Chemical SynthesisChemical SynthesisModule C6Module C6

Chemical synthesisChemical synthesis:: chemical reactions and processes used chemical reactions and processes used to get a desired product using starting materials called reagents. to get a desired product using starting materials called reagents. The products can be useful for a variety of purposes but tend to The products can be useful for a variety of purposes but tend to be either…be either…

• fine chemicalsfine chemicals

a chemical product that is made in relatively small a chemical product that is made in relatively small quantities and is typically high in cost, e.g. a quantities and is typically high in cost, e.g. a flavouring or vitamin, drugs etcflavouring or vitamin, drugs etc

• bulk chemicalsbulk chemicals

A chemical product that is made in large amounts, very cheaply and often used to make other chemicals or to process other materials e.g. bleach, solvents, sulphuric acid etc

Useful chemicals include… Useful chemicals include… Food additives, fertilisers, dyestuffs, paints, pigments and pharmaceuticals;

Learn the formulae of…Learn the formulae of…• Gases: chlorine, hydrogen, nitrogen, oxygen• Acids: hydrochloric acid, nitric acid, sulfuric acid,• Alkalis: sodium hydroxide, magnesium hydroxide• Salts:

– Simple salts…• [MX] sodium chloride, magnesium oxide, potassium chloride • [MX2]calcium chloride,

– More complex salts… [MgCO3] magnesium carbonate, [MgSO4] magnesium sulfate, [Na2CO3] sodium carbonate, [CaCO3] calcium carbonate

Task: For each salt show the ions that are presentTask: For each salt show the ions that are present

More ions…More ions…• If magnesium forms MgIf magnesium forms Mg2+2+ ions and sulfate ions and sulfate

forms SOforms SO442-2- ions then identify the charges ions then identify the charges

on the other ions in the following on the other ions in the following compounds…compounds…– MgOMgO– MgClMgCl22

– MgNOMgNO33

– NaNa22SOSO44

– AlAl22(SO(SO44))33

– CaSOCaSO44

Acidic Acidic substancessubstances……• Dissolve in water to form HDissolve in water to form H++ ions giving a pH of ions giving a pH of

less than 7less than 7• Can be… Can be…

– solids e.g. citric acid, tartaric acidsolids e.g. citric acid, tartaric acid– Liquids e.g. sulfuric acid, nitric acid, ethanoic acidLiquids e.g. sulfuric acid, nitric acid, ethanoic acid– Gases e.g. hydrogen chlorideGases e.g. hydrogen chloride

• Form salts with many other substances such as Form salts with many other substances such as alkalis, hydroxides, carbonates, oxides, metals alkalis, hydroxides, carbonates, oxides, metals

Alkalis:Alkalis:

• Dissolve in water to form OHDissolve in water to form OH -- ions giving a ions giving a pH of GREATER than 7pH of GREATER than 7

• Best examples are sodium hydroxide, Best examples are sodium hydroxide, potassium hydroxide, calcium hydroxidepotassium hydroxide, calcium hydroxide

• Form salts with acidsForm salts with acids• Like acids they can attack skin and other Like acids they can attack skin and other

materialsmaterialsCorrosive when Corrosive when

concentrated and concentrated and irritants when diluteirritants when dilute

Salt formationSalt formation……

• Metal + acid Metal + acid salt + hydrogen salt + hydrogen • Metal carbonate + acid Metal carbonate + acid salt + water + salt + water +

carbon dioxide carbon dioxide • Metal hydroxide + acid Metal hydroxide + acid salt + water salt + water• Metal oxide + acid Metal oxide + acid salt + water salt + water• Alkali + acid Alkali + acid salt + water salt + water

Task: Give an example of each type of reaction Task: Give an example of each type of reaction for salt formationfor salt formation

NeutralisationNeutralisation……

• When the number of When the number of HH++ ions ions is exactly is exactly matched by the number of matched by the number of OHOH-- ions ions to to form a pH of 7form a pH of 7

• HH+ + ++ OHOH- - H H22OO• An alkali can cancel out an acid to form a An alkali can cancel out an acid to form a

salt and the water (shown above)salt and the water (shown above)

Stages in a chemical synthesis of a compound:

• choosing the reaction or series of reactions • risk assessment (chemical and procedural)• working out the quantit ies of reactants to use• carrying out the reaction in suitable apparatus in the right

conditions (such as temperature, concentration or the presence of a catalyst)

• separating the product from the reaction mixture• purifying the product• measuring the yield and checking the purity of the

product

Useful techniques:Useful techniques:

• Dissolving… forming solutions to allow easy mixing of reactants,

• Crystallisation… to purify a sample by the formation of pure crystals from a cooled (often saturated) solution,

• Filtration… to separate solid impurities from a solution OR to obtain the pure crystals,

• evaporation… to remove excess solvent from a solution, • drying in an oven or dessicator… to remove water which

might otherwise bump up the yield and cause an error!• Titration… to find the concentration of an acid (or alkali)

using an alkali (or acid) of a known concentration AND an indicator

TitrationTitration::• Open the tap to let the Open the tap to let the

acid run into the flaskacid run into the flask• Stop the tap at the first Stop the tap at the first

sign of a colour changesign of a colour change• calculate the volume calculate the volume

delivereddelivered• Repeat but add drop by Repeat but add drop by

drop near the volume drop near the volume where the colour change where the colour change occurred to get a greater occurred to get a greater accuracyaccuracy

