AQA Chemistry C3 Part 1

Early 1800s - categorise elements by physical/chemical properties & atomic mass

Newlands' Octaves - every 8th element similar properties BUT left no gaps for unknown elements, so not all had similar properties, mixed up metals & non-metals

Mendeleev - left gaps so similar properties were in the same group, gaps predicted unknown elements - convincing evidence that periodic table useful

NOW - based on electronic structure (in order of atomic number)

Max number of electrons in each shell is:

• 1st = 2
• 2nd = 8
• 3rd = 18,

As you go down group 1:

• Bigger atoms
• More reactive
• More dense
• Lower melting point & boiling point

All:

• Reactive
• Have 1 outer-shell electron
• Form ionic compounds
• Reaction with water produces hydrogen gas (& the solution becomes alkaline)

As you go down group 7:

• Less reactive
• Higher melting & boiling points

All:

• Travel in pairs
• Do both ionic and covalent bonding
• React with metals to form salts (2Al + 3Cl2 ---------> 2AlCl3)
• More reactive ones displace less reactive ones

• Flourine - poisonous, yellow gas
• Chlorine - poisonous, green gas
• Bromine - poisonous, red/brown liquid
• Iodine - dark grey solid, or purple vapour

• Good conductors
• Dense, strong & shiny
• Not very reactive
• High melting points
• Often have more that one ion (Fe2+, Fe3+)
• The compounds are colourful
• Make good catalysts (iron for Haber process)

Properties because of the way the electron shells fill - further away from nucleus shells start to overlap effecting the way shells fill:

Sc: 2,8,9,2     Ti: 2,8,10,2     V: 2,8,11,2     Cr: 2,8,13,1     Mn: 2,8,13,2     Fe: 2,8,14,2  Co: 2,8,15,2     Ni: 2,8,16,2     Cu: 2,8,18,1     Zn: 2,8,18,2

Arrhenius - acids release hydrogen ions (H+) in water & all alkalis for hydroxide ions (OH-) in water. BUT only worked for acids & alkalis that dissolved in water, exceptions (ammonia gas) so scientists didn't believe him

Bronsted and Lowry - acids release H+ ions (proton donors), alkalis accept H+ ions (proton acceptors)

Acidic solutions - the acid molecules dissociate and release lots of H+ ions, which become hydrated (surrounded by water molecules)

Basic solutions - water molecules dissociate into H+ and OH- ions, some base molecules take hydrogen ions from water causing more molecules to dissociate, other bases release OH- ions straight into the solution

Strong Acids (sulfuric, hydrochloric, nitric) ionise/dissociate almost completely in water - lots of H+(aq)

Weak Acids (ethanoic, citric, carbonic) ionise slightly - small numbers of H+(aq)

Strong Alkalis (sodium & potassium hydroxide)

Weak Alkalis (ammonia)

Titrations:

• How much acid needed to neutralise an alkali - concentration
• Alkali in a flask add acid

Indicators:

• Strong acid & strong alkali - any
• Strong acid & weak acid - methyl orange
• Weak acid & strong alkali - phenolphthalein

Step 1 - Calculate number of moles of known substance

Step 2 - Write out equation and ratio moles

Step 3 - Work out the concentration of 'unknown stuff'

Step 1 - Work out relative formula mass for the unknown substance

Step 2 - Work out concentration in grams using the concentration in moles that you just worked out

Water cycle - sun causes evaporation > water carried upwards as warm air rises > cools > water condenses to form clouds > droplets get too big > fall as rain > back to sea

Water dissolves most ionic compounds - water molecules surround the ions and disrupt the bonding, slightly positive hydrogen attracts the negative side & slightly negative oxygen attracts the positive side

These ionic compounds dissolve in water:

• Salts of sodium, potassium & ammonium
• Nitrates
• Chlorides except from silver & lead
• Sulphates apart from barium & lead

Saturated solution can't hold any more solid at that temp

Solubility increases with temp

Cooling causes crystallisation

Higher pressure, more gas dissolves

Lower temp, less gas dissolves

Problems: Won't form lather - more soap, scale on inside of pipes & kettles, thermal insulator - longer for kettle to boil, scum

Caused by: Ca2+ & Mg2+ ions - rainwater runs over rocks

Good points: Ca2+ good for teeth & bones, scale forms protective coating inside pipes - stops lead poisoning

Removing hardness:

• Add sodium carbonate - carbonate joins on to calcium/magnesium to form an insoluble precipitate
• Ion exchange column - swaps calcium & magnesium ions for sodium ones
• Scale can be dissolved by acid

Water must be free of poisonous salts and harmful microorganisms - treated

Treatment:

• Through mesh screen - remove big bits (twigs)
• Treated with ozone/chlorine - kill microorganisms
• Chemicals added - makes solids & microorganisms stick together and fall to the bottom
• Filtered through gravel beds - remove all solids
• pH corrected
• Chlorinated to kill any microorganisms left

Water filters:

• Contain carbon or silver to remove substances from tap water
• Carbon - removes chlorine taste
• Silver - kills bugs