ELEMENTS, ACIDS AND WATER

Chemistry

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  • Created on: 17-05-12 12:28

history of the periodic table

In the early 1800s they could go on atomic mass, they categorised the elements into their physical and chemical properties and their relative atomic mass.

Newlands Law of Octaves was the first good effort - he tried to arrange things more usefully in 1864. He notices that every 8th element had similar properties so he put them in rows of 7. (H, Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl, K, Ca, Cr, Ti, Mn, Fe). The pattern broke down on the third row with transition metals like titanium and iron messing it up. He bought hs ideas to the Chemical Society in 1865 but his work was critized because hs groups contained elements that didnt have similar properties (carbon and titanium), he mixed up metals and non metals (oxygen and iron) and he didnt leave any gaps for elements that hadnt been discovered yet.

Dmitri Mendeleev left gaps and predicted new elements - in 1869, Mendeleev in Russia was armed with about 50 known elements and put them into his table of elements with various gaps. He put the elements in order of atmoic mass but found he had to leave gaps in order to keep elements with similar properties in the same vertical groups. He left some very big gaps. The gaps were clever because they predited the properties of so far undiscovered elements. When they were found they fitted the pattern.

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the modern periodic table

Not all scientists thought that the periodic table was important - mant thought it was just a bit of fun because there wasnt much evidence to suggest that the elements really did fit together in some way. After Mendeleev released his work, newly discovered fitted into the gaps he left. Scientists found that the periodic table could be a useful tool for predicting properties of elements (it worked). In the late 19th century, scientists discovered protons, neutrons and electrons. The periodic table matches up to what has been found about the structure of the atom.

The modern periodic table is based on electronic structure - when electrons, protons and neutrons were discovered, the periodic table was arranged in order of atomic (proton) numbers. All elements were put into groups.

From the periodic table you can predict the elements chemical properties. Electrons in an atom are set out in shells which each correspond to an energy level. Apart from transition metals, elements in the same group have the same number of electrons in their highest occupied energy level. The positive charge of the nucleus attracts electrons and holds them into place; the further from the nucleus the electon is, the less the attraction. The attraction of the nucleus is even less where there are a lot of inner electrons. Inner electrons get in the way of the nuclear charge reducing the attraction. This effect is known as shielding.

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alkali metals

AS YOU GO DOWN GROUP 1, THE ALKALI METALS BECOME:

BIGGER ATOMS ... because theres one extra full shell of electrons for each row you go down. MORE REACTIVE ... because the outer electron is more easily lost, because its further from the nucleus. HIGHER DENSITY ... because the atoms have more mass. The three at the top are less dense than water, LOWER MELTING POINT and LOWER BOILING POINT.

  • The alkali metals are very reactive - they have to be stored in oil and handled with forceps (they burn the skin)
  • They are: lithium, sodium, potassium and a couple more - maybe rubidium and caesium
  • The alkali metals all have ONE outer electron - this makes them very reactive and gives them all similar properties
  • The alkali metals all form 1+ ions - they are keen to lose their one outer electron to form a 1+ ion
  • The alkali metals alway form ionic compounds - there are so keen to lose the outer electrion theres no way they would concider sharing so covalent bonding it out of the question
  • Reaction with water produces hydrogen gas - when lithium, sodium or potassium are put in water they react very vigorously, moving around the surface fizzing. They produce hydrogen, potassium gets hot eough to ignite it. A lighted splint will indicate hydrogen (squeaky pop)
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the halogens

AS YOU GO DOWN GROUP VII, THE HALOGENS HAVE THE FOLLOWING PROPERTIES:

1) LESS REACTIVE, 2) HIGHER MELTING POINT, 3) HIGHER BOILING POINT

  • The halogens are all non-metals with coloured vapours - fluorine is very reactive, poisonous yellow gas, chlorine is fairly reactive, poisonous dense green gas, bromine is a dense, poisonous, red-brown volatile liquid and iodine is a dark grey cyrstalline solid or purple vapour.
  • They all form molecules which are pairs of atoms - F₂Cl₂Br₂I₂
  • The halogens do both ionic and covalent bonding - the halogens from 1-ions when they bond with metals but they form covalent bonds with non-metals to form molecules like hydrogen chloride (HCl)
  • the halogens react with metals to form salts - they react with most metals including iron and aluminium to form salts (or metal halides)
  • more reactive halogens will displace less reactive ones - chlorine can displace bromine and iodine from a solution of bromide or iodide. Bromise will also displace iodine because of the trend in reactivity
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transition elements

Transiton metals make up the big clump of metals in the middle of the periodic table. Transition elements (or transition metals) are typical metals and have poperties you would expect of a 'proper' metal: they are good conductors of heat and electricity, they are very dense, strong and shiny, transition metals are much less reactive than Group 1 metals - they dont react very much with water or oxygen, for example. They are also much denser, stronger and harder than Group 1 metals and have much higher melting poits (except for mercury).

