gcse science chemistry unit C6 cards
- Created by: charlie
- Created on: 08-06-13 22:03
redox reactions
- if electrons are transferred- O I L R I G
- reduction+oxidation happen at the same time
- oxdising agents accepts electrons + gets reduced
- reducing agent dontates electrons + gets oxidised
e;g- iron atoms oxideised to iron (II) when react with dilute acid
- Fe - 2e- = Fe2+
- 2H+ + 2e- = H2
displacement reactions- redox- more reactive metal displaces less reactive metal from compound
- place reactive metal into solution of dissolved metal compound- reactive will displace
- always metal ion that is reduced
- always metal atom that is oxidised
rusting of iron
- rusting is redox reaction
- only happens when in contact with both oxygen + water
- iron+oxygen+water= hydrated iron (III) oxide
- each iron atom loses electron + becomes oxidise to Fe3+
- oxygen gains electron when reacts with iron + becomes reduced to O 2-
prevention methods
- metals combined to prevent rust- alloys- steel(iron+carbon), stainless stell(iron+carbon+chronium)
- oil grease- physical barrier- when moving parts are involved
- painting - physical barrier- large structures+looks attractive
- coat of tine- physical barrier- cheap- when scratched iron loses e in preference +rust
- sacrificial method- glavanising- coat of zinc loses e in preference even when scratched as more reactive- physical barrier
- sacrificial method- blocks of more reactive metal bolted to iron- lose e in preference
electrolysis
splitting up with electricity
- electric current passed through molten/dissolved ionic compound causing it to decompose
- crease a flow of charge through electrolyte
- postive ions ---> cathode + gain e
- negative ions ---> anode + lose e
- when it loses/ gains e become atoms/molecules + discharged at electrode
easier to discharge ions from water than solute
- in aq solution as well as ions from solute there are H+, OH- from water
- easier to discharge ions from water instead of solute- H2 at cathode + O2 at anode
eg. aqueous H2SO4 solution-
- cathode (hydrogen ions)- 2H+ + 2e- = H2
- anode (hydroxide ions) - 4OH- - 4e- = O2+2H2O
electrolysis part 2
copper sulphate electrolysed to for copper+oxygen:
- cathode(copper ions) - Cu2+ + 2e- = Cu
- anode(hydroxide ions) - 4OH- - 4e- = 2H2O
- copped ions easier to discharge form solution over H+
in molten ionic solids- only one source of ions
- ionic solid cant be electrolysed as in fixed positions- molten ionic compounds can
- always broken up into their elements:
- postive metals reduced at cathode-- Pb2+ + 2e- = Pb
- negative ions oxidised at anode-- 2Br- = Br2 + 2e-
amount of product made depends on no. of electrons- (inc. no. of e inc. substance produced)
- electrolysing for long time
- inc. the current
amount of product proportional to time+current Q= I x T
- (Q-charge in coulombs) (I- current in amps) (T- time in seconds)
fuel cells
- hydrogen+oxygen give out energy when they react
- produce water + exothermic reaction- shown via energy level diagram
use fuel + oxygen to produce electical energy:
- hydrogen-oygen fuel cell
- produces water- clean + no pollutants
hydrogen- oxygen fuel cell involve redox reaction:
- electrolyte often solution of potassium hydroxide
- hydrogen into anode + oxygen into cathode
- at -ve cathode O2 - O2 + 4e- + 2H2O = 4OH-
- OH- ions move to +ve anode
- 2H2 + 4OH- = 4H2O + 4e-
- electrons flow through external circuit from anode to cathode
- overall reaction- hydrogen+oxygen=water
fuel cells part 2
hydrogen-oxygen advantages:
- much more efficient than power stations/ batteries greater than 80%
- direct energy transfer + therefore fewer places where heat can be lost
- no moving parts so energy isnt lost by friction + no pollutants only water
disadvantages
- producing hydrogen needed to power takes energy- may even come from burning fossil fuels
- contain posionous catalysts have to be eventually disposed- time money+ enviro problems
used in spacecraft for electricity:
- readily available from rocket fuel tanks as reaction between O2+H2 happens
- light weight, compact, no moving parts, some product used as drinking water, no pollutants
car industry developing fuel cells
- trying to replace petrol/diesel engines, advantage as no pollutants, obtained by electrolysis- renewable
CFC's + ozone layer
history:
- first thought of as great- non-toxic, non-flammable, chemically inert(unreactive), insoluble in water, low BP, + used as collants in refridgerators+ a.c + propellants aersol spray cans
- 1974- chlorine destroyed ozone
- 1985- evidence of dec. ozone levels over antartica
- measurements in upper atmosphere show compunds of broken down ozone
- ozone (O3) in stratosphere absorbing UV light + breaks down (O3=O+O2)
- then joins together to make ozone again (O+O2=O3)
- uv light reaches earth- sun burn, skin cancer, eye cataracts, ageing
some countries have banned CFC's
- took a while for something to happen- research fully reveiwed+evaluated
- 1978- USA, canada, sweden, norway - banned CFC's as aersol propellants
- after ozone hole discovered - countries joined together reduce CFC production + eventually ban CFC's completely
CFC's + ozone layer part 2
free radicals made by breaking covalent bonds:
- covalent bond breaks unevenly to form two ions- H--H = H+ = H-
- H+ has both shared electrons + H- has none
- covalent bond can break evenly each atoms gets one shared e- H--H = H. + H.
