OCR Chemistry Salters F331

• Avagadro constant (NA) - 6.02x1023. It is the number of particles in 1 mole of a substance
• RAM (Ar) - tells you the number of times an atom of an element is heavier than one 12th of Carbon12. 
                 - it is the weighted average of the relative isotopic masses                 for an element (therefore are not always whole numbers)
                 -is the average mass of 100 atoms (of the same element)
• RFM (Mr) - is the sum of the RAM's for each atom in a formula. 
• To obtain one mole of a substance, weigh out the Ar/Mr exactly in grams
• Empirical formula - tells you the simplest ratio of atoms of each element in a compound:
  step 1) divide the mass of each element in the compound by its RAM
  step 2) divide theses answers by the smallest answer you got from step 1
  step 3) this will give you a 1:?... ratio
• Molecular formula - tells you the actual number of atoms of each                                          element in a molecule:
  step 1) :S

 To work out the amount of moles in a substance use: moles = mass(g)/mr

On a nuclear symbol 

• the top number is the elements mass number; it tells you the number of protons and neutrons present
• the bottom number is the atomic number; it tells you the number of protons/electrons (in a neutral atom) present
• protons, neutrons and electrons are all subatomic particles with different properties:

Protons: 

• have a +1 charge
• have a mass of 1 (on an RAM scale)

Neutrons:

• have a neutral charge/charge of 0
• have a mass of 1 (on an RAM scale)

Electrons:

• have a -1 charge
• have a negligible mass (0.00055)

• Isotopes are atoms of the same element with the same atomic number (number of protons) but a different mass number (number of neutrons)
• If you know the relative isotopic masses and the relative abundance of the two isotopes, you can calculate the mixed elements RAM
• The formula is:
                
                (abundance of isotope* RIS) + (abundance of isotope * RIS)

                        ----------------------------------------------------------------------------------
                                                                  100 

• some isotopes are unstable & their nuclei breaks down spontaneously
• this makes them radioactive
• the nuclei breaking down can produce 3 types of ionising emissions:
  1) alpha      2) beta    3) gamma
• all 3 types of emission are dangerous to humans and can cause cancer

1) Alpha - slow moving Helium nuclei, +2 charge, stopped by paper/skin, has                  a low deflection in an electric field

2) Beta - fast moving electrons, -1 charge, stopped by aluminium foil, has a                   high deflection in an electric field 

3) Gamma - high frequency electromagnetic radiation, 0 charge, stopped by                      thick lead sheet, has no deflection in an electric field

• alpha decay is common around heavy elements with atomic number 83+
• the isotope produced by alpha decay will have a mass number 4 units below and a nuclear charge/atomic number 2 units below what it was originally

• during beta decay the mass number of an element remains consistent yet the atomic number increases by one
• in beta radiation, a neutron is converted to a proton and an electron which is emitted

• gamma decay is the emission of energy from a nucleus which is changing from a high energy level to a lower one. no new element is formed.

• this is the joining together of 2 or more nuclei to form a heavier nucleus of a new element
• high temperatures and pressures are needed to overcome the repulsion between the two positive nuclei
• at these temperatures the nuclei are moving so quickly and with greater energy that the repulsive barrier can be overcome
• once the nuclei are close enough, the strong nuclear forces holding the protons and neutrons together in the nucleus take over and the nuclei fuse
• nuclear fusion reactions take place in the gas clouds of stars and form new elements

• the half life is the time it takes for half the number of radioactive nuclei in a sample to decay
• the half life is fixed for any given radioisotope and is not effected by temperature
• the sample never completely decays

• some rock types contain radioactive elements, this means its age can be estimated
• if the original radioisotope decays through a series of daughter products to eventually form a stable, non-radioactive isotope of an element then it may be possible to work out the age of the rock from the mass ratio of parent to daughter product
• for radiometric systems to work as geological clocks:
  1) the half life of the radioisotope must be known accurately
  2) there must've been no movement of parent/daughter isotopes into or out of a mineral since the time of crystallisation of the rock
  3) there must've been no 'resetting' of the 'radiometric clock' (via heating etc.) 

