AS Chemistry Revision-Chemistry for Life
1. Avagadro constant, 6.02 x 10²³
 number of particles in 1 mole of a substance
2. Empirical formula-simplest ratio of atoms of each element in a compound
3. 𝑀𝑜𝑙𝑒𝑠 =
𝑀𝑎𝑠𝑠
𝑀𝑟
4. Model of an atom
Particle
Proton
Neutron
Electron
Mass on relative atomic scale
1
1
Very small (0.00055)
Charge
1+
0
1-
5. Radioactive isotopes
Radiation
What is it?
Alpha α
Helium nuclei
⁴₂He
Electrons
-1
₋₁°e
Electromagnetic none
radiation
Beta β
Gamma ϒ
Relative
charge
+2
How does the
nucleus change?
2 fewer protons
2 fewer neutrons
1 more proton
1 fewer neutron
No change
Stopped by?
Paper or skin
Aluminium
foil
Lead sheet
Deflection in
electric field?
Low
High
None
6.
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7.
Half-life
i. The time taken for half a radioactive isotope/substance to decay.
ii. Not affected my temperature
iii. Can be used to date archaeological artefacts made from living things
 Tracers
i. Radioactive isotopes whose decay is monitored
ii. Can be used to aid diagnosis, followed by a Geiger counter
iii. An isotope should have a half-life which is neither too short or it will decay
before tracing is complete. nor too long or it will persist for too long in the
body, potentially causing harm to the patient.
Absorption spectra
 Coloured background with black lines
 Lines go up
 Lines get closer at higher frequency
 Produced when:
i. Electrons absorb a photon (package of energy) and become excited.
ii. Excited electrons move to a higher energy state- they are promoted
iii. The electromagnetic radiation absorbed by each atom has a definite frequency
related to the difference in energy levels by ∆E = hv
8. Emission spectra
 Black background with coloured lines
 Lines go down
 Lines get closer at higher frequency
 Produced when:
i. Electrons first absorb a photon and become excited
ii. Excited electrons move up to higher energy level
iii. Electrons drop back to lower energy level and emit electromagnetic radiation
iv. The electromagnetic radiation emitted by an atom has a definite frequency
related to the difference in energy levels by ∆E = hv
9. Properties
Structure
Type of bonding
Melting point
Ionic lattice
Ionic
high
Solubility in
water
Usually soluble
Giant covalent
network
covalent
high
insoluble
Simple
molecular
Metallic lattice
covalent
Low
Usually soluble
Electrical
conductivity
Only if molten or
in solution
Wont conduct,
apart from
graphite
Wont conduct
Metallic
high
insoluble
Will conduct
10. Shapes of molecules-Electron densities repel each other as far as possible in space to minimise
repulsion
 6 groups of electrons – octahedral-90°

5 groups of electrons – trigonal bipyramidal-90°/120°

4 groups of electrons:
i. 4 bonding pairs – tetrahedral- 109°
ii. 3 bonding pairs and 1 lone pair – pyramidal- 109°
iii. 2 bonding pairs and 2 lone pairs – bent- 109°

3 groups of electrons – planar triangular - 120°

2 groups of electrons – linear - 180°
11. Reactions of group 2 elements
 Metal + water
 Metal oxide + water
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metal hydroxide + hydrogen
metal hydroxide
Metal hydroxide + acid
Salt + water
Hydroxides are more soluble down the group
Metal oxide + acid
salt + water
Metal carbonate
metal oxide + carbon dioxide
Thermal stability increases down the group .i.e BaCO₃ is less likely to break
down on heating than MgCO₃
12. Heterogeneous catalyst
Group
2
Be
Mg
Ca
Sr
Ba
Ra
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
Are in different physical state to the reactants
Work by:
i. The reactants are absorbed onto the catalyst surface
ii. Bonds between the reactants are weakened and broken
iii. New bonds and compounds are formed
iv. The products formed diffuse off the catalyst surface
Catalyst poison:
i. Poison is strongly absorbed onto the catalyst surface
ii. The poison blocks the catalyst surface, preventing reactants from bonding.
13. Mass spectrometer
 Sample inlet Ionisation area- electrons produced by a heated filament bombard any atoms or
molecules in the sample and knock electrons out. Cations (positive ions) are formed:
X + e¯
X⁺ + 2e¯
Produces fragments.
 Acceleration are- an electric field accelerates any ions so that they all have the same
kinetic enbergy.
 Drift region- contains a vacuum so that the ions do not collide with air molecules. Since
kinetic energy = mass x velocity² and all ions have the same kinetic energy, heavier ions
move through this region more slowly than light ions.
 Ion detector-light ions reach the detector before heavier ions. The information is
converted into a mass spectrum. Only positive ions are detected.
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Molecular ion-the peak with the greatest mass that corresponds to the molecular mass
(Mr) of the sample.
Base peak-the most intense peak.
14. Enthalpy
 Standard enthalpy change of combustion, ∆Hᶱc - the enthalpy change when 1 mole of a
substance burns completely in oxygen under standard conditions (1 atm and 298k).