 Transition metals often have more than one ion - e.g Fe2+ and Fe3+. The different ions usally form different coloured compounds too, Fe2+ usually give green compounds whereas Fe3+ ions usually form red/brown compounts (rust)

The compounds are very colourful - 1) the compounds are colourful due to the transition metal ion they contain e.g. potassium chromate(VI) is yellow, potassium manganate(VII) is purple and copper(II) is sulfate blue. 2) the colour of peoples hair and also the colours in gemstones like blue sapphires and green emerals and the colours in pottery glazes are all due to transition metals. Weathered copper is a colourful green

Transition metals and their compounds all make good catalysts - 1) iron is the catalyst used in the Haber process for making ammonia, 2) manganese(IV) oxide is a good catalyst for the decomposition of hydrogen peroxide, 3) nickel is useful for turning oils into fats for making margarine.

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acids and alkalis

Arrhenius said acids release hydrogen ions in water - studies acids bases in water. His theory was that when mixed with water, all acids release hydrogen ions (an H+ ion is a proton). He said that alkalis form OH- ions (hydroxide ions) when in water. His idea only worked for acids and bases that dissolved in watrer. Ammonia gas can react as a base even when it isnt dissolved in water which is why his ideas werent immediately accepted. In the 1800s when Arrhenius first suggested that molecules ionise in water, scientists didnt believe it was possible. Charged subatomic particles hasnt been discovered yet, so the idea of charged ions seemed very strange.

Lowry and Bronsted said acids are proton donors - (working separately) made things a little more general. They came up with definitions that work for both soluble and insoluble bases -> ACIDS RELEASES H+ IONS - I.E THEY'RE PROTON DONORS. BASES ACCEPT H+ IONS - I.E THEY'RE PROTON ACCEPTORS. These ideas of Lowry and Bronsted were rradily accepted because they explained the behavious of acids and bases in solvents other than water. They were an adaptation of an idea which already kind of worked.

Protons are hydrated in water - In ACIDIC solutions: the acid molecules dissociate releasing lots of H+ ions, they become hydrated (surrounded by water molecules) so called 'hydrated protons'. In BASIC solutions: water molecules can dissociate into H+ and OH- ions althought they almost never do in pure water. Some base molecules like ammonia can take hydrogen ions from water causing more molecules to dissociate and leaving an excess of OH- ions behind. Potassium hydroxide (KOH) release hydroxide ions straight into the solution.

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acids, alkalis and titration

Acids can be strong or weak - strong acids (sulphuric, hydrochloric and nitric) ionise almost completely in water. This means almost every hydrogen atom is releases to become a hydrated proton (so theres loads of H+ (aq) ions). Weak acids (ethanoic, citricm carbonate) ionise only very slightly - only some of the hydrogen atoms in the compount are releases - so only a small number of H+ (aq) ions are formed.

Titrations are used to find out concentrations - they allow you to find out exactly how much acid is needed to neutalise a quantity of alkali (vice versa)

  • Put alkali in a flask with some indicator, the indicator used depends on the strengths on the acid and alkali (Phonolphthalien used for a weak and strong acid and Methyl Orange used to strong acid and weak alkali. If both the acid and the alkali are strong ANY acid-base indicator can be used). Add the acid, a bit at a time to the alkali using a burette giving the flask a regular swirl. Go espcially slowly unless alkali almost neutralised. The indicator changes colour when all the alkali has been neutralised. Record amount of acid used to neutralise the alkali (repeat process!!!)
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titration calcultions

Calculating the concentration:

  • start with 25cm³
  • concentration is 0.1 moles per dm³
  • find from titration it takes 30cm³ of sulphuric acid

Number of moles = Concentration x Volume. (1000cm³ = 1dm³) = 0.1 moles per dm³ x (25/1000) dm³ = 0.0025 moles of NaOH

For everyone two moles of sodium hydroxide you had, there was just one mole of sulfuric acid.

