chemistry revision igcse

Chemistry Revision

AtomsAll chemical elements are made up of atomsEvery atom is made up of smaller particles; neutrons and protons in the atom’s nucleus which is surrounded by electronsAtoms become ions when they gain/lose an electronThe atomic number is the number of protons in the nucleus ( which is the same as the number of electrons)The mass number is the protons + neutrons in the nucleus, the mass of an electron is so small it is considered to be negligible

Atom Mass

A is the relative atomic mass M is the relative molecular mass, For example- H₂O has a relative molecular mass of 18 [(2x1) + 16)For ionic compounds the term relative formula mass is used as there is no separate molecules

r

r

IsotopesIsotopes are different forms of the same elementIf an isotope has more neutrons than protons, it can sometimes be unstable and is likely to decay

Alkali MetalsGroup 1 in the periodic tableMore reactive as you go down the groupLow densitiesIonic compounds +1 chargeAlkaline solutionSoft shiny metalsReact with the oxygen in the air and become tarnished so they are stored in oilReact vigorously with water, they fizz and produce hydrogen gasIdentifying Alkali Metals- identified using the flame testLithium burns with a red colourSodium burns with a yellow/orange colourPotassium burns with a lilac/purple colour

HalogensGroup 7 in the periodic tableMore reactive as you go up the groupReacts with alkali metalsHalogens react with iron- iron wool bursts into flames when it reacts with fluorine gasHalogen displacement reactions mean the more reactive halogen displaces the less reactive halogenIdentifying Halides- identified using the precipitation test using silver nitrateChlorine- white/silverBromine- Pale yellow/creamIodine- yellow

Metal – Physical PropertiesThe following properties applies to copperHigh electrical conductivityHigh thermal conductivityHigh melting point and boiling pointDuctileCorrosion resistantAntibacterialMalleable (can be hammered/squashed into different shapes)Easy to alloy

Metallic Structure

positive

The structure can easily pass the vibration of hot particles along and the free electrons move faster which is why metals are such good conductors

Ionic Compound- Physical Properties

The following properties applies to Sodium ChlorideHigh melting point- because they have strong attractive forces ( lot of energy to separate)Solid sodium chloride does not conduct electricity because the lattice holds to sodium and chloride ions in fixed positions ( Not free to move)but it can conduct electricity does if dissolved in water as the lattice breaks down so the ions move freelyBrittle- if a stress is applied will make the layers moveSoluble in water

Ionic Structure

The sodium lost an electron and the chlorine gained an electron

Simple Covalent Molecule- Physical properties

Low melting point( strong bonds but weak attractive forces)Liquid at room temperaturePoor conductor of electricity Water soluble

Simple Covalent Structure

A covalent bond happens when electrons are shared between to atoms

Giant Covalent SubstanceDiamondTransparent + crystalline (used for jewellery)Extremely hard ( lot of energy needed to break down lattice)Electrical insulatorHigh melting pointGraphiteGrey/black shiny solidVery soft- used in pencilsConducts electricity( along the layer as electrons are free to move but does not conduct across the layers)High melting pointSlippery( hexagonal layers slide across each other

Both different forms of carbon

Structure of diamond and graphite

Carbon atoms each connected to 4 other atoms3-d Lattice based on a tetrahedral unit cell

graphite Diamond

Made up of layersArranged in hexagonal ringsEach carbon atom is connected to 3 other atomsBonds between layers are weakLayers can slide over each other

Carbon NanotubesOne of the stiffest and strongest fibres knownHigh electrical conductivityFormed when graphite layers are rolled up into tubesCovalently bonded hexagonal carbon sheets make it very strongProposed uses for electrical connections in smaller circuitsUsed in bike components, boat hulls and bonding of things like wind-turbinesCould be used for cancer treatment

Smart MaterialsSmart materials are materials that change when there is a change in their surroundings

Thermochromic PigmentsSpecial paints which change colour when there is a change in temperatureMost are base on liquid crystal technology Used in mugs to see hot temperaturesAlso used in T-shirts

Photochromic PigmentsContain organic molecules which will change colour when exposed to light, especially ultraviolet lightUsed in T-shirts and glasses

Smart MaterialsShape-memory polymersPlastics that regain their shape when heatedSomewhere between thermoplastics and thermosetsUsed for sealing around window framesCould be used for car bodies (fix the dent)or stitches which will adjust to tension

Shape-memory alloysMetal alloys that regain their shape when heatedUsed for spectacle frames, surgical plates and wires

HydrogelsPolymers that absorb or expel water and swell or shrink the size it swells/shrinks to depends on changes in PH or temperatureUsed in artificial muscles

