- Created by: Rebecca
- Created on: 21-05-12 15:11
THE PERIODIC TABLE
- Elements are arranged in order of ATOMIC NUMBER. They are arranged in rows so that elements with similar properties are in the same columns, or groups.
- Elements in the same group have the same number of electrons in their outermost shell (except helium). E.g. Group 1 elements have 1 electron in their outermost energy level, Group 2 elements have 2 electrons, etc.
- From left to right across each row, a particular energy level is gradually filled with electrons.
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GROUP 1- THE ALKALI METALS
- There are six metals in this group. As we go down the group, the alkali metals become more reactive.
- They have low melting points. The melting and boiling points decrease as we go down the group.
- They are stored under oil as they react vigorously with oxygen and water.
- When alkali metals react with water a metal hydroxide and hydrogen gas are formed. The metal hydroxide (e.g. lithium hydroxide, sodium hydroxide) dissolves into water to form an alkaline solution, e.g:
Potassium + Water ---> Potassium Hydroxide + Hydrogen
- When alkali metals react with non-metals to form ionic compounds the metal atom loses one electron to form a meral ion with a positive charge. The products are white solids that dissolve in water to form colourless solutions, e.g:
Sodium + Chlorine ---> Sodium Chloride
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GROUP 7- THE HALOGENS
- There are five non-metals in this group. As we go down the group, the melting and boiling points of halogens increase so they become less reactive.
- At room temperature, fluorine and chlorine are gases and bromine is a liquid.
- They exists as molecules made up of pairs of atoms.
- They are brittle and crumbly when solid and poor conductors of heat and electricity.
- Halogens react with metals to produce ionic salts. The halogen atom gains one electron to form a halide ion which carries a charge of -1.
- They react with other non-metallic elements to form molecular compounds.
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THE TRANSITION METALS
- Placed in the centre of the periodic table, between groups 2 and 3.
- It is a block of metallic elements which are:
- Hard and mechanically strong.
- Have high densities and high melting points.
- Are much less reactive than Group 1 metals.
- Form coloured compounds which can be used as pottery glazes and can be seen in weather copper which turns green.
- The ions of some of the elements have different charges.
- The transition elements have similar properties and some special properties because a lower energy level (inner shell) is being filled in the atoms of the elements between group 2 and 3. This is because a third energy level can hold up to 18 electrons once two electrons have occupied the fourth.
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ACIDS AND ALKALIS
- All ACIDS in aqueous solution, dissociate to produce HYDROGEN IONS (H+). The hydrogen ion is simply a proton and in water this proton is hydrated (i.e. chemically bounded to water). It is the presence of the Hydrogen ions that gives the solution its acidic characteristics, e.g. hydrochloric acid:
HCl ---> H+ + Cl-
- All ALKALIS, however, in aqueous solution, dissociate to produce HYDROXIDE IONS (OH-). This time the presence of the hydroxide ions gives the solution its alkaline characteristics, e.g. sodium hydroxide:
NaOH ---> Na+ + OH-
- An acid can be defined as a proton donor and a base can be defined as a proton acceptor.
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STRENGTH OF ACIDS AND ALKALIS
- Acids and alkalis are classified by the extent of their ionisation in water.
- A strong acid or alkali is one that is completely ionised in water (all of the compound dissociates into ions).
- A weak acid or alkali, however, is one that is only partially ionised in water (not all of the compound dissociates into ions)
- The concentration of an aqueous solution is usually expressed by stating how many moles of a particular solute are present in each cubic decimetre of solution. Concentration is measured in moles per cubic decimetre (mole dm-3 or M). One cubic decimetre is the same as 1 litre.
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- Titration is a technique used to find out how much of an acid is needed to neutralise an alkali.
1) A pipette (which has been washed and rinsed with the alkali) is used to measure out a known volume of the alkali. The alkali is then placed in a conical flask and indicator is added.
2) The acid is placed in a burette (which has been washed with the acid), an initial reading of the acid volume is taken.
3) The acid is carefully added to the alkali until the indicator changes colour to show neutrality. A final reading is taken of the volume of acis in the burette which can then be used to calculate to volume of acid added.
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THE WATER CYCLE
- ThHeat energy from the Sun causes water in rivers, lakes and oceans to rise via evaporation.
- As the water vapour rises higher into the atmosphere, it cools and condensation occurs forming droplets of water which collect together to form clouds.
- As the clouds raise higher, the temperature drops furthur and rain is produced when the droplets are big enough.
- The rain then falls onto the land, drains into rivers and flows into the sea, and the cycle begins again.
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- In order to produce water which is good quality and safe to drink the water is passed through a filter bed to remove any solid particles then chlorine gas is added to kill any harmful microorganisms.
- Any water can be distilled to produce pure water which has no dissolved substances in it. This process involves boiling water to produce steam and condensing the steam by cooling it to produce pure liquid water.
- If a solid dissolves in water it is said to be SOLUBLE. The solid which dissolves is called the SOLUTE and water is described as the SOLVENT.
- Dissolving carbon dioxide under high pressure produces carbonated water.