• Use the volumes of both Use the volumes of both solutions and the solutions and the concentration of the acid concentration of the acid to find the concentration to find the concentration of the alkali using a given of the alkali using a given formulaformula

(This could be a sol id

dissolved in water)

Purity and Yield:Purity and Yield:PurityPurity can be checked by… can be checked by…

– Melting point/ boiling pointMelting point/ boiling point– ChromatographyChromatography

YieldYield can be calculated by… can be calculated by…

actual mass of pure sample X 100 = % yieldactual mass of pure sample X 100 = % yieldtheoretical mass expectedtheoretical mass expected

Calculating theoretical yields:Calculating theoretical yields:• Start with a Start with a balanced equationbalanced equation• Divide the mass of a reactant by its Divide the mass of a reactant by its

relative formula massrelative formula mass• Compare ratios of reactant to productCompare ratios of reactant to product• Use Use relative formula massrelative formula mass of product to of product to

find theoretical mass (yield)find theoretical mass (yield)

• Worked example…Worked example…

Mg(s) + 2HCl(aq) Mg(s) + 2HCl(aq) MgCl MgCl22(aq) +H(aq) +H22(g)(g)

• 2.4g of magnesium (Relative atomic mass 2.4g of magnesium (Relative atomic mass AArr=24) is used to make magnesium =24) is used to make magnesium chloride (relative formula mass Mchloride (relative formula mass M rr=95). =95). How much of this salt should we expect?How much of this salt should we expect?

• 2.4/24 = 0.12.4/24 = 0.1• Ratio is 1:1 for magnesium to magnesium Ratio is 1:1 for magnesium to magnesium

chloridechloride• 0.1 X 1 X 95 = 9.5g yield of magnesium 0.1 X 1 X 95 = 9.5g yield of magnesium

chloridechloride

2Mg(s) + O2Mg(s) + O22(g) (g) 2MgO(s) 2MgO(s)

• 16g of oxygen (Relative formula mass M16g of oxygen (Relative formula mass M rr=32) is =32) is used to make magnesium oxide (relative formula used to make magnesium oxide (relative formula mass Mmass Mrr=40). How much magnesium oxide =40). How much magnesium oxide should we expect?should we expect?

• 16/32 = 0.516/32 = 0.5• Ratio is 1:2 for oxygen to magnesium oxideRatio is 1:2 for oxygen to magnesium oxide• 2 X 0.5 X 40 = 40g yield of magnesium oxide2 X 0.5 X 40 = 40g yield of magnesium oxide

Rates of Reaction:Rates of Reaction:• The The raterate or speed of a reaction depends or speed of a reaction depends

on…on…– Concentration Concentration of the reactantsof the reactants– Particle size (surface area)Particle size (surface area)– TemperatureTemperature– If catalysts are presentIf catalysts are present

• The rate is important because…The rate is important because…– If too fast = If too fast = safetysafety– If too slow = too little profit (If too slow = too little profit (economic economic

reasonsreasons))

Measuring rates of reaction:Measuring rates of reaction:• By monitoring the By monitoring the

products formed OR products formed OR the reactants used the reactants used upup

• The The gradientgradient gives gives the ratethe rate

• We can monitorWe can monitor– The volume of gas The volume of gas

formedformed– Change in colourChange in colour– Change in massChange in mass– The formation of a The formation of a

precipitateprecipitate (solid (solid from two solutions)from two solutions)

Formation of a Formation of a product fast (pink)product fast (pink)

Formation of a Formation of a product slowly product slowly

(yellow)(yellow)Using up of a Using up of a reactant (blue)reactant (blue)

Measured Measured property property

e.g. e.g. volume of volume of

gasgas

Explaining Rates:Explaining Rates:• It’s all to do with particles and their It’s all to do with particles and their collisionscollisions• If a collision has enough energy (the activation If a collision has enough energy (the activation

energy) then bonds are broken allowing a energy) then bonds are broken allowing a reaction to occurreaction to occur

• More collisions mean more chance of a reactionMore collisions mean more chance of a reaction• Faster moving particles mean more chance of Faster moving particles mean more chance of

having enough kinetic energy to reacthaving enough kinetic energy to react• We can increase the number of collisions by…We can increase the number of collisions by…

– Increasing the concentration (more particles)Increasing the concentration (more particles)– Increasing the temperature (faster moving particles)Increasing the temperature (faster moving particles)– Adding a Adding a catalystcatalyst

Increasing the concentration:Increasing the concentration:

What would you expect to happen to the rate if we…What would you expect to happen to the rate if we…• Doubled the concentrationDoubled the concentration• Increased it by 100 XIncreased it by 100 X• Decreased it by one thirdDecreased it by one third

Catalysts:Catalysts:

• Speed up the rate by bring reactants Speed up the rate by bring reactants together or by making a simpler reaction together or by making a simpler reaction pathpath

• Are called Are called enzymesenzymes in biological systems in biological systems• Are often transition metals in inorganic Are often transition metals in inorganic

reactions (non-biological)reactions (non-biological)• Are not used up during the reaction… so Are not used up during the reaction… so

can be used againcan be used again