Radiocarbon dating:

• Carbon12 and Carbon14 occur naturally in all living things
• the ratio doesn't change in a living organism but when the organism dies C14 is not replenished
• the ratio therefore gradually increases and can give an estimate of the material's age

Tracers:

• tracers are radioactive isotopes whose decay is monitored
• these isotopes can be used in medicine to aid diagnosis
• the tracer is eaten, injected or drunk and then its pathway is followed using a Geiger counter
• the tracer should have a half life neither too long (or it will persist for too long in the body, potentially causing harm to the patient) or too short (or it will decay before tracing is complete)
• generally, gamma emitters should be used to enable detection as they minimise cell ionisation
• small doses are used to limit exposure, despite the risk of cancer

• an electron in an atom can occupy any one of the fixed energy levels
• in the ground state, the electrons are closest to the nucleus and have the lowest energy
• the  △ energy levels decreases as the electron moves away from the nucleus

Atomic absorption spectra

• an atomic absorption spectrum seen on Earth is the spectrum of     
  rainbow coloured visible light with black lines corresponding to the absorption of energy by electrons 
• these spectra are seen from Earth when atoms in the chromosphere around stars absorb light

1) for an absorption spectra to occur an electron must absorb a photon of    energy
2) excited electrons will be promoted and move up to a higher energy level     - this is what produces lines in the spectrum
3) the electromagnetic radiation absorbed by each of the atoms has a             definitive frequency (v) related to the difference in energy levels by:  △E=hv

Atomic emission spectra

• an emission spectra has a black background with coloured lines in it
• these lines correspond to the emissions of energy by electrons
• an emission spectrum is seen when a chemical burns with a coloured flame

1) electrons absorb a photon, are excited and promoted
2) when the photon is used up the electrons drop back down to lower energy       levels, this is what produces lines in the spectrum
3) the electromagnetic radiation emitted by each of the atoms has a                     definitive frequency related to the difference in energy levels by △E=hv.
    Since △E is different for each transmission (due to the different elements) ,       so is the frequency and therefore the colour of the lines

1) Metallic bonding  =  metal + metal:

• the metal ions are arranged regularly in a lattice
• the outershell electrons are shared by all the ions and are delocalised
• the 'sea' of electrons are free to move meaning metals can conduct electricity

2) Ionic bonding  = metal + non-metal:

• as atoms are usually more stable if they have a full outer shell of electrons, the metal atom will transfer electrons to the non-metal so they all have full outer shell electrons
• this creates charged ions
• the cations and anions are held together in a giant ionic lattice by an electrostatic attraction between the oppositely charged ions

3) Covalent bonding  = non-metal + non-metal :

• to attain a full outer shell of electrons, the 2 non-metal atoms share a pair of electrons
• if both the electrons in a shared pair come from the same atom it's a dative covalent bond i
• the atoms involved in the bond are held together by the electrostatic attractions between the positive nuclei of the 2 atoms and the shared pair of negative electrons

Metallic Bonding - form metallic lattices:

• have high melting/boiling points
• are insoluble
• will conduct electricity

Ionic Bonding - form ionic lattices:

• have high melting/boiling points
• are usually soluble
• will only conduct electricity if molten or in a solution

Covalent bonding - can form simple molecular structures:

• these have low melting points
• are usually insoluble
• wont conduct electricity

• they can also form giant covalent networks:
• these have high melting/boiling points
• are insoluble
• only graphite will conduct electricity

Groups      Bond angles (degrees)            Shape                                                
6                      90                                 octahedryl    
5                      90&120                         triagnol bipyramidal
4                      109.5                            tetrahedryl/pyramidal(1 lone pair)/bent(2)  
3                      120                               planar triangular    
2                      180                               linear

• the shape of a molecule depends on the number of groups of electrons
• an electron group could be a; single bond, double bond, triple bond or a lone pair
• groups of electrons (areas of electron density) repel each other due to their similar charges
• therefore they will arrange themselves to be as far apart in space as possible

- on molecule shape drawings, the block triangle represents electrons coming forward/towards you, the dotted line shows electrons moving away from you, straight lines show electrons which are on the same plane as the nucleus         

• Mendeleev arranged the known elements in order of RAM (now ordered by atomic number)
• Elements with similar physical and chemical properties were in the same group ie metals with low boiling points
• He swapped elements over if he thought they fitted into another group better based on their physical and chemical properties eg Iodine and Tellurium
• He left gaps for elements he thought were yet to be discovered
• He made predictions about the properties of elements yet to be discovered, later they were found to be very close to the actual elements properties.
• This validated his version of the periodic table in the eyes of other chemists. 

• Sample inlet - gases and liquids are injected here but solids are heated to vapourise them
• Ionisation area - a heated filament produces high energy electrons which bombard the atoms/molecules in the sample and knock off electrons forming cations
• Acceleration area - an electric field is used here to accelerate any ions so that they all have the same kinetic energy
• Drift region - there is a vacuum here so that ions do not collide with air molecules (which could change their flight path direction). As kinetic energy = mass * velocity^2 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 ones. A computer system converts info into a mass spectrum and "fragments" them. Only positive ions are detected after this.