Standard enthalpy change of formation, ∆Hᶱf - the enthalpy change when 1 mole of a
product is formed from its constituent elements. Both reactants and products are under
their standard states.

∆Hᶱf = zero, for elements

Assumptions:
i. All energy from burning fuel is transferred to the water
ii. When using solutions, assume the heat capacity of the solution is 4.18Jg¯¹k¯¹
and the density is 1gcm¯³

Working out ∆Hᶱc from experiment:
i. Calculate energy transferred using:
Energy transferred (J) = m x c x ∆T
Mass, specific heat capacity and change in temperature
ii. Calculate the moles of the fuel used using:
𝑀𝑜𝑙𝑒𝑠 =
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑓𝑢𝑒𝑙 𝑢𝑠𝑒𝑑
𝑀𝑟
iii. Enthalpy of combustion (kJmol¯¹)
=
𝑒𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑 (𝑘𝐽)
𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑓𝑢𝑒𝑙 𝑢𝑠𝑒𝑑
15. Bond enthalpies
 Double bonds are shorter than single bonds
 Shorter bonds are stronger than longer bonds
 Shorter bonds have higher bond enthalpies than longer bonds

The data book value for the enthalpy change of combustion of a substance may be
slightly different from the calculated value due to:
i. The bond enthalpy values used are averages
ii. The bond enthalpies are not in standard state/are for the gaseous state
16. Hydrocarbons
 Aliphatic-they have no ring structures, includes:
i. Alkanes
ii. Alkenes
iii. Clycoalkanes
 Aromatic- contain one or more benzene rings in their structure
 Saturated-no double bonds
 Unsaturated-contains one or more double bonds
17. Structural isomerism
 Chain isomerism- i.e. unbranched chains to branched chains
 Positional isomerism- i.e. pentan-1-ol and pentan-2-ol
 Functional group isomerism- same molecular formula but the functional groups are
different. i.e. ethanol and methoxymethane
18. Increasing octane number
 Cracking
i. Produces shorter, branched alkane + alkene
ii. Use of zeolite catalyst
 Reforming
i. Converts alkanes into cycloalkanes + hydrogen
ii. Converts cycloalkanes into arenes + hydrogen
iii. Use of platinum catalyst
 Isomerisaton
i. Produces shorter, more branched alkanes
ii. Use of platinum catalyst + zeolite sieve
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Group 2 oxides (and hydroxides) are bases which allow them to react with carbon dioxide (and
acids) to form group 2 carbonates
Group 2 elements do not readily form 3+ ions:
o The 3rd ionisation enthalpy of group 2 elements would come from a full shell of
electrons, which is very stable and would require a lot of energy to remove a 3rd electron
from.
Solubility of group 2 carbonates decreases down the group
The reactivity of group 2 elements increases down the group
The solubility of group 2 hydroxides increases down the group
The thermal stability of group 2 carbonates increases down the group

Cracking-breaking up long chain molecules into shorter, branched and unsaturated molecules

Zeolites-are good at absorbing small molecules due to small holes that act as a sieve.

Ionisation enthalpies decrease down a group as:
o Outermost electrons is further away from the nucleus down a group
o This makes electrons less firmly held by the nucleus (weaker attraction)
o Therefore electrons require less energy to remove
Isotopes-atoms of the same element that have the same number of proton but different
numbers of neutrons
Symbol for electron in ionisation enthalpies = e¯
Symbol for electron in alpha-particles = ₋₁°e
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Diesel produces more particulates than petrol due to:
o Diesel has a lower operating temperature than petrol
o Longer molecules in diesel
o More oxygen for complete combustion of diesel

Significance of wedges and dotted lines
o Wedges-show bonds in front of the plane of paper
o Dotted lines-shows bonds behind the plane of paper

Changes in type of bonding as a period is crossed from left to right
Metallic > Covalent
Changes in the structure as a period is cross from left to right
Giant metallic > giant covalent network > simple molecular
