C6.2 Planning, carrying out and controlling a chemical synthesis
Chemical synthesis is the procedure used to make useful compounds – this often involves a number of chemical reactions.
There are a number of different stages to any chemical synthesis of an inorganic compound:
A number of simple techniques may be used in the synthesis of a chemical compound:
- Dissolving – Many reactions are carried out using reactants that in solution – this allows the ions of the molecules of reactants to collide with each other and react. Insoluble impurities will remain solid
- Crystallisation – when a product is made as a solution one way to separate it from the solvent is to make crystals. This involves evaporating the solution to a much smaller volume and then leaving it to cool – as the solution cools, crystals form and can be obtained by filtration
- Evaporation – When a solution is heated the solvent evaporates. This can be used prior to crystallisation, or continued until all of the solvent has evaporated, to leave the product as a dry powder.
- Drying – a solid product that has been separated from a liquid reaction mixture by filtration or crystallisation will still contain some of the solvent. It can be dried by placing it in an ovenor into a desiccator.
- Filtration – This technique is used to separate a solid from a liquid – it can be used to obtain product that is free from unreacted chemicals, by-products or solvent
In order to work out how much of each reactant is required to make a known amount of product, you must understand:
- A balanced equation for a chemical reaction shows the relative numbers of atoms and molecules of reactants and products taking part in the reaction
- The relative atomic mass of an element shows the mass of its atoms relative to the mass of other atoms
- The relative atomic masses of elements can be found on the periodic table – the top number
- How to calculate the relative atomic mass
- How to substitute the relative formula masses and data into a given mathematical formula to calculate reacting masses and/or products from a chemical reaction
A balanced symbol equation can be used as the starting point to calculate the quantities of reactants used and the quantities of products made in a reaction.
Step 1: write down the balanced symbol equation
Step 2: work out the relative formula mass
Step 3: work out the masses of reactants and products
Percentage yield compares actual and theoretical yield
Actual yield is mass of pure, dry product
Theoretical yield is maximum possible mass of pure product that could’ve been made using the amount of reactants you started with.
Titration can be used to calculate the concentration of an acid by finding how much alkali is needed to neutralise it or the other way round
❶ Fill a burette with the alkali and take an initial reading of the volume
❷ If the sample is solid it is weighed and dissolved to make up a known volume of solution (usually 100cm3). A pipette is used to measure accurately a volume of this solution (e.g. 10cm3) – this is emptied into a conical flask
❸ Add a few drops of the indicator – e.g. phenolphthalein to the conical flask – the indicator will stay colourless
❹ Place the flask on a white tile under the burette (this will make the colour change easier to identify)
❺ Add the alkali from the burette drop by drop
❻ Swirl the flask to ensure it mixes it well – turn the tap off as soon as the acid changes colour
❼ Record the final burette reading – work out the volume of alkali added by doing:
Volume = final reading – initial reading
The rate of a chemical reaction is the amount of reaction that takes place in a given unit of time – chemical reactions only occur when the reacting particles collide with each other with sufficient energy to react.
The rate of chemical reaction can be found in different way:
❶ Weighing the reaction mixture – if one of the products is gas – you could weigh the reaction mixture at timed intervals as the mass of the mixture will decrease
❷ Measuring the volume of gas produced – using a gas syringe to measure the total volume of gas produced at timed intervals
❸ Observing the formation of a precipitate – this can be done by either watching a cross in order to measure the formation of a precipitate or by measuring colour change using a light sensor.
❹ Observing the loss of colour or loss of precipitate – opposite of ❸
The steeper the line the faster the reaction
The same amount of product is formed from the same amount of reactants irrespective of the rate
There are four important factors that affect the rate of reaction:
Collision theory – Chemical reactions only occur when particles collide with each other with sufficient energy. Increasing temperature causes an increase in the kinetic energy of the particles i.e. they move a lot of faster. This results in more energetic collisions happening more frequently.
When carrying out a chemical synthesis on an industrial scale – there are economic, safety and environment factors to consider:
- The rate of manufacture must be high enough to produce a sufficient daily yield of product
- Optimum conditions should be used that give the lowest cost rather than the fastest reaction or highest percentage
- Care must be taken when using any reactants or products that could harm the environment if there was a leak