Download Chem notes qualitative and quantitative data

Learning intentions
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Understand the difference between quantitative and qualitative data
○ qualitative analysis in chemistry gives the presence or absence of different
chemical components in a sample. {observations made during an experiment}
■ (refers to numbers eg. 178 cm tall - measurements)
quantitative analysis in chemistry gives the amount of different chemical
components present in a given sample. {numerical data}
○ (refers to descriptions eg. big, small)
p.s. you might need to apply both qualitative and quantitative data in design practice.
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Understand the difference between random uncertainties and systematic errors
○ random errors lead to fluctuations around the true value as a result of
difficulty taking measurements. Eg. temperature.
■ (arise due to the limitations of the measuring apparatus)
■ (all measurements must be quoted with an uncertainty (eg. 69.0 plus
or minus 0.3cm cubed)
■ Analog apparatus (can be both random and systematic) - the
uncertainty on measurement is half the smallest division that you take
a reading to.
■ systematic errors lead to predictable and consistent departures from
the true value due to problems with the calibration of your equipment.
Digital Instruments the uncertainty of a measurement is quoted as ± the smallest division
e.g. reading taken with a 2 decimal place electronic balance should be quoted as 2.46 ±
0.0.1g.
*p.s. uncertainties MUST be showcased in the column heading.*
extra notes:
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Uncertainties indicate the size of the random error.
The effects of random uncertainties should mean that the measurements taken will
be distributed on either side of the mean.
The random uncertainties can be reduced by repeating the measurements more
often.
systematic errors:
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A systematic error can be introduced into an experiment due to the apparatus used
or the procedure. Systematic errors are always in the same direction.
For example, heat loss to the surroundings will always cause a measured value for
the enthalpy change of combustion to be less exothermic (exothermic reactions are
reactions or processes that release energy, usually in the form of heat or light. In an
exothermic reaction, energy is released because the total energy of the products is
less than the total energy of the reactants.) than expected.
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The presence of systematic errors can be identified by comparison with accepted
literature values for quantities.
We can use the percentage error to compare the experimental value with the
accepted literature value
percentage errors:
= (experimental value - accepted value/accepted value) x 100
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If the percentage error is greater than the percentage uncertainty due to random
errors the experiment involves some systematic errors, but if the percentage error is
smaller than the percentage uncertainty, any deviation from the literature value can
be explained in terms of random errors, that is, the limitations of the measuring
apparatus.
p.s. when you plan the experiment u need to make sure you get numerical data as this
ensures your experimental value and accepted value. you also need to be specific and state
if there are systematic errors or random errors as well as identify the errors.
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Understand the difference between precision and accuracy
○ Accuracy refers to how closely the measured value of a quantity corresponds
to its “true” value.
■ (refers to how close a measurement is to the actual value of a
particular quantity. low accuracy is due to systematic errors within the
experimental procedure)
Precision expresses the degree of reproducibility or agreement between repeated
measurements. The more measurements you make and the better the precision, the
smaller the error will be.
○ (relates to the reproducibility of results. high precision means that repeat
values are close together and close to the mean).