chemistry unit 3

used to catagorise elements by:

• physical and chemical properties
• relative atomic mass 

protons, neutrons and electrons hadnt been discovered so atomic number didnt exist. 

ELEMENTS WERE ARRANGED IN ORDER OF RELATIVE ATOMIC MASS

NEWLANDS:

- in 1864 he noticed that every eighth element had similar properties

- he put the similar elements into rows of 7 so every 8th were in the same group- newlands octaves

-didnt leave any gaps so the pattern broke down and he was ignored. 

- his groups also contained elements that different have similar properties and mixed NM with M

1869 DMITRI MENDELEEV:

- he ordered the 50 known elements in order of relative atomic mass like newlands but left gaps to keep elements with similar properties in the same group. 

- the gaps predicted properties of undiscovered elements and when they were found they fitted the pattern

- most scientists didnt take the periodic table seriously at first but when new elements were discovered and they fitted the gaps, it provided evidence for the table and more scientists realised that it was useful to predict the properties of an element. 

- when sub-atomic particles were founded, the periodic table matched up to the structure of the atom. 

MODERN PERIODIC TABLE:

- when sub atomic particles were discovered, elements were arranged in order of atomic number. 

- they cant be seen to be arranged in order of electron structure- this is used to predict elements chemical properties. 

- group number= number of electrons in outer shell (other than transition metals)

- the positive charge in the nucleus attracts the electrons and holds them in place. so the further away the electrons are from the nucleus the less of the attraction 

- as well as 'shielding' when inner electrons get in the way of the charge and reduce the attraction 

- so the more energy levels means a greater distance and more shielding so electrons are lost more easily and its harder to gain electrons

- as you go DOWN the group, elements get more reactive as more energy levels so electrons are more easily lost from atom 

- as you go DOWN the group- melting and boiling point decreases

- they are all very LOW IN DENSITY

- they ALL have ONE OUTER SHELL ELECTRON  so they're all very REACTIVE and have SIMILAR PROPERTIES

- because its easier for them to lose one electron than to share electrons to gain a full outer shell- they form IONIC compounds with non-metals 

-they all react vigrously with water to form soluble HYDROXIDES and HYDROGEN GAS 

-as you go DOWN the group the elements get LESS reactive as less attraction from nucleus to gain electrons 

-as you go DOWN, boiling and melting point INCREASE 

-they all exist as diatomic molecules 

-they all have coloured vapours:

• FLOURINE- poisonous YELLOW GAS
• CHLORINE- poisnous DENSE GREEN GAS 
• BROMINE- poisonous RED/BROWN VOLATILE LIQUID 
• IODINE- DARK GREY solid or PURPLE VAPOUR 

- they form ionic bonds with metals

- a more reactive halogen can displace a less reactive one from an aqeuous solution of its salt. 

- good conductors of heat and electricity

-dense, strong and shiny 

-less reactive than group 1 metals

- higher melting points 

-each element can usually form more than one ION- which each give a different coloured compound 

-give COLOURFUL compounds- copper sulfate is blue. colours in gemstones are because of transition metals 

-GOOD CATALYSTS- iron in harber process, nickel in hydrogenation 

SOFT WATER- lather

HARD WATER- scum. so you need to use more soap to get a lather which is more expensive

HARD WATER WHEN HEATED- scale. this blocks pipes and boilers so reduces efficiency of heating systems. also can be expensive as pipes may have to be replaced. scale is a thermal insulator so kettles take longer to boil water. 

HARD WATER:

• contains calcium and magnesium ions
• ions from soluble compounds in rocks are dissolved in the water when water flows over it 
• eg calcium carbonate is made soluble by acid rain reacting with it to form calcium hydrogencarbonate
• hydrogen ions and calcium ions are then released
• calcium ions are good for teeth and bones and hard water can reduce heart disease

TEMPORARY HARDNESS:

- caused by hydrogencarbonate ions from calcium/ magnesium hydrogencarbonate

- when you boil temporary hard water, the calcium hydrogencarbonate thermally decomposes to form insoluble limescale (calcium carbonate). 

