Chemistry PLONKNOTES 2010 EYAS (: Elements Compounds Mixtures - Pure substance: fixed boiling and melting point If not pure (impurities), melting point decreases and boiling point increases. - Moatomic elements: one atom. E.g. diatomic, triatomic, polyatomic. - Cannot be split up into 2 or simpler substances by chemical processes or electricity. - Heat can be used to break down elements (thermal decomposition) - Separated by chemical reactions or by electricity - Physical and Chemical properties different from elements in the compound. - Chemical reaction takes place when formed, usually there is an energy change, and reactants get hot. - Fixed proportion (by mass) - When 2 substances are added together without chemical bonds. - Alloy is an example. - Not fixed in any ratio. - Separated by physical method, e.g. filtration, distillation or chromatography. - Chemical properties are same as components. - No chemical reaction when mixture is formed little energy change. A vapour is a gas that exists at room temperature and pressure. Gas of lighter mass, small size and higher temperature diffuses faster. Chemical reactions Takes place at a molecular level Explosion/flame Rusting New substance formed e.g. HCL + NaOH H2O + NaCL Hints that chemical reactions take place: light, heat, colour change, gas production, odour, or sound. Physical reactions Concerned with energy and states of matter Crushing a can, breaking a bottle. Start and end products are the same despite the difference in appearance. Kinetic Particle Theory - In a heating curve, the constant temperature would be where state changes occur, and both states of the substance are present. - BOILING: throughout liquid, EVAPORATION: only at surface. - A lump of sugar dissolves faster in hot water than cold water (water particles vibrate more vigorously when gaining heat. The sugar particles can then fit in between the water particles, dissolving.) - Ability to conduct differs: o Spaces in between particles o Frequency of collision o More frequent successful collisions: Transfer of heat - TEMPERATURE is the amount of kinetic energy possessed by the particles. 1
- Anything that has mass will have a force of attraction with something next to it (close enough) UNIVERSAL LAW. - Melting is not instantaneous. The outer layer melts first. ATOMS AND MOLECULES - Salt has a very strong bond when it is sitting on your table. It would be nearly impossible to break those ionic bonds. However when you put it in water, the bonds is overcome because of the electrical attraction of the water. - In the presence of other ions, the ionic bonds are weaker because of outside electrical forces and attractions. - Monatomic elements: helium, neon, argon, krypton, xenon and radon. SEPARATION TECHNIQUES - Chromatography o Separate dyes in ink, pigments in plants, amino acids obtained from proteins, identify poisons or drugs and detect traces of banned substances. - Decanting o Separate dense, insoluble solid from a liquid - Filtration o Separate small particles from a liquid (or something dissolved in a solution) o RESIDUE: solid that remains on filter paper o FILTRATE: liquid or solution that passes through. - Evaporating to dryness o Not ideal, any soluble impurities will be left on crystals. o DO NOT HEAT THE SOLUTION: the substance may decompose upon being heated. o Decompose: breaking down to form simpler substance. - Crystallisation o Water is removed by heating (take note: SUBSTANCE MAY DECOMPOSE.) o Cool down hot saturated solution, dissolved solid will be formed as pure crystals. o FAST COOLING: small but many crystals o slow cooling: LARGE BUT FEW CRYSTALS o Solid solubility must vary according to temperature. o Saturated solution: No more solute can be dissolved in the solution. o Obtain PURE substances (purification): only same material, molecules will crystallize. o How does temperature affect? Sugar molecules go in between water molecules (separated into individual molecules small enough) - Distillation o Boiling points at least 20 degrees Celsius apart. 2
o TAKE NOTE: thermometer not dipped into solution o If volatile, put ice to keep temperature low to remain liquid ice bath OR smaller beaker, less evaporating surface. o Separate solvent from solution o Boiling chips ensure smooth boiling. - Separating funnel o Separate immiscible liquids o Use another container in the middle part to dispose of the centre meeting of e.g. oil and water. o SOLVENT EXTRACTION: If substance is soluble in the 2 selected solvents, one must be more soluble than the other: e.g. iodine is more soluble in CCL4 than WATER. Redistribute the solution and shake, so it becomes IODINE + WATER IODINE + CCL4 Repeat to collect more iodine Evaporate the CCL4 to obtain solid iodine (CCL4 is volatile.) Iodine will sublime (become gas) followed by water, so its not possible. - Fractional Distillation o Wider surface area to allow successive condensation to occur. o Lowest boiling point distils over to condenser first. o Higher boiling point condenses along fractionating column and re-enter round-bottomed flask. o Temperature remains constant until the lower boiling point substance has distilled out of the round-bottomed flask. - Magnetic: Magnetic properties - Sublimation: One substance sublimes, the other doesn t. o Substances that sublime: Iodine, Dry Ice, Naphthalene (moth balls), Ammonium chloride. - EXTRA! Centrifugal o Lighter objects will move to the top, dense substances sink to the bottom of the tube. ADSORPTION Accumulation of molecules of a gas to form a thin film on the surface of a solid, it refers to a surface instead of a volume like in absorption. Boiling chips are small, insoluble, porous stones made of calcium carbonate or silicon carbide. They have pores inside which provide cavities to trap air. When a boiling chip is heated, it releases tiny bubbles to ensure even boiling and prevent bumping and boiling over and loss of the solution. ATOMIC STRUCTURE - Proton/atomic number o Number of protons in an atom (electrons also) - Nucleon number 3
