Topic 11: Measurement, data processing and analysis

Define qualitative data

non numerical data obtained from an experiment e.g. observations

Define quantitative data

numerical data

What is the uncertainty on analogue apparatus?

half the smallest division e.g. ±0.5 cm

What is the uncertainty on digital apparatus?

the smallest division e.g. ±1cm

What can be done to reduce random errors from the limitations of measuring apparatus?

repeating the measurements more often

Give the characteristics of systematic errors

• occur due to apparatus used or procedural method

• the error is always in the same direction e.g. heat loss to surroundings will always cause a measured value for enthalpy of combustion to be less exothermic than expected.

How are the presence of systematic errors identified?

• comparing the experimental value to the literature value

• use the percentage error equation

What is the percentage error equation?

experimental value - accepted value / accepted value x 100

What is the percentage uncertainty equation?

absolute uncertainty / value x 100

If the percentage error is greater than the percentage uncertainty…

the experiment involves some systematic errors

If the percentage error is smaller than the percentage uncertainty…

the variation can be explained in terms of random errors and the limitations of apparatus.

Give 2 possible sources of systematic errors and their solutions:

• heat loss to the surroundings; insulate reaction vessel

• solutions did not have stated concentrations; find the concentrations of the solution using a titration

Give two sources of random errors and their solutions:

1. measuring the volumes using a measuring cyclinder; use pipette + repeats

2. temperature reading from thermometer; use temperature probe and increase concs of solutions in order to create a larger temp change

What is the independent variable?

what is changed in the experiment and what is being investigated

What is the dependent variable?

what is measured in the experiment and what is kept the same

What does the Index of Hydrogen Deficiency indicate?

the number of double bond equivalents in a compound.

What is the IHD of one double bond?

1

What is the IHD value does a triple bond have?

2

What is the IHD value for a ring?

1

What is the IHD value for a benzene ring?

4

Give the IHD equation:

½ [2c + 2 - h - x + n]

c = number of carbons

h = number of hydrogens

x = number of halogens

n = number of nitrogens

What is Infrared Spectroscopy and what is it used for?

Infrared spectroscopy is an analytical technique used to determine the bonds/functional groups present in a molecule.

What is the fingerprint region?

any absorption spectra below 1500

Why can functional groups presence not be determined solely by looking at the fingerprint region?

This region is not reliable as the fingerprint region is characteristic of the molecule as a whole and is hard to interpret. It can be used to determine the absence of functional groups but not the absolute presence.

What is the principle of Mass spectrometry?

• an organic compound is bombarded with high energy electrons to produce positive ions.

• positive ions pass through the mass spectrometer where they are separated according to mass and detected.

What is the molecular peak?

The peak with the highest value; this determines the Mr of the organic compound

How must fragments of the organic compound be written?

CH3CH2 ^+

What does NMR stand for?

Nuclear Magnetic Resonance Spectroscopy

What does NMR look at?

this technique looks at the hydrogen environments (specifically H+) in a molecule and requires electromagnetic radiation in the radio frequency

If the hydrogen environments in a molecule are symmetrical…

it counts as only 1 environment

The number of signals on the NMR corresponds to…

the number of proton/hydrogen environments in a molecule

The area under the peak depends on…

the ratio between the number of H atoms in each environment e.g. CH—CH2—CH3 = 1:2:3

How do you count each hydrogen environment?

Hydrogen atoms attached to the same Carbon are in the same environment.

What can be seen in High resolution NMR?

the splitting pattern of hydrogen environments

What causes the splitting patterns?

the number of hydrogen atoms on the adjacent carbon atom.

If there are 0 hydrogen atoms on the adjacent carbon..

the peak is a singlet

If there is 1 hydrogen atom on the adjacent carbon..

the peak is a doublet

If there are 2 hydrogen atoms on the adjacent carbon..

the peak is a triplet

If there are 3 hydrogen atoms on the adjacent carbon..

the peak is a quartet

Why is tetramethylsilane used as a reference standard for chemical shift?

1. it has 12 protons all in the same environment

2. its peak/chemical shift is found at an extreme (lower) so does not interfere with other peaks.

3. it is non-toxic and inert

What is X-ray crystallography used to determine?

X-ray crystallography is used to provide a detailed 3-D structure for a molecule including bond lengths and bond angles.