Module one Flashcards
What is meant by heating under reflux?
Boiling a mixture
AND condensing/cooling the vapour to return it to the
flask
Describe the main steps involved in carrying out the recrystallization of an organic substance
and describe what property of the substance this process depends upon
Dissolve crystals in minimum of hot water
Cool solution (to produce crystals)
Filter off crystals AND wash with cold solvent
the process depends on the substance being more soluble in hot water
than in cold water
devise an experiment to compare the thermal stability of
magnesium carbonate and calcium carbonate.
The students have access to magnesium carbonate powder, lumps of calcium carbonate, calcium
hydroxide powder, distilled water and whatever apparatus they need.
(i)* Describe how the students could carry out their experiment.
You should include in your answer:
• a labelled diagram of the apparatus used to safely heat the carbonate compounds
• the main steps in the experimental procedure and the names of the key apparatus (not
included in the labelled diagram)
• the observations and measurements that should be recorded
• how to ensure the comparison is fair and the results are as accurate as possible.
Method with details
• Crush lumps of calcium carbonate to a powder
using a pestle and mortar
• Add calcium hydroxide to distilled water and filter
• Glass test tube/boiling tube to contain the solid
fitted with a bung carrying a delivery tube which
dips into a solution of calcium hydroxide in a
test tube/boiling tube.
• Tube is heated and is approximately horizontal,
• solid (metal carbonate) and solution (calcium
hydroxide) are clearly labelled in diagram.
Measurements
• Measure time for cloudiness to first appear.
• Measure time taken to obscure a cross on paper
OR alternative way of measuring relative
thickness of ppt (of calcium carbonate)
Fair comparison
• Equal mass of MgCO3 and CaCO3
• Same volume and concentration of calcium
hydroxide solution
(iii) Explain, in terms of the charge densities of the cations, the relative thermal stabilities of
magnesium carbonate and calcium carbonate.
Magnesium carbonate is less thermally stable than
calcium carbonate (because) the smaller magnesium
(Mg2+) ion
distorts the (large) carbonate (CO3
2− ion
Suggest two reasons why the total energy transferred from the spirit burner is different in the
two experiments.
A different number of moles of alcohol are burnt 9
The alcohols have different enthalpy changes of
combustion 9
Describe how the student can ensure that the same amount of energy is transferred from
the spirit burner in the experiment using methanol as is transferred in the experiment
described in (a)(ii).
State the assumption you have made.
Use the same start and end temperatures of water 9
(Assume) the same proportion of energy transferred
to the surroundings is transferred to the water (AW) 9
At the end of the experiments the student notices that there is a black deposit on the bottom of the
beaker.
Suggest what this might be and why it might have been formed.
Black solid is carbon
Due to incomplete combustion (of the fuel)
Name a suitable piece of apparatus (with its size) that could be used to measure 25 cm3 of dilute
sulfuric acid into the conical flask in Part 1.
25/50/100 cm3 measuring cylinder 9
Suggest two reasons why the conical flask in Part 1 was fitted with a bung carrying a capillary
tube, apart from loss of spray.
To allow hydrogen to escape 9
To prevent air getting in (and oxidising Fe2+ to Fe3+) 9
Explain why a ‘2% Mn’ solution made by dissolving potassium manganate(VII) crystals would
be more accurate than a ‘0.5% Mn’ solution made using the same method.
Larger mass of KMnO4 dissolved so) smaller
weighing error
- Describe how the student could use the ‘2% Mn’ solution and a colorimeter to produce a
calibration curve suitable for finding the concentration of manganese in the paper clip solution
Steps (italics indicate details that would differentiate
higher level from lower level answers):
• Dilute 2% Mn solution using burettes and
pipette
• To include range 0.1 to 0.4% includes volumes
to achieve dilution (these could include 2 cm3
of 2% + 8 cm3
of water to give 0.4%, or
details of a serial dilution,)
• Mix each dilution thoroughly to ensure
standard solution
• Makes a zero reading (on the colorimeter)
using water/base solvent
• Selects an appropriate filter/wavelength
chooses green-blue filter OR sets
(spectrophotometer) to green-blue
• Measures absorbance of diluted solutions
ensure each sample is in a clean cuvette
• Plot absorbance against dilution and draw line Specimen
of best fit
How could the student have improved the line drawn on the calibration graph in Part 2 without
doing further experiments?
The line should be drawn through the zero–zero point
The student suggests that the concentration of the MnO4
– ions in the solution made from the
paper clips could be determined by titrating it against a standard solution of Fe2+ ions.
Why would this method not give an accurate result?
The solution contains nitric acid/potassium iodate(VII)
that would also oxidise Fe2+ 9
Suggest another method, other than using a titration or a colorimeter, that the student could use to
find the concentration of Fe2+ ions in a solution made from paper clips.
Use the solution in an electrochemical cell 9
Rules of the table
The following guidelines should be followed when presenting results in tables.
• All raw data in a single table with ruled lines and border.
• Independent variable (IV) in the first column; dependent variable (DV) in columns to the
right (for quantitative observations) OR descriptive comments in columns to the right (for
qualitative observations).
• Processed data (e.g. means, rates) in columns to the far right.
• No calculations in the table, only calculated values.
• Each column headed with informative description (for qualitative data) or physical quantity
and correct units (for qualitative data); units separated from physical quantity using either
brackets or a solidus (slash).
• No units in the body of the table, only in the column headings.
• Raw data recorded to a number of decimal places appropriate to the resolution of the
measuring equipment.
• All raw data of the same type recorded to the same number of decimal places.
• Processed data recorded to up to one significant figure more than the raw data.
