New OCR Gateway specification from
September 2016 Higher tier: grades 9 to 4:
In this and subsequent posts I’m simply going to explain and illustrate each learning objective as they come up in the topics in the new GCSE specification.
I’m giving you my notes from each lesson.
You can really get ahead of your class if you follow this blog and all the posts that will appear here about the new GCSEs over the coming months.
This rejigging of the specification is just that: there is nothing really new here it has all been with us for the past half century at least.
That written in italics is for the higher tier paper only.
C3 Chemical Reactions
C3.1 Introducing chemical reactions
In this and subsequent posts I’m simply going to explain and illustrate each learning objective as they come up in the topics in the new GCSE specification.
I’m giving you my notes from each lesson.
You can really get ahead of your class if you follow this blog and all the posts that will appear here about the new GCSEs over the coming months.
This rejigging of the specification is just that: there is nothing really new here it has all been with us for the past half century at least.
That written in italics is for the higher tier paper only.
C3 Chemical Reactions
C3.1 Introducing chemical reactions
C3.1a use chemical symbols to write the formulae of
elements and simple covalent and ionic compounds
Formulas
A chemical formula refers to the
ratio of moles of elements in a compound.
A simplest or empirical formula gives
the simplest molar ratio of elements in that compound.
e.g. water H2O
This formula tells us that water molecules contain 2 moles of hydrogen
atoms for every mole of oxygen atoms.
The molecular formula tells us
the exact molar ratio of elements in that compound.
e.g. hydrogen peroxide H2O2
In hydrogen peroxide molecules there are two moles of hydrogen atoms for
every two moles of oxygen atoms.
But its empirical formula would be HO since the simplest mole ratio of
hydrogen atoms to oxygen atoms would be 1:1.
Formulas of some common elements
Some elements have their own formulas because their atoms exist in distinct
groups.
The formulae of elements refers to elements that are non-metals:
Hydrogen H2,
Oxygen O2,
Nitrogen N2
Chlorine Cl2
Bromine Br2
Iodine I2
Fluorine F2
Sulphur S8
Phosphorus P4
All other non-metals like the noble gasses are monatomic molecules i.e. one
atom per molecule so Helium He, Neon Ne and so on…….
You just have to memorise these formulas.
It helps to get a blank periodic Table and add them to the element in the
right place.
C3.1d use the formula of common ions to deduce the
formula of a compound
Here is a table of common ions:
Anions
|
Cations
|
||
Name
|
Formula
|
Name
|
Formula
|
Oxide
|
O2—
|
Lithium
|
Li+
|
Sulphide
|
S2—
|
Sodium
|
Na+
|
Hydroxide
|
OH—
|
Potassium
|
K+
|
Nitrate
|
NO3—
|
Magnesium
|
Mg2+
|
Chloride
|
Cl—
|
Calcium
|
Ca2+
|
Bromide
|
Br—
|
Barium
|
Ba2+
|
Iodide
|
I—
|
Aluminium
|
Al3+
|
Sulfate
|
SO4 2—
|
Ammonium
|
NH4+
|
Carbonate
|
CO3 2—
|
Copper(II)
|
Cu2+
|
Ethanoate
|
CH3COO—
|
Iron(II)
|
Fe2+
|
Phosphate
|
PO4 3—
|
Iron(III)
|
Fe3+
|
Zinc
|
Zn2+
|
||
Silver
|
Ag+
|
||
You deduce
the formula of an ionic compound adding the ions of equal and opposite charge
together.
Example 1: Sodium chloride
You need a
sodium ion ( Na+ ) and a chloride ion ( Cl— )
Ask are the
charges the same size?
If they are
as in this example then the formula is a combination of one of each ion.
Sodium
chloride formula: NaCl
Why is
this?
One positive
charge is cancelled out by one negative charge giving a neutral compound.
Example 2: Magnesium bromide
You need a
magnesium ion (Mg2+) and a bromide ion (Br—)
Ask are the
charges the same size?
In this
example the charges are not the same.
We need to
make sure that the number of charges the negative ions carry are the same as
those the positive ions carry.
So a
magnesium ion carries two positive charges but two bromide ions carry two
negative charges.
So the formula
is a combination of one magnesium ion and two bromide ions.
Magnesium
bromide formula: Mg Br2
The two follows the Br symbol to
show there are two
bromide ions for every magnesium ion.
Why is
this?
Two positive
charges on one magnesium ion are cancelled out by two negative charges on two
bromide ions giving a neutral compound.
Example 3: Ammonium sulphate
You need an
ammonium ion (NH4+) and a sulphate ion (SO4 2—)
Ask are the
charges the same size?
In this
example the charges are not the same.
We need to
make sure that the number of charges the negative ions carry are the same as
those the positive ions carry.
So an
ammonium ion (NH4+) carries one positive charge but a
sulphate ion (SO4 2—) carries two negative charges.
So the
formula will be a combination of two ammonium ions (NH4+) and one sulphate ion (SO4
2—)
We write the
formula of ammonium sulphate with a bracket around the sulphate ion because the
sulphate ion contains two or more elements like this:
Ammonium
sulphate formula: (NH4)2SO4
The two follows the NH4
symbol to show there are two
ammonium ions for every sulphate ion.
Why is
this?
Two positive
charges on two ammonium ions are cancelled out by two negative charges on one
sulphate ion giving an electrically neutral compound.
Always put
brackets around ions with more than one element in them unless there is only
one ion in the formula so:
Sodium
hydroxide formula is NaOH but
magnesium
hydroxide formula is Mg(OH)2 and
aluminium
hydroxide’s formula is Al(OH)3
Notice too
that the final formula does not show the charges that the ions carry.
Other
formulas:
Structural formula
This refers
to the structure of organic molecules such as alkanes.
Simple
examples occur at GCSE.
The alkanes
have molecular, structural and displayed formulas
Here are the
first four members of the alkane series with all three types of formula
Alkane
name
|
Molecular
formula
|
Structural
formula
|
Displayed
formula
|
Methane
|
CH4
|
CH4
|
 |
Ethane
|
C2H6
|
CH3CH3
|
 |
Propane
|
C3H8
|
CH3CH2CH3
|
 |
Butane
|
C4H10
|
CH3CH2CH2CH3
|
 |
Methyl propane
|
C4H10
|
CH3CH(CH3)CH3
|
 |
Structural formulas show the atoms connected to
each carbon atom with brackets round the branches in the molecule (see methyl
propane).
Displayed formulas show all the atoms and all the
bonds.
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