0.0025 moles of sodium hydroxide, so ... 0.0025/2 = 0.00125 moles of sulfuric acid

Concentration = Number of moles/Volume = 0.00125 mol / (30/1000) dm³ = 0.041666.. mol/dm³ = 0.0417 mol/dm³

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water

THE WATER CYCLE MEANS WATER IS ENDLESSY RECYCLED:                        The sun causes evaporation of water from the sea, the water vapour is then carried upwards as the warm air rises. As it rises it cools due to the general cooling of the lower part of the atmosphere at higher altitudes. This fall in temperature means the water condenses to form clouds. When the condensed droplets get too big they fall as rain. Then he water runs back into the sea (at some stage the water is going to come into contact with rocks on (or underneath) the ground meaning that the water in different places with dissolve minerals). Then the cycle starts over again.

Waters a solvent - It dissolved many other chemicals: Many substaces dissolve in water that sometimes water is called the universal solvent. Water dissolves into MOST IONIC COMPOUNDS (they surround the ions and disrupt the ionic bonding so the solid structure of the ionic compound gradually falls apart). Water molecules are POLAR (they have a positive hydrogen side and a negative oxygen side). The following ionic compounds dissolve in water:

  • Salts SODIUM (Na), POTASSIUM (K), or AMMONIUM (NH4). All of these dissolve.
  • Nitrates (NO3). All of these dissolve.
  • CHLORIDES (Cl), except for silver and lead.
  • SULFATES (SO4), except for barium and lead. Calcium sulfate is only slightly soluble.

Many substances that exist as small molecules are soluble in water. Many covalent compounds dont dissolve - they dont form ions and their molecules are too big.

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solubility

  • The solubility of a substance in a given solvent is the number of grams of the solute (usually a solid) that dissolve in 100g of the solvent (the liquid) at the particular temperature.
  • The solubility of (solid) solutes usually increases with temperature.
  • A saturated solution is one that cannot hold any more solid at that temperature - and you have to be able to see solid on the bottom to be certain that its saturated.

Solubility curves show when a solution is saturated: it plots the mass of solute dissolved in a saturated solution at various temperatures. The solubility of most solids increase as the temperaure increases.This means that cooling a saturated solution will usually cause some solid to cystallise out (separates from the solution). The mass of the cryslals formed by cooling a solution a certain amount can be calculated from a solubility curve.

All gases are soluble - to some extend: chlorine water is a solution of chlorine gas in water, it is used to bleach in the paper and textile industries and to sterilse water supplies (it kills bacteria). The amount of gas that dissolves depends on the pressure of the gas above it - ther higher the pressure, the more gas that dissolves. Gases become less soluble as the temperature of the solvent increases which is exactly the opposite of solids.

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hard water

Hard water makes scum and scale: hard water wont easily lather with soap. The problem is 'hardness minerals' in the hard water reacting with the soap. It forms furring or scale (mostly calcium carbonate) on insides of pipes, boilers and kettles. Badly scaled up pipes and boilers reduce the efficiency of heating systems and may need to be replaced which is expensive. Scale can eventually block pipes. It is also a bit of a thermal insulator. A kettle with scale on the heating element takes longer to boil than a clean kettle (less efficient).

It is caused by Ca2+ and Mg2+ ions: contains lots of calcium and magnesium ions. You get hard water in certain areas because of the type of rocks there. Hardness often comes from limestone, chalk and gypsum.

Isnt all bad: Ca2+ ions are good for healthy teeth and bones, and scale inside the pipes forms a protective coating. It stops poisionous metal ions getting into drinking water. It also protects iron pipes from rust.

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water quality

Drinking water NEEDS to be good quality! Water is essential for life, but it must be free of poisonous salts and harmful microorganisms which can cause diseases such as cholera. Most of our drinking water comes from reservoirs, water slows into reserviors from rivers and groundwater.

Water from reservois goes to the water treatment works for treatment: the water passes through a mesh machine to remove big bits like twigs. Next its treated with ozone or chlorine to kill microorganisms. Chemicals are added to make solids and microorganisms stick together and fall to the bottom. Sometimes iron is added to remove dissolved phosphates. Bacteria are used to remove nitrates. The water is filtered through gravel beds to remove all the solids. Nasty tastes and odours can also be removed by passing the water through 'activated carton' filters or with 'carbon slurry'. The pH is corrected if the water is too acidic or too alkaline. Water is chlorinated to kill off any harmful microorganisms left.

To monitor water quality, water companies take sample of water. Totally pure water with nothing dissolved in it can be produced by DISTILLATION - boiling water to make steam and condensing the steam. This process is too expensive to produce tap water.

Not everyone has clean water. The WORLD HEALTH ORGANISATION (WHO) and the United Nations estimated in 1995 a billion people in the world dont hae access to clean water. In developing countries ts very expensive to get clean water and have to walk miles to get any water at all. Some water purifying processes can damage the environment.

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