectrum corespond to?

Answer 97

groups of protons (hydrogen atoms) in different chemical environments

Question 98

How do you determine the ratio between the number of hydrogen atoms (protons) in each environment?

Answer 98

measure the vertical height of each step in the intergration trace using a rule and find the ratio between the heights

this is the same as the ratio between the numbers of protons in each environment

Question 99

What do the 3 peaks in the spectrum correspond to?

Answer 99

to three difference chemical environments for the protons (hydrogen atoms) in one molecule of the aldehyde

Question 100

What is said about the hydrogen atoms joined to the same carbon atom?

Answer 100

said to be chemically equivalent

Question 101

What is the area underneath a peak proportional to?

Answer 101

to the number of hydrogen atoms in that envioronment

so the area under the red peak is twice (2H in this environment) the area under the blue peak (1H in this environment); and the area under the green peak (3H in this environment) is 3 times that under the blue peak

Question 102

What can the NMR spectrometer work out the area under each peak to produce?

Answer 102

an integration trace

Question 103

What are the vertical heights of the steps in the integration trace proprotional to?

Answer 103

to the number of hydrogen atoms in each environment

Question 104

What is the horizontal scale on an NMR spectrum?

Answer 104

the chemical shift

given by the symbol δ and has units of parts per million (ppm)

this quantity gives information about the environments that the protons (hydrogen atoms) are in - protons in difference chemical environments have diference chemical shifts

Question 105

What is the first step to working out the number of hydrogen environments?

Answer 105

decide whether or not the molecule is symmetrical

Question 106

If the molecule is not symmetrical, what cad be said about hydrogen atoms on the different atoms in the carbon chain?

Answer 106

they will be in different chemical environments

Question 107

Why is there only 2 different chemical environments for. thehydrogen atoms?

Answer 107

because the molecule is symmetrical

Question 108

What is the ratio of the numbers of hydrogens in each environment?

Answer 108

The heights of the steps in the integration trace are in the ratio 2:3 because there are 4 hydrogen atoms in one environment and 6 hydrogen atoms in the other

Question 109

How many chemical environments does butanone have? ratio?

Answer 109

3
ratio is 3:2:3

Question 110

How many peaks will there be in the NMR spectrum of propan-2-ol?

Answer 110

3 because the molecule is symmetrical

the CH₃ groups are equivalent

the ratio of the numbers of protons in each environment is 6:1:1

Question 111

Why is there only 2 environments in 2-methylpropan-2-ol?

Answer 111

because the CH₃ groups that are attached to the same carbon atom will be equivalent

Question 112

Proton environments more exmples

Answer 112