- Ca(HCO3)2 -----> CaCO3 + H2O + CO2

PERMANENT HARDNESS:

-caused by calcium sulfate 

-add washing soda (NaCO3) to it. the calcium and magnesium ions will react with carbonate ions to form insoluble mag carb or calc carb (this works for temp hard water too)

- 'ion exchange column' containing Na ions will swap wti the Ca or Mg ions in the water- making it soft 

to test for how hard water is, or compare hardness of water- complete a TITRATION 

1. add 50cm3 of water to a conical flask

2. fill a burette with soap solution- let it run through the bottom to get rid of air. measure starting volume 

3. add 1cm3 of soap solution at a time to the conical flask, and after each drop place a bung on the flask and shake for 10 seconds. 

4. repeat the above step until a lather is formed which covers the surface for at least 30 secs

5. read finishing volume and calc how much soap solution was needed 

water has to be free of poisonous salts and harmful microbes to make it safe enough to drink. 

most drinking water comes from reservoirs where water flows into them from rivers 

TREATMENT:

1. water passes through a mesh screen to get rid of big bits eg twigs

2. chemicals are added to make solids and microbes stick together and fall to the bottom

3. water is then filtered through gravel beds to remove the solids

4. chlorine is added to remove any more microbes 

some people buy filters that contain carbon or silver to get rid of the chlorine taste and remove other microbes 

distilled water is made by distillation- boiling water to make steam and condensing it. ££££££

FLOURINE- added to water to help prevent tooth decay 

CHLORINE- added to prevent diseases

- chlorine can cause some types of cancers. also can react with other substances to form toxic- by-products 

-flourine- high doses can cause cancer and bone problems. some people also believe its their own choice whether they want to use flourine 

a reversible reaction is one where the products can themselves react to form the original reactants

- if a reversible reaction is happening in a closed system (no substances can get in or out) equilibrium will be reached. 

- equilibrium means that the amount of reactants and the amount of products will reach a certain balance and stay there. the reaction will still be happening but the effect will be nil as both reactions are happening at exactly the same rate so they cancel eachother out

-position of equilibrium- relative amount of reactants and products. 

- depends on temperature and pressure

TEMPERATURE:

• reversible reactions are always exothermic one way and endothermic the other way. 
• so if you increase the temperature of the reaction, the position of equilibrium moves towards the endothermic side to counteract the change and use up the extra heat. 
• if you decrease the temperature, the position of equilibrium will move towards the exothermic side to counteract the change and release more heat 

PRESSURE:

• both sides of the reactions will have different number of moles on. (greater or fewer number of molecules)
• so if you increase the pressure, position of equilibrium will move towards the side with less moles to counteract change and decrease the pressure again
• if you decrease pressure, position of equilibrium will move towards the side with more moles to counteract change and increase pressure again

- Nitrogen and Hydrogen are used to make ammonia which is used for fertilisers

- N2 + 3H2 -----> 2NH3 -nitrogen is obtained from the air and hydrogen obtained from natural gas 

-because reaction is reversible, unused nitrogen and hydrogen are recycled. ammonia forms as a gas but is cooled in the condenser and liquifies so it can be removed. 