- unpaired electron makes free radical extremely reactive
UV light makes carbon-chlorine bonds in CFCs break:
- CCI2F2 = CCIF2. + Cl.
- O3 + Cl. = ClO. + O2
- ClO. + O3 = 2O2 + Cl.
happens high in atmosphere where UV light stonger then free radicals react with ozone creates chain reaction- one Cl free radical breaking up loads of ozone molecules
CFCs stay in stratosphere for ages- not very reactive only break into Cl where high UV light- each CFC produces one Cl atoms which reacts with thousands of ozone molecules
- Alkanes- dont react with ozone so safe alternative
- HFC's- very similar but dont contain Cl, none of compunds which broken into attack ozone
hardness of water
- hard water makes scum+scale + wont lather with soap
- formes limescale (CaCO) as a thermal insulator- on heating element kettles take longer to boil
hardness caused by Ca2+ + Mg2+
- as water flows over rocks + through soil these ions dissolve in it
- Magnesium sulphate dissovles + calcium sulphate (a little bit) (permanent hardness)
- calcium carbonate- only reacts with acids:
- CO2 dissolves in rain water forming carbonic acid- CO2 + H20 = H2CO3
- so calcium carbonate can react- H2CO3 + CaCO3 = Ca(HCO3)2 (aq) (temporary hardness)
- overall eqation- carbon dioxide+water+calcium carbonate=caclium hydrocarbonate
ways of removing
- temporary hardness- boiling Ca(HCO3)2 (aq)= CaCO3(s) + H2O(l) + CO2(g)
- both types of hardness- adding washing soda (sodium carbonate) - carbonate ions join onto calcium ions to form precip. - Ca2+(aq) + CO3 2- (aq) = CaCO3(s)
- both types of hardness- ion exchange resin- Na2+/H+ ions exchange for Ca2+/Mg2+
hardness of water experiments
- add 100cm^3 of water to concical flask
- add 1cm^3 soap solution to water- put bung on + shake
- repeat till lather formed- record how much soap needed
alcohols
- alcohols have a general formula - C (n) H (2n+1) OH
- methanol--ethanol--propanol--butanol--pentanol...
fermentation produces ethanol (glucose=ethanol+carbon dioxide)
- enzymes found in yeast, temp controlled, prevent O2 getting in otherwise turns to ethanoic acid (vinegar) once reaction stops- fractional distillation for pure ethanol
hydrating ethene produces ethanol
- how it is usually made industrially- ethene+water(steam) = ethanol
- temp of 300d.c + pressure of 70 atmospheres passed over heated catalyst phosphoric acid
fermentation/hydration
- batch process slow inefficient/continuous harsher reaction condition + expensive
- ethanol is renewable from sugar cane/ethene from crude oil
- ethanol isnt pure + needs fractional distillation/much higher purity
- lower atoms economy + yield/low but recycled unused reactants so 95% yields
fats + oils
comes from animals or plants:
- animal fats are: lard, blubber, ghee, cod liver oil (plant fats + oils: walnut oil, coconut oil, olive oil, soya oil )
- fats are solids+oils are liquids at room temp + are both esthers reacting the alcohol glycerol with fatty acids
- important in raw materials for chemical industry- paints, machine lubricants, detergents, cosmetics
emulsions made from oil+water
- oils dont mix in water- immiscible but can make emulsion- small droplets dispersed in another liquid
- milk (oil-in-water emulsion) butter (water-in-oil emulsion)
vegetable oils used to produce biodiesel- provide energy, similar properties to diesel burning in same way so an alternative
fats + oils used to make soaps-
- vegetable oil+alkali makes soap-
- natural fats+oils boiled with sodium hydroxide splitting the fats + oils producing soap + glycerol (saponification)
- first breaks up fat or oil producing glycerol + fatty acids (hydrolysis) then react with sodium hydroxide making soap
- fat+sodium hydroxide = soap + glycerol
using plant oils/fats
unsaturated oils contain C=C double bonds:
- saturated oils have single C-C bonds
- unsaturated oils have double C=C bonds
- monounsaturated fats contain one C=C bond + polyunsaturated more than one C=C
- detected using bromine water- unsaturated decolourises it as additional reaction takes place- double bond opens + colourless from orange compound is formed
unsaturated oils hydrogenated
- hardened reacting with H2 in presence of nickel catalyst at around 60d.c (hydrogenation) reacting with double bonds to open them up
- margarine made from partially hydrogenated vegetable oil- it would be too hard to spread if all bonds were opened up - inc. bad cholesterol in blood + dec. amount of good
vegetable oils in food can affect health
- vegetable o/f- unsaturated however animal o/f saturated- inc. bad cholestrol in blood
- unsaturated- reduce amount of bad cholesterol in blood
detergents
- hydrophilic head (loves drinking water) + hydrophobic tail
- hydrophilic forms strong intermolecular forces with water + hydrophobic with oil
- when detergents come in contact with oil/fat helps lift it out of fabric
dry cleaning uses solvents
- much better at removing stains that wont dissolve in water
- solvent can completely dissolve the grease:
- weak intermolecular forces between solvent + weak intermolecular forces between grease
- solvent applied + intermolecular forces formed between solvent + grease which is surrounded
- when solvent removed so is grease
washing clothes at lower temps saves energy
- biological detergents contain enzymes which are bilogical catalysts- help break down large insoluble molecules
- enzymes work best at lower temp washes 30d.c/40d.c saving money
- higher temps denatured + at cooler temp means delicate clothes can be washed
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