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Chemical Economics
OCR module 14
Chemical formulae
Methane, CH4
Water, H2O
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Carbon
dioxide, CO2
Key
Hydrogen
Ethyne, C2H2
Oxygen
Sulphuric
acid, H2SO4
Carbon
Sulphur
Chemical formulae
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The chemical formulae of a molecule or compound is simply a
way of showing the ratio of atoms in it. For example…
Na
Cl
= sodium chloride (NaCl)
K
I
= potassium iodide (KI)
O
K
N
O
O
= potassium nitrate (KNO3)
Chemical formulae
Try drawing these:
1) Water H2O
2) Carbon dioxide CO2
3) Calcium sulphate CaSO4
4) Magnesium hydroxide Mg(OH)2
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Naming compounds
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Rule 1– If two identical elements combine then the
name doesn’t change
This happens with the following elements:
1) H2
4) F2
2) N2
5) Cl2
3) O2
6) Br2
These elements always go
around in pairs (diatomic
molecules). For example,
hydrogen looks like this:
Naming compounds
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Rule 2 – When two elements join and one is a halogen,
oxygen or sulphur the name ends with ____ide
e.g. Magnesium + oxygen
magnesium oxide
1) Sodium + chlorine
6) KBr
2) Magnesium + fluorine
7) LiCl
3) Lithium + iodine
8) CaO
4) Chlorine + copper
9) MgS
5) Oxygen + iron
10)KF
Naming compounds
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Rule 3 – When three or more elements combine and two of
them are hydrogen and oxygen the name ends with
hydroxide
e.g. Sodium + hydrogen + oxygen
Sodium hydroxide
1) Potassium + hydrogen + oxygen
2) Lithium + hydrogen + oxygen
3) Calcium + hydrogen + oxygen
4) Mg(OH)2
Naming compounds
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Rule 4 – When three or more elements combine and
one of them is oxygen the ending is _____ate
e.g. Copper + sulphur + oxygen
Copper sulphate
1) Calcium + carbon + oxygen
6) AgNO3
2) Potassium + carbon + oxygen
7) H2SO4
3) Calcium + sulphur + oxygen
8) K2CO3
4) Magnesium + chlorine + oxygen
5) Calcium + oxygen + nitrogen
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Conservation of mass in reactions
In any reaction the total mass of products is
the same as the total mass of the reactants
Example 1 – Magnesium oxide and hydrochloric acid
H
Mg
O
H
Cl
Cl
Cl
1 x magnesium, 1 x oxygen, 2 x
hydrogen and 2 x chlorine atoms
Mg
H
Cl
Also 1 x magnesium, 1 x oxygen, 2
x hydrogen and 2 x chlorine atoms
Example 2 – Burning methane
H
H
H
C
H
O
O
O
O
O H
C
O
O
H
H
O
O
H
H
Balancing equations
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Consider the following reaction:
Sodium + water
Na + H2O
Na
+
sodium hydroxide + hydrogen
NaOH + H2
Na
O
H
H
O
H
+
H
H
This equation doesn’t balance – there are 2 hydrogen
atoms on the left hand side (the “reactants” and 3 on
the right hand side (the “products”)
Balancing equations
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We need to balance the equation:
Sodium + water
sodium hydroxide + hydrogen
Na
O
H
Na
+
Na
H
O
O
H
Na
H
O
H
+
H
H
Now the equation is balanced, and we can write it as:
2Na(s) + 2H2O(l)
2NaOH(aq) + H2(g)
H
Some examples
2Mg
O2
2 MgO
Zn
+ 2 HCl
ZnCl2
2 Fe
+ 3Cl2
2 FeCl3
NaOH
CH4
Ca
+
+
HCl
+ 2 O2
NaCl
CO2
+
+
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H2
H2O
+ 2H2O
+ 2 H2O
Ca(OH)2
+
+
H2SO4
Na2SO4
+ 2H2O
2 CH3OH
+ 3 O2
2 NaOH
2 CO2
+ 4H2O
H2
Simple formulae to learn
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Covalent formulae
Ionic formulae
H2O
Water
NaCl
Sodium chloride
CO2
Carbon dioxide
CaCl2
Calcium chloride
NH3
Ammonia
MgO
Magnesium oxide
H2
Hydrogen
HCl
Hydrochloric acid
Sulphuric acid
O2
Oxygen
H2SO4
HNO3
Nitric acid
N2
Nitrogen
NaOH
Sodium hydroxide
SO2
Sulphur dioxide
Ca(OH)2
Calcium hydroxide
CaCO3
Calcium carbonate
Al2O3
Aluminium oxide
Fe2O3
Iron oxide
Atomic Mass
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The “bigger number” on the
periodic table is the “relative
atomic mass”, i.e. how heavy the
atom is compared to a hydrogen
atom (roughly).