Rates of reaction

We can measure the rate of reaction by :-Capturing & measuring the volume of gas produced by a reaction- Measuring the change in mass- Measuring the amount of light passing

through a reaction that is producing a precipitate

Chemical ReactionsChemical reactions occur when molecules/atoms/ions collide but it only happens there is enough energy for the bonds to break and be reformed.Successful collisions per second = collision frequencyFactors that affect the rate of reaction1. Temperature2. Surface Area3. Concentration4. Use of a catalyst

CatalystsThese are substances that increase the rate of reaction but remain chemically unchangedIt reduces the amount of energy needed which will increase the collision frequency

Used in the production of bulk materials eg. Sulphuric acid and food processing

Percentage Composition of Compounds

Calculate the % of copper in copper sulphate, CuSO4 ◦Relative atomic masses: Cu = 64, S = 32 and O = 16◦relative formula mass = 64 + 32 + 4x16 = 160◦only one copper atom of relative atomic mass 64◦% Cu = 64 x 100 / 160 = 40% copper by mass in the compound

Example

Masses of reactants and products

The total mass of the reactants equals the total mass of the products

The Yield of a Chemical reaction

Yield= What was obtainedWhat was expected X 100 %

Example: Mass of carbon monoxide you would expect to reduce 1000g of iron oxide Fe₂O₃ + 3CO 2Fe + 3CO₂

Fe₂O₃ CO

Mass(g) 1000 ? = 525

Mr 160 40

Mass/Mr 6.25 18.75 (3x6.25)

1 : 3

Combustion of methane

The breaking of the bong is endothermic (gives out heatThe formation of the bond is exothermic (takes in heat)

OilCrude oil is a mixture of hydrocarbonsFractional distillation is how you separate different fractions of oilThe larger the molecule the higher the boiling point

Alkanes and Alkenes

Alkanes are saturated hydrocarbons which means they only contain single bondsAlkenes are unsaturated hydrocarbons they have a double bondAlkenes can be produced from alkanes by cracking ( heating the alkane with a catalyst)

Polymers

Examples: polythene , polypropeneMonomers (ie ethene) are used to form polymersEthene can become polyethene by heating it under pressure

Thermosets and thermoplastics

Thermosets are plastics that are resistant to heat. Used for kettles, saucepan handles etc. Have strong cross linkages which hold the structure together and make them resistant to heat make them resistant to heatThermoplastics are plastics that soften when heated. Used for packaging, containers. Made up of polymer chains not linked together so they can slide over each other

Water TreatmentWhen the water is in the reservoir, the large particles will sink to the bottom (sedimentation), the smaller particles will be filtered.The water is chlorinated to kill the bacteria to make it safe to drink. Fluoride is also addedIt is then stored until it is needed

DesalinationDefinition: The removal of salt from sea water so it can be used for drinkingThis is usually done by reverse osmosis which goes through a membrane that lets water molecules through but not saltProblems- Lots of energy required- Produces a lot more greenhouse gases- Expensive- Very salty water is left when fresh water is used- Poor countries that have droughts cannot afford this- Some countries don’t have access to the sea

Extracting waterIf something is dissolved in water, you can extract it by evaporating & condensing To separate a liquid from water you need to use distillation. Different liquids have different boiling points . If you heat an ethanol and water mixture, the boiling point of ethanol is 78C so by heating the mixture at this temperature the ethanol will evaporate and when the ethanol vapour reaches the condenser it will be cooled and become a liquid

ChromatographyPaper Chromatography- a drop of mixture is placed on the Chromatography paper and placed in a solvent used to dissolve pigment. The solvent soaks into the paper and moves upwards. The more soluble will travel with the solvent and move further up the paper.The R value is used to measure this (distance substance travelled/ distance solvent travelled)

f

Gas Chromatography- can be used to detect pollutants in water or air and it is also used to test for banned substances in sports players. The mixture must be in the form of a gas (vaporised or naturally occurring.) The gas passes through a column and different substances are absorbed into an inert solid or liquid. The position the chemical moves along the column is detected electronically

Hard water

Hard water contains dissolved calcium and magnesium ions.Can be temporary or permanent Temporary hard water contains magnesium/calcium hydrogen carbonates . When heated the hardness is removed but it forms calcium carbonate (scale) which clogs up pipes.Permanent hard water contains chlorides/sulphates of calcium and magnesium which does not soften when heatedAdvantages of hard water -Some people prefer the taste-Calcium is good for teeth and bones-Helps reduce heart illness- Some brewers use hard water in beer

Disadvantages of hard water-Difficult to form lather with soap-Scum forms with soap- Scale forms, clogs pipes

and kettles

How to soften hard water1. Boiling- easy and cheap but doesn’t work for

permanent hardness2. Adding Sodium Chloride- prevents calcium and

magnesium ions bonding with washing detergent. Cheap, removes permanent hardness but deposits are still formed

3. Ion exchange column- Column filled with resin which has sodium ions. The calcium/magnesium ions are swapped for the sodium ions and come out of the tube with the water where as the calcium ions remain in the resin. Can treat a large amount of water but are more expensive.