Applying Techniques in Chemistry (HL IB Chemistry)

• There are a number of practical experiments and techniques that you need to be aware of, within the following categories:
  • Volumetric analysis techniques
  • Separation techniques
  • Purification techniques
  • Other techniques and experiments

Volumetric analysis

• Volumetric analysis techniques including:
  • Preparing a standard solution
  • Carrying out dilutions
  • Performing titrations (acid–base titration and redox titration)
• Volumetric analysis is a process that uses the volume and concentration of one chemical reactant (a standard / volumetric solution) to determine the concentration of another unknown solution
• The technique most commonly used is a titration
• The volumes are measured using two precise pieces of equipment:
  • A volumetric or graduated pipette
  • A burette
• Before the titration can be done, the standard solution must be prepared
• Specific apparatus must be used both when preparing the standard solution and when completing the titration, to ensure that volumes are measured precisely

Key pieces of apparatus used to prepare a volumetric solution and perform a simple titration 

Key pieces of apparatus for volumetric analysis include a beaker, burette, volumetric pipette, conical flask and a standard / volumetric flask

Making a standard / volumetric solution

• Chemists routinely prepare solutions needed for analysis, whose concentrations are known precisely
  • These solutions are termed standard solutions or volumetric solutions
• They are made as accurately and precisely as possible using three decimal place balances and volumetric flasks to reduce the impact of measurement uncertainties

How to prepare a standard solution

A standard solution is made by adding a measured mass of solid to a measured volume of deionised water

Carrying out dilutions

• The concentration of a solution is the amount of solute dissolved in a solvent to make 1 dm³ of  solution
  • The solute is the substance that dissolves in a solvent to form a solution
  • The solvent is often water
• A concentrated solution is a solution that has a high concentration of solute
• A dilute solution is a solution with a low concentration of solute
• Concentration is usually expressed in one of three ways:
  • moles per unit volume
  • mass per unit volume
  • parts per million
• A concentrated solution can be diluted to form a dilute solution 
  • For example, diluting 500 cm³ of a stock 1.0 mol dm^–3 standard solution to a 0.5 mol dm^–3 standard solution
    • Take the 500 cm³ of the 1.0 mol dm^–3 standard solution
    • Add 500 cm³ of deionised water
    • There is now 1000 cm³ of a 0.5 mol dm^–3 standard solution
• Serial dilutions are a sequence of dilutions
  • The initial stock solution is typically diluted by a factor of 10, e.g. 100 cm³ of the stock solution added to 900 cm³ of deionised solution
  • This process is then repeated until a solution of the desired concentration is achieved

Performing titrations

• Titrations include acid-base titrations and redox titrations
  • Acid-base titrations involve an acid and a base
  • Redox titrations are more specific reactions involving the reduction and oxidation occurring simultaneously, e.g. the Fe²⁺ / MnO₄^– titration
• The key piece of equipment used in the titration is the burette
• Burettes are usually marked to a precision of 0.10 cm³
  • Since they are analogue instruments, the uncertainty is recorded to half the smallest marking, in other words to ±0.05 cm³
• The endpoint or equivalence point occurs when the two solutions have reacted completely
  • In the case of most acid-base titrations, this can be observed with the use of an indicator
  • For more information about choosing indicators, see our revision note on Choosing an Acid-Base Indicator
  • There are some examples of redox titrations where no indicator is required, e.g. the Fe²⁺ / MnO₄^– redox titration

Using an indicator in titrations

Only a few drops of indicator are added, if necessary, because they are typically weak acids and can influence the results

• The steps in a titration are:
• Measuring a known volume (usually 20.0 or 25.0 cm³) of one of the solutions with a volumetric pipette and placing it into a conical flask
• The other solution is placed in the burette
• To start with, the burette will usually be filled to 0.00 cm³
• If necessary, a few drops of indicator are added to the solution in the conical flask
  • A white tile is sometimes placed under the conical flask while the titration is performed, to make it easier to see the colour change
• The tap on the burette is carefully opened and the solution is added, portion by portion, to the conical flask until the indicator starts to change colour
  • After each portion, the conical flask should be swirled
• As you start getting near to the endpoint, the flow of the burette should be slowed right down so that the solution is added dropwise
• You should be able to close the tap on the burette after one drop has caused a permanent colour change
• Multiple runs are carried out until concordant results are obtained

Recording and processing titration results

• Both the initial and final burette readings should be recorded and shown to a precision of  ±0.05 cm³, the same as the uncertainty

A typical layout and set of titration results

• The volume delivered (titre) is calculated and recorded to an uncertainty of ±0.10 cm³
  • For more information about working with uncertainties, see our revision note on Processing Uncertainties in Chemistry
• Concordant results are then averaged, and non-concordant results are discarded
  • For more information about calculating average titres, see "What is the mean average?" in our Applying General Mathematics in Chemistry revision note
• Appropriate titration calculations are then performed, as shown in our revision note on Concentration Calculations

Separation of mixtures

• The required separation techniques covered in our revision note on Separating Mixtures include:
  • Filtration
  • Simple and fractional distillation
  • Paper chromatography
    • The process of thin layer chromatography is the same as paper chromatography
    • The stationary phase is changed from chromatography paper to a sheet with a fine layer of silica or alumina
    • The mobile phase can still be any liquid solvent
    • Separation is still based on solubility
    • It can be common to use UV light or locating agents, such as ninhydrin, to identify the spots
  • Crystallisation