Relative formula mass, Mr
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The relative formula mass of a compound is blatantly the relative
atomic masses of all the elements in the compound added together.
E.g. water H2O:
Relative atomic mass of O = 16
Relative atomic mass of H = 1
Therefore Mr for water = 16 + (2x1) = 18
Work out Mr for the following compounds:
1) HCl
H=1, Cl=35 so Mr = 36
2) NaOH
Na=23, O=16, H=1 so Mr = 40
3) MgCl2
Mg=24, Cl=35 so Mr = 24+(2x35) = 94
4) H2SO4
H=1, S=32, O=16 so Mr = (2x1)+32+(4x16) = 98
5) K2CO3
K=39, C=12, O=16 so Mr = (2x39)+12+(3x16) = 138
More examples
CaCO3
40 + 12 + 3x16
HNO3
1 + 14 + 3x16
2MgO
2 x (24 + 16)
3H2O
3 x ((2x1) + 16)
4NH3
2KMnO4
3C2H5OH
4Ca(OH)2
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100
80
Calculating the mass of a product
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E.g. what mass of magnesium oxide is produced when 60g of
magnesium is burned in air?
Step 1: READ the equation:
2Mg + O2
2MgO
IGNORE the
oxygen in step 2 –
the question
doesn’t ask for it
Step 2: WORK OUT the relative formula masses (Mr):
2Mg = 2 x 24 = 48
2MgO = 2 x (24+16) = 80
Step 3: LEARN and APPLY the following 3 points:
1) 48g of Mg makes 80g of MgO
2) 1g of Mg makes 80/48 = 1.66g of MgO
3) 60g of Mg makes 1.66 x 60 = 100g of MgO
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1) When water is electrolysed it breaks down into hydrogen and oxygen:
2H2O
2H2 + O2
What mass of hydrogen is produced by the electrolysis of 6g of water?
Work out Mr: 2H2O = 2 x ((2x1)+16) = 36
2H2 = 2x2 = 4
1.
36g of water produces 4g of hydrogen
2. So 1g of water produces 4/36 = 0.11g of hydrogen
3. 6g of water will produce (4/36) x 6 = 0.66g of hydrogen
2) What mass of calcium oxide is produced when 10g of calcium burns?
2Ca + O2
Mr: 2Ca = 2x40 = 80
2CaO
2CaO = 2 x (40+16) = 112
80g produces 112g so 10g produces (112/80) x 10 = 14g of CaO
3) What mass of aluminium is produced from 100g of aluminium oxide?
2Al2O3
4Al + 3O2
Mr: 2Al2O3 = 2x((2x27)+(3x16)) = 204
4Al = 4x27 = 108
204g produces 108g so 100g produces (108/204) x 100 = 52.9g of Al2O3
Another method
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Try using this equation:
Mass of product IN GRAMMES
Mass of reactant IN GRAMMES
Mr of product
Mr of reactant
Q. When water is electrolysed it breaks down into hydrogen and oxygen:
2H2O
2H2 + O2
What mass of hydrogen is produced by the electrolysis of 6g of water?
Mass of product IN GRAMMES
6g
4
36
So mass of product = (4/36) x 6g = 0.66g of hydrogen
Empirical formulae
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Empirical formulae is simply a way of showing how many atoms are in a
molecule (like a chemical formula). For example, CaO, CaCO3, H20 and
KMnO4 are all empirical formulae. Here’s how to work them out:
A classic exam question:
Find the simplest formula of 2.24g of iron
reacting with 0.96g of oxygen.
Step 1: Divide both masses by the relative atomic mass:
For iron 2.24/56 = 0.04
For oxygen 0.96/16 = 0.06
Step 2: Write this as a ratio and simplify:
0.04:0.06 is equivalent to 2:3
Step 3: Write the formula:
2 iron atoms for 3 oxygen atoms means the formula is Fe2O3
Example questions
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1) Find the empirical formula of magnesium oxide which
contains 48g of magnesium and 32g of oxygen.
2) Find the empirical formula of a compound that contains
42g of nitrogen and 9g of hydrogen.
3) Find the empirical formula of a compound containing 20g
of calcium, 6g of carbon and 24g of oxygen.
The Mole
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Definition: A mole is a measure of the number of
particles in a substance. 1 mole is 6x1023 particles.
Molar Mass (g/mol)
Molar mass is the mass of one mole of a substance and is equal
to the relative atomic mass (in grams).
