Revision of C6 Chemical Revision of C6 Chemical synthesissynthesis

Chemical synthesis: chemical reactions and processes used to get a desired product

using starting materials called reagents. The products can be useful for a variety of

purposes but tend to be either…

C6.1 Chemicals are why we need themC6.1 Chemicals are why we need them

Understand the importance of chemical synthesis Understand the importance of chemical synthesis to provide food additives, fertilisers, to provide food additives, fertilisers, dyestuffs, , paints, pigments and pharmaceuticals.paints, pigments and pharmaceuticals.

• fine chemicals

A chemical product that is made in relatively small quantities and is typically high in cost, e.g. a flavouring or vitamin, drugs etc. They are made to high levels of purity. Usually in laboratories.

• bulk 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. Usually made in industries.

Interpret information about the sectors, scale and importance of chemical synthesis in industry and laboratory.

Recall the formulae of ….

Gases: chlorine = Cl2, hydrogen = H2, nitrogen= N2, oxygen = O2

Acids: hydrochloric acid = HCl, nitric acid = HNO3, sulfuric acid = H2SO4,

Alkalis

•sodium hydroxide = NaOH,

•magnesium hydroxide Mg(OH)2

Salts[MX] sodium chloride = NaCl, magnesium oxide = MgO, potassium chloride = KCl

[MX2]calcium chloride = CaCl2,More complex salts… magnesium carbonate = [MgCO3] magnesium sulfate = [MgSO4]; sodium carbonate [Na2CO3]; calcium carbonate [CaCO3]

H ONLY : Work out the formulae of ionic compounds given the charges on the ions.

Compound Positive ion Negative ion Formula

Sodium chloride

Na+ Cl- NaCl

Magnesium chloride

Mg2+ Cl- MgCl2

Calcium oxide Ca2+ O2- CaO

Aluminium oxide

Al3+ O2- Al2O3

Magnesium sulfate

Mg2+ SO42- MgSO4

• If magnesium forms Mg2+ ions and sulfate forms SO4

2- ions then identify the charges on the other ions in the following compounds…– MgO– MgCl2– MgNO3

– Na2SO4

– Al2(SO4)3

– CaSO4

H ONLY Work out the charge on one ion, given the formula of a salt and the charge on the other ion.

harmful / irritant

corrosive

toxic

highly flammable

oxidising

Recall the main hazard symbols, and understand the safety precautions to use when handling hazardous chemicals.

Recall examples of pure acidic compounds which are solid, liquids and gases.

Solids =citric acid & tartaric acid

Liquids =

sulfuric, nitric and ethanoic acids

Gases =hydrogen chloride

Recall common alkalis…

sodium hydroxide

potassium hydroxide

calcium hydroxide.

Recall the pH scale

pH 1 to pH 3 shows that there is a STRONG ACID pH 4 to pH 6 shows that there is a WEAK ACID pH 7 shows that the substance is NEUTRAL pH 8 to pH 10 shows that there is a WEAK ALKALI pH 11 to pH 14 shows that there is a STRONG ALKALI

Recall the use of indicators and pH meters to measure pH.

Indicator Colour in acid

Colour in neutral

Colour in alkali

litmus Red Blue

phenolphthalein colourless pink pink

Universal (a mixture of indicators)

Red Green Purple

A pH meter.

Recall the reactions of acids that produce salts.

Salts can be produced by reacting acids with….. metals

metal oxides

metal hydroxides

metal carbonates

Write balanced equations with state symbols to describe the characteristic reactions of acids

Metal + acid

Metal + acid metal salt + hydrogen

calcium + sulfuric

magnesium + hydrochloric

acid

acidcalciumsulfate

+ hydrogen

magnesiumchloride

+

hydrogen

Ca(s) + H2SO4(aq)

CaSO4(aq)

+ H2(g)

Mg(s)+ HCl(aq) MgCl2(aq)

+ H2(g)

Metal oxide + acid

metal + acid metal salt + water

copper + sulfuric

magnesium + hydrochloric

oxide acid

oxide acid

oxide

coppersulfate

+ water

magnesiumchloride

+

water

CuO(s) + H2SO4(aq)

CuSO4(aq)

+ H2O(l)

MgO(s) 2HCl(aq)+ MgCl2(aq)

+ H2O(l)

Metal hydroxide + acid

metal + acid metal salt + water

potassium + sulfuric

sodium + hydrochloric

hydroxide

acid

hydroxide

acid

hydroxide

potassiumsulfate

+ water

sodiumchloride

+

water

2KOH(aq)

+ H2SO4(aq)

NaOH(aq)+ HCl(aq) NaCl(aq)+ H2O(l)

K2SO4(aq)

+2H2O(l)

Metal carbonate + acid

metal + acid metal salt + carbon + water

copper + sulfuric

magnesium +hydrochloric

carbonate acid

carbonate acid

carbonate dioxide

copper + carbon + water

magnesium + carbon + water

sulfate dioxide

dioxide

chloride

CuCO3(s) + H2SO4 (aq) CuSO4(aq) + CO2(g) + H2O(l)

MgCO3(s) + 2HCl (aq) MgCl2(aq) + CO2(g) + H2O(l)

Recall that the reaction of acid with an alkali to form a salt is a neutralisation reaction.