A mass spectrum's x-axis shows the mass to charge ratio (m/z) and as all ions formed have the same +1 charge, the m/z is the same as the mass of the ion detected.
The y-axis shows the intensity/relative abundance in % 

• an exothermic reaction gives out energy from the system to the surroundings - the temperature of the surroundings increase so the difference in enthalpy is negative
• an endothermic reaction takes in energy from the surroundings to the system - the temperature of the surroundings decreases so the difference in  enthalpy is positive
• The standard enthalpy change of combustion is the enthalpy change when 1 mole of a substance burns completely in oxygen under standard conditions
• The standard enthalpy change of formation is the enthalpy change when 1 mole of substance is formed from its constituent elements (both reactants and products are in their standard state)

• Record the temperature rise (of water) when a known volume of water is heated by the complete combustion of a measured mass of fuel
• The energy transferred (Q) = m * c * difference in T
• M = mass of the water (g)
• C = the waters specific heat capacity (4.18)
• Difference in T = change in waters temperature (K)

• The answer may not be accurate however due to the incomplete combustion of the fuel or heat which may escape to the surroundings
• To prevent heat escaping, use draught shields

Entropy is the measure of the number of ways in which particles can be arranged

• Gases have greater entropy than liquids
• Liquids have greater entropy than solids
• Mixtures (eg solutions) have greater entropy than the unmixed constituents
• If the number of particles increases during the course of the reaction then entropy usually increases

Crude oil is a mixture of different hydrocarbons (compounds of carbon and hydrogen only). The hydrocarbons are separated into fractions (mixtures of compounds with specific boiling point ranges) by the process of fractional distillation. These fractions are used as fuels.

Alkanes:

• All have bond angles 109.5 and are arranged tetrahedrally  
• have names ending in -ane
• are saturated (single bonds only)
• are aliphatic (have no ring structures)
• alkanes react completely with O2 to produce carbon dioxide and water

Cycloalkanes: 

• Name ends in -ane
• are saturated
• are not aromatic (they dont have a benzene ring)

Alkenes:

• Name ends in -ene
• are unsaturated (double bonds)
• are aliphatic

Arenes:

• Name ends in -ene
• are unsaturated
• are aromatic (have 1 or more benzene rings in their structure) 

Meth - 1 carbon (in chain)

Eth - 2 carbons

Prop - 3 carbons

But - 4 carbons

Pent - 5 carbons

Hex - 6 carbons

Hept - 7 carbons

Oct - 8 carbons

Non - 9 carbons

Dec - 10 carbons

• Alcohols contain hydroxyl groups (-OH)
• their names end in -ol
• they react completely with oxygen to give CO2 and H2O
• They require less oxygen for complete combustion than the corresponding alkane as the molecules already have oxygen in them
• they are called oxygenates as they burn more efficiently than alkanes so therefore produce less carbon monoxide
• they are commonly added to petrol to reduce pollution
• 

• Ethers have alkoxy groups (-OR)
• they are also oxygenates for the same reason

structural isomers have the same molecular formula as each other, but different structural formulae.

There are 3 types of structural isomer:

1) Chain Isomerism - the chain lengths are different however the molecular formula is the same. This often occurs in alkanes.

2) Positional Isomerism - the same functional group appears in the isomers, however the positions of each are different ie in alcohols.

3) Functional group Isomerism - the molecular formula is the same for each isomer but the functional groups are different. This can be seen in alcohols and ethers.

Isomerisation at the oil refinery:
When straight chained alkanes are heated in the presence of a platinum catalyst, they become branched alkanes. The branched-chain alkanes have a higher octane number and less tendency to auto-ignite. 

• Autoignition is the spontaneous combustion of a fuel without a spark
• 
• Auto-ignition is undesirable in car petrol engines as it causes:
  -a knocking or 'pinking' sound
  -reduced engine performance
  -engine damage 

• The octane number is the measure of the tendency of a fuel/petrol to auto-ignite
• the higher the octane number the lower the tendency of the fuel to auto-ignite is

In terms of octane number:

• Short chain compounds > long chain compounds
• Branched chain compounds>corresponding unbranched chain compounds
• Cycloalkanes> corresponding straight chained alkanes
• Arenes > cycloalkanes
• Oxygenates > corresponding alkane
• Oxygenates are added to petrols to increase their octane number

The petrochemical industry uses a number of different chemical reactions, cracking, reforming and isomerisation, to increase the octane numbers of the components in petrol.