For example:
1) Carbon has a relative atomic mass of 12, so 1mol of carbon
atoms would have a mass of 12g
2) Aluminium has a relative atomic mass of 27, so 1mol of
aluminium atoms would have a mass of ___g
3) Sodium hydroxide has a relative atomic mass of 40, so
2mols of NaOH would have a mass of ____g
Molar Calculations
Mass (g)
No. of moles =
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N= m
Molar mass (g/mol)
M
Some example questions:
1) Calculate the mass of 4mol of lithium
2) Calculate the mass of 2mol of sodium
3) Calculate the number of moles in 36g of carbon
4) Calculate the number of moles in 88g of carbon dioxide
5) Calculate the number of moles in 27g of water
Percentage yield
Percentage yield =
Actual yield
Predicted yield
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X 100%
Some example questions:
1) The predicted yield of an experiment to make salt was 10g.
If 7g was made what is the percentage yield?
2) Dave is trying to make water. If he predicts to make 15g
but only makes 2g what is the percentage yield?
3) Sarah performs an experiment and has a percentage yield
of 30%. If she made 50g what was she predicted to make?
Making Ammonia
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Guten Tag. My name is Fritz Haber and I won the Nobel
Prize for chemistry. I am going to tell you how to use a
reversible reaction to produce ammonia, a very important
chemical. This is called the Haber Process.
Nitrogen + hydrogen
Ammonia
N2 + 3H2
2NH3
Fritz Haber,
1868-1934
To produce ammonia from nitrogen and hydrogen you
have to use three conditions:
Nitrogen
Hydrogen
•High pressure
•450O C
•Iron catalyst
Mixture of NH3, H2 and
N2. This is cooled
causing NH3 to liquefy.
Recycled H2 and N2
Haber Process Summary
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A low temperature increases the yield of ammonia but is too
slow
A high temperature improves the rate of reaction but
decreases the yield too much
A high pressure increases the yield of ammonia but costs a lot
of money
To compromise all of these factors, these conditions are used:
Nitrogen
Hydrogen
•200 atm pressure
•450O C
•Iron catalyst
Mixture of NH3, H2
and N2. This is
cooled causing NH3
to liquefy.
Recycled H2 and N2
Quiz on acids and alkalis
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Acid, alkali or both???
1) This a pH of less than 7
2) This could kill cells
3) A metal hydroxide (e.g. sodium hydroxide) would be an _____
4) When this reacts with a metal hydrogen is released
5) A metal carbonate (e.g. calcium carbonate) would be an _____
6) This would feel soapy on your skin
7) This could be a corrosive
8) This will turn universal indicator purple
9) This would taste sour
10) This means “a base that can be dissolved”
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Universal Indicator and the pH scale
Universal Indicator is a mixture of liquids that will produce a
range of colours to show how strong the acid or alkali is:
1
2
3
Stomach acid
4
5
Lemon juice
6
7
8
9
10
11
12 13 14
Water Soap Baking powder Oven cleaner
Strong alkali
Strong acid
Neutral
An acid contains hydrogen ions, H+
An alkali contains hydroxide ions, OH-
Neutralisation reactions
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When acids and alkalis react together they will NEUTRALISE
each other:
Sodium hydroxide
Na
Hydrochloric acid
H
OH
The sodium replaces the
hydrogen from HCl
Na
Cl
Sodium chloride
H2O
Water
Cl
Neutralisation experiment
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In this experiment we mixed sodium hydroxide (an _____) and
hydrochloric acid together and they ________ each other. The
equation for this reaction is…
Sodium hydroxide + hydrochloric acid
sodium chloride + water
A ____ was formed during the reaction, and we could have separated
this by __________ the solution. The salt that we formed depended
on the acid:
•Hydrochloric acid will make a CHLORIDE
•Nitric acid will make a _________
•Sulphuric acid will make a _________
Words to use – nitrate, neutralised, alkali, sulphate, salt, evaporating
Making salts
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Whenever an acid and alkali neutralise each other we are
left with a salt, like a chloride or a sulphate. Complete
the following table:
Hydrochloric
acid
Sodium
hydroxide
Potassium
hydroxide
Calcium
hydroxide
Sulphuric
acid
Nitric acid
Sodium chloride
+ water
Potassium
sulphate + water
Calcium nitrate
+ water
Eutrophication
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Eutrophication is when lakes become stagnant due to careless use of
fertilisers like ammonium nitrate. There are six steps:
1) Inorganic fertilisers
used on fields are
washed into the lake
3) This growth causes
overcrowding and many plants die
due to lack of enough light or food
2) The fertiliser causes increased growth in water plants
Eutrophication
4) Microorganisms and
bacteria increase in number due
to the extra dead material
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6) The lack of oxygen
causes the death of fish
and other aquatic animals
Can’t…breathe…
5) These microorganisms
use up the oxygen in the
lake during respiration
Eutrophication
4) Microorganisms and
bacteria increase in number due
to the extra dead material
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6) The lack of oxygen
causes the death of fish
and other aquatic animals
5) These microorganisms
use up the oxygen in the
lake during respiration