OCR Chemistry revision 2 Higher tier

A+ level

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  • Created by: Sophie
  • Created on: 16-04-11 17:25

C2a Paints and pigments

Synthetic dyes introduced a wider variety of colours and are now mainly used.

Paints contain coloured pigments, a solvent and a binding medium.  The solvent thins it while the binding medium hardens when exposed to oxygen

Paint is a mixture, a colloid – solid particles mixed with liquid particles (solvent). The solvent evaporates leaving the colouring, which is no longer a liquid

In emulsion (water-based) paints, the solvent is water; a polymer is used as the binding medium.  The solvent evaporates, the polymer particles fuse into a film

Oil paints use hydrocarbon oil as the solvent, when it evaporates the binding medium oxidises to form the film

Gloss paints used to be made from a hardening oil but it took several days to dry. In modern gloss paints the alkyd resin (binding medium) is a polymer

Some paints contain thermochrmoic pigments which change colour with heat. Others contain phosphorescent pigments, which make it glow in the dark

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C2b Construction materials

Glass is sand heated to a high temp, bricks are made from baked clay, concrete is made from limestone and cement is made from limestone, clay and/or shale (it is mixed with sand and water to harden).  Many materials are extracted from ores.

Reinforcing concrete makes a composite material – combines flexibility of steel and the hardness of concrete, transfers the force sideways.

Sedimentary rocks (limestone) are formed when small sea creatures die, sink, get buried and crushed –fossils can be found in them, they are weak because they are  made of tiny cemented grains which can be separated. Metamorphic rocks (marble) are made by a high temp on other rocks (igneous/sedimentary) which melt and crystallise, there is some fusion of the grains together under high pressure and temps and so is harder. Igneous rocks (granite) are formed when molten rock cools and crystallises either inside the Earth (as magma), or outside (as lava) during volcanic eruptions, these are they hardest because they are crystalline.

Limestone and marble are forms of calcium carbonate.

Thermal decomposition is where a substance is chemically changed by heating into at least to new substances, e.g. CaCO3(limestone or granite) = CaO + CO2.

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C2c Does the Earth move?

  • Geologists study volcanoes to study the Earth and to predict eruptions/earthquakes
  • The crust is rocky, the mantle is cold and rigid nearer the crust but less rigid nearer the core (where convection currents can move molten rocks), the outer core is liquid, and the inner core is solid.  The lithosphere is the outer 100km of the Earth
  • The lithosphere is made of large tectonic plates which can collide (earthquakes) – rocks squeeze together, the oceanic plate is force under, the rocks partly melt into magma (subduction) making an oceanic trench and a mountain chain
  • Lava with low silica is runny, it flows steadily (and so is fairly safe) and quickly cools into iron-rich rocks with very tiny crystals
  • Lava with high silica is not so runny and piles form, the siica stops trapped gas escaping which can cause explosions of lava (huge eruption)
  • Rocks can cool slowly inside the earth, which allows large crystals to form
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C2d Metals and alloys

 

Copper is expensive because: of a huge demand (wires + pipes), limited ores, ores contain a small percentage of pure copper.  Recycling it, is cheaper and makes the resources last longer but is not widely accepted yet. Copper can be heated purify it, as it has a low reactivity and can be split by reduction

It is also purified by electrolysis making it much purer, in copper(II) sulphate.  The copper moves from the impure ore at the anode to the pure at the cathode

Alloys are more useful, and are made to fit the use

Brass is copper + zinc, solder is tin + lead, and amalgam is mainly mercury…

Smart alloys or shape-memory alloys remember the original shape and return if put in hot water or if a current is passed through

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C2e Cars for scrap

Corrosion needs water and oxygen, and is speeded up by salt and acid rain

Rusting is an oxidation reaction and forms hydrated iron(III) oxide

Aluminium doesn’t rust because it quickly forms a layer of aluminium oxide in air

Steel is an alloy of iron and some carbon, it is harder, stronger and less likely to rust

Cars can be made of aluminium and stainless steel, both don’t rust but are more expensive than painted steel cars

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C2f Clean air

Pollutants: carbon monoxide, sulphur dioxide/oxides of nitrogen (acid rain)

Composition of air: O2 = 21%, N = 78%, other gases(including CO2) = 1%

The composition of O2 and CO2 varies slightly, but don’t change much as they are balanced with: photosynthesis against combustion and respiration (carbon cycle) although human impact may change them: deforestation, increased energy consumption, population increase may change

Photosynthesis= 6CO2 + 6H2O ® C6H12O6 + 6O2, respiration is vice versa. simples!

The noble gases are all the elements in group 8

How the atmosphere was formed (most accepted prediction): 1. the original atmosphere contained CO2, steam, methane + ammonia from inside the Earth 2. The steam cooled to form oceans, and some of the CO2 dissolved into the oceans. 3.  Much later simple plant life converted CO2 by photosynthesis; ammonia was converted into nitrogen by bacteria which stayed there due to lack of reactivity.

A catalytic converter can be fitted to exhausts to change CO + NO ® N + 2CO2

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C2g Faster or slower (1)

 

  • Reactions can be sped up with a higher temp./conc./pressure
  • A reaction happens when the particles collide - the more collisions, the faster the reaction, the reaction only happens when the particles have at least a certain amount of energy – the activation energy
  • Increasing the temp. makes the particles move faster so more collisions and also gives the particles more energy so more will be over the activation energy
  • Increasing the concentration (or pressure for gases)  means there are more particles in the same volume so more collisions will happen
  • Changing the rate of reaction wont effect the yield, only the speed will be changed
  • The rate can be measure using the gradient on a product vol./time graph
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C2h Faster or slower (2)

 

  • An explosion is a very fast reaction, made mainly by chemicals but also by fine powders, e.g. flour and so factories follow important guidelines
  • Catalysts are chemicals which are used frequently to speed up reactions without altering the yield, they are specific to the reaction
  • Increasing the surface area (e.g. making a lump, a powder) on a solid also speeds up the reaction as there is a larger are for the particles to collide
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