C6 - Chemical synthesis

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  • Created by: n.anwar
  • Created on: 13-06-16 15:03

Chemical synthesis

<strong>       Chemical synthesis = using simple substances to make new compounds à important for food additives, fertilisers, dyes, paints, medicines

Stages

</strong>       Choosing reaction/s to make required product

<strong>       Risk assessment

</strong>       Working out quantities of reactants to use

<strong>       Carrying out reaction

</strong>       Separating product from mixture

<strong>       Purifying product

</strong>       Measuring yield and checking purity of product by titration

% yield = actual yield/ theoretical yield x 100%

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Acids and alkalis and acid reactions

<strong>       Litmus = red in acids, blue in alkalis

</strong>       Universal indicator = orange/red in acids, green/blue in alkalis

<strong>       Pure acid compounds can be of any state

</strong>       Solid = citric and tartaric acids

<strong>       Liquid = sulphuric, nitric and ethanoic acids

</strong>       Gas = HCl

<strong>       Common alkalis = hydroxides of sodium, calcium and potassium

</strong>       pH scale = measure of how strong an acid or alkali is à measured by pH meter and UI

<strong>       Below 7 = acidic, 7 = neutral, above 7 = alkali

</strong>       Metal + acid à salt + hydrogen

*       Metal oxides + acid à salt + water

Metal carbonate + acid à salt + water + carbon dioxide

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Titrations

<strong>       Titration = measure volume of acid and alkali that exactly react together

</strong>       Indicator added = see when neutralisation happens à suddenly changes colour at end-point of titration

<strong>       Repeated to check results are close together à variations between readings due to small experimental errors

</strong>       Range = highest to lowest readings

*       True value should fall between range of readings

Volumes obtained from titration will always be in same proportion if same concs of solution are used

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Neutralisation

<strong>       Reaction of acid with alkali to form salt = neutralisation reaction

</strong>       Acidic compounds dissolved in water = aqueous hydrogen ions

<strong>       Alkaline compounds dissolved in water = aqueous hydroxide ions

</strong>       H+ (aq) + OH- (aq) à H2O (l)

(http://www.s-cool.co.uk/assets/learn_its/gcse/chemistry/acids-and-alkalis/neutralisation_/g-che-acids-dia05a.gif)

*       Pos ion and neg ion make a salt e.g: NaOH + HClàNaCl + H2O

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Crystallisation

1.       Dissolving = dissolve product in as little as possible hot water

2.       Filtering = filter off solid impurities à solution coming through is filtrate

3.       Evaporating = filtrate crystallises as some water evaporates à cool filtrate as product crystallises

4.       Filtering = filter off crystals, leaving solid impurities in solution

5.       Drying = dry crystals in desiccator or oven

A chemical needs to be pure, some impurities may be harmful

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Measuring rates of reaction

<strong>       Rate of reaction = amount of product produced or reactant used in a certain time à amount per sec à shown by gradient of graph

</strong>       Faster reaction = more economical à reactions need to occur at safe rate

<strong>       Engineers aim = produce most amount in least time by changing conditions

</strong>       Rate can be followed by:

<strong>       Measuring vol of gas made at set times e.g. every 30 secs

</strong>       Measuring decrease in mass of flask as gas leaves reactio

*       Using a colorimeter to follow colour change

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Changing rates of reaction

<strong>       Reaction rates vary with size of solid particles, conc of solutions and temp of mixture

</strong>       More collisions = faster reactions

<strong>       Catalyst = speeds up reactions but isn’t used up

</strong>       Rate of reaction increases with larger surface area of solids with higher conc of solutions à more      collisions 

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