Purification techniques

• The specific purification techniques explicitly stated in the syllabus are:
  • Recrystallisation
  • Melting point determination

Recrystallisation

• Recrystallisation involves dissolving an impure solid in a suitable solvent and then allowing the compound to crystallise out of the solution
  • The recrystallisation product should have a higher purity 
• This process relies on the differences in solubility between the desired compound and the impurities present in the original solid
• For more information about recrystallisation, see our revision note on Separating Mixtures

Melting point determination

• The melting point of a solid is indicative of its purity and identity
• A melting point can be matched to a known substance as a means of identification or confirmation of a desired product
• The proximity of a melting point to the actual data book value can express purity
  • Impurities tend to lower the melting point of a solid
• The melting point range also reveals the degree of purity
  • Pure substances have sharp well-defined melting points
  • Impure substances have a broad melting point range, i.e. a large difference between when the substance first melts and when it completely melts
• The skills needed in performing a melting point test are largely dependent on the specific melting point apparatus you are using:

Different apparatus used to determine the melting point of a sample

• However, there are some common key skills:
  • Correctly preparing the melting point tubes
  • Heating the tubes very slowly
  • Repeating to get a range of measurements (three would be normal)
• The sample solid must be totally dry and finely powdered - this can be achieved by crushing it with the back of a spatula onto some filter paper or the back of a white tile (this absorbs any moisture)
• Use the first tube to find the approximate melting point range and then repeat using a much slower heating rate 

Other experiments and techniques

• Other specific experiments and techniques explicitly stated in the syllabus are:
  • Calorimetry
    • For more information about calorimetry, see our revision note on Calorimetry
  • Electrochemical cells
    • For more information about experiments involving electrochemical cells, see the relevant revision notes in our Electron Transfer Reactions topic
  • Drying to constant mass
  • Reflux
  • Colorimetry / spectrophotometry
  • Physical and digital molecular modelling

Drying to constant mass

• This is used to determine the amount of water (or volatile components) in a substance
  • The initial mass of the substance is recorded, using a balance
  • The substance is placed in an oven / drying chamber and heated at a specific temperature 
  • At regular intervals, the substance is taken out of the oven, allowed to cool and reweighed
  • These steps are repeated until the recorded mass of the substance remains constant
• This is a common technique associated with water of crystallisation in hydrated transition metal compounds

Heating under reflux

• Organic reactions often occur slowly at room temperature 
• Therefore, organic reactions can be completed by heating under reflux to produce an organic liquid
• This allows the mixture to react as fully as possible without the loss of any reactants, products or solvent
  • In distillation, you are trying to separate a chemical or product from a mixture
  • When heating under reflux, you aim to keep all the chemicals inside the reaction vessel
• Example reactions where heating under reflux could be used include:
  • The production of a carboxylic acid from a primary alcohol using acidified potassium dichromate
  • The production of an ester from an alcohol and acid in the presence of an acid catalyst
• The reaction mixture is placed into a pear-shaped or round-bottomed flask
• Anti-bumping granules are added to promote smooth boiling
• The flask is placed in a heating mantle or it can be immersed in a water bath for heating
• Quickfit apparatus is then set up with the condenser clamped vertically in place 
  • The joints of the Quickfit apparatus are commonly greased as with distillation
• A steady and constant stream of water passes through the condenser in a 'water jacket' - it enters at the bottom of the condenser and the drainage pipe removes the water from the top of the condenser
• The flask is indirectly heated and the reaction mixture is allowed to boil 
• Finally, heating stops and the mixture is allowed to cool back to room temperature

Heating under reflux practical equipment

The preparation of ethyl ethanoate involves heating under reflux for about 15 minutes

Colorimetry / spectrophotometry

• Colorimetry and spectrophotometry are techniques used to measure the concentration of substances in a solution based on the absorbance / transmittance of light at specific wavelengths
• Both techniques use the same basic method:
  • A light source emits a beam of light covering a wide range of wavelengths
  • The sample solution absorbs certain wavelengths of light, depending on its composition and concentration
  • The absorbance and transmittance of various wavelengths are then recorded
• The detector on a colorimeter measures the intensity of light which is directly related to the concentration of the solution
  • It is a relatively quick process although not as precise as spectrophotometry, especially with low concentrations or complex mixtures

• The detector on a spectrophotometer measures the absorbance of each wavelength of light
  • The resulting absorption spectrum is plotted, showing the characteristic absorption peaks of the sample
  • The concentration is then determined by comparing this spectrum to a calibration curve
  • Spectrophotometry is highly sensitive and accurate, making it suitable for analysing low concentrations and complex mixtures
  • It is widely used in research, quality control, drug analysis, environmental monitoring and food testing
• For more information about calorimetry, see our revision note on Measuring Rates of Reaction

Physical and digital molecular modelling

• Physical molecular modelling is the creation of three-dimensional models using materials such as plastic balls and sticks (molymods)
  • It serves as a tool to understand molecular geometry, bond angles and the overall spatial arrangement of atoms within a molecule
• Digital molecular modelling uses specialist computer software to generate accurate and detailed 3D models of molecules
  • By giving specific data, such as bond lengths and angles, the software can produce highly accurate representations of molecules, including their electronic structures
  • It allows the study of more complex molecules, especially ones that are challenging to construct
  • It allows observations of molecular movements and reactions in real time
  • Digital molecular modelling provides access to various tools and simulations that can predict:
    • Molecular properties
    • Behaviour in different environments
    • Potential interactions with other molecules
    • These simulations aid researchers in drug design, material science and many other applications