Catalysts are used in 3 processes at the oil refinery to produce petrol components with low tendencies to auto-ignite. The products of these processes have higher octane numbers than the initial reactants:

1) Cracking - A shorter, branched alkane with a higher octane number and an alkene is produced/formed. A zeolite is used as the catalyst in this reaction.

2) Reforming - Alkanes are converted into cycloalkanes and hydrogen. The cycloalkanes are then converted into arenes and hydrogen. The arene has a higher octane number than the alkane and platinum is used as a catalyst. 

3) Isomerism - The product is shorter and more branched than the reactant as the longest carbon chain becomes shorter and branched. Platinum is the catalyst for the reaction and a zeolite is used as a sieve to separate the branched and unbranched isomers due to it being porous (the straight chained isomers fall through and react again to become shorter/more branched whereas the branched chained isomers cannot and can be separated/removed). 

- A catalyst speeds up a chemical reaction but is not used up in it (catalysis)
- Heterogeneous catalysts are a type of catalyst which is in a different physical state to the reactants. They work in 4 stages:

•  Reactants are adsorbed/bond to the surface of the catalyst
•  Bonds between the atoms inside the reactants weaken and are broken
•  New bonds and compounds are formed
•  The reactants diffuse away

- Catalyst poisons stop catalysts working by adsorbing onto their surface and preventing reactants from bonding to it.

-In a catalytic converter in a car, platinum or rhodium act as heterogeneous catalysts in the reaction: 2NO (g) + 2CO (g) --> N2 (g) + CO2 (g) 
- Catalysts in catalytic converters only work when the temperature is high (so not at the start of the car journey when the engine is cold) 

Pollutant         How it's formed              Effects                          Reduced by

Unburnt                Evaporative emissions        Contributes to the formation            Catalytic converters & 
hydrocarbons      & incomplete combustion    of photochemical smog (which        oxygenates (however
                                                                         causes respiratory problems)         this increases CO2)

CO                  Incomplete combustion       Toxic to humans and decreases  (the same as above) Also 
                      of hydrocarbons                the uptake of O2 by the blood        reduced in lean burn engines
                                                                                                                        but this may mean unburnt 
                                                                                                                        hydrocarbons are increased 

CO2               Complete combustion          Causes the greenhouse effect     Hydrogen is the only petrol
                       of hydrocarbons                 & global warming                        alternative to not produce CO2

NOx                Nitrogen from the air              Contributes to PC smog              Reduced by catalytic con
                       reacts with O from the          Causes acid rain                        -verters & lean burn    burn 
                     air in the high temperatures                                                         engines
                      of the car engine 

SOx             Sulfur in petrol reacts with       Causes acid rain which             low sulfur petrol can be 
                   O in the air in the heat of the     errodes limestone buildings        produced by desulfurisation
                   engine                                       & damages forests & lakes
 

Alternative fuel          Is it sustainable           Benefits                    Risks

Diesel                               No, crude oil is running out        Less CO produced than  Produces more NOx
                                                                                           from a petrol engine,       & particulates (which
                                                                                           already sold at petrol     can irritate lungs) than in 
                                                                                           stations                           a petrol engine                   
LPG or autogas             No, crude oil is running out        Less CO, CO2, unburnt     Needs to be stored                                                                                            hydrocarbons & NOx       under pressure so 
                                                                                         than from a petrol engine,   that it's a liquid
                                                                                          petrol engines are easily                                                                                                                                      converted
Ethanol                        Possibly not, large amounts of     Less CO, SO2 & NOx          Highly           flammable                  energy is needed to cultivate       from a car engine, has       flammable
                                     sugar cane for fermentation        a high octane number               
                                     sugar cane absorbs CO2            in growth

Alternative fuel         Is it sustainable                 Benefits                    Risks

Biodiesel                   From waste plant material and         Living things have absorbed     NOx emissions
                                 animal oils/fats so it renewable        CO2, it's biodegradable, less     higher than diesel
                                 but fossil fuels may be used as       CO, CxHy, SO2 and particu       engine
                                 an energy source                            -lates than from diesel engine 

Hydrogen                Only if the electricity needed for       Water is the only product of      Highly flammable, 
                               electrolysis of water is from a           combustion                                high pressure fuel
                               renewable source eg solar cells                                                         tank needed to store
                                                                                                                                            it as a liquid