Explain what happens during a neutralisation reaction.

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

• H+(aq) + OH- (aq) H2O(l)• An alkali can cancel out an acid to form a

salt and the water (shown above)

H+ ions from the acid react with OH- ions from the alkali.

Acidic substances

…• Dissolve in water to form H+ ions giving

a pH of less than 7• Can be…

– solids e.g. citric acid, tartaric acid– Liquids e.g. sulfuric acid, nitric acid,

ethanoic acid– Gases e.g. hydrogen chloride

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

Explain that acidic compounds….

Dissolve in water to produce aqueous hydrogen ions H+(aq)

All acids contain hydrogen – HCl, H2SO4, CH3COOH

Explain that alkaline compounds….

Dissolve in water to produce aqueous hydroxide ions OH-(aq)

Form solutions with pH lower than 7.

Form solutions with pH higher than 7.

Write down the formula of the salt produced given the formula of the acid and the alkali.

acid alkali salt

HCl NaOH NaCl

H2SO4 KOH K2SO4

HCl Ca(OH)2 CaCl2

H2SO4 Mg(OH)2 MgSO4

C6.2 : Planning, Carrying out and controlling chemical synthesis

1. Identify the stages in the chemical synthesis of an inorganic compound.

• choosing the reaction or series of reactions • risk assessment (chemical and procedural)• working out the quantities 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.

Understand the purpose of these 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 remove excess solid.

• Evaporation… to remove excess solvent from a solution

• Drying in an oven or dessicator… to remove water without the risk of wasting yield.

• Titration… to find the concentration of an acid (or alkali) using an alkali (or acid) of a known concentration AND an indicator

Understand the importance of purifying chemicals and checking their purity.

There are three main grades of chemicals :

1. Analytical – this is the most pure (and most expensive!) If a product is to be used in foods/medicines then this grade is needed – eg. Table salt.

2. Laboratory – this is the ‘medium grade’.

3. Technical – this is low grade purity – eg salt for gritting roads.

Understand that a balanced equation for a chemical reaction shows the relative numbers of atoms and molecules of reactants and products.

Mg(s) + 2HCl(aq) MgCl2(aq) +H2(g)

Reactants : 1 x Magnesium atom

2 x Hydrogen atoms

2 x Chlorine atomsAnd, because the equation is balanced, the same number of atoms are present for the products side!The numbers in front of the formula tells you how many molecules there are of it. So above, there are 2 molecules of HCl, and 1 molecule of everything else.

Understand that the relative atomic mass of an element shows the mass of it’s atom relative to the mass of other atoms.

Specifically, the RAM is compared to the mass of Hydrogen.

Be able to use the periodic table to obtain the relative atomic masses of elements.

Look at the periodic table in your planners.

The bottom number is the relative atomic mass.

Calculate the relative formula mass of a compound using the formula and the relative atomic masses of the atoms it contains.

Periodic table :

Ca = 40g C = 12g O = 16g

CaCO3 = 40+12+(3x16)

= 40+12+48

= 100g

What is the relative formula mass of calcium carbonate? Formula of calcium carbonate = CaCO3

What is the relative formula mass of magnesium chloride?

Formula of magnesium chloride = MgCl2From the periodic table:

Mass of Mg = 24g Mass of Cl = 35.5g

MgCl2 = 24 + (2 x 35.5)

= 24 + 71

= 95g

2Mg(s) + O2(g) 2MgO(s)

• 16g of oxygen (Relative formula mass = 32) is used to make magnesium oxide (relative formula mass = 40).

How much magnesium oxide should we expect?

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

O8

16

On periodic table

Mg(s) + 2HCl(aq) MgCl2(aq) +H2(g)

• 2.4g of magnesium (Relative atomic mass = 24) is used to make magnesium chloride (relative formula mass = 95).

