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Advanced Higher Chemistry Definitions > Unit 3 > Flashcards

Unit 3 Flashcards

1
Q

Gravimetric analysis

A

Used to determine the mass of an element or compound in a substance.

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2
Q

Heating to constant mass

A

Heat the substance.

Allow the substance to cool in a desiccator to prevent re absorption of water.

Weigh the substance

Repeat the steps of heating, cooling and weighing until a constant mass is obtained.
(This shows that all the water has been driven off)

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3
Q

Why should a blue flame be used when heating to constant mass ?

A

To avoid build up of soot on the outside of the crucible which may affect mass.

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4
Q

Precipitation steps

A

The substance being analysed is reacted with a reagent to form a precipitate.

The precipitate is then separated from the filtrate.

The filtrate is tested by adding more reagent to ensure completion.

The precipitate is then washed, dried and weighed.

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5
Q

What does the accuracy of precipitation depend on ?

A

The accuracy of the balance used.

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6
Q

Volitalisation

A

Involves heating a substance to drive off any volatile products.
(Use heating to constant mass)

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7
Q

Weighing accurately approximately

A

The mass you measure must be around the desired mass.
But you must have an ACCURATE value for the measured mass.

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8
Q

Standard solution

A

A solution of accurately known concentration.

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9
Q

How to make a standard solution ?

A

Weigh the primary standard accurately.

Dissolve the solid in a small amount of deionised water. Use a stirring flea /rod to help completely dissolve the solid.

Transfer to a standard flask and transfer the washings.

Top up with deionised water to the graduation mark. (The bottom of the meniscus is level with the line)

Stopper flask and invert.

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10
Q

Primary standard properties

A

Must : be soluble in water
be stable in air

         have a reasonably high GFM
         have a high state of purity
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11
Q

Examples of primary standards

A

Hydrated Oxalic acid
Sodium Carbonate
Silver Nitrate

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12
Q

Why is sodium hydroxide not a primary standard ?

A

It has a relatively low GFM, and is unstable in air. (i.e. readily absorbs moisture)

So, before using NaOH in a reaction it must be standardised. This involves doing a separate titration with a primary standard.

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13
Q

How can you tell the end point of a reaction ?

A

By looking for a permanent colour change in a solution, usually done by using an indicator.

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14
Q

Equivalence point

A

The point at which a reaction is just complete.

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15
Q

What is the common reagent used in compleximetric titrations ?

A

EDTA 4+

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16
Q

Colorimetry

A

Used to determine the concentration of coloured substances in a solution.

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17
Q

Steps in colorimetry

A

4/5 standard solutions of the chemical of interest are prepared.

A filter complementary to the colour of the species being tested is used.

Initially use a blank.

Each sample gives an absorbance reading from the device.

Plot a graph of absorbance V concentration

Test the unknown and obtain its absorbance

Use the graph to obtain its concentration

18
Q

What is a common error / uncertainty in colorimetry ?

A

Not choosing the correct filter.

19
Q

Points of good practice in colorimetry

A

The calibration graph should cover the dilution range likely to be used in the determination of the unknown concentration.

The absorbance values of the standard solutions should be measured at least twice and a mean calculated. Rouge results should be discarded.

Calibration graphs should be drawn as lines of best fit.

20
Q

Heating under reflux

A

This is a technique that allows heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping.

21
Q

Steps in heating under reflux

A

The reaction mixture is placed in a round bottomed flask with anti bumping granules.

A condenser is placed in the mouth of the flask and connected to a cold water tap.

The flask is heated using an appropriate heat source.

The vapours produced are condensed back into liquids when they reach the condenser.

22
Q

Condenser

A

Water in the bottom and out the top

23
Q

Vacuum Filtration

A

Involves carrying out a filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate

24
Q

Steps in vacuum filtration

A

The pure recrystallised solid product collects on the filter paper.
The filtrate contains a solution of soluble impurities.

The solid product should be washed with a small volume of COLD SOLVENT to remove any soluble impurities.

The pure solid can now be dried and weighed.

25
Q

Recrystallisation

A

Technique used to PURIFY an impure solid.

26
Q

Steps in recrystallisation

A

An impure solid is gently dissolved in a MINIMUM volume of hot solvent.

Hot filtration of the resulting mixture to remove any insoluble impurities.

Cooling of the filtrate slowly to allow crystals of the pure compound to form, leaving soluble impurities dissolved in the solvent.

Filtering, washing and drying the pure crystals.

27
Q

How is the solvent picked in recrystallisation ?

A

The solvent is chosen so that the compound being purified is completely soluble at high temperatures and only sparing soluble at lower temperatures.

28
Q

Distillation

A

Used for identification and purification of organic compounds.

Identification : determines the boiling point of a compound
Purification : separates a compound from less volatile substances in the mixture.

29
Q

Steps in distillation

A

The reaction mixture is slowly heated.

The compound with the lower boiling point will vapouriest first and as the vapour passes through the condenser, it condenses to a liquid, collected in a comical flask.

The distillate can be purified further by solvent extraction.

30
Q

Solvent extraction

A

Involves isolating a solute from a liquid mixture/solution by extraction using an immiscible solvent in which the solute is soluble.

31
Q

Steps in solvent extraction

A

The extraction solvent is added. The separating funnel is inverted and shaken several times. The stopcock is opened to release pressure.

The 2 immiscible solvents form 2 layers in the separating funnel.

The stopper is removed. The lower layer is run off into a beaker and the upper layer is poured into a second beaker.

32
Q

Why are the steps in solvent extraction repeated ?

A

To maximise the quantity of solute extracted.

33
Q

How to increase the quantity of solute extracted ?

A

Carry out a number of extractions using smaller volumes of solvent.

34
Q

Melting point analysis

A

Used to confirm the identity of a pure product.

35
Q

How mpt analysis works ?

A

If the product is solid we can determine its melting point and compare it with the accepted or literature value.

The determination of mpt of a compound gives an indication of the purity of the compound.

36
Q

What do impurities do in mpt analysis ?

A

They lower the compounds melting point and broaden the temperature range of melting.

37
Q

Steps in mixed melting point analysis

A

Mix a small quantity of the product with some of the pure compound and determine this melting point.

If the 2 compounds are identical, then a mpt of the correct temperature range will be obtained.

If the mpt is lower than expected, then it is likely that the product is either impure or not the same substance.

38
Q

Chromatography

A

A technique used to separate mixtures of substances.

39
Q

Rf value

A

Distance travelled by the sample
_________________________________________________________

Distance travelled by the solvent (i.e. the solvent front)

40
Q

Wavelength eqn

A

Wavelength = 1 / wavenumber

                  = cm
                  = (/100) to get answer in metres
41
Q

Explain the effect infrared radiation has on the bonds within molecules and how this allows different functional groups to be identified.

A

Infrared radiation causes the bonds to stretch, bend and vibrate.

Different functional groups absorb radiation at different wavenumber/wavelength/frequency/energy.

Advanced Higher Chemistry Definitions (4 decks)

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