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Revision of C6 Chemical synthesis
C6.1 Chemicals are why we need them
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…
Understand the importance of chemical synthesis to provide
food additives, fertilisers, dyestuffs, paints, pigments and
pharmaceuticals.
Interpret information about the sectors, scale and
importance of chemical synthesis in industry and
laboratory.
• 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.
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+
Magnesium
Mg2+
chloride
Calcium oxide Ca2+
Aluminium
oxide
Al3+
Magnesium Mg2+
sulfate
Cl-
NaCl
Cl-
MgCl2
O2-
CaO
O2-
Al2O3
SO42-
MgSO4
H ONLY Work out the charge on one ion, given the formula
of a salt and the charge on the other ion.
• If magnesium forms Mg2+ ions and sulfate
forms SO42- ions then identify the charges
on the other ions in the following
compounds…
– MgO
– MgCl2
– MgNO3
– Na2SO4
– Al2(SO4)3
– CaSO4
Recall the main hazard symbols, and understand the
safety precautions to use when handling hazardous
chemicals.
harmful /
irritant
corrosive
highly flammable
toxic
oxidising
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
litmus
Red
Colour
in
neutral
Colour in
alkali
Blue
phenolphthalein colourless pink
pink
Universal (a
mixture of
indicators)
Purple
Red
Green
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
acid
calcium
sulfate
Ca(s) + H2SO4(aq)
CaSO4(aq) + H2(g)
magnesium + hydrochloric
acid
magnesium + hydrogen
chloride
Mg(s) +
HCl(aq)
+ hydrogen
MgCl2(aq) + H2(g)
Metal oxide + acid
metal
oxide
+
acid
copper + sulfuric
oxide
acid
CuO(s) + H2SO4(aq)
magnesium + hydrochloric
oxide
acid
MgO(s) + 2HCl(aq)
metal salt +
copper
sulfate
water
+ water
CuSO4(aq) + H2O(l)
magnesium + water
chloride
MgCl2(aq) + H2O(l)
Metal hydroxide + acid
metal
+
hydroxide
acid
potassium + sulfuric
hydroxide acid
2KOH(aq) + H2SO4(aq)
sodium + hydrochloric
hydroxide
acid
NaOH(aq) + HCl(aq)
metal salt +
water
potassium + water
sulfate
K2SO4(aq) + 2H2O(l)
sodium
+ water
chloride
NaCl(aq) +
H2O(l)
Metal carbonate + acid
metal + acid
carbonate
copper + sulfuric
carbonate acid
CuCO3(s) + H2SO4 (aq)
magnesium +hydrochloric
carbonate
acid
MgCO3(s) +
2HCl (aq)
metal salt + carbon + water
dioxide
copper + carbon + water
sulfate dioxide
CuSO4(aq) + CO2(g) + H2O(l)
magnesium + carbon + water
chloride
dioxide
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.
H+ ions from the acid react with OH- ions from the
alkali.
• 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)
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
Form solutions with pH lower than 7.
Explain that alkaline compounds….
Dissolve in water to produce aqueous hydroxide ions
OH-(aq)
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 atoms
And, 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.
What is the relative formula mass of calcium carbonate?
Formula of calcium carbonate = CaCO3
Periodic table :
Ca = 40g
C = 12g
CaCO3 = 40+12+(3x16)
= 40+12+48
= 100g
O = 16g
What is the relative formula mass of magnesium
chloride?
Formula of magnesium chloride = MgCl2
From 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)
8
16
O
On periodic table
• 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
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.
Percentage
yield =
actual mass of pure sample X 100
theoretical mass expected
So if by experiment, 7.4g of magnesium chloride was
made, when theoretically 9.5g was expected; what is the
percentage yield?
% yield = 7.4 X 100
9.5
= 0.7789 x 100 = 77.8 % yield.
Titrations
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.
Describe how to carry out an acid alkali titration accurately.
• 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)
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?
Method
1. Write a balanced equation for the reaction.
sodium hydroxide + hydrochloric acid
NaOH(aq)
HCl(aq)
sodium chloride + water
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) :Volume
Conc = (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.
It slows down as
it proceeds.
Mass
lost
(g)
It eventually stops when one
of the reactant particles
has run out.
The reaction
starts off quickly
Time (seconds)
Rate graphs and reactant concentrations
Amount of product
All product
Mix of reactant
And product
Reactant Concentration falls
Rate of Reaction falls
Gradient of graph decreases
All reactant
Time
Rates and Graphs
Shallow gradient
Slow reaction
Steep gradient
Fast reaction
Time
Amount of reactant
Amount of product
• These show the increasing amount of product
or the decreasing amount of reactant.
Steep gradient
Fast reaction
Shallow gradient
Slow reaction
Time
Powder
Mass
lost
Chips
Graph shows a
faster reaction
with powder.
Same final
volume of gas
made.
Time
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.
Use simple collision theory to explain how rates of
reaction depend on the concentration of solutions of
soluble chemicals.
• 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
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.