How much of this salt should we expect?• 2.4/24 = 0.1• Ratio is 1:1 for magnesium to magnesium

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

chloride

Calculate percentage yields given the actual and theoretical yield.

actual mass of pure sample X 100 theoretical mass expected

Percentage yield =

So if by experiment, 7.4g of magnesium chloride was made, when theoretically 9.5g was expected; what is the percentage yield?

% yield = 7.49.5

X 100 = 0.7789 x 100

= 77.8 % yield.

An acid-base titration is the determination of the concentration of an acid or base by exactly neutralizing the acid/base with an acid or base of known concentration. This allows for quantitative analysis of the concentration of an unknown acid or soluble base. It makes use of the neutralisation reaction that occurs between acids and bases and the knowledge of how acids and bases will react if their formulas are known.Acid-Base titrations can also be used to find percent purity of chemicals.

Titrations

• Open the tap to let the acid run into the flask

• Stop the tap at the first sign of a colour change

• Note the volume delivered (this is approximate)

• Repeat, but add drop by drop near the volume noted for greater accuracy. Record exact volume of acid needed to get colour change for neutral.

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

(This could be a solid

dissolved in water)

Describe how to carry out an acid alkali titration accurately.

Substitute results in a given formula to interpret titration results quantitatively.

In a titration, 50cm3 of 2M sodium hydroxide was exactly neutralised by 30cm3 of hydrochloric acid.

What is the concentration of hydrochloric acid?Method1. Write a balanced equation for the reaction.

sodium hydroxide + hydrochloric acid  sodium chloride + waterNaOH(aq

) HCl(aq) NaCl(aq) H2O(l

)2. Use the big numbers in front of the formulae ( if any) to work out the proportion of NaOH to HCl.In this case, it is 1:1 so 1 mole NaOH reacts with 1 mole HCl

3. Find out how many moles of sodium hydroxide are present.(moles = [concentration (in M) x volume (in cm3)] : 1000

The number of moles in 50cm3 of 2M sodium hydroxide = (2x50) : 1000

= 0.1 moles of sodium hydroxide.4.From part 1 and 2 we know that 30cm3 of

HCl also contains 0.1 moles.

So to find the concentration of acid you rearrange the formula: concentration = (moles x 1000) : VolumeConc = (0.1 x 1000) : 30

= 3.33 M

Understand why it is important to control the rate of a chemical synthesis.

If it is too fast it could make it unsafe.

(eg could get too hot if exothermic; gas could be produced to quickly and pressure build up)

If it is too slow, then product would be made too slowly, and yield low, so profit too low. (economic factors)

Explain the term ‘rate of chemical reaction’.

This is the speed at which the reaction takes place.

A reaction takes place when reactant molecules collide with enough energy .

Describe ways for following the rate of a reaction.1. By collecting a gas.

2. Weighing the reaction mixture.

3. Observing colour change or precipitate.

Interpret results from experiments that investigate rate of reactions.

Time (seconds)

Mass lost (g)

The reaction starts off quickly

It slows down as it proceeds.

It eventually stops when one of the reactant particles has run out.

Rate graphs and reactant concentrationsA

mou

nt o

f pr

oduc

t

Time

reactants

product

Reactant Concentration falls

Rate of Reaction falls

All product

All reactant

Mix of reactant

And product

Gradient of graph decreases

Rates and Graphs

• These show the increasing amount of product or the decreasing amount of reactant.

Am

ount

of

prod

uct

Time

Am

ount

of

reac

tant

Time

Steep gradient Fast reaction

Shallow gradient Slow reaction

Steep gradient Fast reaction

Shallow gradient Slow reaction

Time

Mass lost Chip

s

Powder

Graph shows a faster reaction with powder. Same final volume of gas made.

Increasing the temperature would have the same effect.

Recall that reaction rates vary with….

• particle size (surface area)

• concentration

• temperature.

Remember – to increase the rate of reaction, there needs to be more successful collisions per second.

Surface area

• The reactions of solids can clearly only take place at the surface of the solid.

• If we break a solid into smaller pieces we get more area and a faster reaction.

Molecules collide with the surface of the solid

Extra surface for molecules to collide with.

• Reactions in solution involve dissolved particles that must collide before reaction is possible.

• The more crowded (concentrated) the solution, the faster the reaction because the frequency of successful collisions increases.

Collisions infrequent Collisions frequent

Use simple collision theory to explain how rates of reaction depend on the concentration of solutions of soluble chemicals.

Understand that catalysts speed up a chemical reaction while not being used up in the process.Because they are not used up, they are recyclable.

For chemical reactions to occur:• Existing bonds have to begin breaking so that new ones can be formed.• The molecules have to collide in such a way that the reacting parts of the molecules are brought together.

Catalysts can help with either or both of these processes.