Chemistry module 3

during the 19th centuary many sceintists were trying to find ways that they could classify the elements

1808- john dalton- he arranged them in order of  mass

1863- john newlands- he arranged them in groupsd of eight

1869- dmitri mendelev- he built on newlands ideas but he left gaps

his ideas were accepted when new elements were discovered that fitted the pattern

the modern table is organised in atomic number

elements in a group have similar properties

Group 1 as you go down the periodic table reactivity increases

-  as the atoms get bigger the single electron in the outer shell is less attracted to the nucleus because the other shells screen it from the inner nucleus.

- these elements from ionic bonds so al compounds are crystals

Group7 as you go down the periodic table reactivity decreases

- as the atoms get bigger an electron added to the outer group is attracted less strongly to the positive nucleus this is because it is further away from the nucleus and the inner shells of electrons screen it from the positive charge of the nucleus.

- they form ionic and covalent bonding

they are all metalically bonded so have electrons that are free to move

good conductors of heat & electricity

except mercury they all have high boiling and melting points

because of their partly filled lower energy shells they can be used as catalysts and they have bright colours

they have more than one ion eg Fe2+ or Fe 3+, this is why we use a (II) and (III) 

acids form H+ ions in water so we call then proton donors

an alkali is a base disolved in water

because the hydroxide ions readily react with the H+ ions we call them proton acceptors

the strength of an acid/ or alkali is measured by much it ionises

the pH scale measures the amount of hydrogen ions in a soultion

strong acids - low pH

nuetral - 7

strong alkali- high pH 

strong acid + strong alkali ~> any indicator

weak acid + strong alkali ~> phenolphthalein

strong acid + weak alkali ~> methyl orange

We describe the concentration of a solute in a solution in terms of the number of moles of solute dissolved in once cubic decimetre of solution. We write these units as moles per cubic decimetre or mol/dm3 for short (1dm3= 1000cm3) we can uses this to work out the concentration of a solution.

To calculate the concentration of a solution

• Calculate the mass of solute in 1dm3 of solution (mass/volume)
• X 1000 to find mass of solute in 1000dm3 of solution
• Convert the mass to moles

To calculate the mass of a solute in a in a certain volume of solution

• Calculate the mass of solute there in 1dm3
• Times by the desired volume

most ionic compounds and gases are soluble in water

we  call the amount of solute that can disolve in a solution at a given temp and pressure the solubility

the solubility of most solids goes up with temperature

teh solubility of most gases goes down with temperature

a saturated solution is where as much solute has been disolved in a solution at a specific temp and pressure

an increase in pressure increases solubility

hard water contains disolved compounds that react with soap to form scum

the substances are normally magnesium and calcium these come from when the water streams over rocks 

scum- formed when the hard water mixes with soap:

sodium stearate (soap) + Ca2 and mg ions --> calcium stearate + na ions

scale formed in the pipes and in kettles

Ca  +  2HCO3 --> CaCO3 + H2O + CO2

method 1-

adding sodium carbonate to hard water precipitates out the calcium and magnesium, so the water can react with the soap. 

method 2- 

ion exchange. these columns contain sodium ions to exchange with the magnesium and calcium ions