o Total number of protons and neutrons in an atom - ISOTOPES: o Atoms of the same element with the same number of protons but different number of neutrons. o Same number of electrons o Isotopes have the same chemical properties but slightly different physical properties. o Chemical reactions only involve electrons. o Physical properties (higher boiling point, density) differ because the relative masses differ. - Would the energy between the first and second atomic model be different? NO. o The further away from the nucleus, the lesser the energy. o Electrons are held in position with the force of attraction between electrons and the nucleus. o The outer and innermost electrons repel. o The inner electrons shield the electrons outside from the full force of the attraction between nucleus and electrons. o Gap between shells are also not uniform. o Electrons are usually not paired unless cannot be helped because they repel. - Why are metallic elements positive? o Easier for the few valence electrons to get rid of them than exert a greater force to pull so many electrons. - PURE METALS are of the same size so it is easy for the layers of atoms to slide over one another. ELECTRICAL CURRENT - A flow of electrons in a particular direction. - In metals, mobile electrons are moving in random directions. - When circuit is closed, electrons move in same direction. - 2 reasons cations are smaller than atoms THINGS TO NOTE: o Electrons are transferred out but protons remain the same. More electrostatic force is allocated to each electron to be pulled towards the atomic nucleus. Since the number of electrons are lesser, more force of attraction is pulling the distance between them to become smaller. o Valence electrons become empty, one shell smaller. o Bromine, mercury and water are the only liquids. Sugar decomposes upon being heated to form carbon (black). Solid has higher melting and boiling point but liquid has lower melting and boiling point. Pure gold can be bitten into hence jewellery is not pure gold. 4
Electricity passing through rare gases produces colourful lights. COMMON MOLECULES: o CH4 methane Ammonia is found in fertilizers and hair dyes. Boiling point of a liquid depends on the atmospheric pressure in the room o Energy is used to push back the atmosphere and this pushing back is dependent on how much pressure is acting on the liquid. o Air pressure is low reaches boiling point faster Reactive metals Unreactive metals QUESTIONS Copper Tribasic means that 1 acid molecule dissolves in water to give 3 H+ ions. WHY DID THE TEMPERATURE REMAIN CONSTANT? o It is boiling and IT IS A PURE LIQUID. WHY STRONG BONDS? o HIGH melting and boiling points ELECTROSTATIC FORCES OF ATTRACTION/IONIC BOND: between oppositely charged ions ELECTROSTATIC FORCES OF ATTRACTION/METALLIC BONDING: between positively charged ion and negatively charged sea of delocalised electrons. CO2 is a non-metallic oxide (other than H2O and CO being neutral) as it is a non-metallic oxide which dissolves in water to form an acidic solution (carbonic acid). GROUP 1: o Low density and hence is soft o Extremely reactive (since they can lose their 1 valence electron very easily) o Reactive with air and water means that IT WILL FORM THE OXIDE VERY QUICKLY WHEN EXPOSED TO AIR (easily oxidized to its ions) and will REACT VIGOROUSLY WITH WATER to give the metal hydroxide (easily loses electrons to form the metal ion). Dissolving is for substances which do not react with the solvent they are placed in. Metal + water Metal hydroxide + hydrogen gas Decomposition we usually look at solid substances. Water is already present in the beginning hence even Bromine as a produce will be AQUEOUS. THERE IS NO WATER PRESENT IN A PURE COMPOUND hence it s LIQUID not AQUEOUS. 1. Collecting hydrogen gas: a. CANNOT DISSOLVE IN WATER SO DOWNWARD DISPLACEMENT OF WATER. b. Hydrogen gas is lighter than air, so delivery tube pointing upwards into an overturned test tube. 5
2. Fractional distillation: SMALL DIFFERENCE, MISCIBLE LIQUIDS. 3. Suggest a reason why one dye moves further than the other dye from the centre of the filter paper. a. The greater the solubility in a solvent, the further the dye will travel. State which one more soluble. 4. In iron, the positively charged ions are held by strong electrostatic forces of attraction and are surrounded by a sea of delocalized electrons in a giant metallic lattice structure. Fe3+ and CL- are oppositely charged and are held by electrostatic forces of attraction in a giant ionic lattice structure. 5. PURE: fixed boiling point, IMPURITY: boil over range of temperatures. 6. MAGNESIUM RIBBON + HCL: BUBBLES, AMMONIUM CHLORIDE + SODIUM HYDROXIDE: damp red litmus paper blue (gas), WATER WITH MAGNESIUM OR AMMONINUM CHLORIDE: NO VISIBLE CHANGE. 7. WATER BATH reduces probability of salt decomposing for CRYSTALLISATION. a. OR use oil controlled crystallisation (slower) 8. Why pencil line and ink spot above the solvent? a. If the line is below the solvent, it may spread among the solvent also and may not definitely travel straight upwards. 9. Why sit the starting line in pencil? a. Pen ink may react with the solvent and it will cause the unreliability of the experiment as it may spread. 10. AIR POCKETS IN WOOD, hence although it is solid(high DENSITY), it can still float. 6