• PRESSURE- there are more moles on the left hand side, so increasing pressure will move position of equilibrium in the forward direction (to counteract change) so yield of ammonia is increased. however too high of pressure is too expensive and dangerous so 200 atmospheres is used
• TEMPERATURE- the forward reaction is exothermic, so decreasing temperature would move position of equilibrium in the forward direction (to give out more heat)- increasing yield of ammonia. however having too low of a temp will mean too slow rate of reaction so 450 degrees used
• iron catalyst used so position of equilibrium can be reached quicker. it allows the temp to be low

alcohols are part of a homologous series, which is where a group of chemicals react in a similar way because they have the same functional group. in alcohols it is -OH 

CnH2n+1OH

METHANOL- CH3OH

ETHANOL- C2H5OH

PROPANOL- C3H7OH

- alcohols are flammable- they burn in water to produce carbon dioxide and water

- the first 3 alcohols all completely dissolve in water to make neutral solutions

- they all react with sodium to form an alkoxide and hydrogen. eg sodium ethoxide 

alcohols used as solvents:

• alcohols can dissolve most substances water can dissolve but also some extras eg oils and fats
• so good solvent in perfumes etc as the oil which gives the scent is dissolved in it 
• methylated spirit is ethanol with chemicals added to it to clean paint brushes and as a fuel. its dyed so people wont drink it 

alcohols used as fuels:

• ethanol is used in spirit burners as it burns cleanly and doesnt smell 
• ethanol can also be mixed with petrol to use in cars- less pollution and cleanly produced 

carboxylic acids are made by oxidising alcohols eg with either oxidising agents or allowing microbes to ferment the ethanol. 

alcohol + oxygen ---> carboxylic acid + water 

they have a -COOH- functional group 

methanoic acid- HCOOH

ethanoic acid- CH3COOH

propanoic acid- C2H5COOH

- They react with carbonates to form carbon dioxide- the salts formed end in -anoate eg  sodium ethanoate

- carboxylic acids dissolve in water to form weak acidic solutions. this is because when they dissolve they only partially dissociate as they only release some H+ ions which make it acidic

- ethanoic acid dissolved in water is vinegar, good solvents (but acidic), make soaps/detergents

esters are formed when an alcohol and a carboxylic acid react together with an acid catalyst (eg sulfuric acid). it also makes water

alcohol forms first part of name and acid forms 2nd part- methyl propanoate is methanol and propanoic acid 

-esters have pleasant smells- usually fruity- and are volatile- ideal for perfumes and aromas 

- but also highly flammable so their volatility makes them dangerous 

- they dont mix well with water but mix well if alcohols and other organic solvents 

ISSUES:

• inhaling esters can irritate mucus membranes 
• easily cause flash fires
• some are toxic in large doses- issues about food additives
• but they arent as volatile or flammable as other organic solvents 

1 mole is 6.023x10^23 because this is how many 12carbon atoms there is in 12g. so if you get that number of atoms for any element, it weighs exactly the same as their relative atomic mass. 

so you use moles when talking about amount of a substance 

concentration is measures in moles/dm3. 1dm3=1000cm3 

the more solute you dissolve in a given volume, the more crowded the solute molecules become so the more concentrated the substance is 

USE TITRATIONS TO FIND OUT CONCENTRATIONS:

1. place a specific volume of either an acid or alkali with a known conc in a conical flask. you also add an indicator of either methyl orange or phenolphthalein 

2. place the other substance whose conc you want to work out in a burette and slowly add to conical flask whilst swirling.

3. when the indicator changes colour, end point is reached. note how much vol you used from burette

1. work out moles of known substance in conical flask by moles= conc x volume. (to convert from volume in cm3 into dm3- divide it by 1000. eg 25cm3 = 0.0025dm3)

2. then look at ratios in equation to work out how many moles of substance in burette you have

3. find concentration by moles divided by volume of substance used 

if they ask for the conc in grams per dm3- first work out moles of the substance then times it by relative formula mass 

you can measure the amount of energy released in a chemical reaction by taking the temperature of the reactants (making sure they are the same), mixing them in a polystyrene cup and then measuring the temperature at the end. 

large amount of heat lost to the surroundings. reduce by putting polystyrene cup in a beaker of cotton wool and putting a lid on it. 

this method works for reacting solids with water or neutralisation reactions. 

a water bath can be used to make the reactants the same temperature

MAKING AND BREAKING BONDS:

- energy must be supplied to break old bonds so its an endothermic process

- energy is released in making new bonds so it is an exothermic process

in an ednothermic reaction, more energy is required to break bonds than released when making bonds. in an exothermic reaction, more energy is released than required to break bonds

FUEL ENERGY IS CALCULATED USING CALORIMETRY

- to measure the amount of energy released when a fuel burns, use the flame to heat up water. 