- Oxygen dissolves in water and this dissolved oxygen in essential for aquatic life.
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- Hard water is water which contains CALCIUM or MAGNESIUM ions, which dissolve in natural water that flows over ground or rocks containing compounds of these elements. These react with soap to form scum.
- The advantages are:
-Hard water is good for your health, e.g. calcium compounds help in the development of strong teeth and bones.
-They help to reduce the development of heart diseases.
- The disadvantages are:
-More soap is needed to form a lather (increased costs).
-Often leads to deposits (called scale) forming in heating systems and appliances like kettles. This reduces their efficiency.
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REMOVING HARDNESS IN WATER
- To make hard water soft, we have to remove the dissolved calcium and magnesium ions contained in it. To do this we can:
- Add sodium carbonate solution (washing soda). The carbonate ions react with the calcium and magnesium ions to form calcium carbonate and magnesium carbonate which precipitate out of soution as they are both insoluble.
- Pass the hard water through an ion-exchange column. The column contains a special resin which supplies hydrogen ions or sodium ions. As the hard water passes through the resin, the calcium and magnesium ions contained in it are replaced by hydrogen or sodium ions from the resin. The calcium and magnesium ions consequently remain in the resin.
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JOULES AND CALORIES
- When any chemical change takes place it is accompanied by an energy change.
- If heat is given out it is an EXOTHERMIC REACTION.
- If heat is taken in it is an ENDOTHERMIC REACTION.
- The unit of measurement for energy is JOULES (J). It takes 4.2 joules of energy to heat up 1g of water by 1C. This amount of energy is called 1 calorie.
- If we eat too much food or don't exercise enough, the excess energy is stored in our bodies as fat, which leads to problems like obesity and heart disease.
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- The method of measuring the relative amounts of heat energy released by burning a substance is called CALORIMETRY.
- To measure the temperature change that takes place when a fuel burns you need to:
- Place 100g of water in a calorimeter and take the water temperature.
- Find the mass of the fuel to be burnt.
- Burn the fuel under the water in the calorimeter for a few minutes and record the temperature change of the water.
- Weigh the fuel again to calculate how much fuel has been used.
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MAKING AND BREAKING BONDS
- In a chemical reaction, new substances are produced. In order to do this, the bonds in the reactants must be broken and new bonds made to form the products.
- Breaking a chemical bond requires a lot of energy and is an endothermic process.
- When a new chemical bond is formed, energy is given out so is an exothermic process.
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ENERGY LEVEL DIAGRAMS
- EXOTHERMIC PROCESSES.
- ENDOTHERMIC PROCESSES.
- ACTIVATION ENERGY
- The energy needed to start a reaction (to break the old bonds).
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- LITHIUM- RED
- SODIUM- YELLOW
- POTASSIUM- LILAC
- CALCIUM- BRICK RED
- BARIUM- APPLE GREEN
- COPPER- BLUE/GREEN
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CARBONATES & DILUTE ACID and THERMAL DECOMPOSITION
- Carbonates react with dilute acids to form carbon dioxide gas. Example:
CALCIUM CARBONATE + HYDROCHLORIC ACID ---> CALCIUM CHLORIDE + CARBON DIOXIDE + WATER
- When copper carbonate and zinc carbonate are heated, a thermal decomposition reaction takes place. This results in a distinctive colour change which enables the two compounds to be identified.
- Copper Carbonate turns BLACK as Copper Oxide is formed.
CuCO3 (green) ---> CuO (black) + C02
- Zinc Carbonate turns YELLOW as Zinc Oxide is formed.
ZnCO3 (white) ---> ZnO (yellow) + CO2
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- Metal compounds in solution contain metal ions. Some of these form precipitates.
Metal Ion + Sodium hydroxide Precipitate Formed Colour
- Aluminium + Sodium hydroxide Aluminium hydroxide White
- Calcium + Sodium hydroxide Calcium hydroxide White
- Magnesium+ Sodium hydroxide Magnesium hydroxide White
- Copper + Sodium hydroxide Copper hydroxide Blue
- Iron (Fe2+)+ Sodium hydroxide Iron hydroxide Green
- Iron (Fe3+)+ Sodium hydroxide Iron hydroxide Brown
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IDENTIFYING DIFFERENT IONS
- SULFATE IONS.
- Add Barium Chloride Solution and dilute Hydrochloric Acid to the solution. A white precipitate of Barium Sulfate will be produced if a sulfate is present.
- NITRATE IONS.
- Add Aluminium Powder and Sodium Hydroxide Solution. If a nitrate is present the nitrate ions are reduced to form ammonia which which turns damp litmus paper blue.
- CHLORIDE, BROMIDE AND IODIDE IONS.
- Add Silver Nitrate Solution and Nitric Acid. A white precipitate will form is silver chloride is present, a cream precipitate for silver bromide and a yellow precipitate for silver iodide.
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- Rapid Results.
- Very sensitive and accurate.
- Small samples can be used.
- Easily automated and computerised.
- Staff do not need to be as highly trained to interpret results.
- Machines are expensive to buy, run and maintain.
- The sample must be completely pure.
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