Chemistry, the icky tricky mathy bits


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Mass Numbers


ATOMIC NUMBER is the number of protons/electrons.
MASS NUMBER is the relative mass of an atom (neutrons+protons)

Electrons are disincluded from the Mass Number because they are so light.

ISOTOPES are atoms of the same element which have different numbers of neutrons.
(eg Carbon12 and Carbon14)

Sometimes extra neutrons makes a nucleus unstable and consiquentially radioactive.

Isotopes have different:
PHYSICAL PROPERTIES (different mass number and possible radioactivity.
CHEMICAL PROPERTIES (all three hydrogen isotopes DEUTERIUM, TITRIUM, react with oxygen to make water.

The RELATIVE MASS of protons and neutrons is 1.

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Masses and Moles

Moles = Relative atomic mass in grams / Relative formula mass in grams.
1 Mole = 1 mole of carbon atoms.
A mole of any substance always contains the same number of particles.

Moles = Mass (or in some cases % of mass) / Ar (relative atomic mass)

Mr = Relative formula mass (Ar of compounds)

Ar=mass of an atom compared to Carbon 12

Electrons are so light that they are forgotten, and so ions have the same Ar as their element.

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Percentages and Formulae

Percentage of 'x' in a compound is:

mass of x
-------------------------------------- X 100 = ??? %
total mass of compound

Emperical formula = Knowing the % composition of a compoud and working out the ratio of atom numbers in that compound.

Molecular formula = The number of atoms in one molecule.

eg: 9g of aluminium react with 35.5g of chlorine...
9/27 = 1/3 mole of aluminium ... and ... 35.5/35.5 = 1 mole of chlorine
Meaning the simplest ratio is 3(Cl) : 1 (Al)
Thus AlCl3

If given the percentage, assume the mass is 100g, so 76% would become 76g.

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Yield and Atom Economy

Percentage yeild = amount produced
---------------------------- x100
max amount possible

Percentage atom economy = Relative formula mass of useful product
---------------------------------------------------------- x100
Relative formula mass of all products

Yeild is lost because:

  • Reaction may be reversible (as products form, they re-react)
  • Some reactants may give unexpected products
  • Some product may remain in apparatus
  • Some reactants may be impure
  • Some reactions produce more than one product, and separation may be difficult

Atom economy = resultant useful product (aim is to achieve highest possible)

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Reversible reactions

Reversible Reaction = Products of a reaction can react together to produce origional reactants.

Closed system = no reactants or products can get in or out (more products than reverted reactants.) To keep up, reverting rate increases and forward reaction decreases until equalibrium is achieved.

Exothermic reaction..

  • Increase temp = decrease yield.

Endothermic reaction..

  • Increase temp = increase yield.
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The harber process

The Harber process provides us with a way of turning the nitrogen in the air into ammonia or many reasons, the most important being nitrate fertilisers for healthy plant growth.
Raw materials are:

  • Nitrogen from the air
  • Hyrdrogen from natural gas (containing mainly methane, CH4)

Nitrogen and hydrogen are purified and passed over a high temp/pressure iron catalyst.
This reaction for ammonia is reversible, so to reduce the reverse reaction we have to remove amonia by cooling and liquefying it ASAFormed. We recycle the H and N.

If a reaction produces larger volume of gases..

  • Increase in pressure decreases yeild.

If a reaction produces smaller volume of gases..

  • Increase in pressure increases yeild.
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some typos/spelling mistakes, good otherwise. =D

Good luck to all doing the chemistry papers!!

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