1. put 50cm3 of water in a copper can and record its temperature 

2. weigh the spirit burner and the lid and then light the flame under the copper can and let the fuel burn until the temperature of the water has reached 50 degrees. 

3. put out the flame and measure the final temperature of the water

4. weight the spirit burner and lid again 

work out energy transferred by= mass of water (50)x SHC of water x temp change of water. 

then divide this by mass of fuel burnt to get energy released per gram 

consequences of using fuel for energy is global warming and as non renewable fuels run out, prices will increase

to find out overall energy change:

add together all of the bond energies for the bonds broken

add together all of the bond energies for the bonds made 

bonds broken- bonds made is overall energy change. 

if the energy change is -ve the reaction is exothermic, as the products are at a lower energy than the reactants

if the energy change is positive the reaction is endothermic as reactions are at a higher energy level 

ACTIVATION ENERGY:

• the minimum amount of energy needed by particles to break their bonds and react 
• a catalyst provides a different pathway for a reaction where the activation energy is lowered

HYDROGEN AND OXYGEN:

• hydrogen and oxygen react to produce water which is clean
• the reaction is exothermic 
• so you can get something useful out of this reaction either in a combustion engine or fuel cell

ENGINES:

• hydrogen gas can be burnt in oxygen in engines to power the vehicle
• its very clean burning so less pollution
• you need a special expensive engine, its hard to store and you have to extract hydrogen from something 

FUEL CELLS:

• a fuel cell is an electrical cell thats supplied with a fuel and oxygen and uses energy from the reaction to generate electricity 
• fuel cells down run down or need charging, it will release energy as long as fuel is supplied
• fuel cells are being developed to replace car engines
• reduces pollution and need for crude oil but hard to store etc

FLAME TESTS IDENTIFY METAL IONS:

some compounds of metals burn with a characteristic colour, so you can test for various metal ions by putting the compound in the flame and seeing what colour the flame turns:

• clean a wire loop by dipping it into HCL and then washing with water
• then dip into sample of compound and put wire loop in the clear blue part of the bunsen flame (hottest part) 
• LITHIUM- CRIMSON
• SODIUM- YELLOW
• POTASSIUM- PURPLE
• CALCIUM- RED
• BARIUM- GREEN 

METAL IONS FORM A COLOURED PRECIPITATE:

if you react a compound with NAOH, a metal hydroxide will form. most metal hydroxides are insoluble so a precipitate should form- which has a characteristic colour

• CALCIUM-WHITE
• MAGNESIUM- WHITE
• ALUMINIUM- WHITE AT FIRST- then it redissolves in excess NaOH to form a COLOURLESS SOLUTION
• COPPER- BLUE
• IRON II- GREEN
• IRON III- BROWN 

to distinguish between calcium and magnesium, place white precipitate under flame and if it burns red its calcium 

TESTING FOR CARBONATES:

• carbonates react with dilute acids to form carbon dioxide. so the limewater test is used where co2 is bubbled through limewater to see if it turns cloudy 

TESTING FOR HALIDES:

• add dilute nitric acid to remove carbonate ions and then silver nitrate solution
• CHLORIDE- WHITE precipitate of silver chloride
• BROMIDE- CREAM precipitate of silver bromide
• IODIDE- YELLOW precipitate of silver iodide 

TESTING FOR SULFATES:

• add to nitric acid to remove carbonate ions and then to barium chloride solution
• if a white precipitate of barium sulfate is formed the original compound was a sulfate.